is time travel legal

The Enlightened Mindset

Exploring the World of Knowledge and Understanding

Welcome to the world's first fully AI generated website!

Is Time Travel Illegal? An In-depth Look at the Legal Implications of Time Travel

By Happy Sharer

Introduction

Time travel is a concept that has been present in science fiction for decades. From Doctor Who to Back To The Future, these stories often explore the idea of travelling through time, either for entertainment or for more serious purposes. But is time travel actually illegal? This article will explore the legal implications of time travel, examining the laws that govern it and looking at the potential consequences of breaking those laws.

Examining Laws Across Time & Space: Is Time Travel Illegal?

When it comes to time travel, there are few laws that explicitly state whether it is allowed or not. Instead, the legality of time travel is determined by the laws that govern space and time. These laws vary from country to country, but they all have one thing in common: they prohibit any activity that may interfere with the normal flow of time.

For example, in the United States, federal laws prohibit the use of time travel devices without a license. In the UK, the Offences Against the Person Act 1861 makes it illegal to use time travel technology. This means that, even if you have the right equipment, you could still be breaking the law if you attempt to travel through time.

However, the legality of time travel is not always clear cut. Some countries have laws that specifically address the issue, while others simply refer to laws that cover other areas such as space exploration or physics. This means that the legality of time travel can vary depending on where you are located.

Exploring the Legal Implications of Time Travel: Is It Allowed?

The legal implications of time travel depend largely on the laws of the country in which you are travelling. For example, in some countries, the laws governing time travel are stricter than those governing space exploration. This means that, even if you have the right equipment, you could still be breaking the law if you attempt to travel through time.

In addition to the laws that govern time travel, there are also ethical considerations to take into account. For instance, if you were to travel back in time, you could potentially disrupt the timeline and cause irreparable damage to the future. This could mean that, even if you were able to travel through time legally, the consequences could be far-reaching and devastating.

Can You Break the Law by Travelling Through Time?

The answer to this question depends largely on the laws of the country in which you are travelling. In some countries, time travel is strictly prohibited, while in others it is only illegal when used for malicious purposes. In either case, it is important to understand the laws governing time travel in your area before attempting to travel through time.

In addition to understanding the local laws, it is also important to consider the ethical implications of time travel. As mentioned above, travelling through time could have far-reaching consequences, and it is important to consider these carefully before deciding whether or not to attempt time travel.

Is Time Travel a Crime? An Analysis of Current Laws

In most countries, time travel is not considered a crime. However, there are some exceptions. For instance, in the United States, the Federal Bureau of Investigation (FBI) considers time travel to be a form of tampering with evidence, and thus it is illegal. In the UK, the Offences Against the Person Act 1861 makes it illegal to use time travel technology for malicious purposes.

In addition to the laws governing time travel, there are also potential repercussions for those who attempt to travel through time. These can include fines, imprisonment, and even death. It is therefore important to consider the potential consequences of time travel before attempting it.

Time travel is a fascinating concept, but it is important to understand the legal and ethical implications before attempting it. Laws governing time travel vary from country to country, but they generally prohibit any activity that interferes with the normal flow of time. In addition, there are potential repercussions for those who attempt to travel through time, including fines, imprisonment, and even death.

If you are considering travelling through time, it is important to research the laws in your area and to consider the potential consequences before making a decision. By understanding the legal and ethical implications of time travel, you can make an informed decision about whether or not to attempt it.

(Note: Is this article not meeting your expectations? Do you have knowledge or insights to share? Unlock new opportunities and expand your reach by joining our authors team. Click Registration to join us and share your expertise with our readers.)

Hi, I'm Happy Sharer and I love sharing interesting and useful knowledge with others. I have a passion for learning and enjoy explaining complex concepts in a simple way.

Related Post

Exploring japan: a comprehensive guide for your memorable journey, your ultimate guide to packing for a perfect trip to hawaii, the ultimate packing checklist: essentials for a week-long work trip, leave a reply cancel reply.

Your email address will not be published. Required fields are marked *

Expert Guide: Removing Gel Nail Polish at Home Safely

Trading crypto in bull and bear markets: a comprehensive examination of the differences, making croatia travel arrangements, make their day extra special: celebrate with a customized cake.

An official website of the United States government.

Here’s how you know

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• American Rescue Plan
• Coronavirus Resources
• Disability Resources
• Disaster Recovery Assistance
• Equal Employment Opportunity
• Guidance Search
• Health Plans and Benefits
• Registered Apprenticeship
• International Labor Issues
• Labor Relations
• Leave Benefits
• Major Laws of DOL
• Other Benefits
• Retirement Plans, Benefits and Savings
• Spanish-Language Resources
• Termination
• Unemployment Insurance
• Veterans Employment
• Whistleblower Protection
• Workers' Compensation
• Workplace Safety and Health
• Youth & Young Worker Employment
• Breaks and Meal Periods
• Continuation of Health Coverage - COBRA
• FMLA (Family and Medical Leave)
• Full-Time Employment
• Mental Health
• Office of the Secretary (OSEC)
• Administrative Review Board (ARB)
• Benefits Review Board (BRB)
• Bureau of International Labor Affairs (ILAB)
• Bureau of Labor Statistics (BLS)
• Employee Benefits Security Administration (EBSA)
• Employees' Compensation Appeals Board (ECAB)
• Employment and Training Administration (ETA)
• Mine Safety and Health Administration (MSHA)
• Occupational Safety and Health Administration (OSHA)
• Office of Administrative Law Judges (OALJ)
• Office of Congressional & Intergovernmental Affairs (OCIA)
• Office of Disability Employment Policy (ODEP)
• Office of Federal Contract Compliance Programs (OFCCP)
• Office of Inspector General (OIG)
• Office of Labor-Management Standards (OLMS)
• Office of the Assistant Secretary for Administration and Management (OASAM)
• Office of the Assistant Secretary for Policy (OASP)
• Office of the Chief Financial Officer (OCFO)
• Office of the Solicitor (SOL)
• Office of Workers' Compensation Programs (OWCP)
• Ombudsman for the Energy Employees Occupational Illness Compensation Program (EEOMBD)
• Pension Benefit Guaranty Corporation (PBGC)
• Veterans' Employment and Training Service (VETS)
• Wage and Hour Division (WHD)
• Women's Bureau (WB)
• Agencies and Programs
• Meet the Secretary of Labor
• Leadership Team
• Budget, Performance and Planning
• Careers at DOL
• Privacy Program
• Recursos en Español
• News Releases
• Economic Data from the Department of Labor
• Email Newsletter

Travel Time

Time spent traveling during normal work hours is considered compensable work time. Time spent in home-to-work travel by an employee in an employer-provided vehicle, or in activities performed by an employee that are incidental to the use of the vehicle for commuting, generally is not "hours worked" and, therefore, does not have to be paid. This provision applies only if the travel is within the normal commuting area for the employer's business and the use of the vehicle is subject to an agreement between the employer and the employee or the employee's representative.

Webpages on this Topic

Handy Reference Guide to the Fair Labor Standards Act - Answers many questions about the FLSA and gives information about certain occupations that are exempt from the Act.

Coverage Under the Fair Labor Standards Act (FLSA) Fact Sheet - General information about who is covered by the FLSA.

Wage and Hour Division: District Office Locations - Addresses and phone numbers for Department of Labor district Wage and Hour Division offices.

State Labor Offices/State Laws - Links to state departments of labor contacts. Individual states' laws and regulations may vary greatly. Please consult your state department of labor for this information.

April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

If you liked this, you may like:

Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

Disclosure statement

Peter Watson received funding from NSERC. He is affiliated with Carleton University and a member of the Canadian Association of Physicists.

Carleton University provides funding as a member of The Conversation CA.

Carleton University provides funding as a member of The Conversation CA-FR.

View all partners

• Bahasa Indonesia

Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

• Time travel
• Stephen Hawking
• Albert Einstein
• Listen to this article
• Time travel paradox
• Arthur Eddington

Research Fellow – Magnetic Refrigeration

Centre Director, Transformative Media Technologies

Postdoctoral Research Fellowship

Social Media Producer

Dean (Head of School), Indigenous Knowledges

• Gameumentary
• Review in 3 Minutes
• Design Delve
• Extra Punctuation
• Zero Punctuation
• Area of Effect
• Escape the Law
• In the Frame
• New Narrative
• Out of Focus
• Slightly Something Else
• Escapist Staff
• Privacy Policy
• Terms of Use
• Affiliate Policy

Time Travel Torts: How Law Gets Dicey When Dealing with Groundhog Day

Time travel is weird . We saw it in All You Zombies where the lead character became his own mother, father, lover, kidnapper, and bartender. We also saw it in Primer , which is seemingly impossible to understand, even after reading a book’s worth of flow charts , essays , and attempted explanations . What makes time travel so confusing is that it warps the laws of physics and requires us to revisit everything we thought we knew about the world.

But time travel doesn’t just warp the laws of physics — it warps the law itself. In the past, I’ve explained how time travel obliterates statutes of limitations and how the criminal justice system is powerless in the face of time travel murder. This week, I’m going to take it one step further to explain a legal time paradox — or how time travel can turn widely accepted legal principles on their head.

For the purposes of this article, I’ll be discussing instances in which time travelers have the ability to repeatedly travel back in time, to change the timeline based on previous time trips, and to observe the consequences of their actions. This would apply to the kinds of time travel seen in Groundhog Day , 11/22/63 , and Prince of Persia: The Sands of Time , but it would not apply to the time travel seen in The Terminator , A Connecticut Yankee in King Arthur’s Court , or Avengers: Endgame .

Understanding the Legal Principles

There are two legal principles that are relevant to this analysis. The first principle is that the law does not require individuals to help strangers in need. For example, drivers are not required to help hitchhikers reach their destination, nor is a person required to call an ambulance if they see a person collapse. (I considered this rule in my previous column on transporter liability ). Instead, requirements to help others arise from preexisting relationships. To list just a few examples, parents have an obligation to help their children, doctors have an obligation to help their patients, and innkeepers have an obligation to protect their patrons. Generally speaking, in the absence of a special relationship, the only obligation one has is an obligation not to harm someone else. We can call this the Hippocratic principle. (Lawyers call it “duty.”)

The second principle is that, when it comes to avoiding harm, the law is concerned with predictability rather than personal responsibility. As an example, suppose Alice drives to a restaurant to pick up a carryout order. Alice doesn’t expect to be away from her car very long, so she leaves the keys in the ignition. While Alice is away, Bob steals her car, drives for a few blocks, and then collides with another driver. In several states, the law would hold Alice responsible for the damage caused by Bob — even though Alice did not steal a car and did not drive recklessly.

Why? Because (according to the courts) it is reasonably foreseeable that a thief would steal a readily available car and reasonably foreseeable that a thief would drive negligently in his efforts to flee. Thus, it was negligent of Alice to leave her keys in the car, and she can be held liable for any damage that would reasonably follow from that mistake.

In other words, the fact that the harm was imposed by another person is not as important as the fact that it was reasonably foreseeable that the harm would take place. We can call this the foreseeability principle. (Lawyers call it “proximate cause.”)

Applying Law to Time Travel (and Groundhog Day)

The Hippocratic principle and the foreseeability principle are well-established and can be applied consistently and (mostly) predictably. Time travel, though, changes everything.

What makes time travel special is that time travelers have access to more information than regular people — a time traveler can take an action, see how it plays out, and then, if the outcome is not desirable, go back and try again. Thus, in the world of a time traveler, the concept of “reasonable foreseeability” breaks down. There is no need for time travelers to balance risks or conduct a cost-benefit analysis, since any negative consequence or event can be reversed with the flip of a switch.

Groundhog Day shows us just how powerful this knowledge can be — Phil Connors was trapped in the same day for 10 years , and as he approached the end of the loop, he figured out how to prevent numerous deaths , enact the perfect bank robbery , and seduce 90% of women in the town , among other things. A person in that kind of situation would have perfect knowledge — virtually any action would have known and certain consequences, and a time traveler would know how each and every action would impact — for better or worse — those around him.

This means that a time traveler could be held responsible for all of the consequences of his actions, regardless of how attenuated those consequences are from the underlying action. As a simple example, consider a scenario in which Phil greets Alice on a street corner. Because of the brief conversation, Alice is delayed by a few seconds in her commute to work and as a result gets run over by careless driver Bob. If Alice had not spoken with Phil, she would have avoided Bob entirely.

In the normal world, we would say that Bob is entirely to blame. But if Phil is in a time loop, then he would know that his conversation would lead to Alice’s death and could avoid it in future loops. Thus, as far as Phil is concerned, talking with Alice is a reasonably foreseeable cause of her death — even though, from the perspective of everyone else, the consequence seems completely divorced from the underlying action.

Time travel also poses a problem for the Hippocratic principle. The “do not harm others” principle presupposes the existence of a baseline status, as if to say, “You have an obligation not to act in a way that would harm a person, as measured relative to the person’s status in the absence of your action.” Time travel throws that baseline status into question.

Suppose that during the first course of events (i.e., in the first day of the time loop), Phil takes an action that causes (through some chain of events) Alice to avoid an injury. In time loop B, Phil knows that through his actions in the “normal” course of events, Alice would have avoided an injury. Thus, in a sense, Phil knows that he will cause her injury if he does not take the action. Put differently, time travel raises the question of which timeline should be used to measure someone’s baseline status (i.e., which timeline you should use to figure out whether your actions have harmed another). A few potential answers come to mind:

• What the timeline would look like without the time traveler.
• What the timeline looked like after the first iteration.
• Evaluate each iteration independently, according to the normal rules.
• The best timeline for each person.

It is not easy to choose between those (or any other) options. This is similar to the problem we encountered when considering time annihilation as a crime — when a time traveler has the ability to switch at will between one timeline and another, there is no principled basis to favor one timeline over another. Here, however, the problem is more challenging, since a time traveler’s complete mastery over events means that a time traveler is responsible for everything that happens — and does not happen — in a particular timeline. In other words, a time traveler is responsible for all events, because a time traveler controls all events (provided that the time traveler has the ability to impact those events in at least one of the infinite potential timelines).

Or, put differently, when time travelers enter their last time loop, they necessarily decide — through their actions — which of an infinite number of timelines to enact. In this way, time travelers can be said to have caused any harm or injury experienced in their chosen timeline. In this sense, one could argue that, in a world of time travel (and particularly time loop time travel), one’s obligation not to harm others amounts to an obligation to help others avoid harm, since any decision not to avoid harm would be equivalent to a decision to impose that harm.

So far, my consideration of time travel has been limited to the Groundhog Day scenario — a time loop with countless iterations that does not take a physical toll on the time traveler from loop to loop. The analysis applies just as much to less extreme scenarios, but the implications of the analysis will not be as severe.

For example, time travelers with only three iterations of a time loop will have a much better understanding of how their actions affect others relative to non-time travelers — but their knowledge will not be anywhere near as developed as an infinite time-looper. Thus, three-loop time travelers can be held to a higher standard of “reasonable foreseeability” than an average person, but they cannot be held to the same standard as a Phil Connors-like time traveler. In essence, the less knowledge a time traveler has about the consequences of their actions, the less responsible they are for those consequences.

Likewise, in a scenario with just a few iterations, the overall state of the timeline would still be outside of the time traveler’s control, so the Hippocratic principle would mostly apply in the same way as it does in a world without time travel. (Of course, the more timelines a time traveler has to choose from, the stronger the argument that the traveler’s selection of a timeline amounts to a decision to impose the harm in that timeline.) The specifics of the scenarios and of one’s legal obligations will vary based on the number of time jumps, the physical toll of those jumps on the time traveler, the cognitive abilities of the time traveler, and the duration of the time window (that is, repeating a day is much different than repeating a lifetime).

Again — time travel is weird . It leads to unanticipated consequences and counterintuitive results. But law is also weird and also leads to unanticipated consequences and counterintuitive results. In 1995, a man was sentenced to life in prison for stealing a piece of pizza; Brian Dement is serving a life sentence in prison, despite the fact that DNA evidence conclusively proves he is innocent; police think that saggy pants create probable cause for a search. The list goes on.

It stands to reason that when you cross the streams of time travel and law, “You’re going to see some serious shit .” And that’s exactly what we’ve seen — for time travelers, the rule that says you can only be held responsible for the foreseeable would hold time travelers responsible for everything . The rule that says you have no obligation to help strangers says that time travelers must help strangers.

On one hand, these rules are fantastic — it shows that a hypothetical era of time travel would usher in a new age of moral responsibility and high moral character. On the other hand, for people like me who dream of time travel, these new rules provide yet another obstacle on the road to time mastery. People often wonder: If time travel exists, why haven’t we seen any time travelers yet? Now we know the answer: The legal fees and liability associated with time travel are just too high — not even the most adventurous of explorers would dare risk that kind of legal exposure.

Plus.Maths.org

Is time travel allowed?

In brief: The laws of physics allow members of an exceedingly advanced civilisation to travel forward in time as fast as they might wish. Backward time travel is another matter; we do not know whether it is allowed by the laws of physics, and the answer is likely controlled by a set of physical laws that we do not yet understand at all well: the laws of quantum gravity. In order for humans to travel forward in time very rapidly, or backward (if allowed at all), we would need technology far far beyond anything we are capable of today.

Travelling forward in time rapidly

Albert Einstein's relativistic laws of physics tell us that time is "personal". If you and I move differently or are at different locations in a gravitational field , then the rate of flow of time that you experience (the rate that governs the ticking of any very good clock you carry with you and that governs the aging of your body) is different from the rate of time flow that I experience. (Einstein used the phrase "time is relative"; I prefer "time is personal".)

Example of Twins Paradox

Florence rides in a space ship that accelerates outward from Earth on a straight line with one Earth gravity of acceleration, $g=9.81 m/s^2$ for a time $T_F/4$ (as measured by her), then decelerates at $g$ for a time $T_F/4$, winding up at rest relative to Earth but very far away. Florence then accelerates back toward Earth at $g$ for a time $T_F/4$ and decelerates at $g$ for $T_F/4$, winding up at rest on Earth. When the trip is finished and Florence meets her Earth-bound twin Metheuselah, she has aged by $T_F$ while he has aged by a larger amount, $T_M$. A fairly simple calculation (see [7] ) using the laws of special relativity gives the following expression for Methuselah's aging in terms of Florence's: $$T_M = (2 c/g) [exp(gT_F/4c)- exp(-gT_F/4c)].$$ Here $exp(x) = e^x$ is the exponential function and $c$ is the speed of light (about 299.8 million m/s). This relationship is plotted in the following figure:

Click here for a larger version of the graph.

If Florence's clocks and aging report a round trip time of 10 years, Methuselah will have aged by 25 years. If Florence aged 30 years, Methuselah will have aged 4,500 years. If Florence aged 88 years, Methuselah will have aged 14 billion years, which is the current age of our Universe! Unfortunately, no known rocket fuel, not even thermonuclear fusion, is capable of producing the sustained multi-year-long acceleration required for such a trip.

This personal character of time allows one person to travel forward in time much faster than another, a phenomenon embodied in the so-called twins paradox . One twin (call him Methuselah) stays at home on Earth; the other (Florence) travels out into the Universe at high speed and then returns. When they meet at the end of the trip, Florence will have aged far less than Methuselah; for example, Florence may have aged 30 years and Methuselah 4,500 years. (The twin that ages least is the one who undergoes huge accelerations, to get up to high speed, slow down, reverse direction, then accelerate back and slow to a halt on Earth. The twin who leads the sedate life ages the most.)

A massive black hole is another vehicle for rapid forward time travel: If Methuselah remains in orbit high above the event horizon of a massive black hole (say, one whose gravitational pull is that of a billion suns) and Florence travels down to near the event horizon and hovers just above it for, say, 30 years and then returns, Methuselah can have aged thousands or millions of years. This is because time flows much more slowly near a black hole's event horizon (where the acceleration of gravity is huge) than far above it (where one can live sedately).

These time travel phenomena have been tested in the laboratory. Muons — short-lived elementary particles — travelling around and around in a storage ring at 0.9994 of the speed of light, at the Brookhaven National Laboratory on Long Island, New York, have been seen to age 29 times more slowly than muons at rest in the laboratory. And atomic clocks on the surface of the Earth have been seen to run more slowly than atomic clocks high above the Earth's surface — more slowly by about 4 parts in 10 billion.

Travelling backward in time: chronology protection

We physicists have been working hard since the late 1980s to understand whether the laws of physics allow backward time travel. We do not have a definitive answer yet, but the likely answer has been summarised by Stephen Hawking, in his Chronology Protection Conjecture (see [1] ): The laws of physics always conspire to prevent anything from travelling backward in time, thereby keeping the Universe safe for historians.

We physicists have identified two mechanisms that might protect chronology: (1) The exotic material that is required in the manufacture of any time machine might be forbidden to exist, by the laws of physics — forbidden to exist in the large amounts that time machines always require. (2) Time machines might always self-destruct, explosively, when one tries to activate them.

These mechanisms (1) and (2) are descriptive translations of mathematical results that we physicists have derived using the laws of physics expressed in their own natural language: mathematics. The sentences (1) and (2) capture the essence of our calculations, but crucial details are lost in translation. For anyone who wishes to struggle to understand those details, good places to start are a recent beautiful but highly technical review article by John Friedman (see [2] ), and a much less technical but older and slightly outdated article by Matt Visser (see [3] ).

I shall illustrate these chronology-protection mechanisms by an example of a time machine that my students Mike Morris and Ulvi Yurtsever and I invented and explored mathematically in 1989: a time machine based on wormholes. (This is just one of many time-machine designs that have been studied. For others see Visser's review, [3] )

Me crawling through a wormhole whose length is only a few centimetres but circumference is about that of my belly. (From my book Black Holes and Time Warps , [4] , where you can find a more detailed description of this time machine.)

A wormhole-based time machine: A wormhole is a hypothetical tunnel through hyperspace that links one place in our Universe (e.g. my office at Caltech) to another place (e.g. the Caltech football field). Each end of the wormhole (each mouth ) looks like a crystal ball. Staring into it, one sees a distorted image of objects at the other end. Looking into the mouth in my office, I see the football field, distorted; someone on the football field, looking into the mouth there, sees me and my office, distorted. The wormhole (tunnel) might be only 3 metres long, so if I enter the mouth in my office and then travel just 3 metres through the tunnel, I emerge from the other mouth, onto the football field 300 metres from my office.

A wormhole as viewed from a higher-dimensional hyperspace. Our Universe is the two-dimensional sheet. The wormhole is a short cut through hyperspace from one location in our sheet (our Universe) to another.

Suppose, now, that a creature from an extremely advanced civilisation carries the football-field mouth out into the Universe on a "twins paradox" trip. When that mouth returns, it may have aged by only one second while the mouth in my office aged by one day. The wormhole has become a time machine: If I enter one mouth and travel through it for only a few seconds, I emerge from the other mouth one day in the future. Travelling through it in the other direction, I emerge one day in the past! (See [4] .)

Exotic Matter and Vacuum Fluctuations: We do not know whether the laws of physics permit wormholes. We do know, however, that a wormhole will implode so quickly that nothing can traverse it, unless it is held open by gravitationally repulsive forces that can only be produced by exotic matter. By the phrase "exotic matter" I mean matter that has negative energy and therefore anti-gravitates, i.e. repels.

The quantum laws of physics do permit exotic matter to exist, and it has been created in the laboratory in very tiny amounts: in the so-called Casimir vacuum between two electrically conducting plates, and in the so-called squeezed vacuum that is generated by optical physicists using nonlinear crystals.

The key to this negative energy is the fact that empty space (the vacuum ) is filled with tiny fluctuations of all kinds of matter and fields that exist in the Universe. It is impossible to make these fluctuations go away. They are a consequence of the quantum mechanical uncertainty principle : if, at one moment of time, there are no fluctuations at all of (for example) the electromagnetic field, then the rate of change of the fluctuations must be infinitely large and a moment later the fluctuations will be enormous. The product of the strength of the fluctuations and the magnitude of their rate of change is always bigger than a certain limit, given by the uncertainty principle. As a result, fluctuations are always present. We call them vacuum fluctuations because they are a property of the vacuum, i.e. of otherwise empty space.

The laws of quantum physics say that vacuum fluctuations produce no gravity — or perhaps only an exceedingly tiny amount of gravity: the gravity that is accelerating the expansion of the Universe. In other words vacuum fluctuations may be responsible for the so-called cosmological dark energy . But that dark energy is so tiny (10 -121 in dimensionless numbers ) that it is irrelevant for my discussion of time machines; so I shall say that the quantum fluctuations produce no gravity at all.

Or, rather, they produce no gravity under normal circumstances. One can devise ways, in fact, to make one region of empty space lend some of its vacuum fluctuations to an adjacent region. (This is what experimental physicists do with the Casimir vacuum and with the squeezed vacuum.) When this happens, the lending region is left with a negative amount of gravitating energy, and the borrowing region gets positive gravitating energy. The quantum laws place tight constraints on the amount of fluctuational energy that can be loaned. The larger the size of the lending region, the less energy it can loan and therefore the less negative its energy can become. This is true in the Casimir vacuum, in the squeezed vacuum, and in all other variants of exotic matter. The laws of physics dictate it.

These constraints on the amount of negative gravitating energy might be severe enough to prevent one from ever accumulating enough of it to prevent a wormhole from imploding (see [5] ). The reason is that regions of space which do the borrowing and the devices which catalyse the borrowing might always have so much positive energy of their own, that their attractive gravity counteracts the negative energy's repulsive gravity, and triggers all wormholes to implode. If that is the case, then wormhole-based time machines are forbidden: one can never travel through a wormhole before it implodes. (John Friedman and colleagues have called this topological censorship , see [2] .)

My personal guess is that these constraints on exotic matter do not prevent wormholes from being held open and thus do not protect chronology, but I could well turn out to be wrong. To learn the truth, we physicists must develop a deeper understanding of quantum theory in warped spacetime than we now have — i.e. a deeper understanding of the combined laws of quantum theory and general relativity, the laws of quantum gravity.

Time machine self destruction: If it turns out that wormholes can be held open, then doing so is not enough to guarantee that an ultra-advanced civilisation can convert a wormhole into a time machine via a twins-paradox trip (carrying one mouth out into the Universe at high speed and then back). There is a second obstacle that must be surmounted — time machine self destruction:

Time machine self destruction: As the right wormhole flies back toward the left, at the end of its twins-paradox trip, vacuum fluctuations flow through the wormhole then out through the space between them, returning to their starting point at the moment they left. Their gravitating energy grows extremely large, and perhaps destroys the wormhole at the moment it becomes a time machine. (Figure adapted from my book Black Holes and Time Warps , [4] .)

As the travelling mouth is returning to Earth, there comes a first moment when its wormhole can be used to travel backward in time. The first thing that can do so, and thereby meet itself before it left, is an entity that enters one mouth, exits from the other before it entered, and then flies through the Universe back to its starting point at the highest possible speed, the speed of light — arriving back at the first mouth at precisely the moment it started its trip. Even if no light or other light-speed radiation travels on this round-trip time-travel route, vacuum fluctuations will always do so. They cannot be stopped. Upon arriving back at their starting point at the very moment when they left, the vacuum fluctuations will pile up on top of their younger selves. The result is a duplicate of every fluctuation, and then, with another round trip, a quadrupling of every fluctuation, and so forth. The bottom line, according to a calculation that I did with my postdoc Sung-Won Kim in 1990, is an explosive flow of gravitating fluctuational energy through the wormhole at precisely the moment when time travel is first possible — at the moment of time machine activation [4] .

Will this explosive fluctuational energy destroy the wormhole and thence the time machine? At first Kim and I thought the wormhole could survive. However Stephen Hawking gave strong arguments to the contrary, in his seminal 1991 research paper on chronology protection. The explosion is very likely to destroy the time machine when it is first activated, Hawking argued — and not just this time machine, but any time machine that even the most advanced civilisation might conceive and build. Over the next few years many other physicists weighed in, with analyses of other time machine designs, and it began to look like Hawking might be wrong: a sufficiently clever design might protect a time machine from self destruction. Then in 1996 Bernard Kay, Marek Radzikowski and Robert Wald developed a powerful mathematical proof that the version of the laws of quantum physics which we were all using to analyse time machine self destruction are incapable of revealing the explosion's outcome. The outcome is held tightly in the grip of the laws of quantum gravity, which we do not yet understand fully.

The fate of any time machine?

Hawking and I have a long history of bets with each other, about unsolved mysteries in physics. But we are not making a bet on this one, since for once we are on the same side. When we physicists have mastered the laws of quantum gravity (Hawking and I agree), we will very likely discover that chronology is protected: the explosion always does destroy any time machine, when it is first activated.

In June 2000, on the occasion of my 60th birthday, Hawking presented me with a tentative analysis of the explosion's outcome, using his own tentative version of the laws of quantum gravity. His conclusion: if I try to use a very advanced civilisation's wormhole to travel backward in time, the quantum mechanical probability that I will succeed is one part in 10 60 ; see Hawking's article in my birthday party book, [6] . That's an awfully small probability of surviving the explosion. Given the opportunity to try, I would not take the risk.

Other time machines: It is amazing what we can learn from the laws of physics, when we understand them well. One famous example is the laws' absolutely firm insistence that it is impossible to construct a perpetual motion machine, even if one has all the tools of an exceedingly advanced civilisation. Another example is a proof by Hawking that to make a time machine, no matter how one goes about it, one must use exotic matter — matter with negative energy — as an integral part of the device; wormholes illustrate this, but it is true in general. And a third example is the proof by Kay, Radzikovsky and Wald that the laws of physics as we now know them will break down whenever a time machine is activated, no matter how one designs the machine. Again wormholes are just one example. Hawking's theorem, and that of Kay, Radzikovsky and Wald, tell us that the fates of all time machines are held tightly in the grip of the laws of quantum gravity.

Progress in the quest to understand quantum gravity has been substantial over the past two decades. Complete success will come, I am convinced, within the next two decades or so — and it will bring not only a clear understanding of whether backward time travel is possible, but also an understanding of many other mysteries, including how our Universe was born (see the Plus article What happened before the Big Bang?).

• Add new comment

Ray said...

I know a little GR; but no Quantum Gravity and very little QM. I interpret the article as saying that the consensus is: Quantum Gravity can not be fit into Godel's time-closed solution to GR. Is that correct? In that case the global topology would constrain the deeper theories. Put another way Quantum Mechanics can not be embedded/formulated on an arbitrary manifold. These are questions; not statements. Ray

Quantum_Flux said...

It is the same thing for observer 1 to rapidly travel into the past as it is for observer 2 to rapidly travel into the future.

@Quantum_Flux: always travelling to the future...

"It is the same thing for observer 1 to rapidly travel into the past as it is for observer 2 to rapidly travel into the future."

No, it is not. Going back in time would imply that the time order of the meetings as seen by observer 2 is the reverse of the meetings order as seen by observer 1. As presently "understood", if meeting 2 is later than meeting 1 for one observer then it will also be later for the other observer.

I read references 2,3 3 is quite readable. 2 is tougher and I am not through. The idea that some local properties can't be extended globally in some topologies is not as strange as it might seem at first glance; there are other examples. I do have doubts about some of the reasoning; but that doesn't fault the reasoning just the presumptions. I think its possible that the "energy conditions" are not the right analysis tool. Something along the lines of Entropy (Maxwell's daemon ) might be sharper in the mathematical sense. Ray

The Grin Reaper said...

Th relation to Casimir vacuums was fascinating. So is the solidarity of Hawking's argument. Although the possibilities of time travel would lead to the age old grandfather paradox, and I am not sure as to what would be a right explanation. Maybe the Copenhagen interpretation of splitting states to maintain Quantum decoherence. Cannot say anything conclusively.

The Possibility of Time Travel

If you understood the cause of Gravity, then you would know that time travel is possible as I do. Jump off a building or out of an aeroplane and you don't need fuel to accelerate. Simply creating a G-field in front of a craft would cause the craft to 'fall' towards that field which would in turn be projected further ahead causing an exponential acceleration curve up to and far exceeding the speed of light. One must also realize that only one half of any trip could be accelerated towards and the remainder must be decelerated in the opposite direction to arrive at the destinations relative speed.

The Possibility of Flawed Logic

It looks like somebody doesn't understand relativity. Our current understanding of physics dictates that we cannot travel faster than the speed of light. Even with an extremely fast acceleration, from at least one reference frame, if an object has mass, it will never reach the speed of light. Accelerating an object with a mass's speed to the speed of light would require an infinite (read: extremely massive) amount of energy that we just aren't capable of producing. Sure putting a gravitational field in front of an object would produce a force, but just how massive would would the object have to be to produce a gravitational field that produces enough force to actually accomplish something within a limited amount of time? It would appear that the only hypothetically feasible object to use would be a black hole, and even those are not known well enough to do anything useful with them. It's useless talking about how we are going to accomplish time-travel if we don't even have all of the (mathematical) tools available to evaluate the situation.

Is Time-travel allowed?

Time-travel is not possible for the good reason that time does not exist, so I would answer no(!) if someone asked me that question. Please refer to http://www.spacetime.nu Brgds! Bo Nyberg

A very clear and enjoyable

A very clear and enjoyable article. Thanks. - Neal Asher

early entry into quantum physics

I am a student in 12th grade. I really do not have the mathematical advantage nor the complete skills related to quantum physics, but yes timetravel and quantum gravity is something I am immensely interested in and have been reading about this for a very long time. I found this article very interesting and it has left me wondering what would be going on in the mind of "physics" when it came into existence. Why will it not want us to do something and why permit something else. And if the laws of physics break down, and maybe if I am the one who's behind it, wont I have the freedom to make new laws of my own which permit time travel? Its like I find a way to break something to the ground and build it up again as per my fancy? And well, I have been going through a few books on quantum physics and I would love it if someone can suggest a book on mathematics which will help supplement my present reading. thanks!!!

Time seems not to be proven here, just assumed

A very interesting article but I have to politely disagree. Firstly because no proof of times existence is given or referred to. Specifically no proof is given that as things move change and interact, a thing called time needs to exist, or 'passes'.

From the outset the suggestion "The laws of physics allow members of an exceedingly advanced civilisation to travel forward in time as fast as they might wish." May be seriously flawed.

Relativity, as in GPS systems for example, does indeed show that fast moving objects 'change' more slowly. And as they do so it can be said the surrounding matter effectively changes more quickly. Thus a fast moving astronaut might return to earth finding it very different than might be expected. But unless proof is given that time, and 'the future' actually exist, all this proves is that matter exists, moves, changes and interacts 'now' -at different rates under different conditions. And not, IMO, that time, the future, or time travel may exist.

(Note, in 'electrodynamics' Einstein only -states- that a rotating hand on a numbered dial (a 'watch') shows the existence and passing of a thing called time. But logically, no matter how many people assume this is correct, without actual, scientific, experimental proof, all such a device shows is that motors can rotate hands. And thus SR only shows that moving motors etc run more slowly than (relatively) stationary ones).

M.Marsden (Auth-A Brief history of Timelessness )

i think worm hole 'time travel' can be deconstructed Timelessly

In this article the wormhole is suggested as a possible time travel device. The only problems suggested being that exotic matter would be required, or it might necessarily self destruct re the laws of physics. i.e be practically impossible.

but unless it is proven that there is indeed a past and/or future then perhaps Relatvity just shows how matter might be warped, stretched, and dilated in its rates of change - but not over a thing called time.

If this is the case then the worm hole 'billiard ball' paradox (for example) can be explained 'timelessly', thus...

Time travel, Worm hole, billiard ball' paradox, Timelessly. (re Paul Davies- New scientist article) https://www.youtube.com/watch?v=Wc5cRGOGIEU

Matthew Marsden

Time is indeed a cultural construct. This is in part demonstrated by differing notions of time in different cultures.

Part of the strength of modern physics is that it has allowed useful mathematical calculation of some of what is entailed in passing through a wormhole. Part of the challenge is that this infrastructure provides restrictive overhead for properly understanding and moving toward a deeper knowledge. One obvious example for both of these comments is the apparent requirement of a physical process for creating a time travel machine, and for keeping the wormhole open.

The actual resolution of this matter may emerge from use of different mathematics and innovative experimental procedures. An attitude of open inquiry will be very important. Some time ago, as a graduate student in physics, I posited to a leading physicist at MIT that there might be two dimensions to time, or that time might behave differently than we think it does. He was outraged, and the conversation ended there, but now Kip Thorne and colleagues have done some of the work implied. So my hat is off to them. I think significant answers are just around the corner, and it will take considerable meticulous effort to get there.

Kip Thorne's work in the area has come a long way in very few years.

John Carlton-Foss

Time.University

I recently purchased two websites Spacetime.University and Time.University which I plan to post content related to Spacetime and Quantum Time mathematics. For now most of my tutorials etc are on the website Spacetime.University. I am a big fan of Kipe Thorn's ideas. Some of Kip Thorne's ideas get a mention. Any feedback is appreciated.

My project is very new so any suggestions such as websites to link to or comment on content would be appreciated.

We are all time travellers as time is just a measure of the rate at which the universe is running down, i.e. increasing entropy, so is always in that direction.

If time travel was allowed in the reverse direction then we would need to send an observer back to experience it as such. Unfortunately this would also mean that, as far as this observer is concerned, it would have to put the WHOLE of the universe, i.e. every particle, each and every one in its original position and each with its original momentum and ground energy as they were back at the time the observer makes the journey INCLUDING the particles that make up the observer! This might be the reason for the observer (the time machine) vanishing in a violent explosion as its constituent parts tear of to their various origins at that distant time and place.

I am well aware that I am missing the theoretical knowledge and the mathematics to describe what the above words mean but I am pretty sure this is the way it works no matter how many mathematical fuddles are used to get over this simple concept of why we cannot fight the whole of the universe and the universal increase in entropy as we are part of it and cannot stand apart from it such a way that we are not torn to shreds as our atoms are dispersed through time and space to regain their positions at the time we seek to visit. This is what it really means to go back in time to a universe as it was, back then. If you had at your disposal a universe full of energy, the knowledge about all its constituent particles, and the wit to control all of this, then you might, just, have a chance.

OR, am I missing something?

I am also aware, as are others in the earlier comments, that what we called time is not a real thing it is just the measure of the rate of change of entropy which we also know can vary, relatively, in the forward direction simply by accelerating, relative, to another body such that our rate of change of time is different to the body that is not accelerated. Time appears to be a construct of the mind to enable us to comprehend change and exchange ideas about such change in much the same way as we agree about colours as we can never be certain that the colours we perceive are the same as those perceived by others. Time also flows differently according to mood which also seems to suggest it is a construct of the mind and not a real phenomenon.

There have been designs for interstellar ram-jets that collect hydrogen nuclei using huge electrostatic collector/compressors to fuse these together to make heavier nuclei which they expel out of the back at very near the speed of light thus propelling the ship at, say, a constant 1g using the energy from the, free, interstellar hydrogen fuel. This, if perfected, could make the Nebulae in Andromeda about a lifetime away for the travellers but about 2.5million years, each way, for those living on Earth. There is, of course, the ablation and radiation due to the extremely high energy particle flux (blue shifted, to overcome - maybe we would have to convert a trillion tonne asteroid into a spaceship - or, just maybe, it is all just a pipedream! In any case this is still time travel into the future, not the past.

I would be very much obliged for a critique on the above ideas as I have long (50 years or more) wondered why I have not seen this idea posited anywhere else as it seems such a reasonable rebuttal to the question of the possibility of time travel into the past.

John Barton Wood

Leeds, West Yorkshire

Time travel

As long as we continue to think of the 'arrow of time' time travel will not be achieved, at least not in the sense we wish. For Time Travel proper, Time itself must exist as a genuine entity as does does Space and therefore have separate dimensions of its own.

The Ethics and Morals of Time Travel: Tackling the Time Traveler’s Dilemma

The ethical and moral implications of time travel are complex and multi-layered. In recent years, discussions about the feasibility of time travel have become more prevalent, leading to questions about whether or not it would be ethical to travel through time.

Many experts argue that traveling through time could have far-reaching implications, such as altering the course of history, causing paradoxes, and creating significant ethical dilemmas. Some argue that time travel could allow for the correction of past injustices, while others argue that it could create new ones.

Ultimately, the question of whether time travel is ethical depends on a variety of factors, including the reason for time travel, its potential consequences, and the impact it could have on individuals and society as a whole.

Credit: www.leisurebyte.com

The Concept Of Altering The Past

The concept of time travel has been a source of fascination for humankind for as long as we have dreamt about our future. With the advent of technology and modern science, the idea of traveling through time is gradually moving from the realm of science fiction to scientific possibility.

Altering past events may seem intriguing at first glance, but it raises significant ethical and moral dilemmas. Here are some points to keep in mind when considering the idea of changing past events:

The Butterfly Effect And Its Consequences

The butterfly effect is a concept that suggests that small changes in a complex system can have significant and unpredictable effects. In the context of time travel, this concept takes a whole new meaning. Even the slightest alteration of a past event can have an enormous impact on future events, causing unexpected and potentially dangerous consequences.

For instance, what if someone went back in time and prevented the birth of a famous inventor, such as thomas edison or steve jobs? The world, as we know it, could be vastly different, or may not even exist at all.

The consequence of the butterfly effect is not limited to the advancement of technology or history. It could also result in the loss of life, cultural changes, or the destruction of the environment. Consider the outcome if someone went back in time and killed adolf hitler before he gained power.

While it may seem like a noble act, would it fundamentally change the philosophies that fueled the nazi party? Would that same person be responsible for creating another dictator in the future? The butterfly effect raises more questions than answers, and we may never be able to predict the outcomes of our actions when we meddle with time travel.

Analysis Of The Effect Of Altering Historical Events On Society, Culture, And Moral Values

An action as seemingly innocuous as going back in time to buy a particular painting or artifact could have a profoundly negative impact on society, culture, and moral values. Imagine someone traveled back in time and took the mona lisa from the louvre in the 19th century.

While it could significantly benefit one individual, it would rob the world of a masterpiece that is enjoyed and celebrated for its beauty and cultural significance. Additionally, hypothetically speaking, if someone went back in time and saved john f. kennedy from assassination, it could set a dangerous precedent of altering historical events that changed the trajectory of our culture and values.

The Paradoxical Nature Of Changing The Past And The Resulting Confusion And Chaos

The idea of time travel inherently creates a paradox, as the ability to change the past undermines the very essence of time itself. If someone went back in time and killed their grandfather, what would happen to their existence? Would they cease to exist, or would they have created a different universe entirely?

This paradox and uncertainty surrounding the idea of time travel can lead to a world of confusion and chaos.

The concept of changing the past presents a host of ethical and moral dilemmas, including the butterfly effect, the impact of changing historical events on society and culture, and the resulting paradoxical nature of changing the past. While it may be tempting to correct our mistakes or improve our future, we may need to make peace with the fact that time is a one-way street, and changing it would bring more unknown and unintended risks than rewards.

The Responsibility Of Time Travelers

Analyzing the responsibility that time travelers bear for the consequences of their actions.

The very notion of time travel brings up several moral and ethical dilemmas that we as a society must address. One such dilemma involves analyzing the responsibility that time travelers bear for the consequences of their actions. Here are some key points to consider:

• Time travelers must recognize the impact of their actions on the course of history. Any significant deviation from the timeline can have catastrophic consequences in the present and future.
• Time travelers have a responsibility to ensure that they do not disrupt the course of natural events in a way that could cause harm to future generations.
• Time travelers must consider the implications of their actions on future generations and act accordingly to minimize negative outcomes.
• Time travelers must be prepared to accept responsibility for their actions and the consequences that may arise from them.

The Moral And Ethical Considerations Of Using Knowledge Gained From The Future For Personal Gain

Using knowledge gained from the future for personal gain is a compelling prospect, but it raises several moral and ethical questions, including:

• The question of whether it is fair to use knowledge obtained from future generations for one’s benefit in the present day.
• The question of whether this creates an unjust advantage, as knowledge of future events gives someone an unfair advantage over others who do not have access to that future information.
• The question of whether using future knowledge in the present day can change the future, impacting the lives of those who sent the traveler back in time.
• The ethical implications of using future knowledge in the present day must be carefully considered to avoid incurring future harm.

The Impact Of Time Travel On The Freedom Of Choice And The Concept Of Fate

The concept of time travel also challenges our ideas of fate and the extent of human agency. Here are some key points to consider:

• Time travel raises the question of whether our actions are predestined, or whether we have free will, as we can change the course of events in the past and future.
• This creates unique ethical challenges since our present-day actions can have a significant impact on the lives of future generations, and our choices may be seen as morally questionable by those who sent us back in time.
• The ability to time travel also means that our actions may have consequences that we cannot foresee, making it important to consider the wider ramifications of our actions.

Time travel raises complex ethical and moral considerations that require us to consider the consequences of our actions carefully. Factors like responsibility, personal gain, and the nature of time and free will all warrant careful and thoughtful reflection.

Time Travel Regulation

Time travel is undoubtedly one of the most fascinating concepts that humanity has ever come up with. With movies like back to the future and interstellar gaining global popularity, the idea of “bending” time is both exciting and unsettling. However, time travel is not just a science-fiction concept, and it carries profound ethical and moral implications.

Therefore, regulating time travel is crucial to prevent any paradoxes or unforeseen repercussions. In this section, we will analyze the current regulations and legal frameworks for time travel, the role of governments and international organizations, and the debate surrounding the need for stricter regulations and moral guidelines.

Analysis Of The Current Regulations And Legal Frameworks For Time Travel

At the moment, time travel is still a theoretical concept, and there are no explicit legal frameworks or regulations in place. However, some countries have proposed some guidelines to prevent any misuse of this technology, such as denmark’s proposed “time traveling ethics commission.

” The commission aims to investigate, analyze, and possibly regulate any possible time travel attempt. There are several reasons why we need these regulations, such as:

• The butterfly effect: The butterfly effect suggests that a tiny change in the past can cause drastic future changes. Hence, we need regulations to prevent the possibility of a paradox and any other unforeseen consequences.
• The possibility of changing history: Time travel could allow people to influence the course of history, which could be dangerous. Therefore, we need guidelines to prevent any ill-intended attempt to modify events and the course of history.

The Role Of Governments And International Organizations In Regulating Time Travel

As the concept of time travel becomes more plausible, many governments and international organizations have raised concerns about it and have attempted to introduce regulations to prevent any unforeseen consequences. Here are some of the reasons why governments and international organizations need to regulate time travel:

• National security: Time travel could compromise national security, and governments need to regulate it to prevent any potential threats.
• Human rights abuse: Time travel could also be used for human rights abuse, such as meddling in the affairs of other nations or even genocide.
• Environmental impact: Time travel could have a significant impact on the environment, especially if it affects natural resources and ecosystems. Hence, we need regulations to prevent any exploitation of natural resources.

The Debate Surrounding The Need For Stricter Regulations And Moral Guidelines

Despite the current regulations and legal frameworks, there is still a debate surrounding the need for stricter regulations and moral guidelines for time travel. Some argue that regulations are essential to prevent any misuse of this technology, while others believe that it should be entirely banned.

Here are some of the reasons why we need stricter regulations and moral guidelines:

• Preventing abuse: Stricter regulations and moral guidelines can prevent any unethical intent of time travel, such as meddling in personal affairs or changing history to suit personal interests.
• Unforeseen consequences: Stricter regulations can prevent any unforeseen consequences of time travel, such as the butterfly effect or creating alternate timelines.
• Transparency: Stricter regulations can ensure transparency concerning the development, testing, and application of time travel technology.

Time travel is a fascinating concept with significant ethical and moral implications. As we move towards making time travel a reality, we need to have strict regulations and moral guidelines to prevent any potential misuse of this technology. Governments and international organizations need to take action in creating these regulations to ensure the safety and security of all.

Frequently Asked Questions On The Time Traveler’S Dilemma: Ethics And Moral Implications

What is the time traveler’s dilemma.

The time traveler’s dilemma refers to the ethical and moral implications of time travel. It raises questions about altering the past, the butterfly effect, and the responsibility of the time traveler.

What Are The Moral Implications Of Time Travel?

Time travel can have severe moral implications, such as altering historical events, changing the course of history, and affecting innocent people’s lives. The time traveler must take responsibility for the consequences of their actions.

Can Time Travel Lead To Unintended Consequences?

Yes, time travel can lead to unintended consequences as altering even the smallest detail in the past can cause a significant impact on the present and future. The butterfly effect must be considered before taking any action in the past.

The dilemma of time travel ethics and moral implications is not a simple one. It requires a thoughtful and deliberate approach to understand the intricacies involved in such a scenario. As we have seen, the butterfly effect can have drastic consequences on the future.

It is important to be mindful of how our actions today can affect generations to come. The concept of time travel raises important questions about our responsibility towards the future. As we explore the possibility of time travel, we must also consider the implications it would have on our sense of morality and ethics.

A strong code of ethics is crucial to safeguarding the interest of humanity as a whole. We must tread carefully and thoughtfully in the realm of time travel, lest we inadvertently change the very course of history itself.

Passport Specialist, Tech fanatic, Future explorer

Unleashing Efficiency with Virtual Assistant Technology in the Workplace

Exploring the fascinating mechanics of time travel, leave a comment cancel reply.

Save my name, email, and website in this browser for the next time I comment.

Can You Time Travel Without Paradoxes? Physicists Compute and See Its Logical Possibility

While each person is traveling through time at a one second per second rate, it's safe to say that no one has managed to travel through it and end up in the past or future. However, scientists are still fascinated by the theoretical possibilities.

Science Alert  notes how several movies show that moving through time can lead to issues with universal laws. For example, if one goes back in time to stop parents' from meeting in the first place, how is it possible to exist and to time travel?

This head-scratcher is dubbed the grandfather paradox. However, a few years back, Germain Tobar, a physics student from the University of Queensland, worked around the numbers in order to calculate how time travel would be possible without such kind of paradoxes. According to Big Think , Tobar teamed up with physicist Dr. Fabio Costa from the same university.

ALSO READ: Is Time Travel Real? TikTok User Who Claims To Be From 2671 Warns Seven People Will Mysteriously Fall From the Sky Next Year

Is It Possible To Time Travel Without Paradoxes?

Their study was published in Classical and Quantum Gravity  last 2020.

Tobar mentions how there are certain physicists who believe in these possibilities. He also notes, however, that this may be logically difficult to embrace because it would affect one's freedom to decide. This means that one may time travel but be unable to perform anything that could lead to a paradox.

He further notes that the prominent general relativity theory of Einstein predicts time travel and time loops.

Space-Time Can Theoretically Adapt

Science Alert notes how their calculations revealed that space-time may adapt with its own self in order to dodge such paradoxes. With this, time travel may possibly be consistent and exempted from such occurrences. However, Big Think notes that it would need all outputs except that of two specific space-time regions. In such a case, even if timelike loops that are closed are present, entities may have preserved freedom to decide without leading to a paradox.

The University of Queensland  also reports how Tobar stated that classical dynamics states that if one knows a system's state in a certain period, it may reveal the system's complete history. He also states how this may have widespread applications, such as enabling humanity to deploy rockets to different planets.

The work of Tobar may be quite difficult for those who are not mathematicians. However, the approach focuses on the deterministic processes' influence across various regions within the continuum of space-time. It also shows how time-like curves that are closed may still coincide with classic physics and free will.

The study also smoothed out an issue with a different hypothesis pertaining to how time travel may be possible but travelers would have restrictions in order to prevent paradoxes from taking place. In such a model, time travels have freedom while paradoxes cannot happen.

Time Travel Remains Elusive

Nevertheless, though these numbers may theoretically work out, bending both space and time in order to move to the past is quite elusive. Costa expressed, however, that the various mathematical processes that they noted revealed that time travel, while preserving one's free will, is logically possible to happen without any paradoxes.

RELATED ARTICLE: Is Time Travel Possible? Science Studies How to Make It Happen, But It Might Not Be the Way You Think

Check out more news and information on Quantum Physics  in Science Times.

Most Popular

Cognify: Revolutionary Prison Concept Uses AI and Brain Implants to Fast-Track Criminal Rehabilitation

Cigar-Shaped Glowing UFO Captured on Military-Grade Night Vision in Montana; What Does This Mean for UFO Research?

Unveiling the Future of Precision Medicine: A Closer Look at Nuvectis Pharma's Innovative Approach

‘Planet Killer’ Asteroid To Skim Past Earth This Week; Here’s What To Know About the Mountain-Sized Space Rock

Three-Fingered 'Alien Mummy' Shares Interesting Similarities and Differences With Humans, New Study Reveals

Latest stories.

‘Mask Fishing’ of TikTok’s Psychological Impact; Are People More Attractive With a Mask On?

Pooping on the Moon Will Be a Messy Business of Waste Management and Disposal; How Will NASA Address This?

Earth on the Midst of Sixth Major Extinction Crisis; Scientists Identify Key Areas That Need Protection

Gold Membrane Revolutionizes Surface Study With Enhanced Raman Spectroscopy: A New Era in Surface Analysis?

Honeybees Can Detect Lung Cancer Through Human Breath, MSU Research Finds; Could These Tiny Pollinators Be Heroes of Medicine?

Subscribe to the science times.

Sign up for our free newsletter for the Latest coverage!

Recommended Stories

China’s Chang’e 6 Mission To Bring First Lunar Samples Obtained From the Far Side of the Moon

Satellites Made From Metal-Halide Perovskites Could Self-Heal When Exposed to Space Radiation, Study Reveals

NASA’s JWST Unravels Mysteries of a Supermassive Black Hole Which Can Give Clues About the Early Universe

Astronauts Stranded in ISS Due to Malfunctioning Boeing Starliner; NASA To Send Rescue Missions To Fix the Capsule and Return the Crew

• Table of Contents
• Random Entry
• Chronological
• Editorial Information
• About the SEP
• Editorial Board
• How to Cite the SEP
• Special Characters
• Advanced Tools
• Support the SEP
• PDFs for SEP Friends
• Make a Donation
• SEPIA for Libraries
• Entry Contents

Bibliography

Academic tools.

• Friends PDF Preview
• Author and Citation Info
• Back to Top

Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

• Adams, Robert Merrihew, 1997, “Thisness and time travel”, Philosophia , 25: 407–15.
• Arntzenius, Frank, 2006, “Time travel: Double your fun”, Philosophy Compass , 1: 599–616. doi:10.1111/j.1747-9991.2006.00045.x
• Asimov, Isaac, 1995 [2003], Gold: The Final Science Fiction Collection , New York: Harper Collins.
• Baron, Sam and Colyvan, Mark, 2016, “Time enough for explanation”, Journal of Philosophy , 113: 61–88.
• Belot, Gordon, 2005, “Dust, time and symmetry”, British Journal for the Philosophy of Science , 56: 255–91.
• Benovsky, Jiri, 2011, “Endurance and time travel”, Kriterion , 24: 65–72.
• Berkovitz, Joseph, 2001, “On chance in causal loops”, Mind , 110: 1–23.
• Black, Max, 1956, “Why cannot an effect precede its cause?”, Analysis , 16: 49–58.
• Brier, Bob, 1973, “Magicians, alarm clocks, and backward causation”, Southern Journal of Philosophy , 11: 359–64.
• Carlson, Erik, 2005, “A new time travel paradox resolved”, Philosophia , 33: 263–73.
• Carroll, John W., 2010, “Context, conditionals, fatalism, time travel, and freedom”, in Time and Identity , Joseph Keim Campbell, Michael O’Rourke, and Harry S. Silverstein, eds., Cambridge MA: MIT Press, 79–93.
• Craig, William L., 1997, “Adams on actualism and presentism”, Philosophia , 25: 401–5.
• Daniels, Paul R., 2012, “Back to the present: Defending presentist time travel”, Disputatio , 4: 469–84.
• Deutsch, David and Lockwood, Michael, 1994, “The quantum physics of time travel”, Scientific American , 270(3): 50–6.
• Dowe, Phil, 2000, “The case for time travel”, Philosophy , 75: 441–51.
• –––, 2001, “Causal loops and the independence of causal facts”, Philosophy of Science , 68: S89–S97.
• –––, 2003, “The coincidences of time travel”, Philosophy of Science , 70: 574–89.
• Dummett, Michael, 1964, “Bringing about the past”, Philosophical Review , 73: 338–59.
• Dwyer, Larry, 1977, “How to affect, but not change, the past”, Southern Journal of Philosophy , 15: 383–5.
• Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
• Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press.
• Effingham, Nikk and Robson, Jon, 2007, “A mereological challenge to endurantism”, Australasian Journal of Philosophy , 85: 633–40.
• Ehring, Douglas, 1997, “Personal identity and time travel”, Philosophical Studies , 52: 427–33.
• Elliott, Katrina, 2019, “How to Know That Time Travel Is Unlikely Without Knowing Why”, Pacific Philosophical Quarterly , 100: 90–113.
• Fulmer, Gilbert, 1980, “Understanding time travel”, Southwestern Journal of Philosophy , 11: 151–6.
• Gilmore, Cody, 2007, “Time travel, coinciding objects, and persistence”, in Oxford Studies in Metaphysics , Dean W. Zimmerman, ed., Oxford: Clarendon Press, vol. 3, 177–98.
• Goddu, G.C., 2003, “Time travel and changing the past (or how to kill yourself and live to tell the tale)”, Ratio , 16: 16–32.
• –––, 2007, “Banana peels and time travel”, Dialectica , 61: 559–72.
• Gödel, Kurt, 1949a [1990a], “An example of a new type of cosmological solutions of Einstein’s field equations of gravitation”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 190–8; originally published in Reviews of Modern Physics , 21 (1949): 447–450.
• –––, 1949b [1990b], “A remark about the relationship between relativity theory and idealistic philosophy”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 202–7; originally published in P. Schilpp (ed.), Albert Einstein: Philosopher-Scientist , La Salle: Open Court, 1949, 555–562.
• Godfrey-Smith, William, 1980, “Travelling in time”, Analysis , 40: 72–3.
• Gorovitz, Samuel, 1964, “Leaving the past alone”, Philosophical Review , 73: 360–71.
• Grey, William, 1999, “Troubles with time travel”, Philosophy , 74: 55–70.
• Hafele, J. C. and Keating, Richard E., 1972a, “Around-the-world atomic clocks: Observed relativistic time gains”, Science , 177: 168–70.
• –––, 1972b, “Around-the-world atomic clocks: Predicted relativistic time gains”, Science , 177: 166–8.
• Hales, Steven D., 2010, “No time travel for presentists”, Logos & Episteme , 1: 353–60.
• Hall, Thomas, 2014, “In Defense of the Compossibility of Presentism and Time Travel”, Logos & Episteme , 2: 141–59.
• Hanley, Richard, 2004, “No end in sight: Causal loops in philosophy, physics and fiction”, Synthese , 141: 123–52.
• Harrison, Jonathan, 1980, “Report on analysis ‘problem’ no. 18”, Analysis , 40: 65–9.
• Hawking, S.W., 1992, “Chronology protection conjecture”, Physical Review D , 46: 603–11.
• Holt, Dennis Charles, 1981, “Time travel: The time discrepancy paradox”, Philosophical Investigations , 4: 1–16.
• Horacek, David, 2005, “Time travel in indeterministic worlds”, Monist (Special Issue on Time Travel), 88: 423–36.
• Horwich, Paul, 1975, “On some alleged paradoxes of time travel”, Journal of Philosophy , 72: 432–44.
• –––, 1987, Asymmetries in Time: Problems in the Philosophy of Science , Cambridge MA: MIT Press.
• Ismael, J., 2003, “Closed causal loops and the bilking argument”, Synthese , 136: 305–20.
• Keller, Simon and Nelson, Michael, 2001, “Presentists should believe in time-travel”, Australasian Journal of Philosophy , 79: 333–45.
• Kiourti, Ira, 2008, “Killing baby Suzy”, Philosophical Studies , 139: 343–52.
• Le Poidevin, Robin, 2003, Travels in Four Dimensions: The Enigmas of Space and Time , Oxford: Oxford University Press.
• –––, 2005, “The Cheshire Cat problem and other spatial obstacles to backwards time travel”, Monist (Special Issue on Time Travel), 88: 336–52.
• Lewis, David, 1976, “The paradoxes of time travel”, American Philosophical Quarterly , 13: 145–52.
• Loss, Roberto, 2015, “How to Change the Past in One-Dimensional Time”, Pacific Philosophical Quarterly , 96: 1–11.
• Luminet, Jean-Pierre, 2011, “Time, topology, and the twin paradox”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press. doi:10.1093/oxfordhb/9780199298204.003.0018
• Markosian, Ned, 2004, “Two arguments from Sider’s Four-Dimensionalism ”, Philosophy and Phenomenological Research , 68: 665–73.
• Markosian, Ned, 2020, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio , 12: 137–65.
• Maudlin, Tim, 2012, Philosophy of Physics: Space and Time , Princeton: Princeton University Press.
• Meiland, Jack W., 1974, “A two-dimensional passage model of time for time travel”, Philosophical Studies , 26: 153–73.
• Mellor, D.H., 1998, Real Time II , London: Routledge.
• Meyer, Ulrich, 2012, “Explaining causal loops”, Analysis , 72: 259–64.
• Miller, Kristie, 2005, “Time travel and the open future”, Disputatio , 1: 223–32.
• –––, 2006, “Travelling in time: How to wholly exist in two places at the same time”, Canadian Journal of Philosophy , 36: 309–34.
• –––, 2008, “Backwards causation, time, and the open future”, Metaphysica , 9: 173–91.
• Monton, Bradley, 2003, “Presentists can believe in closed timelike curves”, Analysis , 63: 199–202.
• –––, 2009, “Time travel without causal loops”, Philosophical Quarterly , 59: 54–67.
• Nerlich, Graham, 1981, “Can time be finite?”, Pacific Philosophical Quarterly , 62: 227–39.
• Ney, S.E., 2000, “Are grandfathers an endangered species?”, Journal of Philosophical Research , 25: 311–21.
• Price, Huw, 1996, Time’s Arrow & Archimedes’ Point: New Directions for the Physics of Time , New York: Oxford University Press.
• Putnam, Hilary, 1975, “It ain’t necessarily so”, in Mathematics, Matter and Method , Cambridge: Cambridge University Press, vol. 1 of Philosophical Papers , 237–49.
• Reinganum, Marc R., 1986, “Is time travel impossible? A financial proof”, Journal of Portfolio Management , 13: 10–2.
• Riggs, Peter J., 1991, “A critique of Mellor’s argument against ‘backwards’ causation”, British Journal for the Philosophy of Science , 42: 75–86.
• –––, 1997, “The principal paradox of time travel”, Ratio , 10: 48–64.
• Savitt, Steven, 1994, “The replacement of time”, Australasian Journal of Philosophy , 74: 463–73.
• –––, 2005, “Time travel and becoming”, Monist (Special Issue on Time Travel), 88: 413–22.
• Sider, Theodore, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Clarendon Press.
• –––, 2002, “Time travel, coincidences and counterfactuals”, Philosophical Studies , 110: 115–38.
• –––, 2004, “Replies to Gallois, Hirsch and Markosian”, Philosophy and Phenomenological Research , 68: 674–87.
• –––, 2005, “Traveling in A- and B- time”, Monist (Special Issue on Time Travel), 88: 329–35.
• Simon, Jonathan, 2005, “Is time travel a problem for the three-dimensionalist?”, Monist (Special Issue on Time Travel), 88: 353–61.
• Slater, Matthew H., 2005, “The necessity of time travel (on pain of indeterminacy)”, Monist (Special Issue on Time Travel), 88: 362–9.
• Smart, J.J.C., 1963, “Is time travel possible?”, Journal of Philosophy , 60: 237–41.
• Smeenk, Chris and Wüthrich, Christian, 2011, “Time travel and time machines”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press, online ed. doi:10.1093/oxfordhb/9780199298204.003.0021
• Smith, Joseph Wayne, 1985, “Time travel and backward causation”, Cogito , 3: 57–67.
• Smith, Nicholas J.J., 1997, “Bananas enough for time travel?”, British Journal for the Philosophy of Science , 48: 363–89.
• –––, 1998, “The problems of backward time travel”, Endeavour , 22(4): 156–8.
• –––, 2004, “Review of Robin Le Poidevin Travels in Four Dimensions: The Enigmas of Space and Time ”, Australasian Journal of Philosophy , 82: 527–30.
• –––, 2005, “Why would time travellers try to kill their younger selves?”, Monist (Special Issue on Time Travel), 88: 388–95.
• –––, 2011, “Inconsistency in the A-theory”, Philosophical Studies , 156: 231–47.
• –––, 2015, “Why time travellers (still) cannot change the past”, Revista Portuguesa de Filosofia , 71: 677–94.
• –––, 2017, “I’d do anything to change the past (but I can’t do ‘that’)”, American Philosophical Quarterly , 54: 153–68.
• van Inwagen, Peter, 2010, “Changing the past”, in Oxford Studies in Metaphysics (Volume 5), Dean W. Zimmerman (ed.), Oxford: Oxford University Press, 3–28.
• Vihvelin, Kadri, 1996, “What time travelers cannot do”, Philosophical Studies , 81: 315–30.
• Vranas, Peter B.M., 2005, “Do cry over spilt milk: Possibly you can change the past”, Monist (Special Issue on Time Travel), 88: 370–87.
• –––, 2009, “Can I kill my younger self? Time travel and the retrosuicide paradox”, Pacific Philosophical Quarterly , 90: 520–34.
• –––, 2010, “What time travelers may be able to do”, Philosophical Studies , 150: 115–21.
• Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press.
• Williams, Donald C., 1951, “The myth of passage”, Journal of Philosophy , 48: 457–72.
• Wright, John, 2006, “Personal identity, fission and time travel”, Philosophia , 34: 129–42.
• Yourgrau, Palle, 1999, Gödel Meets Einstein: Time Travel in the Gödel Universe , Chicago: Open Court.

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

causation: backward | free will: divine foreknowledge and | identity: over time | location and mereology | temporal parts | time | time machines | time travel: and modern physics

Copyright © 2024 by Nicholas J.J. Smith < nicholas . smith @ sydney . edu . au >

• Accessibility

Support SEP

Mirror sites.

View this site from another server:

• Info about mirror sites

The Stanford Encyclopedia of Philosophy is copyright © 2024 by The Metaphysics Research Lab , Department of Philosophy, Stanford University

Library of Congress Catalog Data: ISSN 1095-5054

Ideal timing for travel insurance purchase

Advantages of purchasing travel insurance early.

• When to buy annual travel insurance 

When is it too late to buy travel insurance?

• Frequently asked questions

When to Buy Travel Insurance: Timing Your Purchase Perfectly

Affiliate links for the products on this page are from partners that compensate us (see our advertiser disclosure with our list of partners for more details). However, our opinions are our own. See how we rate insurance products to write unbiased product reviews.

• Purchase travel insurance when traveling internationally or on long and expensive trips.
• Buying travel insurance right after booking your trip is best, as some coverage options are time-sensitive.
• Cancel For Any Reason policies and coverage for pre-existing coverage require early purchase.

Travel insurance protects you against financial losses and medical emergencies while on a trip. You'll want to book travel insurance when going on long, expensive, or international trips. However, the exact timing of your purchase is also important.

Generally, you'll want to buy travel insurance soon after you book your trip to get the maximum coverage out of your policy. Here's what you need to know about travel insurance and when you should buy it.

Though you can insure your trip anytime after booking, the best time to purchase travel insurance is immediately after booking your trip.

For one, unexpected circumstances may arise between when you book your trip and when you leave. If an injury or inclement weather cancels your trip before you have a chance to purchase travel insurance, you may not get a refund. Purchasing travel insurance early reduces the chances of that happening.

Michelle Osborn, owner of boutique travel agency Outta Here Travels , says, "Most travel insurance policies have a time limit of when you can purchase to get the maximum benefits." 

It's important to note that you do not have to have every detail of your trip planned before purchasing travel insurance. Most policies are flexible, so you can add details and update your costs in the days leading up to the trip. Don't let some unplanned details in your trip prevent you from purchasing travel insurance.

When you purchase travel quickly after booking, you'll have additional coverage options.

• Purchase cancel for any reason coverage , which reimburses you for 60% to 75% of your costs if you back out on your travel plans for a reason not usually covered. CFAR policies must be purchased within 15 to 21 days of putting down the first deposit on your trip. You can find our picks for the best CFAR travel insurance here.
• Guarantee coverage for pre-existing conditions. Many travel insurers won't cover pre-existing conditions unless you purchase coverage within two weeks of booking your trip.
• Cancel your travel insurance if you're not happy with it. Most plans allow you to cancel your insurance and get a refund — as long as you do so within 15 days and your trip hasn't started.

If you cannot purchase your travel insurance right after booking, you should still qualify for a policy if your trip hasn't been canceled. Most companies allow you to buy insurance until the day of your trip. You just won't have access to CFAR or pre-existing condition coverage.

When to buy annual travel insurance 

Annual travel insurance , sometimes referred to as multi-trip insurance, covers you for all your trips over a 12-month period. This type of travel insurance may be a good option if you regularly travel for work or have a handful of trips, particularly international trips, planned for the next 12 months.

"Travel insurance should definitely be purchased anytime you travel out of the United States," Osbon says. "The main reason is most US health insurance policies don't cover treatment internationally."

You may want to buy annual travel insurance just before your first trip begins. Since these plans last 365 days, you can stretch your policy to cover the most travel. However, this may limit your coverage, so tread carefully. For example, you may not be able to get medical coverage for pre-existing conditions.

"In the case of a pre-existing medical condition, you'll need to meet a few requirements to be covered," says Jeff Rolander, director of claims at Faye Travel Insurance . "Faye's travel protection covers pre-existing conditions as long as you purchase your plan within 14 days of your initial trip deposit and are medically able to travel when you purchase your plan."

You should be able to purchase travel insurance any time before the date of your trip. Once the day of your departure rolls around, the window will close. You also can't purchase travel insurance during your trip or after an injury or loss has already happened.

The moral of the story: It's always better to buy sooner rather than later. 

"Right when you book your flights or hotel stay is when you should get your trip covered," Rolander says. "The sooner you buy coverage for your trip, the sooner your coverage starts."

When to buy travel insurance frequently asked questions

You should purchase travel insurance within two weeks of booking your trip to receive maximum benefits, especially pre-trip cancellation coverage.

While you can purchase travel insurance up until the day of departure, doing so may limit certain benefits. Additionally, last-minute travel insurance won't cover pre-existing health conditions. 

Lengthier and more expensive trips generally call for travel insurance because you have more to lose. Purchasing travel insurance earlier mitigates the potential damage of a cancellation.

Most travel insurance policies allow for modifications or additions within a specific period after purchase, but it's important to check the policy terms.

Purchasing travel insurance early is generally beneficial for comprehensive coverage, but you may see your premiums rise if you make significant changes to your travel plans.

• Real estate

• Main content

Mobile Menu Overlay

The White House 1600 Pennsylvania Ave NW Washington, DC 20500

FACT SHEET: President   Biden Announces New Actions to Keep Families   Together

Since his first day in office, President Biden has called on Congress to secure our border and address our broken immigration system. As Congressional Republicans have continued to put partisan politics ahead of national security – twice voting against the toughest and fairest set of reforms in decades – the President and his Administration have taken actions to secure the border, including:

• Implementing executive actions to bar migrants who cross our Southern border unlawfully from receiving asylum when encounters are high;
• Deploying record numbers of law enforcement personnel, infrastructure, and technology to the Southern border;
• Seizing record amounts of fentanyl at our ports of entry;
• Revoking the visas of CEOs and government officials outside the U.S. who profit from migrants coming to the U.S. unlawfully; and
• Expanding efforts to dismantle human smuggling networks and prosecuting individuals who violate immigration laws.

President Biden believes that securing the border is essential. He also believes in expanding lawful pathways and keeping families together, and that immigrants who have been in the United States for decades, paying taxes and contributing to their communities, are part of the social fabric of our country. The Day One immigration reform plan that the President sent to Congress reflects both the need for a secure border and protections for the long-term undocumented. While Congress has failed to act on these reforms, the Biden-Harris Administration has worked to strengthen our lawful immigration system. In addition to vigorously defending the DACA (Deferred Action for Childhood arrivals) policy, the Administration has extended Affordable Care Act coverage to DACA recipients and streamlined, expanded, and instituted new reunification programs so that families can stay together while they complete the immigration process.  Still, there is more that we can do to bring peace of mind and stability to Americans living in mixed-status families as well as young people educated in this country, including Dreamers. That is why today, President Biden announced new actions for people who have been here many years to keep American families together and allow more young people to contribute to our economy.   Keeping American Families Together

• Today, President Biden is announcing that the Department of Homeland Security will take action to ensure that U.S. citizens with noncitizen spouses and children can keep their families together.
• This new process will help certain noncitizen spouses and children apply for lawful permanent residence – status that they are already eligible for – without leaving the country.
• These actions will promote family unity and strengthen our economy, providing a significant benefit to the country and helping U.S. citizens and their noncitizen family members stay together.
• In order to be eligible, noncitizens must – as of June 17, 2024 – have resided in the United States for 10 or more years and be legally married to a U.S. citizen, while satisfying all applicable legal requirements. On average, those who are eligible for this process have resided in the U.S. for 23 years.
• Those who are approved after DHS’s case-by-case assessment of their application will be afforded a three-year period to apply for permanent residency. They will be allowed to remain with their families in the United States and be eligible for work authorization for up to three years. This will apply to all married couples who are eligible.  
• This action will protect approximately half a million spouses of U.S. citizens, and approximately 50,000 noncitizen children under the age of 21 whose parent is married to a U.S. citizen.

Easing the Visa Process for U.S. College Graduates, Including Dreamers

• President Obama and then-Vice President Biden established the DACA policy to allow young people who were brought here as children to come out of the shadows and contribute to our country in significant ways. Twelve years later, DACA recipients who started as high school and college students are now building successful careers and establishing families of their own.
• Today’s announcement will allow individuals, including DACA recipients and other Dreamers, who have earned a degree at an accredited U.S. institution of higher education in the United States, and who have received an offer of employment from a U.S. employer in a field related to their degree, to more quickly receive work visas.
• Recognizing that it is in our national interest to ensure that individuals who are educated in the U.S. are able to use their skills and education to benefit our country, the Administration is taking action to facilitate the employment visa process for those who have graduated from college and have a high-skilled job offer, including DACA recipients and other Dreamers. 

Stay Connected

We'll be in touch with the latest information on how President Biden and his administration are working for the American people, as well as ways you can get involved and help our country build back better.

Opt in to send and receive text messages from President Biden.

Eugene Levy's The Reluctant Traveler Gets Apple TV+ Future Revealed

The future of The Reluctant Traveler is now confirmed. For fans of the Apple TV+ series, it's excellent news.

Per Deadline , The Reluctant Traveler has been renewed for Season 3 at Apple TV+ . Eugene Levy will return to star and executive produce the third season, which will follow the American Pie star on a "truly global adventure." Levy also addressed the renewal in a statement.

Presumed Innocent Review: Apple TV+ Legal Thriller Is Out of Place and Time

The Apple TV+ series Presumed Innocent is based on Scott Turow's 1987 novel, and the Jake Gyllenhaal-led drama feels like it belongs in that era.

“I really appreciate what this show is trying to do for me," he said. "But to be a real seasoned traveler you need to have a strong sense of adventure and curiosity, and I’m ashamed to say over the past two seasons I’ve developed neither. But I have to admit I’m having a lot of fun putting in the effort. So the beat goes on, apparently.”

The Reluctant Traveler Season 2 Wrapped in April

The documentary series debuted in February 2023, and its second season wrapped up in April 2024. Its concept sees Levy traveling to hotels around the world to explore the cultures surrounding them, paying trips to locations including Costa Rica, Finland, Italy, Japan, South Africa, the United States, and more. The show's future following its second season has since been unclear, but the green light has now officially come in.

The Reluctant Traveler is streaming on Apple TV+ .

Source: Deadline

The best time to book flights for the cheapest airfare in 2024

Editor's Note

• The best days to fly for the cheapest domestic and international fares are midweek.
• Aim to book domestic flights one to two months ahead of time. 
• For international flights, plan on booking three to five months ahead of the trip.

At TPG, one of our key goals is to help you find the cheapest airfare . However, your ability to find valuable airline deals depends on a few factors.

Flights during the peak of summer and, of course, the holidays are often much more expensive. Still, there are ways to hedge your bets and get better deals than others. That said, some of the common tips people tout are flat-out false.

Have you ever heard that booking a flight on a certain day of the week will yield the cheapest results? Perhaps you're thinking of a cousin who claims he found a great deal when he checked on airfare prices at 12:01 a.m. on a Wednesday. Maybe your friend swears by setting a reminder on their phone for 1 p.m. on a Thursday when shopping for flights.

There's just no truth to that legend.

While there is no shortage of rumors (and "studies"), here's what airfare experts say about how timing influences airfare prices so you can maximize airfare savings.

Is there a day that's cheaper than others to book a flight?

It's a question we get asked all the time: "What is the cheapest day to book a flight?" There's a common misperception that certain days are better for finding flight deals. However, research shows that this is a myth.

There is no magic day to book, but there are some sweet booking windows when airlines lower prices, according to Lindsay Schwimer, a consumer travel expert at the booking app Hopper .

Hopper relies on 10 years of data and 80 trillion flight prices to recommend the best time to book specific routes and dates.

"Typically, we tell travelers for domestic trips to start monitoring prices three to four months in advance of a trip," Schwimer said. "Expect to book one to two months in advance."

For international trips, the booking window is a bit larger. You should start monitoring six to seven months prior to the trip and book it three to five months in advance. That's the recommendation from both Hopper and TPG.

"When you're traveling internationally, planning ahead is key to getting the cheapest airfare," Hayley Berg, Hopper's lead economist, said in another interview. "Travelers often book international flights too far in advance or too last minute, overpaying significantly for their tickets."

Remember that the strategy changes pretty dramatically if you book with points and miles . Airlines often open award space for coveted business- and first-class seats either when the schedules open or at the last minute.

As for whether there's a particular day that's cheaper than the rest for booking, Schwimer mentioned that "there's no one day or golden rule when to book."

Berg echoed this sentiment.

"There's a common myth that 'booking on a Tuesday' will guarantee a traveler the best price," she said. "The reality is prices change so often and depend on the route, the travel dates, etc., that there isn't one day that guarantees you the best price."

What's the best day to travel?

Although airfare prices fluctuate based on when you book, midweek travel is generally cheaper than flying on weekends. That said, Sundays can also be a sweet spot.

"Flying midweek can save you nearly $100 off your ticket," Schwimer said. "So when you're thinking about when you want to travel, try shifting your dates midweek versus flying over the weekend. If you can be flexible and book either a Tuesday or Wednesday versus a Friday or Saturday, you're gonna save significantly off your trip."

Berg had similar thoughts.

"Travelers who fly midweek, usually Wednesday, can save an average of $56 per ticket on domestic airfare throughout the year," she said. "Midweek savings spike over $60 per ticket during busy spring break and summer vacation months, while flying midweek over the holidays can save you $100 or more."

Schwimer also had some good advice to price the first flight of the day. Often, you'll find those early departures can save you some big bucks. It's not easy to set the alarm for 4 a.m., but it's easier to wake up when you know you're saving money.

"We always recommend booking the first flight of the day, not only to avoid delays and cancellations but also in terms of price," she said.

Additionally, according to Bob Harrell — an airfare consultant at Harrell Associates who analyzes airline pricing — you might also find airfare prices fluctuate Monday through Friday. That's because airlines look at customer demand and monitor booking trends to decide whether to open or close certain types of fares and whether to raise or lower existing fares.

So, how can you use this information to your advantage when buying airline tickets?

Try using a price monitoring tool to get real-time updates to help you decide when to travel for the lowest fare. Tools like Google Flights and Hopper's "Watch This Trip" can help you track the price of your desired route before booking; they will indicate or directly notify you when prices are at the lowest for your trip.

When to book holiday travel

Best time to book flights for summer trips.

It's not too late to look for deals for summer travel.

Berg said that prices tend to drop on average one to two months before departure, but lately, we are seeing a lot of last-minute travel deals — even to Europe.

"Prices will change frequently," Berg said. "Don't wait too long to book, as prices can increase by almost double in the last three weeks ahead of takeoff."

If you are already looking ahead to next summer, Hopper recommends monitoring flight prices at least three to four months in advance and then booking one to two months before the intended travel date.

If you plan to head abroad next summer, the booking window is naturally further out, particularly for longer-haul transpacific flights.

"For trips to Asia and Oceania, we recommend booking five to seven months before takeoff," Berg said. "For Europe and other regions of the world, we recommend booking anywhere from three to six months before departure to get the best prices."

Best time to book flights for spring break

The best time to book your spring break trip is similar to the timeline for summer trips. You should begin monitoring prices for your desired destination three to four months in advance, depending on when your (and your family's) time off falls. Set Google Flights alerts now for upcoming trips you have in mind.

For example, if your spring break falls in March, the ideal time to book is before early February. Flexibility will also help here.

Remember that demand surges during the spring break period after the doldrums of winter. Many people wait too long to book those trips and end up paying more than they need to.

Hopper says airfare prices will fall between one and three months before departure and rise rapidly in the last three to four weeks before spring break.

Best time to book flights for Thanksgiving and Christmas

The cheapest holiday airfare deals are generally available in September and October.

"We tell people to start monitoring in August," Schwimer said, "but our typical book-by date is mid-October, so usually around the 14th of October." She added that "no later than Halloween is usually our book-by date for both Thanksgiving and Christmas."

Similarly, a Google Flights booking trends report suggested that average flight prices around Christmas were lowest 71 days before departure.

Prices will only rise as the holidays approach, so your best bet is to book sooner rather than later.

Best time to book flights for New Year's Eve

Like Christmas flights, Hopper says you should try to lock in bookings for New Year's by Halloween at the latest.

That is true for "really any of the major winter holidays," Berg said. "We recommend booking further in advance. I would book that New Year's Eve trip as well before Halloween."

What about hotels?

Hotels are a special category where you can often wait until much later to score good deals.

"We often see good deals last minute for hotels in big cities. Something to keep in mind ... especially for those holiday weekends," Schwimer told TPG. "And you know that [during] the Christmas holiday period, you can often find some really great last-minute deals in big cities if you wait.

"Obviously, that's not always ideal in terms of planning," Schwimer continued, "but we have seen some really great deals in that last-minute window in big metro cities."

Bottom line

While there is no magical time or day to book airfare for the best prices, your best bet is to fly midweek, specifically on Wednesdays, for both domestic and international travel. Flying over the weekend can come at a significant price premium, though Sundays have occasional deals.

Also, try booking domestic flights one to two months ahead of time.

For international flights, your best bet is to aim to book three to five months ahead of the trip.

If you are using airline miles , remember that it's never too soon to start checking and booking.

Related reading:

• Best airline credit cards
• 6 real-life strategies you can use when your flight is canceled or delayed
• The best credit cards for booking flights
• The best credit cards to reach elite status
• What are points and miles worth? TPG's monthly valuations

Advertisement

Supported by

How Biden’s New Immigration Policy Works

The new policy will give some 500,000 people a pathway to citizenship.

• Share full article

By Hamed Aleaziz

President Biden’s new immigration policy protects some 500,000 people who are married to U.S. citizens from deportation and gives them a pathway to citizenship.

The election-year move comes just two weeks after Mr. Biden imposed a major crackdown at the U.S.-Mexico border, cutting off access to asylum for people who crossed into the United States illegally.

The policy announced on Tuesday is aimed at people who have been living in the United States for more than a decade and have built their lives and families here.

Here is how it works:

Why do the spouses of American citizens need protection?

Marrying an American citizen generally provides a pathway to U.S. citizenship. But people who crossed the southern border illegally — rather than arriving in the country with a visa — must return to their home countries to complete the process for a green card, something that can take years. The new program allows families to remain in the country while they pursue legal status.

Who is eligible?

There are roughly 1.1 million undocumented immigrants married to U.S. citizens in the United States, according to Fwd.us , an immigration advocacy group, but not all of them are eligible for the program.

The spouses must have lived in the United States for 10 years and have been married to an American citizen as of June 17. They cannot have a criminal record. Officials estimate that the policy will provide legal status and protections for about 500,000 people. The benefits would also extend to the roughly 50,000 children of undocumented spouses who became stepchildren to American citizens.

When will the program take effect?

Biden administration officials said they expected the program to start by the end of the summer. Those eligible will then be able to apply for the benefits.

Why is President Biden doing this now?

Mr. Biden is trying to strike a tricky balance on immigration, which is a serious political vulnerability for him. Polls show Americans want tougher policies. Just two weeks ago, Mr. Biden announced a crackdown on asylum at the southern border.

His new policy, giving hundreds of thousands of immigrants new legal protections, is a way for him to answer the calls from the progressive base of the Democratic Party, which has accused the White House of betraying campaign promises to enact a more humane approach to immigrants.

Hamed Aleaziz covers the Department of Homeland Security and immigration policy. More about Hamed Aleaziz

Money blog: Big supermarket wants to rival Aldi and Lidl with 400 new convenience stores

Welcome to the Money blog, your place for personal finance and consumer news and advice. Let us know your thoughts on any of the topics we're covering using the comments box below.

Wednesday 26 June 2024 18:19, UK

• Morrisons to open hundreds more convenience stores
• Qatar Airways voted world's best airline this year - see where the others ranked
• Quorn to start blending meat into some products
• Popular energy supplier ranked worst for customer service - in big reversal from last year

Essential reads

• Cheap Eats:  Great British Menu legend outlines his top picks in North Yorkshire
• Women in Business : 'Her baby was choking' - How accident in cafe and £400 turned into a genius business idea that's about to go global
• Money Problem : 'I bought a new car but it's been back to dealership six times with same fault - what can I do?'
• Barcelona to ban all holiday apartment rentals by 2028
• Basically... Guarantors
• Best of the Money blog - an archive

Ask a question or make a comment

The supermarket has announced plans to open about 400 more of its Morrisons Daily convenience shops.

It is part of plans to take on discount retailers Aldi and Lidl, according to Retail Gazette . 

Morrisons said it wanted to increase its total number of smaller shops to 2,000 in 2025, although it has not yet given a list of locations. 

The supermarket has seen its like-for-like sales, excluding fuel and VAT, rise by 4.1% over the three months to 28 April. 

Chief executive Rami Baitieh said he was "pleased with the overall performance" of the business in the second quarter. 

Drivers are being warned this week's heatwave brings risks of an unexpected £180 bill. 

Car mechanics from Prestone said the hot weather could cause significant damage to windscreens - meaning you'll have to shell out for a replacement. 

"Slightly counterintuitively, it isn't the heat directly that causes damage, it is the fluctuation in temperature that could cause damage," they said. 

"When this happens to brittle materials such as glass, it can cause them to explode or crack; this is called thermal shock. To stop your windscreen from accumulating a lot of heat, avoid parking in direct sunlight. Consider parking under a tree, in a garage or other forms of shade."

Deliveroo shares have risen following reports US rival Doordash held takeover talks with the business. 

Doordash flagged an interest in taking over the UK's Deliveroo last month, but talks ended as the two sides couldn't agree on the value of the deal, according to Reuters news agency. 

The London-listed company's share price jumped by 6% to 136p this morning, although later eased to 3.7% higher. 

Deliveroo has seen its value fall by more than 50% since debuting on the stock market in March 2021. 

New details have been unveiled for Universal's first UK theme park - including plans for the attraction to be open 365 days a year.

Universal Destinations & Experiences - which is owned by Sky's parent company Comcast - has bought land near Bedford as it  plans to build  Europe's largest  theme park  with millions of visitors per year, as well as a 500-room hotel and dining area.

Read the full story here ...

The building society is the latest to slash rates, after HSBC yesterday and Barclays on Monday. 

It will be lowering two, three and five-year rates by up to 0.21% tomorrow. 

It follows a reduction in swap rates - which dictate how much it costs lenders to lend.

So what does this mean for the market more broadly? 

Here's what the experts told Newspage... 

Emma Jones, managing director at Whenthebanksaysno.co.uk: "They're falling like dominoes now. Things are suddenly looking considerably brighter for the property market as we enter the second half of the year. Enquiries are up sharply this week, which suggests the improved weather may also have lifted spirits and confidence."

David Stirling, independent financial advisor at Mint Mortgages and Protection: "We've not exactly hit the bargain basement for borrowers just yet but the competition for new business from the banks has definitely heated up this week. For some the Bank of England decision in August is very hotly anticipated and could hopefully lead to an incredibly busy autumn."

Dariusz Karpowicz, director at Albion Financial Advice: "While these minor decreases are certainly a step in the right direction, the average rate for a two or five-year fix remains high. It's good to see rates moving down, but let's not get carried away. There are still too many unknowns on the horizon. The upcoming election, fluctuating house prices and geopolitical uncertainties all add layers of complexity to the market."

No one could have failed to notice the rocketing price of olive oil over the last couple of years - and it's been just as bad in Spain, where it is an integral part of the Mediterranean diet. 

The Spanish government has now said it will temporarily ditch the sales tax on olive oil to help consumers cope. 

Spain is the world's leading producer and exporter of olive oil, and last year Spanish households consumed an average of six litres per person. 

However, the country's agriculture ministry says prices have surged by 272% since September 2020. 

The Money team wrote this piece in March about why that's happening... 

The Spanish government had already cut the sales tax on olive oil from 10% to 5% as part of an anti-inflation package.

Now they've said no sales tax will be applied from July until September, when it will be taxed at 2% until the end of the year. 

From then on, it will be taxed at 4%. 

Spanish treasury minister Maria Jesus Montero said the decision reflects "the importance of olive oil in the Mediterranean diet and a healthy lifestyle". 

The "Oscars of the aviation industry" has taken place and Qatar Airways has come out on top. 

Qatar Airways was named the overall winner, as well as first in the business class, business class lounge and Middle East categories, at Skytrax's World Airline Awards last night. 

It's the eighth time it has been awarded the best in class. 

Last year's winner, Singapore Airlines, came second, followed by Emirates, ANA All Nippon Airways and Cathay Pacific. 

British Airways was the highest-ranking UK airline in 13th place.

Europe's best airline was named as Turkish Airlines, while the continent's top low-cost airline was Spanish company Volotea. 

The rankings are based on airline passenger satisfaction surveys across more than 100 countries between September 2023 and May 2024. 

Other notable names to make the top 100 were: Ryanair (63), easyJet (66), Jet2.com (68) and Tui Airways (83).

These are the airlines that made the top 20: 

1. Qatar Airways 2. Singapore Airlines 3. Emirates 4. ANA All Nippon Airways 5. Cathay Pacific Airways 6. Japan Airlines 7. Turkish Airlines 8. EVA Air 9. Air France 10. Swiss International Air Lines 11. Korean Air 12. Hainan Airlines 13. British Airways 14. Fiji Airways 15. Iberia 16. Vistara 17. Virgin Atlantic 18. Lufthansa 19. Etihad Airways 20. Saudi Arabian Airlines. 

By James Sillars , business reporter

Two issues are dominating the behaviour of investors at the moment: US inflation and the snap parliamentary election in France.

Uncertainty over the two has been driving a topsy-turvy performance on stock markets as any scrap of evidence over the potential timing of a US interest rate cut is seized upon.

A sticky inflation picture across the pond – as we have witnessed here – has pushed back Federal Reserve and therefore market expectations for a reduction in borrowing costs.

The next big number awaited is the latest personal consumption expenditures price index, a closely watched inflation indicator at the US central bank.

A weaker than expected number could see a boost for shares globally after a volatile few weeks.

Following a 0.4% decline yesterday, the FTSE 100 opened 0.4% up at 8,285. 

Among the wider shares doing well were those of AO World, up more than 3%.

The online electricals retailer raised growth targets for its current financial year after profits over the 12 months to the end of March beat market expectations.

The pound and dollar have benefitted amid struggles for the euro over the past couple of weeks.

Much of that has been down to election speculation in France where Marine Le Pen's National Rally (RN) is leading first-round polling.

The election was called by President Emmanuel Macron after RN's strong gains in the recent European elections, sparking a renewed market focus on the French economy and the potential knock-on effects for the euro area.

France has a debt to GDP ratio of 110%, meaning its debt is more than the value of its annual output.

There is a budget deficit of 5%. EU rules allow for just 3%.

The market's worry, according to analysts, is that if the far-right RN were to win big in the first round, then voters may take a tactical turn to the left in the second.

It is a concern for France and the wider euro, they said, because a left-wing alliance influence in government would seek even greater public spending commitments than RN has made.

French government borrowing costs have soared since the election announcement.

Earlier this month, the risk premium France pays for its debt on top of Germany's neared levels last seen in 2012.

Quorn has been known for decades as a producer of meat-free alternatives - but in a new venture, it will be blending actual pork sausages with fake meat. 

The company has said it wants to reach consumers who are cutting down on their meat consumption but who are not going entirely vegetarian or vegan, according to a report in The Grocer . 

The new products include burgers and sausages, and will contain Quorn's mycoprotein blended with meat. 

They will be available by the end of the year within the NHS and from some food service operators. 

Marco Bertacca, Quorn's chief executive, told the publication that appealing to meat eaters "represents the majority of people, and so it is a massive opportunity to decarbonise part of the food system and improve public health".

"Once upon a time we were effectively competing with the meat industry – only making products that were alternatives to theirs, and encouraging people to switch," he said.

"There have been attempts in the past to make products like burgers and sausages with a blend of meat and plant-based ingredients like soya and pea protein, but the products have not delivered for consumers."

It told Sky News that it is still in the development stages of its catering partnerships but it has had "fantastic" feedback on the quality of the products and impact on sustainability targets so far. 

Livestock farming accounts for about 15% of global greenhouse emissions, according to FAO data. 

Every Wednesday we ask Michelin chefs to pick their favourite Cheap Eats where they live and when they cook at home. This week we speak to the highly celebrated Shaun Rankin from Michelin-starred Shaun Rankin at Grantley Hall - who won the Great British Menu dessert course in 2009.

Hi Shaun , c an you tell us your favourite places in North Yorkshire  where you can get a meal for two for less than £40?

North Yorkshire is bountiful in its selection of brilliant places to dine and we're incredibly lucky to have so many eateries on our doorstep.

My first recommendation would be the Blue Lion , a traditional country pub nestled within the picture-perfect village of East Witton in the Yorkshire Dales. Enjoy a scenic walk through the surrounding countryside before stopping off for lunch – their light lunch menu includes personal favourites such as croque madame with a rocket salad and Black Sheep-battered fish and chips.

If you're heading in to explore the characterful charm of Ripon, a trip to Oliver's Pantry is a must. They do great coffee alongside a delicious all-day brunch, an array of sweet treats and their seasonal lunch menu packed with fresh homemade dishes.

What's your go-to cheap meal at home?

Chicken pesto pasta with mushrooms and broccoli - a delicious, easy dish to prepare and packed with nutrients.

My tip for this dish is to buy a whole chicken and roast it whole – it's so much more cost-effective than buying chicken breast or thigh, and you can enjoy several meals from just the one bird. With the leftovers I'll prepare a simple soup by making a chicken stock from the carcass, or perhaps a comforting bowl of chicken egg fried rice with the meat, so nothing is wasted. 

We've spoken to lots of top chefs and bloggers - check out their cheap eats from around the country here...

Those looking to drive to polling stations can park for free this general election.

JustPark are offering drivers 30 minutes of free parking near polling stations on 4 July.

They will need to pre-book online or via the JustPark app, however. 

"We believe that everyone should have the opportunity to vote, but know that on the day it isn't always that straightforward," Mary Corrie, managing director of Just Park UK, said.

"By offering free parking on election day, we hope to make it easier for all voters, especially those with mobility issues or who are short on time, to find a place to park near their polling station. 

"This is the second time we have offered this service, following its great success in the 2019 election, and we are confident it will help to get people to the polls once again."

EDF and Utilita have been ranked the worst suppliers for customer service, as ratings across the industry reached their lowest point, according to Citizens Advice .

One of the highest-scoring suppliers this time last year, EDF fell to the very bottom of the table after its average call waiting times jumped from just under a minute to more than five minutes.

Average customer ratings between January and March for all firms fell by 10.5% compared with the same period in 2021.

"Citizens Advice has long called for Ofgem to be given stronger powers to hold suppliers to account on customer service. That must include tackling the complaints backlog before next winter," said Citizens Advice chief executive Dame Clare Moriarty.

At the bottom of the table, both Utilita and EDF scored 2.1 out of five stars, followed closely by British Gas on 2.4.

Dale Vince's Ecotricity topped the rankings with 3.8, followed by Outfox the Market and Ovo Energy, both scoring 3.3.

Among the other big suppliers, E.ON Next scored 3.0, Scottish Power 2.9 and Octopus 2.5.

An EDF spokeswoman said: "We recognise our call answer times haven't been up to the high standards we set ourselves and we're committed to doing better."

The company has recruited and trained more people at its Sunderland, Brighton and Exeter offices, she said.

She added EDF were nearing the end of a "complex transfer of our residential customers to a new IT system, resulting in more customers getting in touch as we navigate this process".

Utilita said: "We must not underestimate how savvy consumers are today. They will realise the star rating contradicts other market-wide supplier assessments – including those of Ofgem, Trustpilot and Which? – where Utilita performs consistently well.

"As such, we look forward to ongoing dialogue to close the gap between the perception the Citizens Advice star rating gives, and the reality other rankings provide."

Be the first to get Breaking News

Install the Sky News app for free