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Is time travel even possible? An astrophysicist explains the science behind the science fiction

Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County

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Adi Foord does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

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Is Time Travel Even Possible? An Astrophysicist Explains The Science Behind The Science Fiction

If traveling into the past is possible, one way to do it might be sending people through tunnels in space..

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the  laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s  theory of special relativity  suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the  speed of light  – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is  6 minutes and 5 milliseconds younger .

Time isn’t the same everywhere.

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves  wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be  incredibly challenging  to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by  throwing a dinner party  where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he  pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the  James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

Adi Foord is an Assistant Professor of Astronomy and Astrophysics at the University of Maryland, Baltimore County. This article is republished from  The Conversation  under a  Creative Commons license . Read the  original article .

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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time? 

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative. 

Learn more: Best travel insurance

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said. 

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said. 

She also mentioned the anecdote of  Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

What did Albert Einstein invent?: Discoveries that changed the world

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USA TODAY is exploring the questions you and others ask every day. From "How to watch the Marvel movies in order" to "Why is Pluto not a planet?" to "What to do if your dog eats weed?" – we're striving to find answers to the most common questions you ask every day. Head to our Just Curious section to see what else we can answer for you. 

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.

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Do you believe in time travel? I’m a skeptic myself — but if these people’s stories about time travel are to be believed, then I am apparently wrong. Who knows? Maybe one day I’ll have to eat my words. In all honesty, that might not be so bad — because the tradeoff for being wrong in that case would be that time travel is real . That would be pretty rad if it were true.

Technically speaking time travel does exist right now — just not in the sci fi kind of way you’re probably thinking. According to a TED-Ed video by Colin Stuart, Russian cosmonaut Sergei Krikalev actually traveled 0.02 seconds into his own future due to time dilation during the time he spent on the International Space Station. For the curious, Krikalev has spent a total of 803 days, nine hours, and 39 minutes in space over the course of his career.

That said, though, many are convinced that time dilation isn’t the only kind of time travel that’s possible; some folks do also believe in time travel as depicted by everything from H. G. Wells’ The Time Machine to Back to the Future . It’s difficult to find stories online that are actual accounts from real people — many of them are either urban legends ( hi there, Philadelphia Experiment ) or stories that center around people that I’ve been unable to verify actually exist — but if you dig hard enough, sincere accounts can be found.

Are the stories true? Are they false? Are they examples of people who believe with all their heart that they’re true, even if they might not actually be? You be the judge. These seven tales are all excellent yarns, at any rate.

The Moberly–Jourdain Incident

In 1901, two Englishwomen, Anne Moberly and Eleanor Jourdain , took a vacation to France. While they were there, they visited the Palace of Versailles (because, y’know, that’s what one does when one visits France ). And while they were at Versailles, they visited what’s known as the Petit Trianon — a little chateau on the palace grounds that Louis XVI gave to Marie Antoinette as a private space for her to hang out and do whatever it was that a teenaged queen did when she was relaxing back then.

But while they were there, they claimed, they saw some… odd occurrences. They said they spotted people wearing anachronistic clothing, heard mysterious voices, and saw buildings and other structures that were no longer present — and, indeed, hadn’t existed since the late 1700s. Finally, they said, they caught sight of Marie Antoinette herself , drawing in a sketchbook.

They claimed to have fallen into a “time slip” and been briefly transported back more than 100 years before being jolted back to the present by a tour guide.

Did they really travel back in time? Probably not; various explanations include everything from a folie a deux (basically a joint delusion) to a simple misinterpretation of what they actually saw. But for what it’s worth, in 1911 — roughly 10 years after what they said they had experienced occurred — the two women published a book about the whole thing under the names Elizabeth Morison and Frances Lamont simply called An Adventure. These days, it’s available as The Ghosts of Trianon ; check it out, if you like.

The Mystery Of John Titor

John Titor is perhaps the most famous person who claims he’s time traveled; trouble is, no one has heard from him for almost 17 years. Also, he claimed he came from the future.

The story is long and involved, but the short version is this: In a thread begun in the fall of 2000 about time travel paradoxes on the online forum the Time Travel Institute — now known as Curious Cosmos — a user responded to a comment about how a time machine could theoretically be built with the following message:

“Wow! Paul is right on the money. I was just about to give up hope on anyone knowing who Tipler or Kerr was on this worldline.

“By the way, #2 is the correct answer and the basics for time travel start at CERN in about a year and end in 2034 with the first ‘time machine’ built by GE. Too bad we can’t post pictures or I’d show it to you.”

The implication, of course, was that the user, who was going by the name TimeTravel_0, came from a point in the future during which such a machine had already been invented.

Over the course of many messages spanning from that first thread all the way through the early spring of 2001, the user, who became known as John Titor, told his story. He said that he had been sent back to 1975 in order to bring an IBM 5100 computer to his own time; he was just stopping in 2000 for a brief rest on his way back home. The computer, he said, was needed to debug “various legacy computer programs in 2036” in order to combat a known problem similar to Y2K called the Year 2038 Problem . (John didn’t refer to it as such, but he said that UNIX was going to have an issue in 2038 — which is what we thought was going to happen back when the calendar ticked over from 1999 to 2000.)

Opinions are divided on whether John Titor was real ; some folks think he was the only real example of time travel we’ve ever seen, while others think it’s one of the most enduring hoaxes we’ve ever seen. I fall on the side of hoax, but that’s just me.

Project Pegasus And The Chrononauts

In 2011, Andrew D. Basiago and William Stillings stepped forward, claiming that they were former “chrononauts” who had worked with an alleged DARPA program called Project Pegasus. Project Pegasus, they said, had been developed in the 1970s; in 1980, they were taking a “Mars training class” at a community college in California (the college presumably functioning as a cover for the alleged program) when they were picked to go to Mars. The mode of transport? Teleportation.

It gets better, too. Basiago and Stillings also said that the then- 19-year-old Barack Obama , whom they claimed was going by the name “Barry Soetero” at the time, was also one of the students chosen to go to Mars. They said the teleportation occurred via something called a “jump room.”

The White House has denied that Obama has ever been to Mars . “Only if you count watching Marvin the Martian,” Tommy Vietor, then the spokesman for the National Security Council, told Wired’s Danger Room in 2012.

Victor Goddard’s Airfield Time Slip

Like Anne Moberly and Eleanor Jourdain, senior Royal Air Force commander Sir Robert Victor Goddard — widely known as Victor Goddard — claimed to have experienced a time slip.

In 1935, Goddard flew over what had been the RAF station Drem in Scotland on his way from Edinburgh to Andover, England. The Drem station was no longer in use; after demobilization efforts following WWI, it had mostly been left to its own devices. And, indeed, that’s what Goddard said he saw as he flew over it: A largely abandoned airfield.

On his return trip, though, things got… weird. He followed the same route he had on the way there, but during the flight, he got waylaid by a storm. As he struggled to regain control of his plane, however, he spotted the Drem airfield through a break in the clouds — and when he got closer to it, the bad weather suddenly dissipated. But the airfield… wasn’t abandoned this time. It was busy, with several planes on the runway and mechanics scurrying about.

Within seconds, though, the storm reappeared, and Goddard had to fight to keep his plane aloft again. He made it home just fine, and went on to live another 50 years — but the incident stuck with him; indeed, in 1975, he wrote a book called Flight Towards Reality which included discussion of the whole thing.

Here’s the really weird bit: In 1939, the Drem airfield was brought back to life. Did Goddard see a peek into the airfield's future via a time slip back in 1935? Who knows.

Space Barbie

I’ll be honest: I’m not totally sure what to do with thisone — but I’ll present it to you here, and then you can decide for yourself what you think about it. Here it is:

Valeria Lukyanova has made a name for herself as a “human Barbie doll” (who also has kind of scary opinions about some things ) — but a 2012 short documentary for Vice’s My Life Online series also posits that she believes she’s a time traveling space alien whose purpose on Earth is to aid us in moving “from the role of the ‘human consumer’ to the role of ‘human demi-god.’”

What I can’t quite figure out is whether this whole time traveling space alien thing is, like a piece of performance art created specifically for this Vice doc, or whether it’s what she actually thinks. I don’t believe she’s referenced it in many (or maybe even any) other interviews she’s given; the items I’ve found discussing Lukyanova and time travel specifically all point back to this video.

But, well… do with it all as you will. That’s the documentary up there; give it a watch and see what you think.

The Hipster Time Traveler

In the early 2010s, a photograph depicting the 1941 reopening of the South Fork Bridge in Gold Bridge, British Columbia in Canada went viral for seemingly depicting a man that looked… just a bit too modern to have been photographed in 1941. He looks, in fact, like a time traveling hipster : Graphic t-shirt, textured sweater, sunglasses, the works. The photo hadn’t been manipulated; the original can be seen here . So what the heck was going on?

Well, Snopes has plenty of reasonable explanations for the man’s appearance; each item he’s wearing, for example, could very easily have been acquired in 1941. Others have also backed up those facts. But the bottom line is that it’s never been definitively debunked, so the idea that this photograph could depict a man from our time who had traveled back to 1941 persists. What do you think?

Father Ernetti’s Chronovisor

According to two at least two books — Catholic priest Father Francois Brune’s 2002 book Le nouveau mystère du Vatican (in English, The Vatican’s New Mystery ) and Peter Krassa’s 2000 book Father Ernetti's Chronovisor : The Creation and Disappearance of the World's First Time Machine — Father Pellegrino Ernetti, who was a Catholic priest like Brune, invented a machine called a “chronovisor” that allowed him to view the past. Ernetti was real; however, the existence of the machine, or even whether he actually claimed to have invented it, has never been proven. Alas, he died in 1994, so we can’t ask him, either. I mean, if we were ever able to find his chronovisor, maybe we could… but at that point, wouldn’t we already have the information we need?

(I’m extremely skeptical of this story, by the way, but both Brune’s and Krassa’s books swear up, down, left, and right that it’s true, so…you be the judge.)

Although I'm fairly certain that these accounts and stories are either misinterpreted information or straight-up falsehoods, they're still entertaining to read about; after all, if you had access to a time machine, wouldn't you at least want to take it for a spin? Here's hoping that one day, science takes the idea from theory to reality. It's a big ol' universe out there.

A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time. 

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future. 

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly. 

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them? 

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers 

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible. 

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time 

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox 

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on … 

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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How I Became Obsessed With Accidental Time Travel

The web is awash with ordinary peoples’ stories of “time slips.” Their real magic is what they can tell us about our relationship to time.

By Lucie Elven

This year, I turned 30, a development that came with a breathless sense of dread at time’s passing. It wakes me up in the early mornings: Nocturnal terror breaks through the surface of sleep like a whale breaching for air. My ambition and fear kick in together until I get up, pour myself some water and look out the window at the squid-ink sky and the string of lights along my neighbors’ houses. I lie down again after finding firmer mental ground, dry land.

So when a guy that my friend was seeing evangelized about “time slips” — a genre of urban legend in which people claim that, while walking in particular places, they accidentally traveled back, and sometimes forward, in time — I was a ripe target. Curious and increasingly existential, I Googled these supposed time slips. I found a global community of believers building an archive of temporal dislocations from the present. These congregants gathered in corners of the internet to testify about how, in the right conditions, the dusting of alienation that settles over the world as we age can crystallize into collective fiction.

I was initially skeptical of the vague language that time-slip writers employed to convey experiences I already found dubious: too many uses of foggy words like “blunder” and “sporting”; detail lavished on varieties of hats encountered. But I was drawn in by their secretive tone — I sensed that sharing these anecdotes was compromising, even shameful (“People would laugh at you,” one poster wrote). Disapproval became attraction, and I returned to the message boards throughout the summer.

Here’s a classic that, like the best of these stories, was related secondhand on a paranormal blog: In a Liverpudlian street in 1996, an off-duty policeman named Frank was going to meet his wife, Carol, in a bookshop called Dillons when “suddenly, a small box van that looked like something out of the 1950s sped across his path, honking its horn as it narrowly missed him.” More disorienting still, Frank “saw that Dillons book store now had ‘Cripps’ over its entrance” and that there were stands of shoes and handbags in the window instead of new fiction. The only other person not wearing midcentury dress was a girl in a lime green sleeveless top. As Frank followed her into the old women’s wear boutique, “the interior of the building completely changed in a flash”; it was once again a bookshop.

I found a global community of believers building an archive of temporal dislocations from the present.

As with a spell of déjà vu, the experience was short-lived, and time was regained. According to the blogger’s detective-like report, Cripps “was later determined” to have been a business in the 1950s. In response to Frank’s slip, posters have told their own or related accounts they’ve heard from others: “This happened to my ex-boss, Glyn Jackson in London, England,” one begins. “Glyn’s story is Highly believable as Glyn is person who lacks imagination on such a scale that he could not put together a grade one story for English to save his life.” And on it goes.

I have never appreciated stories about the passage of time. I resent that I won’t ever get back the hours of my life that Richard Linklater stole with “Boyhood” — his two-and-three-quarter-hour film, shot over a 12-year period in which time is the force that overwhelms everything, not least the idea that our own actions drive our life stories. There’s a whole lot of unwelcome profundity there.

Time-slip anecdotes, though fashioned out of the ambient dread of living with the ticking clock, are childlike in their sense of wonder. They are light, playful and irrational, as frivolous and folky as a ghost story if it were narrated by the confused ghost instead of the people it haunts. One poster, as a girl, used to see a woman in a blue bathrobe in her room: “Her hair was long and messy, a reddish brown. I didn’t see her face because she was usually turned away. I used to mistake her for my mom.” Years later, grown up, the poster’s daughter slept in her former bedroom. “One day I realized ... I was wearing the same blue bathrobe,” the mother writes. Paranormal trappings aside, this story speaks to the feeling of whiplash brought on by time’s passing.

Slipping can be significant, as any Freudian will tell you, and these narratives are riddles whose answers might tell us about our relationship to time. I have begun considering the message boards on which they are exchanged to be narrow but important release valves, allowing posters to talk about the feelings that arise from being time-bound: depression, midlife crises, the dysmorphia of living in a human body. What ailed Miss Smith, whose car slid into a ditch after a cocktail party, and who witnessed “groups of Pictish warriors of the late seventh century, ca. 685 AD,” if not an understanding of her smallness in history’s vast expanse? Why did two academics, famous in the time-slip community for writing a book about spotting Marie Antoinette in the Versailles grounds, encounter trees that looked lifeless, “like wood worked in tapestry”? Perhaps in that instant, like the last queen of France’s Ancien Régime, they felt radically out of joint with their present moment.

If you suspend disbelief, you’ll find these threads constitute a philosophical inquiry about the place of the spirit in our physical beings. They debate the merits of subjectivity and objectivity and question the idea that time is a one-lane highway to death. These writers argue that our past and future can suffuse our present, unveiling an epic dimension of our quotidian existences in moments when we slip and, like Frank, feel eternity.

Lucie Elven is a writer whose first book of fiction, “The Weak Spot,” was published this year in the United States by Soft Skull Press and in Britain by Prototype.

Background photograph: George Marks/Getty Images

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Time Travel Proof: The Mounting Evidence Of A Broken Timeline

Time traveling tourists & other things.

Evidence of time travel is not something to be ignored — these periodic blips of stories, pictures, and artifacts are nothing less than blatant signs that our universe is in peril. The space-time continuum is breaking apart!

Eight years ago, I shared the first batch of this evidence. A handful of photographs and video clips that many believed were proof that time travel not only existed, but that individuals from the future had already visited us. Some, it was claimed, had even journeyed back to ancient times.

But they left footprints. They dropped artifacts. They were mistakenly caught on camera. Others deliberately shared knowledge of their strange voyages.

Others, if their stories are to be believed, were not quite so deliberate — they were like you or me. Ordinary people caught in time slips, temporarily whisked away to other eras, or perhaps even other universes.

Who were these alleged time travelers? These are some of their stories…

• The 1917 Photograph

The Mummy’s Modern Boots

• Project Pegasus & Nikola Tesla’s Jump Room
• A Watch Found In A Chinese Tomb
• Time Traveler Bridge Photo
• Charlie Chaplin Time Traveler
• An 1800s CD Case
• The Time Slip Hotel
• A Mysterious Near-Miss

Time Traveler In 1917 Photograph

It’s an odd thing when something, or someone, doesn’t quite fit in. At first, you may not notice. But upon closer inspection, the anomaly becomes clear.

So is the case with this seemingly ordinary photograph taken in 1917 Canada, found in a 1974 book titled The Great History of Cape Scott . It shows a group of people sitting upon the rocks of a beach, but among the crowd is a man who appears suspiciously out of place.

Many believe his clothes do not match those of the other beach-goers. Wearing a tee shirt, shorts, and with a hair style that seems quite modern in comparison, the mysterious individual hits all the check boxes for a person displaced in time.

In the photo, others even appear to look at him, perhaps in confusion.

This particular time traveler is colloquially known as the “surfer dude.”

Others, however, raise incredible questions.

For some time now, many have wondered about the existence of the so-called “Adidas Mummy,” the preserved body of a 30 to 40-year-old woman discovered in the Altai mountains region of Mongolia in 2016. She had been buried there for about 1,100 years.

Those studying her found that she had likely died of a “blow to the head.” However, her feet drew the most attention.

She wore what looked uncannily like modern day shoes, perhaps resembling a pair of snowboarding boots, as the Daily Mail mused in 2017 . They were made of felt, with splashes of bright red, and “knee length” with leather soles.

Their unique design, likened to Adidas, led many to question if perhaps the ancient Mongolian woman was in fact a time traveler. Perhaps, even if the shoes themselves were not from current times, they were made, or possibly inspired, by someone from the future utilizing the same style.

Among her other belongings was what archaeologists described as a “beauty kit,” including a mirror, a comb, and a knife. However, scientists brushed off any claims that she was a time traveler, though they did say the style of the boots was “very modern.”

Nikola Tesla’s Jump Room

For years, Andrew Basiago has told the story of Project Pegasus, the alleged covert time travel initiative that was funded and maintained by the United States government throughout the 1960s and 70s. But perhaps the most interesting detail of his peculiar story is the existence of the so-called Jump Room.

It was in this room that Basiago claimed participants such as himself performed their secretive time travel experiments.

Built using designs by the late Nikola Tesla, recovered from his New York apartment shortly after his death, the room housed what some would call a teleportation machine. It consisted of a “shimmering curtain” made of “radiant energy,” a special, and allegedly as-yet-undisclosed, form of energy that is “latent and pervasive” throughout the cosmos. It is, if this story is to be believed, what makes time travel possible.

The curtain stood between two elliptical booms, and by passing through it, would-be travelers could journey across vast distances — and time itself. Basiago claims to have visited Gettysburg using the jump room, and that the experiments even took him as far as the planet Mars.

Beyond that, there are few other details about the mysterious Jump Room, and no physical proof remains. Did it really exist?

Evidence Of Time Travel In A Chinese Tomb?

A story from 2008, originally published by none other than the Daily Mail , claimed that Chinese archeologists found a watch in a 400-year-old Si Qing tomb in Shangsi County, China. This story then went extremely viral, after I shared my original article on it (“Are These Images Proof Of Real Time Travel,” published March 22, 2012). The images, which are still scattered around out there (source unknown), show a group of archeologists examining a stone block within a room, as well as the singular image of someone holding up a tiny piece of metal in the shape of a watch.

Allegedly, the timepiece was “frozen” at 10:06, with the word “Swiss” engraved on its back. However, the object looked much more like a sculpted piece of stone than an actual functioning watch, not to mention the fact that its circumference appeared smaller than that of a finger.

If I were charitable, I could say that perhaps 400 years ago some people carved this to look like a watch someone had seen, perhaps one worn by a visiting time traveler. That, or I’m incorrect and it is an actual watch and the time traveler was just very tiny. Maybe we all do shrink in the future, like the Shrinking Man Project suggests we should. You never know! Moving on!

Time Traveler Visits A Bridge?

A strange photo, once available at the Virtual Museum Of Canada (which has since been decommissioned), appeared to show a group of people attending the reopening of the South Fork Bridge in 1941. The bridge was located in Gold Bridge in British Columbia, Canada, and quite a few people showed up for it. But like the 1917 Canadian surfer dude mentioned earlier, someone in this photo seemingly doesn’t belong.

Who was this oddly out-of-place individual, wearing what looks like modern clothing all the way back in the 1940s? Who knows? The photo went absolutely viral some time around 2010, with the man in question becoming dubbed the “Time Traveling Hipster.”

What’s fun about this one is that, time traveler or not, it’s a real photo. The man certainly stands out from his cohorts – he’s wearing sunglasses, possibly a hoodie or light jacket, and what appears to be a branded t-shirt. He’s also holding a relatively small camera. Was he simply ahead of his time? Or out of it? We can’t be sure, but but the story of this photograph is one to remember. Next!

Lights! Camera! Action!

Next up we have two, count them two, short videos which many claim show time travelers unwittingly caught in the act of…talking on the phone!

Indeed, smartphones are the bane of any would-be time traveling tourist, as they seem like the most obvious thing to be spotted in photos, videos, and even paintings .

The above video, uploaded October 27, 2010, currently has over 4.3 million views, and contains a clip from a special feature found on the DVD edition of the Charlie Chaplin film The Circus. It’s of the movie’s premiere at Grauman’s Chinese Theatre in 1928.

But what’s so special about it? What’s worth 4.3 million views on YouTube? Well, you see that woman in the dark coat walking behind zebra? What’s she holding up to their ear? Is it a cellphone? Or was she just scratching her head, like the rest of us?

Meanwhile, from over in 1938 Massachusetts, we have the following video that some also suggest proves the existence of smartphones long before their time (or have at least mused about the possibility). A crowd of people exit a DuPont factory, and one woman is seen holding something to her ear.

Smartphones, brushes, other handheld rectangular objects of a dubious nature. Our universe is one filled with mystery, and some questions may forever remain unanswered. Onward!

A Compact Disc Case In The 1800s?

A painting from the 1800s appears to show a man holding what some have described as a fancy CD box. You can see another man depicted to be lifting up what looks like a square plastic “sleeve” that you’d place individual CDs into. The earliest form of plastic didn’t exist until the mid-1800s, and Compact Discs wouldn’t arrive on the scene until the 1980s.

Of course, it’s possibly this is just an ordinary box made for ordinary 1800s-era items.

Safety Not Guaranteed?

In 1997, an unusual classified ad appeared in Backwoods Home Magazine:

“WANTED: Somebody to go back in time with me. This is not a joke. P.O. Box 322, Oakview, CA 93022. You’ll get paid after we get back. Must bring your own weapons. Safety not guaranteed. I have only done this once before.”

As you can imagine, some people found that odd, and it naturally wound up on the Internet. In 2012, they even made a film based on it called, well, Safety Not Guaranteed .

The Time Slip Hotel: Proof That Even Buildings Time Travel?

It’s not unusual to, on occasion, find yourself at what you might describe as a “strange” place, especially if you happen to be in a country you’ve never visited before. Two couples who vacationed to Spain in the late 1970s know this all too well. In fact, their story has become something of a time slip legend.

In 1979, they were on their way to Spain, leaving England and venturing through France. While near Montelimar, they decided to stop and search for a hotel. Their original choices were packed, but eventually they did find one — an odd place, an old two-story building with the simple word “HOTEL” above the entrance.

They got a room, and stayed for the night, despite everything seeming particularly old-fashioned. The bed was hard, the place didn’t have a telephone, and there was no glass in the windows. Likely out of bemusement, they took a number of photographs.

When they went down to the dining room the next morning, they had a chance to see the hotel’s other customers. They too looked strangely out of place, dressed in outmoded clothing, two in old uniforms. Strangest of all was the bill for their stay — 19 francs, far less than they had anticipated.

All in all, it was a somewhat strange experience, but not unpleasant. They’d found a place to rest for the night, and continued on their way to Spain, where they presumably had a decent time. It wasn’t until they returned to France and looked for the hotel again that they knew something was truly amiss.

The hotel didn’t exist.

This wasn’t a matter of the hotel simply shutting down, or their being unable to find it again. It simply was not there. They even asked around in Montelimar, and no one had ever heard of it.

When they returned to England, the mystery deepened: Their photographs of the hotel were gone. Not only was there no proof that the hotel existed, but there was no proof that they’d ever been there at all.

Their odd tale was later featured on an episode of the television series Strange But True .

Is One Man’s Near Miss Proof of Time Travel?

In 2019, a very curious video from Turkey showed a man nearly meeting his fate, only to be saved in the strangest way by a mysterious passerby.

According to Demirören News Agency , the event happened in late February in the Turkish city of Adana. Serdar Binici was standing outside of his store when another man, who happened to be walking by, tapped his left shoulder.

Binici, for reasons even he claims to not understand, instinctively looked over to his right, and in that moment a truck drove by, and its rear metal door swung open toward his head. Binici was able to move out of the way just in time.

The story was featured by news outlets in Turkey, and was eventually shared on Reddit .

During a follow-up interview, Binici questioned how and why the events played out as they did. Why did he look over his opposite shoulder, and not the one he’d been tapped on? Why did the stranger tap his shoulder at all? Binici supposes the stranger may not, himself, know why any of this happened. Perhaps, he thinks, it was all some kind of divine intervention.

However, the peculiarity of this video has led many to speculate that the passing stranger wasn’t just some random person, but actually a time traveler — possibly one on a mission from the future, striving to put right what once went wrong. Could this incident be yet more proof of real time travel?

Somewhere In Time

Even if these stories and strange photographs are not evidence of anything, these tales are part of our shared paranormal folklore, and have become online urban legends. Some, like the tale of Rudolph Fentz, date back decades. And while others are easily disproved, they stand as fun little anecdotes that do make you wonder, what if?

At the end of the day, it’s all just a bunch of wibbly-wobbly, timey-wimey…stuff. Best not to take it too seriously. Still, if you’re looking for more on possible time travelers, consider checking out the following:

• Rudolph Fentz , an accidental time traveler from a series of short stories
• John Titor , the alleged time traveler from 2036
• The HDR , an alleged time traveling device powered by crystals
• A Wormhole Under A Kitchen Sink?

Rob Schwarz

Pocket-lint

The best images of time travellers from throughout history.

Every now and then an image appears online which people claim shows a time traveller somewhere they shouldn't be. Are they real though?

Every now and then an image appears online which people claim shows a time traveller somewhere they shouldn't be. But are they just cases of people letting their imaginations run wild?

We've rounded up some of the best and most interesting images of time travellers throughout history. Some turned out to be plain fakes or cases of mistaken identities, but others are certainly intriguing.

Which have you seen before?

• The most famous ghost photographs ever taken
• 35 of the most famous alien, monster and unexplained photographs ever taken

The time travelling hipster

This photo was snapped in 1941 at the re-opening ceremony for the South Fork Bridge in British Columbia.

If you look carefully, on the right-hand side you can see an unusually dressed man in what appears to be modern clothing, sporting sunglasses at a time when most were wearing hats and smart jackets.

Many argue this is a time traveller, while others have countered that he's simply a man with a fashion sense ahead of his time. Snopes has shown his clothing is relevant to the time and the area, but it's still great to imagine.

World Cup celebrations

This photo comes from the 1962 World Cup and shows the celebrations as the Brazilian team lifts the trophy.

If you look closely though, you'll see in the bottom centre of the image what looks like someone with a mobile phone snapping a photo of the event.

Could this be a time traveller as well? A bit odd to think someone in the future might have a flip phone , but then they have been making a comeback recently and we know folding phones are about to be big too .

The time travelling sun seeker

This image from 1943 apparently shows British factory workers escaping to the seaside for a break during the midst of wartime. The clothes and beachwear of most people certainly fit the era, but in the centre of a frame appears to be a man dressed like Mr Bean checking his mobile phone.

Or maybe it's a time travel device? Likely a bit of a stretch or a case of overactive internet imagination, but we still enjoy the thought. Maybe there are no public beaches in the future?

Mohawk time traveller

This image from 1905 appears to show the usual happenings of the time - including workers and a banana boat delivering its goods.

However, if you look near the edge of the boat you can spy a man in a white shirt with what appears to be a Mohawk-style haircut. A very unusual haircut for the time and possible proof of a time traveller? Who can say?

Film footage captured during the recording of Charlie Chaplin's 1928 silent film "The Circus" appears to show a lady dressed all in black, wearing a hat and walking around the set talking on her mobile phone.

The footage is a little iffy as is the idea that anyone could be talking on a mobile device in the 1920s, but it's certainly got some suggesting it might be proof time travellers are among us.

The ancient astronaut sculpture

In Salamanca, Spain, there's a cathedral with multiple sculptures carved into its sides. One such sculpture appears to show the likeness of a modern-day (or perhaps futuristic) astronaut.

Considering the cathedral's construction dates back to 1513, people have taken this as proof that time travellers made their way back to that time. However, the truth is the astronaut is merely a modern addition to the artwork carried out by Jerónimo García de Quiñones during renovations in 1992.

Time travelling celebrities

There's an interesting trend of people who closely resemble folks from a bygone era. This could just be a spooky coincidence, but maybe it's proof that time travel is possible.

Perhaps these celebrities are living a double life in another century. Here, Marxist-Leninist Revolutionary Leader Mahir Cayan who was born in 1946 and died in 1972 is shown to bear a striking resemblance to TV star Jimmy Fallon. Is Jimmy Fallon living a double life as a revolutionary communist? Seems hilariously unlikely.

A man and his mobile phone

Some claim that this oil painting by Pieter de Hooch, which was lovingly crafted in 1670 appears to show a young man holding his mobile phone. In an age where such a thing would probably have seen him burnt at the stake, this one is hard to believe.

A description of the image also suggests the young man is a messenger and that's a letter in his hand, not a phone, but it's still nice to let your imagination run wild once in a while. We've often wondered what it would be like to be able to travel back to simpler times to see what life was like for ourselves.

The Adidas trainers mummy

A couple of years ago, an ancient mummy was unearthed by archaeologists digging in Mongolia. At the time, it was suggested the funky-looking footwear she was wearing bore a striking resemblance to Adidas trainers. More evidence of a time traveller visiting ancient times? Investigation of the body dated it around 1,100 years old. That's one heck of a blast through the past.

However, further unearthing showed the woman was more likely to have been a Turkic seamstress which might explain the fresh kicks. She was found with an ancient clutch bag, a mirror, a comb, a knife and more. But no mobile phone.

The time surfer

Another image of an out-of-place individual that people have latched on to as proof that time travel is a reality.

This image dates back over 100 years and shows some smartly dressed Canadians sitting on the side of a hill.

On the left-hand side though, sits a young man in what appears to be a t-shirt and shorts with ruffled hair. He was quickly referred to as the surfing time traveller due to how unusual his attire is. Others have suggested people in the photo appear shocked by his appearance, even pointing out the woman on the right who seems to be gesturing in his direction. Again, this a bit of a stretch as would a time traveller really go through time dressed like that?

A visitor to wartime Reykjavík

This photograph apparently shows a scene from downtown Reykjavík in 1943.

In the heart of wartime, soldiers and sailors can be seen everywhere in the streets among civilians. The man circled though, appears to be on a mobile phone.

We've really got a theme going with these smartphone using time travellers. Who is he calling? And how? And if he is a time traveller, why is he not in Berlin trying to assassinate Hitler?

The dabbing WWII soldier

There's an apparent theme to these time traveller photos that not only includes smartphone users, but also people visiting the second world war.

In this image, a young soldier is seen dabbing, a dance move that became popular around 2014, but certainly wasn't known in wartime.

Of course, it turns out this photo isn't an image of a time traveller, but rather just an image of some actors from 2017's blockbuster Dunkirk . The fact that most of the soldiers are smiling should also be a bit of a giveaway with this one.

Greta Thunberg

In 2019, the internet discovered a photograph from 1898 which showed three children working at a gold mine in Canada's Yukon territory.

The image seemed to show a girl with an incredible likeness to the young climate activist Greta Thunberg. Does this make Thunberg a time traveller who's come through time to save the planet? Weird year for her to choose, but it's a nice idea.

A woman clutching a smartphone (1860)

The painting " The expected one " from 1860, by Ferdinand Georg Waldmüller appears to show a woman walking along a rough path, about to be accosted by an adoring young man clutching a pink flower.

A close look though and you'll see she appears to have her attention firmly glued to a modern smartphone. Is this woman actually a time traveller?

Vladimir Putin

A few years back, a number of images surfaced online that seemingly showed Russian President Vladimir Putin snapped over various decades without ageing. Either proof that he's a time traveller or perhaps just immortal?

If true, he's incredibly patriotic, with each image showing him serving his country in one way or another. Though it's more likely to just be a strong likeness.

The AI time traveller

Here's a time traveller with a difference. Stelfie the Time Traveller has been using AI to travel through time. Or at least to give the illusion of doing so.

This creative individual has been using Stable Diffusion to insert the likeness of a modern man into ancient civilisations including Egypt when the pyramids were being constructed, Rome with the centurions and the land of the dinosaurs . It's fun to imagine these as being real, though if you look closely they're clearly AI-generated. As this artificial intelligence improves we'll no doubt get even better images like this. Interestingly even the character taking the selfies isn't real here, but is also made using AI.

ORIGINAL RESEARCH article

Conversational time travel: evidence of a retrospective bias in real life conversations.

• 1 Department of Psychology, University of Zurich, Zurich, Switzerland
• 2 University Research Priority Program “Dynamics of Healthy Aging”, University of Zurich, Zurich, Switzerland
• 3 Department of Psychology, University of Arizona, Tucson, AZ, United States

We examined mental time travel reflected onto individuals’ utterances in real-life conversations using a naturalistic observation method: Electronically Activated Recorder (EAR, a portable audio recorder that periodically and unobtrusively records snippets of ambient sounds and speech). We introduced the term conversational time travel and examined, for the first time, how much individuals talked about their personal past versus personal future in real life. Study 1 included 9,010 sound files collected from 51 American adults who carried the EAR over 1 weekend and were recorded every 9 min for 50 s. Study 2 included 23,103 sound files from 33 young and 48 healthy older adults from Switzerland who carried the EAR for 4 days (2 weekdays and 1 weekend, counterbalanced). 30-s recordings occurred randomly throughout the day. We developed a new coding scheme for conversational time travel: We listened to all sound files and coded each file for whether the participant was talking or not. Those sound files that included participant speech were also coded in terms of their temporal focus (e.g., past, future, present, time-independent) and autobiographical nature (i.e., about the self, about others). We, first, validated our coding scheme using the text analysis tool, Linguistic Inquiry and Word Count. Next, we compared the percentages of past- and future-oriented utterances about the self (to tap onto conversational time travel). Results were consistent across all samples and showed that participants talked about their personal past two to three times as much as their personal future (i.e., retrospective bias). This is in contrast to research showing a prospective bias in thinking behavior, based on self-report and experience-sampling methods. Findings are discussed in relation to the social functions of recalling the personal past (e.g., sharing memories to bond with others, to update each other, to teach, to give advice) and to the directive functions of future-oriented thought (e.g., planning, decision making, goal setting that are more likely to happen privately in the mind). In sum, the retrospective bias in conversational time travel seems to be a functional and universal phenomenon across persons and across real-life situations.

Introduction

Live in the here and now – so goes a common credo. However, one of the most remarkable skills of humans is not their ability to have their minds set on the present, but, rather, to engage in mental time travel. The human cognitive apparatus is a powerful time travel machine, allowing us to almost effortlessly project ourselves into the future to simulate possible future events, as well as put ourselves back into the past to relive our past experiences ( Suddendorf and Corballis, 2007 ). Recently, psychologists have started to emphasize that memory (e.g., autobiographical memory) and prospection (e.g., future-oriented thought) are closely related phenomena that share many common qualities (e.g., Schacter et al., 2012 ; Klein, 2013 ; Rasmussen and Berntsen, 2013 ). Thinking or talking about our past and future are such natural, moment-to-moment activities that we do not notice or wonder how often we recall our memories or imagine our future in everyday life. Psychologists have started to explore this prevalence question using a range of self-report methods (e.g., diary method, experience-sampling) with a focus on participants’ thoughts. Here, we used, for the first time, an ecological behavioral observation method that is free of self-report to examine the prevalence of mental time travel behavior in everyday conversations (i.e., conversational time travel). Using the Electronically Activated Recorder (EAR; Mehl et al., 2001 ), we unobtrusively and intermittently sampled snippets of ambient sounds and speech from participants’ natural lives, and extracted information about their moment-to-moment conversations.

The first goal of the current research was to develop and validate a new, naturalistic observation approach to studying mental time travel reflected in everyday conversations. We listened to and coded participants’ recorded utterances in terms of whether they (a) had a time reference or not and (b) were about the self versus others. We tapped onto mental time travel by focusing on those utterances that were about the self with a time reference. In two studies, we validated our coding scheme using a text analysis program and with adult samples representing different age groups and countries.

The second goal of the current research was to examine how often people engage in conversational time travel, and when they do, how often they talk about their past versus future. There is some work on how much people think about their personal past versus future in everyday life (e.g., Klinger and Cox, 1987 ; Berntsen and Jacobsen, 2008 ; Gardner and Ascoli, 2015 ), but no work on how much they talk about their past versus future. The solitary nature of thinking versus the social nature of talking should have different effects on mental time travel (e.g., Kulkofsky et al., 2010 ), which has methodological and theoretical implications. Humans spend 32–75% of their waking time with other people ( Mehl and Pennebaker, 2003 ). That is, much of human behavior occurs in a social context, therefore we examined, for the first time, mental time travel in the context of conversations. We unobtrusively observed and objectively coded the overt behavior of talking in everyday life to examine mental time travel reflected in people’s utterances.

Prevalence of Past- and Future-Oriented Thoughts in Everyday Life

Previous studies measuring the incidence of subjective thoughts and experiences have typically used variants of the original experience-sampling method (ESM; Csikszentmihalyi et al., 1977 ). One type of ESM is event-contingent sampling (e.g., diary method; Berntsen, 2007 ) in which diary entries are prompted by participants through introspection and detection of the occurrence of a target event. The second type is signal-contingent sampling , which requires people to evaluate the presence of a targeted experience when prompted by a randomly timed signal (e.g., Pasupathi and Carstensen, 2003 ). One important advantage of these methods is their high ecological validity.

Using the diary method, D’Argembeau et al. (2011) explored the frequency of thinking about the personal future in everyday life. They asked participants to report whenever they realized that they were thinking about their future and found that participants reported experiencing, on average, 59 future-oriented thoughts on a typical day. In contrast, Rasmussen and colleagues (i.e., Rasmussen and Berntsen, 2011 ; Rasmussen et al., 2015 ) examined the frequency of thinking about the personal past (i.e., autobiographical memories) using the diary method. They made a distinction between voluntarily thinking about memories versus involuntary memories (which spontaneously pop up without deliberate search) and compared their frequency. They showed that participants self-reported recalling on average 7–8 voluntary and 20–22 involuntary autobiographical memories per day. Taken together, these two studies suggest that young adults think about their personal future twice as much as their past. Berntsen and colleagues ( Berntsen and Jacobsen, 2008 ; Finnbogadottir and Berntsen, 2013 ), however, did not replicate this finding. They used the diary method to examine involuntary mental time travel and compared the frequency of involuntary memories and involuntary future-oriented thoughts. They found that involuntary memories were as frequent as involuntary future-oriented thoughts in daily life (around 22 per day).

In a signal-contingent experience sampling study, Gardner et al. (2012) examined the frequency of thinking about the personal past (i.e., autobiographical memories) in everyday life. Via random prompts throughout the day, they asked young adults to report whether they were thinking about a specific autobiographical memory at that moment or not. They found that the probability of being caught while recalling a specific autobiographical memory was 15%. However, in a second study using the same method, Gardner and Ascoli (2015) found this probability to be 10%. It was unclear to the authors why this small discrepancy occurred, but they suggested it might be due to the investigation of both past- and future-oriented thoughts in the second study. They found that participants thought about their future about 13% of the time. The second study’s results are in line with an early signal-contingent experience sampling study: Klinger and Cox (1987) have shown that people rated 12% of their momentary thoughts as focused on their past and 12% on their future (versus 67% on their present). However, Felsman et al. (2017) found a large difference between past- and future-oriented thoughts. Via text messages throughout the day, they asked participants to report which of the following would best characterize their thoughts: past-, present- or future-focused. They found that people reported focusing much more on the future (26%) than the past (8%), with present as the most frequent category (66%).

Signal-contingent experience sampling has been used to examine involuntary thoughts, as well, particularly mind-wandering. Mind-wandering is defined as a shift of attention from a primary task in the present toward internal information or self-generated thought, such as autobiographical memories ( Smallwood and Schooler, 2006 ). Song and Wang (2012) examined the temporal orientation of mind wandering by randomly prompting participants throughout the day and asking whether they were mind wandering or not, and if mind wandering, whether they were thinking about past, future, present or atemporal events. They found a prospective bias such that participants were mind wandering about the future (40.53%) twice as much as the past (21.53%). We should note that this prospective bias has been repeatedly shown in laboratory studies of mind wandering (e.g., Smallwood et al., 2009 ; Baird et al., 2011 ; Stawarczyk et al., 2011 ). However, researchers have identified some factors that affect the temporal orientation of mind wandering, with some eliminating the prospective bias, such as manipulating the experimental settings (e.g., response options and cues; Jackson et al., 2013 ; Vannucci et al., 2017 ), and controlling for participant characteristics such as familiarity with the task ( Smallwood et al., 2009 ) and mood ( Poerio et al., 2013 ).

In sum, all studies reviewed above are conducted in the real world, focused on thoughts (retrieved voluntarily and/or involuntarily) and based on self-report (i.e., diary method and signal-contingent experience sampling). They have resulted in two different findings on the prevalence of thinking about the personal past versus future: Some reported that future-oriented thoughts occur almost twice as frequently as past-oriented thoughts, whereas others reported similar proportions of both.

Temporal Orientation

Another line of research that is relevant for our work is time perspective or temporal orientation. Temporal orientation refers to relatively stable individual differences in the relative emphasis one places on the past, present, or future ( Zimbardo and Boyd, 1999 ). Temporal orientation has been widely examined in relation to personality traits (e.g., Zhang and Howell, 2011 ), academic outcomes (e.g., Horstmanshof and Zimitat, 2007 ), risky behaviors (e.g., Daughterty and Brase, 2010 ), and health outcomes (see Stolarski et al., 2015 for reviews). Temporal orientation is usually assessed with surveys, such as the Zimbardo Time Preference Inventory (ZPTI; Zimbardo and Boyd, 1999 ) and Balanced Time Perspective Scale (BTPS; Webster, 2011 ). Jason et al. (1989) interviewed women (mean age = 31) and asked them to rank their past, present and future by the amount of thinking time devoted to each temporal focus. Women self-reported thinking about their present 41% of the time, their future 38% of the time and their past 21% of the time. This finding is similar to others (reviewed above) showing that future-oriented thoughts occur twice as much as past-oriented thoughts.

There is only one study on temporal orientation that is not based on self-report: Park et al. (2016) have created a novel language-based measure of temporal orientation: They have developed a model to automatically classify individuals’ social media messages as oriented toward the past, present or future (model accuracy = 72%). They used the model to classify over 1.3 million Facebook status updates (i.e., short text messages) written by 5,372 individuals aged 13–48. They found that 65% of messages were present-oriented, 19% were past-oriented and 16% were future-oriented. This result does not fit with the questionnaire findings above and presents an equal proportion of past- and future-oriented messages.

In conclusion, studies based on self-report (i.e., diary method, signal-contingent experience sampling, questionnaires) and one study based on a linguistic analysis of social media messages ( Park et al., 2016 ) have resulted in two findings: a prospective bias versus an equal proportion of past- and future-oriented thoughts. All of these studies have focused on thoughts, therefore, we can conclude that future-oriented thoughts tend to dominate our private mental worlds compared to past-oriented thoughts.

In contrast, our social worlds might be dominated by past-oriented thoughts: Humans spend one fifth of their waking time in spontaneous conversation ( Dunbar, 1998 ) and a significant portion of this time is dedicated to talking about past events ( Eggins and Slade, 1997 ; Dessalles, 2018 ). According to Desalles (2007) , the function of recalling the past is to accumulate stories that are relevant to tell in conversation. He claims that events that are memorable are exactly those that are good for narrating. Similarly, Mahr and Csibra (2018) argue that the main function of remembering is communication. They claim that social interactions require the justification of entitlements and obligations, which is possible only by reference to past events. In sum, these theoretical accounts highlight the importance of recalling the personal past in conversations. Therefore, we examined, for the first time, mental time travel in conversations and explored whether there is a retrospective bias in talking behavior, in contrast to the prospective bias observed in thinking behavior (e.g., Felsman et al., 2017 ).

Overview of the Present Studies

The most important novelty of this work is its naturalistic observation approach to studying spontaneous, everyday conversations unobtrusively and with minimal participant burden. We used the Electronically Activated Recorder in both studies to collect random snippets of everyday conversations. The EAR is a portable audio recorder that intermittently records brief snippets of ambient sound and speech ( Mehl et al., 2001 ). It captures acoustically detectible aspects of participants’ environments, such as their locations, activities and social interactions ( Mehl, 2017 ). The strength of the current work is its attempt to increase ecological validity through sampling from a wide range of natural situations: We obtained a huge sample size by collecting more than 32,000 sound snippets.

The EAR has been used with good acceptance and compliance ( Mehl, 2017 ), in all age groups ( Bollich et al., 2016 ; Demiray et al., 2017 ) with healthy and clinical populations (e.g., Robbins et al., 2014 ). The psychometric properties of EAR-observed conversational behavior have been established in prior research with student ( Mehl and Pennebaker, 2003 ) and adult populations ( Bollich et al., 2016 ). Study 1 has been approved by the Institutional Review Board of the University of Arizona, and Study 2 was approved by the Ethics Committee of the University of Zurich. We have implemented a series of safeguards to protect participants’ privacy and to ensure data confidentiality. First, the EAR recorded only a small fraction of the day (e.g., 2.5% when sampling 30 s). Second, participants had the opportunity to review their recordings and erase any files they did not want on record, before the investigators accessed the data. Third, in order to protect bystanders, we encouraged participants to wear the EAR visibly (with large warning stickers on them) and to readily mention the study to others. Finally, although sound files included bystanders’ utterances, we only coded and analyzed the utterances of our participants (for a detailed discussion of EAR privacy and confidentiality policies, see Mehl, 2017 ; Robbins, 2017 ).

In order to examine mental time travel as reflected in participants’ utterances, we developed a novel coding scheme: We, first, coded whether participants’ utterances were time-dependent (i.e., had a reference to time) or time-independent (e.g., semantic memory such as “The name of the restaurant is Satchel’s”; Suddendorf et al., 2009 ). Next, we coded whether time-dependent utterances were about the self (i.e., autobiographical) or about others (e.g., vicarious memories; Pillemer et al., 2015 ). Finally, in order to tap onto mental time travel, we focused on the autobiographical, time-dependent utterances: We coded for “personal past” when the participant was talking about personally experienced past events (e.g., “I visited my grandparents last week”). “Personal future” was about anything that will/might or not happen in one’s future (e.g., “Next year I’m starting my MA degree”). Finally, when the participant was talking about their current activity, task or situation, we coded for “present” (e.g., “This show is boring, let’s change the channel”).

In Study 1, we validated our coding scheme using the Linguistic Inquiry and Word Count (LIWC), which is currently the most extensively validated text analysis tool in the social sciences (LIWC; Pennebaker et al., 2007 ). In Study 2, we validated the coding scheme with different samples. Previous studies on mental time travel have mostly focused on (1) college students or young adults, (2) one culture, with no cross-cultural comparisons, (3) experiences of a single temporal focus, such as only autobiographical memories or only future-oriented thoughts (e.g., Pasupathi and Carstensen, 2003 ; Mace, 2004 ; Kvavilashvili and Fisher, 2007 ; Schlagman and Kvavilashvili, 2008 ; Schlagman et al., 2009 ; D’Argembeau et al., 2011 ; Rasmussen and Berntsen, 2011 ; Gardner et al., 2012 ). Important and novel aspects of the current work is the inclusion of (1) participants that represent the whole adult life span, (2) participants from two countries, and (3) both past- and future-oriented utterances. We compared the prevalence of past- and future-oriented utterances across young, middle-aged and older adults in the United States and Switzerland. Study 1 examined the utterances of healthy spouses of breast cancer patients over a weekend (United States), and Study 2 examined the utterances of healthy young and older adults over 4 days (Switzerland). In addition to sampling such a wide range of individuals, one novel achievement of this work is its sampling from the universe of real-life situations.

This study is part of a larger project on American couples coping with breast cancer. Breast cancer patients and their healthy spouses were recruited at the Arizona Cancer Center, as described in earlier work that examined cancer conversations of couples ( Robbins et al., 2014 ). For the purposes of our research, we focused only on the spouses’ utterances. The reason we used this dataset is that it was the only readily available dataset with Ear transcripts that we could use to develop our coding scheme.

The first goal of Study 1 was to validate our coding scheme using the LIWC ( Pennebaker et al., 2007 ). We used LIWC to count specific words in participants’ utterances. We first compared utterances manually coded as time-dependent and those coded as time-independent in terms of the following LIWC variables: future-tense and past-tense. We expected time-dependent utterances to include significantly more verbs with tense than time-independent utterances. Second, we compared autobiographical (self-related) and others-related utterances in terms of personal pronouns: We expected self-related utterances to include more 1st person singular and plural pronouns, whereas others-related utterances to include more 2nd and 3rd person pronouns. Finally, utterances coded as personal past, personal future and present were compared in terms of their verb tense. We expected, for example, utterances about the personal past to include more verbs with the past tense than utterances about the present and personal future.

The second goal of Study 1 was to examine the prevalence of mental time travel in participants’ utterances, and to specifically compare the frequency of past- versus future-oriented utterances. Recent theories on episodic memory ( Desalles, 2007 ; Mahr and Csibra, 2018 ) suggest that the main function of remembering the past is communication. Past research on autobiographical memory emphasizes significant social functions of memories showing that people recall their personal past to provide material for conversation ( Pasupathi et al., 2002 ), to update others about what is ongoing in their life ( Webster, 2003 ), to create/enhance intimacy in relationships ( Alea and Bluck, 2007 ), to elicit empathy for others ( Bluck et al., 2013 ) and to teach and inform others ( O’Rourke et al., 2017 ). In contrast, future-oriented thinking is shown to serve directive functions such as planning, decision making, problem solving, goal intention and goal achievement (e.g., Szpunar, 2010 ; D’Argembeau et al., 2011 ; Schacter et al., 2017 ). Such directive functions should be inherently private and more likely to occur when people are thinking alone ( O’Rourke et al., 2017 ). For example, Kulkofsky et al. (2010) have shown that private reminiscence favors directive functions (which guide current and future behavior), whereas social contexts are associated with memories that have higher social functions. Thus, we expected to observe significantly more autobiographical memories (i.e., past-oriented utterances) than future-oriented utterances in the social setting of conversations with others. That is, we expected a retrospective bias in conversational time travel in contrast to the prospective bias observed in mental time travel (e.g., Jason et al., 1989 ; Song and Wang, 2012 ).

Materials and Methods

Our sample of real-life situations included 9,010 sound snippets collected from 51 healthy spouses. Out of 51 spouses, 44 were male (86%). Participants were on average 59 years old (Range: 26–94, SD = 14). Eighty-two percent of participants were Caucasian (n = 42), 15% Latin American (n = 8), and 2% Asian (n = 1). All participants were in a marriage-like relationship, and were primarily English speaking. Each couple received $150 for their participation.

The first study session usually occurred on a Friday afternoon. All participants, first, gave written informed consent in accordance with the Declaration of Helsinki. They, then, completed a set of questionnaires as part of the larger study, and were provided with an introduction to the EAR. They were told that the device should be worn as much as possible over the weekend during their waking hours. They were informed that the EAR would record 50 s of ambient sound at a time for a total of approximately 10% of their waking hours. Participants were informed that the snippets would be recorded without their awareness and they should proceed with their normal, everyday life as much as possible. They were also told the EAR would cease recording during sleeping hours. All participants were explicitly told they would have an opportunity to review all recordings prior to anyone listening to them and to erase any files they did not want on record. Following that weekend, typically on the Monday, the EAR devices were collected from the participants and another battery of questionnaires, which included demographics and medical information, was administered. Participants were debriefed and given a password-protected Cd containing all of their sound files to review. There were over 9,000 sound files collected and of those only one participant deleted just one file.

The EAR was software programed on an Hp iPaq 100 handheld computer. The device was set to record 50 s every 9 min. This sampling rate has been established in previous studies as yielding stable estimates of habitual daily behavior ( Mehl et al., 2012 ). The device was housed in a protective case affixed to participants’ waistlines, and an external microphone (Olympus Me-15) was attached to participants’ lapels. The EAR was preprogrammed to not record for 6 h during the participants’ predefined normal sleep hours, starting 30 min after they indicated they typically go to sleep. The EAR recorded participants’ waking days, from the time the participant received the device until they went to sleep on Sunday. This yielded an average of 176 (Sd = 57) valid, waking sound files (approximately 2.4 h of data per participant), which was defined as a file where the participant was wearing the Ear with no technical difficulties, while the participant was awake.

EAR-Derived measures: coding of sound files

All sound files were listened to, transcribed and coded by trained coders. Files were coded, as part of the larger project, for whether the participant was talking or not. For the goals of the current study, we developed a coding scheme for the temporal focus of participants’ utterances (See Table 1 for examples, and note that we make all coding guidelines available upon request to interested researchers). We first differentiated between time-dependent versus time-independent utterances. Time-independent utterances had no reference to time and included semantic memory (i.e., general knowledge about the world, such as “Paris is the capital of France”) and personal comments, beliefs, preferences, attitudes about anything in general (e.g., “He’s really nice”). Time-dependent utterances included a reference to time (i.e., past, present, and/or future) and were divided into autobiographical (self-related) and others-related categories. Autobiographical utterances were about the personal past, present moment and personal future, whereas utterances about others focused on others’ past and others’ future. Personal past refers to talking about personally experienced past events: These could be specific events (that happened at a particular place and time), repeated events (e.g., “I used to go to the gym every day”), extended events (e.g., “our 2-week vacation last Christmas”), and long periods of life (e.g., “When I lived in the United States”; Conway et al., 2004 ). In contrast, others’ past refers to talking about other people’s past experiences (i.e., the participant did not experience the event himself/herself). Personal future refers to anything that will/might or will/might not happen in one’s future (e.g., “We will not go to the movies”). Others’ future refers to talking about other people’s future experiences, which the participant is not personally involved in (e.g., “They might go skiing next week”). Finally, utterances about the present refer to talking about the current activity, task or situation. This also includes extremely recent past and extremely close future, which is connected to the present moment (e.g., “I just washed the potatoes and I am going to cook the veggies now”). There is no “others’ present” category, as the participant has to be there to observe others’ present activity, which automatically involves the participant’s present. Utterances such as “David is at the cinema” were coded as “time-independent,” as semantic knowledge.

TABLE 1. Examples of each coding category.

All coding categories were dichotomous, indicating presence (1) or absence (0) of a temporal focus. In addition, each sound file was coded in a TIME column with 1 = personal past, 2 = others’ past, 3 = present, 4 = personal future, 5 = others’ future, 6 = time-independent (See Table 2 for examples). Categories were not mutually exclusive, such that any 50-s sound file might include any combination of temporal foci. For example, if one talked about both personal past and others’ past within the same sound file, they received a 1 for both temporal categories and a “1–2” for the TIME variable. Each sound file was double-coded by two coders. We calculated inter-rater reliability by using the TIME variable, but not the single temporal focus variables separately: The two coders agreed on the TIME variable 64.12 % of the time. This calculation of inter-rater reliability was much stricter than calculating inter-rater reliability for each temporal focus separately: It is less likely to obtain agreement in the TIME variable, especially in specific cases such as a coding of “1–3–4,” than obtaining agreement separately in single columns (e.g., separately for 1 = personal past, 3 = present, 4 = personal future). Nevertheless, all sound files that showed a disagreement between the two coders were re-listened to and the disagreement was resolved through discussion.

TABLE 2. Examples of the coding scheme.

Text analyses

Transcriptions of utterances were analyzed using LIWC ( Pennebaker et al., 2007 ). LIWC software is one of the most widely used and best-validated text analysis tool in psychological science (e.g., Pennebaker et al., 2003 ; Tausczik and Pennebaker, 2010 ). LIWC analyzes text word-by-word and categorizes it into different linguistic (e.g., pronouns, prepositions) and psychological categories (e.g., emotion words, social words). It creates a percentage of word use (specific category/total number of words) by categories for each participant. In the current study, we used the following categories: past-tense, future-tense, present-tense and all personal pronouns.

A total of 4,100 sound files included participant speech (45.5% of valid sound files). We were unable to code for temporal focus in 747 sound files (18%) due to the brevity or vagueness of speech. The average number of words in these transcripts was four (e.g., “The what? Oh yes,” “Me, um, I guess”) and there were many cases with information that could help identify participants (e.g., names). We excluded 115 sound files (3.4 %) that were related to cancer in order to examine only ordinary daily conversations. Analyses were conducted with the remaining 3,238 sound files: In order to run the following analyses of variance, this dataset with one sound file on each row (sound-level dataset) was converted into a person-level dataset (one row is one participant) which aggregated data on the person level. Note that we make all data available upon request to interested researchers.

Validation of the Coding Scheme

The first goal of Study 1 was to validate our coding scheme using the Linguistic Inquiry Word Count ( Pennebaker et al., 2007 ). Participants’ verbatim EAR transcripts were submitted to LIWC. We first compared utterances manually coded as time-dependent and those coded as time-independent in terms of their verbs with past-tense and future-tense. We conducted a repeated-measures MANOVA and found that time-dependent utterances included significantly more verbs with past tense ( M = 4.58, SD = 1.38) than time-independent utterances ( M = 2.62, SD = 2.19); F (1,45) = 29.64, p < 0.001, η p 2 = 0.40. Similarly, they included significantly more verbs with future tense ( M = 2.18, SD = 0.68) than time-independent utterances ( M = 0.78, SD = 0.99), F (1,45) = 62.41, p < 0.001, η p 2 = 0.58. That is, utterances that we had coded as time-dependent included more verbs with past and future tense than utterances coded as time-independent, which validated our coding.

Second, we compared autobiographical (self-related) and others-related utterances in terms of the number of their personal pronouns. We aggregated the number of pronouns on the person level, conducted a repeated-measures MANOVA and confirmed our expectations: We found that self-related utterances included significantly more 1st person singular pronouns ( M = 5.29, SD = 1.42; F (1,43) = 132.58, p < 0.001, η p 2 = 0.76) and 1st person plural pronouns ( M = 1.22, SD = 0.75) than others-related utterances (singular: M = 1.19, SD = 1.89; plural: M = 0.12, SD = 0.45), F (1,43) = 75.31, p < 0.001, η p 2 = 0.64. In addition, we found that the number of 2nd person pronouns ( M = 5.85, SD = 4.98), 3rd person singular pronouns ( M = 3.64, SD = 3.73), and 3rd person plural pronouns ( M = 2.01, SD = 3.27) in others-related utterances was significantly higher than the number of 2nd person pronouns ( M = 3.60, SD = 1.31), 3rd person singular pronouns ( M = 1.41, SD = 1.01), and 3rd person plural pronouns ( M = 1.01, SD = 0.55) in self-related utterances, F s(1,43) ranged 4.21–8.32, η p 2 ranged 0.09–0.29, p s < 0.05. That is, utterances that we had coded as autobiographical were more about the self with pronouns such as “I,” “me,” “we,” and “us,” whereas others-related utterances were more about second and third persons (e.g., “you,” “he,” “she,” “they,” and “him”).

Finally, we validated our conversational time travel coding by comparing utterances manually coded as personal past, personal future and present in terms of their verb tense. We conducted a repeated-measures MANOVA and found, as expected, that utterances coded as personal past included a significantly higher number of verbs with past tense ( M = 8.39, SD = 2.61) than utterances coded as personal future ( M = 0.88, SD = 0.98) and present ( M = 1.83, SD = 0.90), F (1,39) = 294.72, p < 0.001, η p 2 = 0.88. In contrast, we found that utterances coded as personal future included a significantly higher number of verbs with future tense ( M = 2.48, SD = 2.06) than utterances coded as personal past ( M = 0.66, SD = 0.75) and present ( M = 0.69, SD = 0.48), F (1,39) = 28.25, p < 0.001, η p 2 = 0.42. Finally, we confirmed that utterances coded as present included a significantly higher number of verbs with present tense ( M = 15.62, SD = 2.41) than utterances coded as personal past ( M = 8.60, SD = 2.62) and personal future ( M = 13.92, SD = 4.76), F (1,39) = 34.47, p < 0.001, η p 2 = 0.47. In sum, all of our expectations regarding our coding categories were confirmed and we succeeded in validating the coding scheme with LIWC.

Frequency of Past- Versus Future-Oriented Utterances

The second goal of Study 1 was to examine the prevalence of mental time travel in participants’ utterances, and to compare the frequency of past- versus future-oriented utterances. In order to calculate percentages, we used the sound files that included only a single temporal category (e.g., only personal past, only present or only future) and excluded those that involved more than one temporal focus (e.g., sound file that includes both personal past and personal future). This allowed us to take “sound file” as the unit of analysis and use those sound files that had a single temporal category to clearly count the frequencies of purely past- versus future-oriented sound files.

There were 2,297 sound files that included only one temporal category (Figure 1 , top row). Out of these, 17.5% were time-independent and included utterances presenting semantic memory or personal preferences, ideas and beliefs (Figure 1 , second row). Out of the sound files that were time-dependent, 93% were about the self and 7% were about other people (Figure 1 , third row). Utterances about others were further categorized as others’ past ( N = 92, 68.7%) and others’ future ( N = 42, 31.3%). Sound files that included self-related utterances were further divided into past (17.8%), present (72.9%), and future categories (9.3%) to present mental time travel (Figure 1 , bottom row). That is, participants talked about their personal past in 13.6% of all their sound files and about their future in 7.2% (This means they engaged in conversational time travel in 20.8% of their sound files).

FIGURE 1. Study 1: frequencies and percentages for each temporal category. All sound files (100%) refers to all sound files that include speech with no technical problems.

We ran a repeated-measures ANOVA to compare the number of past-, present-, and future-oriented utterances. For this analysis, we used the aggregate person-level amount of talking about the past, present versus future. We found that people talked significantly more about their past ( M = 6.08, SD = 4.53) than their future ( M = 3.17, SD = 2.60), t (51) = -5.47, p < 0.001. Furthermore, pairwise comparisons showed that present-oriented utterances ( M = 24.71, SD = 14.51) were significantly more frequent than both past- and future-oriented utterances, F (2,50) = 78.69, p < 0.001, η ρ 2 = 0.76.

We observed, over a weekend, the daily conversations of healthy spouses of breast cancer patients and developed a coding scheme for the temporal focus of their utterances. The first goal of the study was to validate our coding scheme using a text analysis tool: We succeeded and showed that utterances manually coded as (1) time-dependent versus time-independent, (2) self-related versus others-related, and (3) past-, present- versus future-oriented were indeed different from each other in terms of the words they included.

The second goal of the study was to explore the prevalence of conversational time travel in everyday life and to compare the frequency of past- and future-oriented utterances. Our coding scheme first revealed that individuals mostly produced time-dependent utterances (82.5% of all sound files). Semantic information and general comments about the world occurred in only 17.5% of the sound files. Second, we found that individuals talked in a self-referential way most of the time: 77% of all sound files and 93% of time-dependent sound files included autobiographical utterances. In contrast, participants talked about other people in only 5.8% of the sound files. This suggests that vicarious memories ( Pillemer et al., 2015 ) and vicarious future-oriented utterances (e.g., Grysman et al., 2013 ) are quite rare in daily conversations. This is the first study to examine the prevalence of vicarious thoughts about others and to explore them in everyday life, therefore these findings may inspire future work.

Finally, we examined mental time travel as reflected in autobiographical utterances and found that 13.6% of sound files were about the personal past, whereas 7.2% were about the personal future. That is, people talked about their personal past almost twice as much as their personal future, and the difference was significant. This is in line with our expectation of a retrospective bias in the social setting of conversations (e.g., Eggins and Slade, 1997 ). Participants referred to their past much more than their imagined future while interacting with others. This is in contrast to previous work on private thoughts: While thinking, people seem to focus more on the future than the past (e.g., Andrews-Hanna et al., 2010 ) or focus equally on both (e.g., Klinger and Cox, 1987 ; Gardner and Ascoli, 2015 ). One explanation might be that recalling past events (i.e., autobiographical memories) may be more useful than simulating future events in social interactions (e.g., Desalles, 2007 ). We know that talking about memories serves social functions such as creating/enhancing feelings of intimacy, feeling empathy toward others, creating/enhancing conversation, teaching and giving advice (e.g., Alea and Bluck, 2003 ; Webster, 2003 ; O’Rourke et al., 2011 , 2013 , 2017 ). In contrast, prospection may be more functional while thinking, as private thoughts tend to serve higher directive functions such as setting goals, planning and decision making (e.g., Kulkofsky et al., 2010 ; Szpunar, 2010 ; D’Argembeau et al., 2011 ). For example, Rasmussen and Berntsen (2013) asked participants in the laboratory to remember two events from their past and to imagine two events from their future, and to rate each event on their perceived functions. Past events were rated higher than future events on the social function, as well as on their frequency of being shared with others. Cole et al. (2016) also asked participants to recall past events and imagine future events using a laboratory paradigm and found that people reported thinking about future events more often than past events. In sum, we believe that the social nature of conversations creates an efficient context for memories to be recalled in everyday life.

Present was the most frequent category, with 60% of all sound files being about the current activity or situation. This shows that while people are talking, more than half of their utterances are focused on what they are actually doing or observing (i.e., goal pursuit, Klinger, 2013 ). This is in line with previous work: In two experience-sampling studies, individuals rated 66% and 67% of their momentary thoughts as focused on the present ( Klinger and Cox, 1987 ; Felsman et al., 2017 , respectively). Similarly, Park et al. (2016) showed that 65% of participants’ social media messages were present-oriented. In sum, present orientation is found to occupy about 60–67% of both our thoughts and utterances, as assessed with three different methodologies.

Study 1 had some limitations. The sample included partners of cancer patients. This may have biased the situation samples toward a present- or past-orientation. However, only 3.4% of situation samples included conversations about the cancer, which we eliminated from our analyses, therefore we assume that there should be a minimal bias. Still, it is an open question to which degree the situation samples would differ with a population that is not associated with cancer. Furthermore, most of the participants were men and middle-aged. Therefore, in Study 2, we tried to obtain more gender-balanced samples from both young and late adulthood. A second limitation was that sound files were collected over a weekend. Although 2 days of EAR sampling has proven to yield reliable data (e.g., Mehl and Pennebaker, 2003 ), it is important to show that our findings are not an artifact of sampling situations insufficiently or sampling situations over a weekend. Therefore, in Study 2, we collected data across 1 weekend and 2 weekdays, with a counterbalanced order. Another limitation was that our inter-rater reliability calculation was overly strict, which led to a lower agreement between coders than expected. In Study 2, we used the same strategy for consistency across studies, but also used a less strict way of calculation. Finally, we had to exclude from the analyses all sound files with multiple temporal foci (e.g., both past- and future-oriented utterances in one sound file), as our unit of analysis was the “sound file.” In Study 2, we used the same strategy for consistency across studies, but also ran additional analyses with all sound files without any exclusions.

In Study 2, we validated our coding scheme with two new samples from a different country. We observed healthy young and older adults in Switzerland for 4 days. Our goal was to examine whether the coding scheme used in Study 1 would lead to similar results with participants (1) from different age groups, (2) from Switzerland who speak a different language (i.e., Swiss German), and (3) who were observed for a longer period of time that also included weekdays.

We expected our finding on conversational time travel to be replicated: Past-oriented utterances should outnumber future-oriented utterances independent of age group, country of origin (and language) and sampling rate of EAR. In terms of age effects, Park et al. (2016) found that across all age groups (between ages 13–48), the rank order of past, present and future orientation remained the same: Present-oriented social media messages were the most frequent, followed by past-oriented and then future-oriented messages (the difference between past and future was very small). However, there were some differences in the relative proportion of each orientation across age. We expected to find similar results, with a retrospective bias in conversational time travel for all age groups. There are no cross-cultural studies on mental time travel, but we did not anticipate country of origin to have a major impact, as mental time travel is a universal human ability ( Suddendorf and Corballis, 2007 ). In terms of sampling rate effects, Gardner and Ascoli (2015) tested different sampling intervals in their experience-sampling study (e.g., weekend versus weekday, early in day versus late in day) and found no significant effect on the prevalence of past- versus future-oriented thoughts. We also did not expect sampling rate to affect our results.

Our sample of real-life situations included 9,827 sound snippets collected from 33 young adults (19–31 years, M = 23.76, SD = 3.03; 10 men, 23 women) and 13,276 sound snippets collected from 48 healthy older adults (62–83 years, M = 70.54, SD = 4.65; 22 men, 26 women). Participants were recruited via the participant pool of the Gerontopsychology Lab at the University of Zurich, via flyers in university buildings and advertisements in a local newspaper, and through snowball sampling used by a research assistant. All participants lived in Switzerland and spoke Swiss German. Young participants were mostly university students, with number of years of education ranging between three and 17 ( M = 12.18, SD = 2.32). Sixty-nine percent of them were single, whereas 31% were in a long-term relationship.

Older participants were healthy with no record of neurological or psychiatric illness and lived independently. 60% were married (with 4 couples within the sample) and 40% were divorced, widowed or single. Forty-six percent lived alone, 44% lived with one person in the same household and the remaining 10% lived with more than one person in the same household. Number of years of education ranged between seven and 25 ( M = 10.55, SD = 3.02). An inclusion criterion for the study was a minimum score of 27 on the Mini Mental State Examination (MMSE; Folstein et al., 1975 ) and all participants were above this cut-off score ( M = 29.2, SD = 0.84). Older participants were compensated with 50 Swiss Francs, whereas young participants could choose between 50 Swiss Francs and research credits.

Participants met the researchers for an introduction session, after which data collection with the EAR started. Data collection spanned four consecutive days. Finally, participants met with the researchers again for a feedback session.

Introductory laboratory session

Participants came to the Psychology Institute for the first session, typically held on a Wednesday or a Friday afternoon. Six older participants were visited at home for their convenience. Participants were given instructions on the study, asked to sign an informed consent form and to complete questionnaires including demographic and psychological measures. All questionnaires were administered in a group setting except for the MMSE which was administered privately. Next, participants received their assigned iPhone with its protective case and charging cable. They were asked to think of the iPhone as a “recorder,” as it was set to “Airplane mode” and locked with only the EAR application on. They were reminded to carry the iPhone as much as possible over the next 4 days during their waking hours. They were told that the EAR would record 30 s of ambient sounds at a time, and that they would not be aware of when the EAR was recording, so that they could continue their normal lives. They were also informed that they would have the opportunity to review and delete any sound files at the end of the study, before anyone listened to them.

EAR data collection

Data collection spanned 2 weekdays and 1 weekend in counterbalanced order: 46 participants started data collection on a weekday (Thu, Fri, Sat, Sun) and 35 participants started on a weekend (Sat, Sun, Mon, Tue). Over these 4 days, participants carried the iPhone either clipped to their waistline or in their pockets. They did not have to do anything with the iPhone other than carrying it and charging it every night. Participants also filled out a short diary each day, in which they reported their main activities throughout the day and indicated when they were and were not carrying the EAR and whether they preferred any sound files from a certain time slot to be deleted due to privacy reasons.

Final laboratory session

After 4 days of data collection, typically on a Monday or a Wednesday, participants returned to the Psychology Institute or were revisited at home. The researcher collected the iPhones, the charging cables and the diaries, and administered a second questionnaire packet. The packet included psychological measures, as well as a questionnaire in which participants evaluated their experience with carrying the iPhone (e.g., degree to which they and others were aware of the EAR, degree to which carrying the iPhone changed their behavior). While participants filled out the questionnaires, the researcher downloaded the recorded sound files onto a lab computer and checked whether there were any problematic files. As participants had the right to listen to their sound files, the researcher burned a CD that included all of their files. Participants could either review their sound files in the lab and permanently delete any files they wished to have deleted, or they could receive the CD to review at home and inform the researcher within 10 days about any deletion requests. In the young group, 9 participants deleted between 1 and 40 sound files, 87 in total. In the old group, 6 participants deleted between 2 and 25 sound files, 46 in total.

Each participant was provided with an iPhone 4S which had the EAR application installed (version 2.3.0). The app was programmed to record 30-s sound snippets every 15 min, but with 100% randomization so that recordings were randomly distributed throughout the day (72 per day). The app was active for four consecutive days, 18 h per day with a blackout period between midnight and 6 AM each day (72 days × 4 days = 288 recordings per participant). In total, only 2.5 % of the participant’s day (i.e., 36 min) was recorded, which kept possible intrusions into participants‘ private lives on a minimal level. The iPhone was set to “Airplane mode” and locked with a screen-lock password, therefore the participants could not access the EAR settings or use the phone for other purposes. Participants were instructed to charge the iPhone overnight, but as a reminder the phone calendar was programmed to automatically beep every evening at 9 PM.

Similar to Study 1, each sound file was coded in terms of whether the participant was talking or not, and if talking, for the temporal focus of the participants’ utterances (Table 1 ). All coding categories were dichotomous (1 versus 0) indicating presence or absence of a category. Similar to Study 1, we also had the TIME variable (1 = personal past, 2 = others’ past, 3 = present, 4 = personal future, 5 = others’ future, 6 = time-independent). In Study 2, we improved this variable and made it much more fine-grained by adding all possible combinations of temporal foci (1–2 = personal past and others’ past, 1–3 = personal past and present, 1-2-3 = personal past, others’ past and present, and so on). Furthermore, we created a new DOMINANT TIME variable, which categorized every sound file that includes more than one temporal focus in terms of which temporal focus is best represented (Table 2 ). This new variable allowed every sound file (every unit of analysis) to have a single temporal focus, which allowed us to include all sound files in our analyses.

All sound files were listened to and coded by two trained coders. Similar to Study 1, when we used the strict strategy of calculating inter-rater reliability using the TIME variable, reliability was 62.12%. However, in this study, we also used a lenient strategy: We calculated inter-rater reliability separately for each temporal focus which led to higher agreement between the coders (Personal past = 90.88%, Others’ past = 94.77%, Present = 77.43%, Personal future = 94.87%, Others’ future = 96.92%, Time-independent = 80.83%). All sound files that showed a disagreement between the two coders were re-listened to and the disagreement was resolved through discussion among the two coders.

Preliminary Analyses

In the young sample, a total of 2,087 sound files included participant speech (21%). We were unable to code for temporal focus in 167 sound files (8%) due to the brevity or vagueness of speech. Of the remaining 1,920 sound files, 255 included more than one temporal focus (13%). The remaining 1,665 sound files included only a single temporal focus. The older sample had 2,590 sound files with (21%) participant speech. Out of these, temporal focus was unidentifiable in 336 files (13%). Of the remaining 2,254 files, 315 included more than one temporal focus (14%). The remaining 1,939 files included only a single temporal focus.

Major Analyses

The goal of Study 2 was to use our validated coding scheme to examine the prevalence of past- versus future-oriented utterances and to replicate Study 1 results (i.e., retrospective bias in conversational time travel). Analyses were conducted in two ways: (1) Similar to Study 1, with sound files that included only a single temporal focus, and (2) with all sound files that included both single and multiple temporal foci, by using the new DOMINANT TIME variable.

Young adults

(1) Similar to Study 1, we first ran analyses with only the sound files that included a single temporal focus. We found exactly the same percentages for the young adults’ time-dependent versus time-independent, and self-related versus others-related sound files (Figure 2 , first three rows). That is, similar to Study 1 participants, young Swiss adults referred to time in 82.6% of their sound files and talked about semantic memory or personal comments in 17.4%. Again similar to Study 1, out of the time-dependent sound files, 92% were about the self and 8% were about others. Utterances about others were further divided into others’ past (79.4%) and others’ future (20.6%). Sound files that included self-related utterances were further divided into past (14.6%), present (80.5%) and future categories (5%) to present mental time travel (Figure 2 , bottom row). This is where young adults diverged slightly from Study 1 participants. They talked about their personal past in 11% of all their sound files and about their future in about 4% (14.9% of total conversational time travel in their sound files). We conducted a repeated-measures ANOVA to compare the aggregated person-level amount of past-oriented utterances with future-oriented utterances and found that young adults talked significantly more about their past ( M = 5.58, SD = 4.21) than their future ( M = 1.85, SD = 1.58), F (2,31) = 58.16, p < 0.001, η ρ 2 = 0.079.

FIGURE 2. Study 2, Young adults: frequencies and percentages for each temporal category. All sound files (100%) refers to all sound files that include speech with no technical problems.

Similar to Study 1 participants, young adults’ utterances about others were divided into others’ past (78.5%) and others’ future (21.5%). Once more, present was the most frequent category (61%, M = 30.97, SD = 17.30), significantly more frequent than both past- and future-oriented utterances, pairwise comparisons: t ranges -8.71 to 10.09, p s < .001. As expected, the retrospective bias in conversational time travel was replicated with the same rank order of present, past and future orientation ( Park et al., 2016 ).

(2) Next, we calculated percentages with the new DOMINANT TIME variable and used all sound files, including those with multiple temporal foci. We found almost the same percentages as in Figure 2 (See Supplementary Figure 1A ). The only difference was that the percentages slightly increased for conversational time travel: Personal past was the dominant temporal focus in 12.7% of the sound files, whereas personal future was the dominant temporal focus in only 5.5% of the sound files (as opposed to 11.1% versus 3.8% in Figure 2 ). We conducted a repeated-measures ANOVA and found that young adults talked significantly more about their past ( M = 7.36, SD = 4.66) than their future ( M = 3.21, SD = 2.56), F (2,31) = 55.07, p < 0.001, η ρ 2 = 0.078. In summary, this shows that the two different ways of calculating percentages led to similar results.

In addition, we created Venn diagrams of autobiographical, time-dependent utterances to take a more detailed and closer look at conversational time travel frequencies. As depicted in Figure 3 , young adults referred to their personal past ( N = 315) much more than their personal future ( N = 139).

FIGURE 3. Study 2, Young adults: basic Venn diagrams for the frequencies of autobiographical, time-dependent utterances.

Older adults

(1) Similar to Study 1, we first ran analyses with only the sound files that included a single temporal focus. We found that older adults had very similar percentages to the young (Figure 4 ). Ten percent of older adults’ sound files were about the personal past, whereas only 2.7% were about the personal future (Figure 4 , bottom row). We were unable to conduct a repeated-measures ANOVA due to the non-normal distributions of the difference scores of each temporal focus (i.e., past-present, present-future, future-past) as shown by Shapiro–Wilk normality tests, W s ranged between 0.89 and 0.95, p s < 0.05. Therefore, we ran the non-parametric equivalent, Wilcoxon signed-rank test. We found that older adults talked significantly more about their past ( Mdn = 3.00) than their future ( Mdn = 1.00), V = 61.5, p < 0.001, r = -0.48.

FIGURE 4. Study 2, Older adults: frequencies and percentages for each temporal category. All sound files (100%) refers to all sound files that include speech with no technical problems.

Similar to Study 1, present (66%, M = 26.85, SD = 17.04) was significantly more frequent than both past- and future-oriented utterances, V ranges 1173–1176, p s < 0.001. As expected, the retrospective bias in conversational time travel was replicated with the same rank order of present, past and future orientation ( Park et al., 2016 ).

We also examined the interaction between age group (young, old) and temporal focus (past, present, future), which was non-significant, F (2,78) = 0.58, p = 0.56. This suggests that the rank order of present, past and future orientation was similar across the two age groups. Finally, we calculated these percentages separately for weekdays and weekends. For both age groups, the percentages are highly similar to the original percentages (See Supplementary Table 1 ). Therefore, we can conclude that the retrospective bias holds similarly in both weekdays and weekends.

(2) Next, we calculated percentages with the new DOMINANT TIME variable and used all sound files, including those with multiple temporal foci. We found almost the same percentages as in Figure 4 (See Supplementary Figure 1B ). Similar to young adults’ results, the only difference was that the percentages slightly increased for conversational time travel: Personal past was the dominant temporal focus in 11.2% of the sound files, whereas personal future was the dominant temporal focus in only 4% of the sound files (as opposed to 10.1% versus 2.7% in Figure 2 ). We conducted a repeated-measures ANOVA and found that older adults talked significantly more about their past ( M = 5.25, SD = 5.25) than their future ( M = 1.88, SD = 1.42), F (2,46) = 53.85, p < 0.001, η ρ 2 = 0.070. Thus, we can conclude that, for both young and older adults, the two different ways of calculating percentages led to highly similar results.

Finally, we created Venn diagrams of older adults’ autobiographical, time-dependent utterances. As depicted in Figure 5 , older adults referred to their personal past ( N = 347) much more than their future ( N = 114). In conclusion, the retrospective bias was confirmed with Swiss older adults, similar to Swiss young adults and American adults.

FIGURE 5. Study 2, Older adults: basic Venn diagrams for the frequencies of autobiographical, time-dependent utterances.

This study aimed to validate our coding scheme and to replicate Study 1 results. Hence, it built on Study 1 in three ways. First, we recruited both men and women, and obtained more gender-balanced samples. This helped us to validate our coding scheme with different samples and to test whether the retrospective bias observed in Study 1 (i.e., with mostly middle-aged men) would generalize to these samples. Indeed, we showed that the results were highly similar across American and Swiss adults from different age groups.

Second, we used different EAR sampling rates across the two studies and tested whether this would have an impact on the results. The duration (50 versus 30 s) and the distribution of recordings (every 9 min versus random) did not influence the results. The advantage of Study 2 was that we collected data across 4 days (i.e., 1 weekend similar to Study 1 plus 2 weekdays). We found no difference between the weekend and weekdays in terms of the prevalence of past- versus future-oriented utterances. This shows that people are more likely to talk about their past than their future on both weekends and weekdays.

Third, we built on Study 1 with new analyses that did not exclude sound files with multiple temporal foci. That is, we conducted analyses with (1) sound files that included only a single temporal focus, and (2) all sound files with single and multiple temporal foci (by using the new DOMINANT TIME variable). The two sets of analyses revealed very similar percentages for both young and older adults. In addition, analyses of variance showed the same results with past-oriented utterances being significantly more frequent than future-oriented utterances. This is an indicator of the robustness of our findings. In sum, for young and older adults, 10.1–12.7% of their sound files were about their personal past, whereas only 2.7–5.5% of their files were about their personal future. The retrospective bias in conversational time travel was replicated with the same rank order of present, past and future orientation as in Study 1.

General Discussion

This work is the first to examine mental time travel reflected in everyday conversations and to introduce the term “conversational time travel.” It is also the first to examine mental time travel using a naturalistic observation method. We used the EAR to observe the overt behavior of talking, rather than focusing on private thinking as has been done in previous work (e.g., Gardner et al., 2012 ). Much of human behavior and cognition occurs in social settings ( Mehl and Pennebaker, 2003 ), therefore we aimed to investigate whether the prevalence of conversational time travel is different from private mental time travel. Using the EAR also allowed us to evade possible limitations of the self-report method (e.g., memory errors, response biases, participant burden; Scollon et al., 2003 ), and to develop an ecological, objective and standard way of assessing conversational time travel. Furthermore, it allowed us to sample both across real-life situations and across individuals to maximize the diversity of situations to establish ecological validity.

The first goal of this research was to develop and validate a coding scheme for conversational time travel. We validated our scheme with a text analysis program (i.e., LIWC). We showed that utterances that we manually coded as time-dependent included more verbs with past and future tense than utterances coded as time-independent (i.e., semantic memory and general comments). Second, utterances that we coded as autobiographical were more about the self with first person pronouns (“I,” “we,” and “us”), whereas others-related utterances included more second and third person pronouns. Finally, we validated our mental time travel categories: Utterances coded as personal past included the highest number of verbs with past tense; utterances coded as personal future included the highest number of verbs with future tense and present-oriented utterances included the highest number of verbs with present tense. In sum, we succeeded in validating the whole coding scheme with LIWC.

Next, we validated our coding scheme with participants from (1) different age groups (i.e., young, middle-aged, old), (2) two countries that speak different languages (i.e., English and Swiss German), and (3) different EAR sampling designs. Across these different person samples, we acquired the same inter-rater reliability (with strict strategy in Study 1: 64.12% and Study 2: 62.12%). In Study 2, we also used a lenient strategy, which led to very high agreement between coders for conversational time travel (personal past = 90.88%, personal future = 94.87%). This suggests that our coding scheme is robust and reliable across different persons and situation samples. Furthermore, we achieved highly similar results across our person samples, which indicates that our results did not vary due to inconsistencies in coding across studies.

Retrospective Bias in Conversational Time Travel

The second goal of the current research was to compare the prevalence of past- and future-oriented utterances across young, middle-aged and older adults in the United States and Switzerland. Our results first revealed that individuals mostly produced time-dependent utterances in everyday life conversations (82.5–85.3% of all sound files across all samples). Semantic information and general comments about the world occurred in only 14.7–17.5% of the recorded situations across samples ( Suddendorf et al., 2009 ). This suggests that time mattered greatly for everyone while communicating with others. This is not surprising, as time is an inescapable aspect of our life-space ( Lewin, 1939 ) that shapes our lives, including our social interactions ( Webster, 2011 ).

Second, we found that individuals talked in a self-referential way across most of the situations: 76.2–79.1% of all sound files included autobiographical utterances across person samples. More specifically, 93% of time-dependent sound files included autobiographical utterances across samples. These percentages show that the majority of participants’ utterances were both time-dependent and autobiographical indicating that people tend to talk mostly about “self in time.” In contrast, participants talked about other people in only 5.8–6.4% of the sound files across all person samples. This suggests that vicarious memories ( Pillemer et al., 2015 ) and vicarious future-oriented utterances (e.g., Grysman et al., 2013 ) occur quite rarely in conversations. Bryant et al. (2013) , using signal-contingent sampling, also found that individuals experienced a higher number of self-related thoughts than others-related thoughts. Future research should further investigate the significance and functions of vicarious thoughts and utterances about others.

Finally, we examined mental time travel as reflected in participants’ autobiographical utterances and found that 10.1–13.6% of their sound files were about the personal past, whereas 2.7–7.2% were about the personal future. That is, individuals across samples talked about their personal past two to three times as much as their personal future. This is in line with our expectation of a retrospective bias in the social setting of conversations, and in contrast to previous work on private thoughts: While thinking, individuals seem to focus more on their future than their past (e.g., Song and Wang, 2012 ). Future-oriented thinking serves directive functions such as planning, decision making, problem solving, goal intention and goal achievement (e.g., Szpunar, 2010 ; D’Argembeau et al., 2011 ; Schacter et al., 2017 ). For example, Barsics et al. (2016) examined the functions of emotional future-oriented thoughts and found that participants self-reported four major functions: to plan actions, form intentions (i.e., to set goals), make decisions, and regulate emotions. Twenty percent of emotional future-oriented thoughts were rated as not functional and 5% were reported to involve other kinds of functions, such as daydreaming. Cole and Berntsen (2016) showed that participants’ future representations were more frequently related to their goals (i.e., current concerns) than their autobiographical memories. Furthermore, future-oriented mind wandering is found to be more self-related and directive than past- and present-oriented mind wandering ( Baird et al., 2011 ; Stawarczyk et al., 2011 ). All of these results show that future-oriented thoughts do not tend to serve social functions. Therefore, they are not highly frequent or relevant in social interactions. They are more useful when people are thinking alone, as directive functions seem to be inherently private ( Kulkofsky et al., 2010 ; O’Rourke et al., 2017 ).

In contrast, past research on autobiographical memories underlines significant social functions of memories showing that people recall their past to provide material for conversation ( Hyman and Faries, 1992 ; Pasupathi et al., 2002 ), to create/enhance intimacy in relationships ( Alea and Bluck, 2007 ), to elicit empathy for others ( Bluck et al., 2013 ) and to teach and inform others ( O’Rourke et al., 2017 ). For example, Demiray et al. (2017) examined how and why older adults reminisced about their past in real-life conversations. They coded participants’ utterances that included reminiscence in terms of their functions and found that reminiscence served mainly social functions (i.e., conversation, teaching) and did not serve any directive functions (e.g., problem solving, death preparation). Therefore, it is not surprising for us to have found a retrospective bias in conversational time travel: Social settings and cues seem to trigger the recall of autobiographical memories.

Indeed, Vannucci et al. (2017) showed, in spite of the widely observed prospective bias in mind wandering, that using external verbal cues in the experimental task changed the nature of mind wandering: They found that task-irrelevant verbal cues directed the temporal orientation of mind wandering toward the past. In the Verbal-cues group, 44.5% of mind wandering episodes were categorized as memories and 18.3% as future-oriented thoughts. In contrast, in the No-cues group, 28.3% were classified as memories, whereas 38.7% as future-oriented thoughts. Furthermore, Mazzoni et al. (2014) found that more involuntary memories were elicited when verbal cues rather than pictorial cues were presented, whereas there was no difference between the effects of verbal and pictorial cues on other spontaneous (and non-memory) thoughts. More generally, it has been shown that external/environmental cues primarily trigger past-oriented thoughts ( Berntsen and Jacobsen, 2008 ; Maillet and Schacter, 2016a ). All of these findings suggest that spontaneous past-oriented thinking is affected by external cues (rather than internal cues, such as mood), and especially by verbal cues ( Plimpton et al., 2015 ). This link between environmental cues and past-oriented thinking may be an important adaptive mechanism that allows individuals to relate the current situation to similar events experienced in the past, which might support adaptive behavior ( Maillet and Schacter, 2016b ). Conversations are strong verbal cues, which might be one factor underlying the retrospective bias we discovered in conversational time travel. In contrast, spontaneous future thinking is mainly related to and triggered by private concerns, being less dependent on external stimuli ( Klinger, 2013 ; Cole and Berntsen, 2016 ).

In sum, the retrospective bias in conversational time travel seems to be a universal phenomenon across situations and persons (e.g., Suddendorf and Corballis, 2007 ; Suddendorf et al., 2009 ), as all of our samples revealed very similar percentages. Although coming from different countries, age groups and research designs, all samples focused on their past much more than their future during conversations. Across all age groups, the retrospective bias in conversational time travel was replicated with the same rank order of present, past and future orientation ( Park et al., 2016 ). Past work shows that the frequency of recalling the personal past does not vary by age ( Webster, 1999 ; Pasupathi and Carstensen, 2003 ; Gardner and Ascoli, 2015 ). Our results on talking behavior are in line with this finding on thinking. Gardner and Ascoli (2015) found that older adults thought about their future twice (21%) as much as young adults (10%). We found, however, that older adults were quite similar to younger individuals in terms of the frequency of talking about the personal future. These findings contradict with the socioemotional selectivity theory ( Carstensen, 1992 ), which states that older adults have a less positive and open-ended future time perspective than young adults ( Demiray and Bluck, 2014 ). This suggests that one’s subjective and global perspective of their future may not be associated with how much they think or talk about their future in everyday life. Thus, future studies could examine conversational time travel via both subjective self-report and objective observation.

Methodological Issues in Measuring the Prevalence of Mental Time Travel

Previous studies measuring the incidence of subjective thoughts have typically used the experience-sampling method. However, event-contingent sampling (i.e., diary method, Berntsen, 2007 ) has some limitations. For example, in the case of examining involuntary autobiographical memories (spontaneously popping in mind), the method requires that the participant first understands what qualifies as an involuntary memory. Next, when a memory comes into awareness, the participant must retrospectively identify the experience as “memory retrieval” (Note that some may not be sufficiently activated to pass the awareness threshold; Barzykowski and Staugaard, 2017 ; Vannucci et al., 2017 ). Then, the participant must decide that the recollection is something worth reporting in the diary. All of these requirements create a cognitive burden to the participants and the risk that many memories may go undetected or ignored due to demotivation or exhaustion ( Hintzman, 2011 ; Vannucci et al., 2014 ; Barsics et al., 2016 ). Finally, informing participants about the phenomenon of interest may bias them toward thinking more about the past or toward voluntarily monitoring their thoughts ( D’Argembeau et al., 2011 ; Barzykowski and Niedźwieńska, 2016 ). Indeed, Barsics et al. (2016) showed, in their diary study, that participants reported having experienced more thoughts than usual because they were requested to record them. Due to these limitations, we do not think that the diary method is the ideal method to examine the natural frequency of past- versus future-oriented thoughts.

Signal-contingent sampling is advantageous over the diary method in that it allows for a random sampling of experiences and avoids expectancy effects ( Scollon et al., 2003 ). It is considered the gold standard for the assessment of cognitive or behavioral processes in everyday life, since recall biases and heuristic biases are minimized ( Shiffman et al., 2008 ). However, participant burden is still an issue and assessments may be reactive. Some participants have reported that the signals interrupted their thoughts, which might have led to confusion and possible misratings in the questionnaire ( Bryant et al., 2013 ). Similar to event-contingent sampling, making participants aware of study aims might affect their responses. For example, asking participants to perform a mental check at each signal on whether they had been thinking about a memory or not ( Gardner et al., 2012 ) might alter their experience. Indeed, research shows that participants who were asked to selectively report memories did this to a greater extent than participants asked to report any type of thought ( Vannucci et al., 2014 ; Barzykowski and Niedźwieńska, 2016 ; Barzykowski and Staugaard, 2017 ).

Therefore, automatized and unobtrusive methods that do not reveal study aims and that minimize participant burden, such as the EAR, are advantageous while examining observable phenomena that do not require self-report. They maximize ecological validity, as huge amounts of data can be collected without experimenter or participant burden, and contextual influences on experience can be detected ( Mehl, 2017 ). However, although the EAR is an ideal method to examine conversations, it cannot be used to assess thoughts. Thus, signal-contingent sampling method and the EAR should be combined as two strong ecological methodologies with different advantages ( Mehl et al., 2012 ). This should create a uniquely powerful way of studying thought processes in natural habitats with the fine-grained multi-method approach.

Limitations

One limitation of the current study is its sole dependence on the coding and analysis of overt speech data. A multi-method approach that also collects self-reports from participants could inform us about what is happening in participants’ minds. Experience-sampling method (merged with the EAR) could help us understand how and why individuals are engaging in conversational time travel in certain situations. This method would allow us to examine both thinking and talking behaviors within the same study and to compare how these two modalities shape mental time travel. A strength of our study, however, is that it demonstrates that meaningful information can be derived from the observation of real-life verbal activities. This may allow us to include (older) persons in research who may feel overly burdened or are unable to reliably self-report information and are, so far, excluded from research.

One limitation of our coding scheme is that it does not differentiate between self-related versus others-related time-independent utterances. This distinction was not within the scope of the current study, however, future research could enhance the coding scheme with two separate dimensions for temporal focus (e.g., past, present, future, none) and subject (e.g., self, others, none).

Another limitation is that we have taken a between-persons approach and neglected the within-person dynamics of conversational time travel. The retrospective bias in conversational time travel seems to be a universal phenomenon, however, there are individual differences in how much people talk about their past or future ( Demiray et al., 2017 ). Future work should focus on within-person variability in mental/conversational time travel across situations and examine the impact of context on the frequency, characteristics and functions of thinking and talking about the past versus future. For example, Study 2 did not include middle-aged adults, who are active in the workforce and who may be using work-related language throughout the weekdays that is mostly time-independent (e.g., semantic information). Such contextual effects (e.g., conversation partners; Demiray et al., 2017 ) and the topic of conversations should be examined in future research. Finally, our older sample included 4 couples, whose data may be dependent on each other. However, it is highly unlikely that duplicate sampling of the same 30-s sound-snippets occurred, as recordings were 100% randomly distributed.

The current research has introduced the term “conversational time travel” and examined its prevalence in everyday life. It seems that individuals, across widely varying real-life situations, talk two to three times more about their personal past than their future. This retrospective bias in conversational time travel highlights the social functions of recalling and sharing the personal past with others. Talking about past experiences seems to be an adaptive behavior that helps us to connect with others and to survive in this social world.

Ethics Statement

Study 1 has been approved by the Institutional Review Board of the University of Arizona, and Study 2 was approved by the Ethics Committee of the University of Zurich. All participants, first, gave written informed consent in accordance with the Declaration of Helsinki.

Author Contributions

BD developed the research concept and the research design. MRM conducted Study 1 for a larger project with prior students. BD conducted Study 2, collected the data, performed the data coding, and analyses and drafted the manuscript. BD and MRM worked together on the interpretation of results and on framing the manuscript. MM provided the critical revisions on the manuscript.

This research was supported by a grant from the University of Zurich, Forschungskredit (Grant No. K-63213-03-01) awarded to BD and by the National Institute of Health (Grant No. R03CA137975).

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We would like to thank Andra Arnicane, Isabel Berwian, Annika Martin, Zoe Waelchli, Marianne Mischler, Mirjam Imfeld, Marion Hauert, and Elisabeth Schoch for their assistance in data collection, data cleaning, and coding.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpsyg.2018.02160/full#supplementary-material

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Keywords : Electronically Activated Recorder, mental time travel, autobiographical memory, future-oriented thought, retrospective bias, conversations, real life

Citation: Demiray B, Mehl MR and Martin M (2018) Conversational Time Travel: Evidence of a Retrospective Bias in Real Life Conversations. Front. Psychol. 9:2160. doi: 10.3389/fpsyg.2018.02160

Received: 17 July 2018; Accepted: 22 October 2018; Published: 13 November 2018.

Reviewed by:

Copyright © 2018 Demiray, Mehl and Martin. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Burcu Demiray, [email protected]

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

10 Most Compelling Pieces Of Evidence That Prove Time Travel Exists

What we don't talk about when we talk about time travel.

What we talk about when we talk about time travel: going back to kill Hitler, Back To The Future, treading on butterflies causing irreparable damage to history, Doctor Who, whether or not it's ethical to use it to cheat on the lottery, and Bill & Ted's Excellent Adventure.

What we don't talk about when we talk about time travel: whether it actually exists. Or will exist. Or has existed. We don't know, this wibbly wobbly, timey wimey stuff kinda messes with our tenses a little.

The idea of being able to travel backwards or forwards through history has appeared in countless forms through pop culture, conversation, and daydreams for ages, but how come we don't discuss whether it'll actually ever be real? Does it just seem too beyond feasibility?

Well, we're here with good news! Not from the future, sadly, just in the grimly predictable present. A grimly predictable present that includes quantum physics, Higgs Bosons and other science-y things we don't quite understand but apparently have something to do with a conceivable way for us to travel through time. No less a genius than Stephen Hawking spent years looking for a reason that time travel couldn't exist, only to find the concept didn't contravene any laws of physics, eventually admitting "time travel may be possible, but it is not practical".

Plus, there's the fact that the history of time travel in our culture has frequently strayed into the non-fiction - so long as you're inclined to believe the possible crackpots that are discussing it - all of which adds up to some pretty compelling pieces of evidence that prove time travel is real.

Ten, in fact.

Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/