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What Are Wormholes, and Could They Be the Answer to Time Travel?

Wormholes, cosmic tunnels also known as einstein-rosen bridges, are a staple of science fiction. could they allow real-world humans to travel back in time.

The sci-fi landscape is littered with wormholes. From Douglas Adam's Hitchhiker's Guide to the Galaxy and Rick and Morty to the Marvel Cinematic Universe, these theoretical constructs allow characters to zip between distant points in the universe as easy as stepping through a doorway.

An Einstein-Rosen bridge is the simplest kind of wormhole. And while it can, in theory, allow you to meet a new friend from a distant part of the universe, there are some important reasons why it won’t let you travel back in time.

Black Holes, White Holes and Wormholes

Let’s start with everybody’s favorite astronomical mystery: a black hole . Despite their fearsome reputation, they’re actually rather simple creature. They have a point of infinite density, known as the singularity, in their centers. They are surrounded by a boundary called the event horizon.

The event horizon doesn’t exist in the same way that the surface of a planet exists. Instead it’s just a mathematical line in the sand that tells you one thing: if you cross within that special distance, you’re trapped forever, because you’ll have to travel faster than the speed of light to escape.

Read More: 'Fuzzballs' Might Be the Answer to a Decades-Old Paradox About Black Holes

And that’s it. That’s a black hole. A singularity and an event horizon. All things that cross the event horizon will never escape back into the universe – things go in and never come out.

Mathematically we can also define the polar opposite of a black hole, which is conveniently called a white hole. White holes also have a singularity, but their event horizons act differently. Anything already on the outside of a white hole (like, the entire universe) can never, ever cross within it, no matter how hard it tries. And anything already inside the white hole will find itself ejected from it faster than the speed of light.

Now when we take a black hole and a white hole and connect their singularities together, we get an entirely new kind of object: an Einstein-Rosen bridge , better known as a wormhole.

Read More: Astronomers Found a Baffling Black Hole That Existed 13 Billion Years Ago

What Is a Wormhole?

Wormholes are essentially hollow tubes through space and time that can connect very distant regions of the universe. A star may be thousands of light-years away, but a wormhole can connect that star to us with a tunnel only a few steps long.

Wormholes also have the somewhat mystical ability to allow backwards time travel. If you take one end of the wormhole and accelerate it to a speed close to that of light, it will experience time dilation — its internal “clock” will run slower than the rest of the universe.

That will cause the two ends of the wormhole to no longer be synchronized in time. Then you could walk in one end and end up in your own past. Voilà: time travel.

Read More: Is There a Particle That Can Travel Back in Time?

Can Humans Travel Through Wormholes?

There's just one, tiny, teensy problem with this setup: Einstein-Rosen bridges are indeed wormholes, but the entrance to the wormhole sits behind the black hole event horizon. And the number one rule of black hole event horizons is that once you cross them, you’re never allowed to escape. Ever.

Once you pass through a black hole event horizon, you are forced towards the singularity, where you are guaranteed to meet your gruesome end. In other words, once you enter an Einstein-Rosen bridge, you will never escape.

So, the unfortunate truth with Einstein-Rosen bridges is that while they appear to be magical doorways to distant reaches of the universe, they are just as deadly as black holes. When you enter you can meet other travelers who have fallen in from the other side, and you could even carry on a conversation…briefly, before you both struck the singularity.

There have been attempts to stabilize Einstein-Rosen bridges and make them traversable by somehow getting their entrances to sit outside the event horizon. So far the only way we know how to do this is with exotic matter. If you threaded the wormhole tunnel with matter that had negative mass, then in principle you could have a not-deadly-at-all wormhole.

Alas, negative matter does not appear to exist in the universe, and so our wormhole — and time travel — dreams will have to remain as mere mathematical fantasies.

Read More: What Did Einstein's Theories Say About the Illusion of Time?

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Scientists Have Determined How to Travel Back in Time With a Ring Wormhole

• A recent study claims to have calculated a potential method of time travel.
• It involves a highly theoretical object called a “ring wormhole,” which is a type of wormhole that connects two regions of space, like a portal.
• Ring wormholes had previously been theorized to be portals to other universes, and researchers now propose they could act as time machines as well.

But that hasn’t stopped scientists from trying to figure out how we maybe could, someday, jump around out of order in the time stream. And recently, a team of theoretical physicists published a paper on exactly what laws of physics could be stretched just far enough to make it happen.

The key to the whole idea is wormholes —specifically, a type of wormhole called a ring wormhole. Now, wormholes are already entirely theoretical, so this discussion is going to get weird. And ring wormholes get even weirder than “normal” wormholes.

Your average, run-of-the-mill wormholes, as we tend to think of them, are basically holes punched in the fabric of spacetime by the immense gravity of black holes. The gravity well at the center of these objects is so intense that scientists have often theorized they could act as tunnels to another universe , or another time.

But ring wormholes aren’t black-hole dependent. Instead, the (again, highly theoretical) objects are caused by circles of mass that have negative energy, something only made possible by the strange effects of the quantum realm . This circle of negative energy would basically create a portal to another universe without the need to go through a black hole tunnel.

“You could go through and not even notice that you went to another universe,” Andrei Zelnikov, one of the authors on the recent paper, told New Scientist .

The paper —published in the journal Physical Review D by Zelnikov and his team—puts forth calculations that claim a ring wormhole could not only act as a universe-to-universe teleport, but as a time machine .

According to the heavy-duty number-crunching, the ring wormholes could generate something called a “closed timelike curve” if one “mouth” of the wormhole near a bunch of mass and the other “mouth” was far away from any significant amount of mass . If the conditions are right around the mouths of the wormhole, the closed timelike curves generated are then able to turn a portal into a time machine.

“The time machine is a natural consequence of the wormhole existing,” Toby Wiseman, a professor of theoretical physics at the Imperial College London who was not a part of the study, told New Scientist . “Apart from the crazy matter that makes up the wormhole, there’s nothing too wild being postulated here, and then the consequence is something even more crazy.”

You can decide for yourself if proposing a method for time travel is “too wild.” But wild or not, scientists remain dedicated to truly understanding all of the laws of time and space —and exactly how we can bend them to make the coolest things possible.

Jackie is a writer and editor from Pennsylvania. She's especially fond of writing about space and physics, and loves sharing the weird wonders of the universe with anyone who wants to listen. She is supervised in her home office by her two cats.

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Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "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."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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Is time travel possible? An astrophysicist explains

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.

Related: The speed of light, explained

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.

Time travel 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.

This article first appeared on the Conversation. You can read the original here .

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April 26, 2023

Is Time Travel Possible?

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

By Sarah Scoles

yuanyuan yan/Getty Images

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Can We Travel Through Time to the Past?

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Going back in time to visit an earlier era is a fantastic dream. It's a staple of SF and fantasy novels, movies, and TV shows. Who wouldn't like to go back and see the dinosaurs or watch the birth of the universe or meet their great-great grandparents? What could possibly go wrong Could someone travel to a previous era to right a wrong, make a different decision, or even completely alter the course of history? Has it happened? Is it even possible?

There are a lot of questions about travel into the past, but not very many solutions. The best answer science can give us right now is: it's theoretically possible. But, no one has done it. 

It turns out that people time travel all the time, but only in one direction: from the past to the present and moving into the future . Unfortunately, no one has any control over how quickly that time passes and nobody can stop time and continue to live. It seems that time is a one-way street, always moving forward.

This is all right and proper. It also fits with Einstein's theory of relativity because time only flows in one direction—forward. If time flowed the other way, people would remember the future instead of the past. That sounds very counter-intuitive. So, on the face of it, traveling into the past seems to be a violation of the laws of physics.

But not so fast! It turns out that there are theoretical considerations to take into account if somebody wants to build a time machine that goes back to the past. They involve exotic gateways called wormholes, or some science fictional-sounding creation of gateways using a technology not yet available to science. 

The idea of building a time machine, like those often depicted in science fiction films, is likely the stuff of dreams. Unlike the traveler in H.G. Wells's Time Machine, no one has figured out how to build a special carriage that goes from now to yesterday. However, astrophysics gives us one possible pathway: one could possibly harness the power of a black hole to venture through time and space. How would that work?

According to general relativity , a rotating black hole could create a wormhole —a theoretical link between two points of space-time, or perhaps even two points in different universes. However, there's a problem with black holes. They've long been thought to be unstable and therefore un-traversable. However, recent advances in physics theory have shown that these constructs could, in fact, provide a means of traveling through time. Unfortunately, we have almost no idea what to expect by doing so.

Theoretical physics is still trying to predict what would happen inside the wormhole, assuming one could even approach such a place. More to the point, there's no current engineering solution that would allow us to build a craft that would let make that trip safely. Right now, as it stands, once a ship enters the black hole, it's going to get crushed by incredible gravity. The ship, and everyone aboard are made one with the singularity at the heart of the black hole.

But, for the sake of argument, what if it were possible to pass through a wormhole? What would people experience? Some suggest it would probably be a lot like Alice falling through the rabbit hole. Who knows what we would find on the other side? Or in what time frame? Until someone can devise a safe way to make that trip, we aren't likely to find out.

The idea of traveling into the past raises all sorts of paradoxical issues. For instance, what happens if a person goes back in time and kills their parents before they can conceive their child? Lots of dramatic stories have been built around that one. Or, the idea that someone could go back and kill a dictator and change history, or save the life of a famous person. An entire episode of Star Trek was built around that idea.

It turns out that the time traveler effectively creates an alternate reality or parallel universe . So, if someone did travel back and prevent someone else's birth, or murdered someone, a younger version of the victim would never come to be in that reality. And, it might or might not carry on as if nothing had changed. By going back in time, the traveler creates a new reality and would, therefore, never be able to return to the reality they once knew. (If they then tried to travel into the future from there, they would see the future of the new reality, not the one they knew before.) Consider the outcome of the movie "Back to the Future". Marty McFly changes reality for his parents back when they were in high school, and that changes his own reality. He gets back home and finds his parents aren't quite the same as when he left. Did he create a new alternate universe? Theoretically, he did.

This brings us to another issue that is rarely discussed. The nature of wormholes is to take a traveler to a different point in time and space . So if someone left Earth and traveled through a wormhole, they could be transported to the other side of the universe (assuming they are even still in the same universe we currently occupy). If they wanted to travel back to Earth they would either have to travel back through the wormhole they just left (bringing them back, presumably, to the same time and place), or journey by more conventional means. 

Assuming the travelers would even be close enough to make it back to Earth in their lifetimes from wherever the wormhole spat them out, would it still be the "past" when they returned? Since traveling at speeds approaching that of light makes time slow down for the voyager, time would proceed very, very quickly back on Earth. So, the past would fall behind, and the future would become the past... that's the way time works flowing forward ! 

So, while they exited the wormhole in the past (relative to time on Earth), by being so far away it's possible that they wouldn't make it back to Earth at any reasonable time relating to when they left. This would negate the whole purpose of time travel altogether. 

Possible? Yes, theoretically. Probable? No, at least not with our current technology and understanding of physics. But perhaps someday, thousands of years into the future, people could harness enough energy to make time travel a reality. Until that time, the idea will just have to stay relegated to the pages of science-fiction or for viewers to make repeated showings of Back to the Future. 

Edited by Carolyn Collins Petersen .

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The Missing Piece: Are Wormholes the Key to Time Travel?

Time travel -- it's a concept we're all too familiar with. From Hollywood Movies (shout out to Back to the Future) to theoretical physics, many theories have circulated regarding the bewildering topic of time travel. However, recently students at the California Institute of Technology1 have shed new light on a relatively dated theory: that wormholes may be the missing piece in the seemingly unsolvable puzzle behind time travel. So, step aside Doc Brown, because we're going to explore how a unique and mysterious part of our universe- the wormhole- connects to traveling through time!

Just like many principles in Physics, this concept finds its base in Einstein's Theory of General Relativity (for some background on this topic, view our previous blog post, "What happens to Matter inside a Black Hole?"). Basically, the theory attributes the characteristics of a wormhole to its incredibly high velocity. Similar to a black hole, a wormhole would compress an entering object to the size of a singularity and accelerate it to immeasurable speeds through the "throat" of the wormhole to the other side of the funnel. So, for now, think of a wormhole as a black hole that has a start and end point.2

Now, this is where the physics becomes complex. We know that a wormhole can basically "teleport" compressed objects at super speeds from one end to another, but how does that explain the time travel aspect? Well, the concept of time dilation can explain that. Time dilation is the relative difference in the passage of time between two entities due to a stark difference in either gravity or relative velocity. In the case of the wormhole, both are in play, which only compounds the effects. For a tangible example to grasp, let's look to the astronauts aboard the International Space Station. After 6 months aboard the ISS (where there is less gravity, but greater velocity of movement), these astronauts have actually aged .007 seconds less than the crew stationed on Earth.3 While this seems insignificant, we have to remember that because the velocity is increased but the gravity is decreased, each is actually mitigating the effects of the other. However, in a wormhole, where the effects of both gravity and velocity are exponentially amplified AND working in tandem, their relativistic effects are enormous.4

This graphic portrays an accurate depiction of velocities' effect on time dilation: http://upload.wikimedia.org/wikipedia/commons/7/72/Nonsymmetric_velocity_time_dilation.gif

Now that we have an understanding of time dilation, we want to leave you with this example. Imagine two test subjects at each side of a wormhole in the year 1995. One mouth of the wormhole has been accelerated to a point where relativistic effects (i.e. time dilation) apply significantly and the other mouth is stationary. When the two subjects pass through their respective mouths, they end up at where their counterpart stood... but at different times! The accelerated mouth would land its test subject in, let's say, 2000. However, the stationary mouth would place its test subject in a later year - we'll call it 2005- since the accelerated mouth exists in a different time caused by time dilation. The time travel becomes evident when we consider the following: what if the test subject who landed in 2005 stepped back into the mouth? They would end up in the year 2000, or 5 years in the past! A bit of manipulation, therefore, could technically transport this subject to any point in time between 2000 and 2005.5

Unfortunately, scientists have been unable to totally grasp how we could adapt the power of a wormhole into time machine-esq technology. We have yet to even theorize how we could accelerate a wormhole to relativistic speeds. Given that this is another theoretical aspect of our known universe, testing is extremely limited and all postulates have very little opportunity for tangible proof. What is certain, though, is that the advances we've made thus far in the field have brought us several pieces closer to solving the time traveler's puzzle!

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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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Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

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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Have wormhole, will travel -- maybe even back in time

The concept of a time machine typically conjures up images of an implausible plot device used in a few too many science-fiction storylines. But according to Albert Einstein's general theory of relativity, which explains how gravity operates in the universe, real-life time travel isn't just a vague fantasy.

Traveling forward in time is an uncontroversial possibility, according to Einstein's theory. In fact, physicists have been able to send tiny particles called muons , which are similar to electrons, forward in time by manipulating the gravity around them. That's not to say the technology for sending humans 100 years into the future will be available anytime soon, though.

Time travel to the past, however, is even less understood. Still, astrophysicist Eric W. Davis of the EarthTech International Institute for Advanced Studies at Austin argues that it's possible. All you need, he says, is a wormhole , which is a theoretical passageway through space-time that is predicted by relativity. [ Wacky Physics: The Coolest Little Particles in Nature ]

"You can go into the future or into the past using traversable wormholes," Davis told LiveScience.

Where's my wormhole? Wormholes have never been proven to exist, and if they are ever found, they are likely to be so tiny that a person couldn't fit inside, never mind a spaceship.

Even so, Davis' paper, published in July in the American Institute of Aeronautics and Astronautics' journal, addresses time machines and the possibility that a wormhole could become, or be used as, a means for traveling backward in time.

Both general-relativity theory and quantum theory appear to offer several possibilities for traveling along what physicists call a "closed, timelike curve," or a path that cuts through time and space — essentially, a time machine.

In fact, Davis said, scientists' current understanding of the laws of physics "are infested with time machines whereby there are numerous space-time geometry solutions that exhibit time travel and/or have the properties of time machines."

A wormhole would allow a ship, for instance, to travel from one point to another faster than the speed of light — sort of. That's because the ship would arrive at its destination sooner than a beam of light would, by taking a shortcut through space-time via the wormhole. That way, the vehicle doesn't actually break the rule of the so-called universal speed limit — the speed of light — because the ship never actually travels at a speed faster than light. [ Warped Physics: 10 Effects of Traveling Faster Than Light ]

Theoretically, a wormhole could be used to cut not just through space, but through time as well. 

" Time machines are unavoidable in our physical dimensional space-time," David wrote in his paper. "Traversable wormholes imply time machines, and (the prediction of wormholes) spawned a number of follow-on research efforts on time machines."

However, Davis added, turning a wormhole into a time machine won't be easy. "It would take a Herculean effort to turn a wormhole into a time machine. It's going to be tough enough to pull off a wormhole," he told LiveScience.

That's because once a wormhole is created, one or both ends of it would need to be accelerated through time to the desired position, according to general relativity theory.

Challenges ahead There are several theories for how the laws of physics might work to prevent time travel through wormholes .

"Not only do we assume (time travel into the past) will not be possible in our lifetime, but we assume that the laws of physics, when fully understood, will rule it out entirely," said Robert Owen, an astrophysicist at Oberlin College in Ohio who specializes in black holes and gravitation theory.

According to scientists' current understanding, keeping a wormhole stable enough to traverse requires large amounts of exotic matter, a substance that is still very poorly understood.  

General relativity can't account for exotic matter — according to general relativity, exotic matter can't exist. But exotic matter does exist . That's where quantum theory comes in. Like general relativity, quantum theory is a system for explaining the universe, kind of like a lens through which scientists observe the universe. [ Video – How to Time Travel ]

However, exotic matter has only been observed in very small amounts — not nearly enough to hold open a wormhole. Physicists would have to find a way to generate and harness large amounts of exotic matter if they hope to achieve this quasi-faster-than-light travel and, by extension, time travel.

Furthermore, other physicists have used quantum mechanics to posit that trying to travel through a wormhole would create something called a quantum back reaction.

In a quantum back reaction, the act of turning a wormhole into a time machine would cause a massive buildup of energy, ultimately destroying the wormhole just before it could be used as a time machine.

However, the mathematical model used to calculate quantum back reaction takes into account only one dimension of space-time.

"I am confident that, since (general relativity) theory has not failed yet, that its predictions for time machines, warp drives and wormholes remain valid and testable, regardless of what quantum theory has to say about those subjects," Davis added.

This illustrates one of the key problems in theories of time travel: physicists have to ground their arguments in either general relativity or quantum theory, both of which are incomplete and unable to encompass the entirety of our complex, mysterious universe.

Before they can figure out time travel, physicists need to find a way to reconcile general relativity and quantum theory into a quantum theory of gravity. That theory will then serve as the basis for further study of time travel.

Therefore, Owen argues that it's impossible to be certain of whether time travel is possible yet. "The wormhole-based time-machine idea takes into account general relativity, but it leaves out quantum mechanics," Owen added. "But including quantum mechanics in the calculations seems to show us that the time machine couldn't actually work the way we hope."

Davis, however, believes scientists have discovered all they can about time machines from theory alone, and calls on physicists to focus first on faster-than-light travel. 

"Until someone makes a wormhole or a warp drive , there's no use getting hyped up about a time machine," Davis told LiveScience.

Accomplishing this will require a universally accepted quantum gravity theory — an immense challenge — so don't go booking those time-travel plans just yet.

Email  [email protected] or follow her @JillScharr . Follow LiveScience @livescience , Facebook   and  Google+ . Original article on LiveScience.

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

Published: Nov 13, 2023

By: Magazine Editor

Written by Adi Foord , assistant professor of physics , UMBC

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

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. https://www.youtube.com/embed/QeRtcJi3V38?wmode=transparent&start=0 Telescopes are a kind of time machine – they let you peer into the past.

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.

This article is republished from The Conversation under a Creative Commons license. Read the original article and see more than 250 UMBC articles available in The Conversation.

Tags: CNMS , Physics , The Conversation

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Wormholes, time travel and quantum gravity

By Ian Redmount

28 April 1990

THE PAST few years have seen an explosion of interest in what theoretical physicists call ‘wormholes’. These are tunnels in the geometry of space and time, connecting otherwise distant or completely disconnected regions of the Universe. In fact, there have been two explosions in two almost unrelated subjects. One is macroscopic wormholes, the kind that science fiction writers or sufficiently advanced civilisations might use for space travel across cosmic distances. The other is microscopic wormholes, on a scale 20 orders of magnitude smaller than an atomic nucleus. At this scale, space and time should obey the rules of quantum physics rather than classical laws.

Neither type of wormhole is a new idea. Theorists have known about large-scale wormholes for more than 70 years – since shortly after Albert Einstein put forward the general theory of relativity, which relates gravity to the geometry of space and time. For 30 years, physicists have conjectured that microscopic wormholes might play a crucial role in understanding the structure of elementary particles or in developing a quantum theory of gravity. Recently, however, researchers have found that both types of objects may have some remarkable, previously unsuspected properties. Large-scale wormholes could provide a relatively easy means of travelling backwards in time, with all the potential complications that entails. Microscopic wormholes might, through their contribution to the quantum mechanics of the Universe, deter mine the values of all the constants in all the laws of physics. Much of this wormhole work is speculative and some very controversial, but that is why these subjects have generated such excitement lately.

Michael Morris and Ulvi Yurtsever, and their PhD thesis adviser…

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How to time travel with wormholes

Brian Greene ,   professor of physics and mathematics at Columbia University and co-founder of the World Science Festival , explains what we know about time travel so far. Following is a transcript of the video.

Brian Greene: I’m Brian Greene, professor of physics and mathematics at Columbia University and co-founder of the World Science Festival.

It's critical that you realize that there are two types of time travel , and they are radically different. Time travel to the future? Definitely possible.

We know how to do it because Einstein showed us the way over a hundred years ago. It’s surprising how few people actually really know about this in their bones. He showed that if you go out into space and travel near the speed of light, and you turn around, and you come back, your clock will be ticking off time more slowly. So, when you step off it's going to be the future on planet Earth. You will have time traveled into the future.

He also showed that if you hang out near a nice strong source of gravity — a neutron star, a black hole — and you kind of get right near the edge of that object, time also for you would slow down real slow relative to everybody else. And therefore, when you come back to Earth, for instance, it'll again be far into the future.

This is not controversial stuff. Any physicist who knows what they're talking about agrees with this. But the other kind of time travel — to the past is where the arguments start to happen because many of us don't think that time travel to the past is possible.

The main proposal that people at least consider worthy of attention for traveling to the past does make use of a weird concept called wormholes . A wormhole is something that really … Albert Einstein again discovered. The guy has like got his name written over everything in this field.

It's a bridge, if you will, from one location space to another. It's kind of a tunnel that gives you a shortcut to go from here to here. Now he discovered this in 1935 but it was subsequently realized that if you manipulate the openings of a wormhole — put one near a black hole or take one on a high-speed journey — then time of the two openings of this wormhole tunnel will not take off at the same rate, so that you will no longer just go from one location in space to another, if you go through this tunnel — through this wormhole — you'll go from one moment in time to a different moment in time. Go one way, you'll travel to the past, the other way, travel to the future.

Now again, we don't know if wormholes are real. We don't know if they are real whether you'll be able to go through them. So, there are all sorts of uncertainties here. Most of us think that you're not going to actually go on a whirlwind journey through a wormhole to the past. But it's still not ruled out.

More from Science

• Main content

Traversable wormholes are possible under certain gravity conditions

If you want to come back from a trip through a wormhole, you'll need some weird gravity.

Interested in scooting through a wormhole, the ultimate cheat-code through space and time? Perhaps you'd like to hop from star system to star system across the universe without breaking a sweat? But first, you'd better make sure your wormhole is traversable.

"Any traveler trying to cross a wormhole that does not satisfy this will be crushed inside as the tunnel collapses," João Rosa, a physicist at University of Tartu, in Estonia, told Live Science.

Rosa is attempting to virtually "build" a stable, traversable wormhole, one that can be safely crossed without the theoretical passageway collapsing or trapping its occupant. And he has recently found that it is indeed possible, but only if we tweak our understanding of gravity . 

Related: Physicists just released step-by-step instructions for building a wormhole

The problem of traversability

Wormholes are deceptively easy to build — on paper. You start with a theory of gravity. In Einstein's theory of general relativity , gravity defines the relationship between matter and energy, and space and time. The trick to building a wormhole is to find a configuration of matter and energy that allows you to form a tunnel — usually called a "throat" in wormhole research — bridging two distant points in space.

In principle, that throat can be as long or as short as you want, but the more interesting wormholes happen when the throat is far shorter than the normal distance between two points, making the wormhole a convenient shortcut. Oh, and wormholes can also act as time machines, sending you into the future or past, depending on how they're constructed.

The idea of such a whimsical shortcut has captured the imaginations of scientists and science-fiction writers for decades. 

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Get the world’s most fascinating discoveries delivered straight to your inbox.

"The possibility to visit other stars (or even other galaxies), possibly finding alien civilizations, and the possibility to revisit the past or not having to wait for the future have been part of the human imagination and fantasy for a long time, and wormholes provide a (relatively) simple and unified solution for both of these problems," Rosa told Live Science in an email.

But wormholes constructed based on the criteria laid out by general relativity suffer a major problem: They're not actually traversable. The entrances of general relativity wormholes are hidden behind event horizons, which are one-way barriers in space. That means if you were to enter the wormhole, you could never leave, which would defeat the purpose.

The other problem is that they are ridiculously unstable. The moment even a single photon, or light particle, enters the throat, the whole wormhole catastrophically collapses before that packet of light can escape.

Bummer. 

A new gravity

In order to solve these problems in general relativity and stabilize a wormhole, a cosmic traveler must fashion the wormhole out of an incredibly exotic ingredient, a form of matter that has negative energy or negative mass. Negative mass (also known as exotic matter) is exactly what it sounds like: If something weighs negative 10 pounds, it would have negative mass. Scientists have not observed negative mass anywhere in the universe. Negative energy is slightly more attainable, which is just a condition where the energy in a particular location is negative relative to its surroundings, but that can only be achieved at microscopic, quantum scales.

Related: 8 ways you can see Einstein's theory of relativity in real life

"What happens is that these gravitational effects needed to guarantee the traversability of the wormhole happen naturally if one modifies gravity." João Rosa

"The presence of this matter is essential as it prevents the wormhole throat from collapsing upon a traveler, but it is also problematic," Rosa explained. "It presents a negative average energy density, an extremely rare characteristic of matter in the universe that is only observed in very specific situations at the quantum level."

Since such matter is so rare, building an entire wormhole out of the exotic matter would seem impossible in our universe.

But all this talk about wormholes is predicated on general relativity. And while that theory of gravity has survived every single observational and experimental test thrown at it over the last century, we know that general relativity isn't the last word on gravity. Relativity is incapable of describing the centers of black holes , the earliest moments of the universe and the link between itself and quantum physics.

So maybe a new, improved theory of gravity would allow wormholes to exist.

Making the trip

That's exactly what Rosa investigated in a new paper published online July 29 to the preprint journal arXiv , meaning the study has yet to be reviewed by peers in the field.

Rosa employed a tweaked form of gravity called generalized hybrid metric-Palatini gravity. This theory of gravity is built on general relativity, but allows more flexibility in relationships between matter and energy, and space and time.

Earlier research had found that traversable wormholes might be possible in this modified theory of gravity, but they still required negative energy outside the wormhole throat. Rosa found that by layering the entrances to the wormholes with double thin shells of regular matter, the wormhole becomes traversable without any negative energy. 

Related: Science fact or fiction? The plausibility of 10 sci-fi concepts

"What happens is that these gravitational effects needed to guarantee the traversability of the wormhole happen naturally if one modifies gravity, and exotic matter [matter with negative mass] is no longer needed to serve this purpose," Rosa explained.

What's next? Rosa wants to test this particular theory of modified gravity. "This is just a very small step towards the final goal: One must now use experimental data and observations (e.g. gravitational waves and trajectories of stars near the center of the Milky Way ) to test and (hopefully) confirm the validity of these theories," Rosa wrote.

— Interstellar space travel: 7 futuristic spacecraft to explore the cosmos — Stephen Hawking's most far-out ideas about black holes — 10 wild theories about the universe

While general relativity has so far explained all measurements of gravity (including gravitational waves and the vicinity of black holes), the story isn't over. Future observations might find a crack in that venerable theory, and if generalized hybrid metric-Palatini gravity better explains cosmic observations, then wormholes made for travel might be possible.

But the questions wouldn't end there. Wormholes can also act as time machines, so a viable wormhole solution would mean that time travel into the past is possible; that, of course, raises all sorts of difficult problems (like the so-called " grandfather paradox " and questions about causality). Knowing for sure that traversable wormholes could exist wouldn't just make our sci-fi dreams come true, it would totally upend our understanding of physics.

Originally published on Live Science.

Paul M. Sutter is a research professor in astrophysics at  SUNY Stony Brook University and the Flatiron Institute in New York City. He regularly appears on TV and podcasts, including  "Ask a Spaceman." He is the author of two books, "Your Place in the Universe" and "How to Die in Space," and is a regular contributor to Space.com, Live Science, and more. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy. 

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Small Wormholes Are Boring Through the Fabric of Space-Time, Physicists Say

This new candidate for dark energy has scientists intrigued., getting wormer.

Researchers suggest that tiny wormholes could be punching microscopic holes through the fabric of space-time, Live Science reports , thereby driving the accelerating expansion of the universe.

For decades, scientists have used a hypothetical form of energy called dark energy to explain why the universe isn't just expanding, but doing so at a rate that accelerates over time.

Over the years, scientists have floated many ideas to explain dark energy, ranging from it hiding in black holes to not existing at all .

And now, as detailed in an April paper in the journal Physical Review D , an international team of researchers is proposing a new candidate: unfathomable numbers of subatomic wormholes that connect points in space and are continuously being destroyed and reborn.

Tunnel Vision

According to the researchers' calculations, ten billion wormholes per cubic centimeter per second could account for the observed rate of the universe's expansion.

"Although our result was derived on the grounds of Euclidean quantum gravity... it is likely that our modification may hold for other quantum gravity theories as well," University of Thessaly doctoral student and coauthor Stylianos Tsilioukas told Live Science .

The team argued that their candidate for dark energy aligns with observations of the actual universe.

"According to our proposal dark energy can change as time flows," Tsilioukas added. "This is a major advantage because recent observations suggest that the rate of expansion of the universe is different in recent times than it was in the early universe."

Of course, it's just a theory — and more research is needed.

"We are working right now on a model which calculates the rate of wormhole formation," Tsilioukas said. "The research seems promising and we hope to publish the results very soon."

More on wormholes: Dark Matter May Be a Deformed Mirror Universe, Scientists Say

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Americans continue to rack up credit card debt, hitting a record $1.14 trillion

By Khristopher J. Brooks

Edited By Anne Marie Lee

Updated on: August 6, 2024 / 4:52 PM EDT / CBS News

U.S. consumers collectively owe a record $1.14 trillion in credit card debt, figures released Tuesday by the Federal Reserve Bank of New York show . That's $27 billion  more than  the $1.13 trillion in credit card debt they carried during the second quarter of 2024.

The high tally comes amid  concerns of an economic downturn  triggered by modestly rising unemployment, and as soaring costs in food , housing and auto rates continue to drain household budgets. Americans have  increasingly been relying on credit cards  to make ends meet, with 6 in 10 adults, or 60%, using credit cards to buy groceries in 2023, according to a May  report  by the Urban Institute.

Prolonged debt also plays a factor in the historic amount as more Americans fall behind on their credit card bills. About 7.18% of cardholders fell into delinquency in the second quarter, up from 5% in the previous quarter, Fed statistics show. 

"More people are carrying more debt for longer periods of time," Ted Rossman, senior industry analyst at Bankrate, said in a statement. 

Americans paid down some of their credit card debt in 2020, using pandemic-related federal stimulus funds, Rossman noted. But beginning in 2021, credit card balances "rocketed upward by 48%, fueled by a post-pandemic boom in services spending as well as high inflation and high interest rates," he added. 

Debt owed on mortgages and auto loans also climbed by $77 billion and $10 billion, respectively, according to the Fed's data. Total consumer debt grew to $17.8 trillion. 

Rising interest rates

Compounding the problem is the soaring price of plastic, as interest rates on credit cards also continue to reach record highs. The average interest rate on a new credit card is now at 24.84%, the highest since LendingTree started tracking rates in 2019. 

To be sure, both credit card debt and credit card interest rates could decline if the Federal Reserve  decides to cut  its benchmark rate in September — or perhaps even sooner . 

The time for a rate cut "is approaching, and if we do get the data we hope we get, then reduction of our policy rate could be on the table at our September meeting," Fed Chairman Jerome Powell said last week. 

• Could the Fed enact an emergency rate cut before its next meeting? Here are the odds.

The Federal Reserve's benchmark rate doesn't translate directly into movements in credit card annual percentage rates, or APRs, but because card rates are tied to the prime rate used by banks, which closely follows the Fed's federal funds rate, there is an indirect correlation.

In other words, credit card companies could lower their APRs in response to a rate cut by the Fed, LendingTree credit analyst Matt Schulz  told CBS News , providing some relief for borrowers in the months ahead.

• Credit Card Debt

Khristopher J. Brooks is a reporter for CBS MoneyWatch. He previously worked as a reporter for the Omaha World-Herald, Newsday and the Florida Times-Union. His reporting primarily focuses on the U.S. housing market, the business of sports and bankruptcy.

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32 Movies Great Movies About Time Travel With Completely Different Rules

Prepare for some serious stipulations.

Is there ever a bad time to watch a time travel movie? Some of the best sci-fi movies in history have tackled this frequently explored topic, and new wrinkles in the fabric of the concept have made the subject more exciting over time. So why don’t we take a look at the different rules these flights of fancy have introduced? Should you be stuck in a time loop, we apologize if this list is starting to get old.

Back To The Future

Everyone loves to talk about how Back to the Future’s time travel works , but there’s one aspect we take for granted throughout the whole trilogy. Doc Brown ( Christopher Lloyd ) may have given Marty McFly ( Michael J. Fox ) the keys to travel through the past, present, and future; but you seriously need to consider the exact spot you’re traveling to. Otherwise, you might find yourself altering history in some intriguing ways. R.I.P. Twin Pines Mall. 

Time After Time

A novel adventure starring the father of time travel H.G. Wells (Malcolm McDowell), Time After Time actually introduced an interesting mechanic to temporal transport. Let’s just say that if you don’t use the Time Machine properly, you could find yourself stuck in your final destination. Or worse, falling through the time-space continuum, without a way back home.

The Terminator

The Terminator's time travel will forever be a head-scratcher, as the existence of John Connor is the ultimate ontological paradox. How else can you explain Kyle Reese (Michael Biehn) being sent back to the 1980s to save the world…and make sure the person who sent him is born in the first place?

Star Trek IV: The Voyage Home

The oldest method of time travel in the Star Trek movies, 1986’s The Voyage Home saw Captain Kirk ( William Shatner ) and his crew trying to save the whales through a time heist. This wouldn’t have been possible if it wasn’t for the Enterprise crew using a Klingon Bird of Prey, a slingshot orbit around the sun, and a lot of engineering power to do it.

2004’s Primer is still hotly discussed among time travel aficionados, and it’s not hard to see why. The shenanigans in this test case involve multiple versions of a singular traveler (Shane Carruth) existing in a single timeline, which creates one of the most chaotic timelines ever depicted.

The Adam Project

Story-wise, The Adam Project is pretty cozy when it comes to how it handles time travel. But when it comes to traveling in style, the older Adam Reed ( Ryan Reynolds ) has a Time Jet that’s specifically coded to his DNA! Not many temporal travelers HAVE that, and it prevents so many mistakes other adventures of this sort use for story purposes.

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Avengers: Endgame

How Avengers: Endgame’s time travel works is rather unorthodox, to be honest. Instead of overwriting the past into a more pleasing result, the MCU’s finest are only allowed to use it in the name of stealing/returning the Infinity Stones. Timelines can still create tangent histories, and 2014 Gamora takes over for her slain variant in the films, but you can’t stop “The Snap.”

Lost In Space

If all time travelers had the device Older Will Robinson (Jared Harris) built in 1998’s Lost in Space , they’d have it made. While only one person can travel at a time, exact coordinates in time and space are required; so you can go to a very specific spatial location on the timeline. 

The Butterfly Effect

The Butterfly Effect's time travel works on rules similar to that of Quantum Leap . Evan ( Ashton Kutcher ) can indeed change history, but it’s only within his own life’s timeline. Unfortunately, thanks to the multiple trips leading to continued alterations to the fabric of events, it all adds up in terms of severe physical wear and tear. 

Hot Tub Time Machine

Hot Tub Time Machine is a very special case when it comes to time travel. To be fair, the comedy ensemble franchise gets points for having its protagonists travel only within their own bodies. As for how one can actually travel with said titular device, apparently you need an energy drink, the right hot tub with the right temperature, and some convenient writing. 

Real-time bodily damage. That’s probably one of the most unique additions to Looper’s usage of time travel , as we see people incur damage in the past, only for it to show up on their future selves. Poor Seth ( Paul Dano ) demonstrated that lesson the hard way in Rian Johnson ’s sci-fi masterpiece. 

Star Trek: Generations

What if you could wish really hard to create an alternate timeline? Or what if you could send yourself back to your best memory, and never leave? That’s what The Nexus from Star Trek: Generations could do, and both Captains Kirk (William Shatner) and Picard ( Patrick Stewart ) got a taste of that sweet life, before ultimately using their new power to stop the villainous Dr. Soren (Malcolm McDowell). 

Bill And Ted’s Excellent Adventure

If you ever want to bring a figure from history home for dinner in the present, do it in the universe of Bill and Ted’s Excellent Adventure . There are little to no consequences, especially when it comes to our heroes (Alex Winter and Keanu Reeves) whisking away two medieval princesses to become betrothed in the 1980s. Seriously, how did that not start a war?

The Time Traveler's Wife

“Chrono Impairment” is a seriously rare affliction, but it’s enough of a headache that it prevents Rachel McAdams and Eric Bana’s clock-crossed lovers from ever enjoying a normal life. Such is the nature of The Time Traveler's Wife , which invented that affliction to send Bana’s character Henry on unpredictable trips at unforeseeable intervals throughout his life. 

Indiana Jones And Dial Of Destiny

For Indiana Jones and the Dial of Destiny's time travel to actually happen, the world of Harrison Ford’s iconic archeologist needed specific hardware. Aided by some very precise calculations to try and take the Nazis to where they were trying to go, it wasn't as simple as jumping into a car and gunning it to 88 miles per hour. 

Star Trek: First Contact

For a franchise that uses temporal transit as much as the Star Trek series does, there sure are a lot of different ways to go back in time. And if you’re not satisfied with The Voyage Home’s method of a slingshot orbit around the Sun, then you can always do what Star Trek: First Contact did. While I wouldn’t personally recommend waiting for a Borg invasion to cause a temporal wake you can just hitch a ride on; you do you. 

Ok, so technically Tenet’s shenanigans involving time is “time inversion,” rather than time travel. Which only makes the journey, and the resulting reality The Protagonist (John David Washington) lives in all the more complicated. It also makes for some classic Christopher Nolan mind melts.

Would this really be a sci-fi party if author Michael Crichton didn’t show up? Timeline’s time travel is a lot of fun, if you consider using a “human fax machine” to send yourself to medieval times “fun.” In which case, try not to abuse it too much, as every trip has the chance to leave you with transcription errors in your reassembled DNA. Again, we’re working with a fax machine here.  

Somewhere In Time

It’s the moment you’ve been waiting for Christopher Reeve fans! Somewhere in Time just had to be on this list, as it's pretty unique in how it sends a person back through the ages. In the case of Reeve’s playwright Richard, all he needs is a really powerful hypnotic focus to zoom back to 1912. 

Star Trek (2009)

It’s kind of fitting that the 2009 Star Trek reboot would use time travel, given that the series has continually danced with that concept on TV and in movies. For this J.J. Abrams-directed venture, the destructive and inexact force of a black hole is what’s used to accidentally alter time so vastly that William Shatner turns into Chris Pine.

Déjà Vu

Tony Scott’s 2006 action-thriller Déjà Vu is a big movie with a relatively limited scope. With intelligence gathering, and ultimately one human transport, that can only go as far back as four and a half days, Denzel Washington’s work was kind of cut out for him on this caper.

The Tomorrow War

The Chris Pratt-starring time travel ensemble adventure The Tomorrow War has some pretty huge stipulations when it comes to recruiting an army for the future. The largest among them was, of course, you had to be dead according to the records of the future hellscape that pitted humanity against some very nasty creatures.

X-Men: Days Of Future Past

Going from here to there in the then and now in X-Men: Days of Future Past requires a serious amount of power. With Wolverine (Hugh Jackman) going back to his past body, the key to how it all happens lies in the phasing abilities of Kitty Pryde (Elliot Page). So this story uses a very physical, and incredibly vulnerable, method to execute its vision.

Out of all the time travel universes we’ve seen on screen, perhaps the one I feel the most sorry for is the one shown in 12 Monkeys . The basic rule of this Bruce Willis epic’s temporal transit is “hope for the best,” thanks to the method of being shot through time and intending to land in the right place going wrong more often than you think.

Midnight In Paris

Reminiscent of many other vehicular-based time travel films like Back to the Future , any character that travels through time in Midnight In Paris just needs to catch the right ride, at just about Midnight. The experience is bespoke to whoever is traveling, as the period of time that suits them best also dictates the method of transportation provided.

The Final Countdown

Dropping an aircraft carrier from the 1980s into the moments before Pearl Harbor, The Final Countdown delivers a moral dilemma plenty of time travelers have tangled with. But the real difference with this underrated sci-fi movie is the fact that the time-traveling storm that is responsible for the trip is inescapable. You’re going home, whether you want to or not.

Sharing a similarity with the romantic classic Somewhere In Time , Richard Curtis’ About Time allows any potential traveler to jump into the past with merely intense concentration. However, certain caveats are in play, like the recommendation of not traveling past certain life milestones, or the fact that only the men of the Lake family can actually use this gift.

Donnie Darko

Donnie (Jake Gyllenhaal) can’t exactly travel through time in Donnie Darko , but he does have a special temporal ability that’s kind of funny and kind of sad. With the ability to open a wormhole between the present and the past, Mr. Darko can send objects through time; the skill that gives Richard Kelly’s movie its bittersweet ending. 

Kate & Leopold

Kate & Leopold’s usage of a localized time portal is a method as old as time. However, the big difference with this Meg Ryan/Hugh Jackman rom-com is that the journey Leopold (Jackman) takes to the “future” of 2001 robs us all of elevators. Also, there’s a ticking clock on this specific portal’s usage, which only complicates things further.

Groundhog Day

Perhaps the movie that nailed the time loop into the consciousness of the world, Groundhog Day brought us a charming Bill Murray rom-com mixed with a time travel story. Its misanthropic lead needed to change, even as the world around him stayed the same. The rest was sci-fi history in the making. 

Happy Death Day

What happens if you make a Groundhog Day-style time loop into a deadly game? You get a movie like Happy Death Day , in which our initially unlikable lead Tree (Jessica Rothe) is being stalked through a single-day time loop. The big kicker in this variant is that, unlike your standard time loop, Tree has a finite number of cycles before she possibly dies for good.

Where does one start with director Joseph Kahn’s Detention? Well, how about the fact that the teenagers in play (including a pre- Hunger Games Josh Hutcherson) use a stuffed bear as a time travel capsule? Or the fact that a mother/daughter pair can body swap on a permanent basis, and with no consequences? 

And with that, our supreme sampling of time travel trips has come to a close. Which more than likely has left you with a want to watch some of these movies again, or for the first time. That's totally natural, because this is a subgenre that always leaves us with one question: is there ever a bad time to watch a time travel movie?

Mike Reyes is the Senior Movie Contributor at CinemaBlend, though that title’s more of a guideline really. Passionate about entertainment since grade school, the movies have always held a special place in his life, which explains his current occupation. Mike graduated from Drew University with a Bachelor’s Degree in Political Science, but swore off of running for public office a long time ago. Mike's expertise ranges from James Bond to everything Alita, making for a brilliantly eclectic resume. He fights for the user.

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Screen Rant

The 25 best time travel k-dramas, ranked.

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10 Must Watch K-Dramas That Aren’t Romances (& Where To Stream Them)

20 steamy k-dramas you should watch today, 10 k-dramas with heartbreaking plot twists.

• Time travel in K-dramas elevates romantic, detective, and historical stories with sci-fi or fantasy twists.
• While some K-dramas overuse time travel, the best ones cleverly use the trope to tell captivating and unique stories.
• Shows like "A Time Called You" and "Twinkling Watermelon" explore time travel to unravel mysteries and change fates in heartwarming ways.

The idea of traveling through time is a powerful storytelling tool popularized by Hollywood movies, anime series, and the best time travel K-dramas . For K-dramas, the past often involves the Joseon era, riddled with political distress, turmoil, and drama. In some cases, K-dramas get creative and use time travel to solve crimes, like a heinous murder from the past, or with clues unraveling through time. In others, time travel is the narrative catalyst for an epic romance story of lovers separated by centuries.

Not surprisingly, many of the best K-dramas involving time travel use the concept to elevate romantic stories with a sci-fi or fantasy twist. In fact, this happens so much that K-drama audiences largely agree that it's become an overused plot device. That said, whether they are mainly romantic, detective, or historical K-dramas , there are dozens that cleverly use time travel to continue telling captivating stories, and the best time travel K-dramas use the trope to tell some of the most unique stories on the small screen.

25 A Time Called You

Released in 2023, a time called you.

Not available

Based on the Taiwanese TV series Someday or One Day, A Time Called You is a South Korean romantic fantasy drama that centers around a young woman named Koo Yeon-jun, grieving her boyfriend's passing. Koo Yeon-jun's life is thrown for a loop when she wakes up in the 1990s - in an eighteen-year-old girl's body named Min-ju. As she tries to solve the mystery and return home to her original body, her life is complicated further when she meets a young man who looks identical to her former lover.

A Time Called You is actually a remake of a Taiwanese series called Someday or One Day , and it features more science fiction than just time travel. It also includes the characters jumping into the bodies of others and having their own doppelgangers. There is a lot going on, but it makes for a fascinating watch.

The series follows two sets of characters played by Jeon Yeo-been and Ahn Hyo-seop who see both their pasts and futures controlled by an old cassette player. The song on the tape player helps to control aspects of their lives, but not everything, leading to a lot of moving parts in the story. There’s even a serial killer subplot for those who like their K-dramas with a little mystery.

The only reason the series isn’t truly the best time travel K-drama is that the finale of the series doesn’t offer a full measure of closure, leaving some things open-ended for the audience.

24 Again My Life

Released in 2022.

Again My Life isn’t the typical time travel K-drama. Rather than feature someone traveling straight into another time, the person is actually murdered early in the series, but then, the main character comes back as a college student . This actually works in his favor since in his previous life, he was a prosecuting attorney, so he’s already extremely intelligent and excels in the academic program, allowing him to spend his free time figuring out who set out to kill him.

Because this is one time travel series that is more of a thriller, there’s also an action element to it. Lee Joon-gi, who stars as the lead, has a black belt in taekwondo, and he does many of his own action scenes in the show instead of a stunt double. He won an award for his work on the show at the SBS Drama Awards after the show’s release.

23 Twinkling Watermelon

Twinkling watermelon.

Twinkling Watermelon is a South Korean television series that follows the lives of a group of high school friends who form a band. As they navigate the complexities of adolescence, they encounter challenges, build relationships, and discover their passions. Featuring a mix of music and drama, the show delves into themes of friendship, ambition, and self-discovery.

There are shades of Back to the Future in this time travel K-drama as a teenage boy (Ryeoun) ends up in his father’s high school years. The boy and his father (Choi Won-young as an adult and Choi Hyun-wook as a teenager) don’t see eye to eye when it comes to music. While he wants to pursue a career as a musician, his father doesn’t think it’s a good idea. Their fight is the catalyst for the time travel experience.

While in the past, he finds that his father is desperate to connect with another young woman, not one who becomes his mom. He also realizes that his father isn’t hearing impaired as he is as an adult. He decides to try to prevent the accident that causes the hearing loss as well as make sure his father ends up with his mother, even as he’s forming a band with him.

It’s a heartwarming story about getting to know one’s parents and understanding how one decision can change everything.

22 Lovely Runner

Released in 2024, lovely runner (2024).

Lovely Runner is a TV series that follows the story of a young athlete who rises to fame in the world of track and field. As the protagonist navigates the highs and lows of competitive sports, the series explores themes of perseverance, sacrifice, and personal growth.

Those who enjoy K-Pop, athletics, and love stories will certainly enjoy Lovely Runner . The series sees a former superstar named Sun-jae (Byeon Woo-seok) lives a life of mostly isolation while his career explodes. Im-sol (Kim Hye-yoon) is a superfan who has been supporting his career. In their 30s, however, they both wonder what their lives would have been like if they had done things differently. Of course, they also have a past connection.

When one of them dies, the other gets a chance to travel to the past to change their future, and it's revealed that they have both been doing everything they can to protect one another, even if it means their own lives might never be the same. The theme of the show is self-sacrifice as it explores what lengths someone will go to for someone they care about.

Interestingly, Beyon Woo-seok is not a trained K-pop singer, even though there are many artists who have crossed over into the acting world. Despite that, the song “Sudden Shower,” which was recorded for use in the show, charted, demonstrating that he’s got what it takes to lead a boy band.

21 Reborn Rich

The last few years have produced a lot of hugely successful K-dramas that have reached an international audience. Reborn Rich , however, was hugely successful at home as well. It was the most-watched K-drama in South Korea in 2022.

Like Again My Life , this series sees someone who is murdered get a chance to start a new life in a new body, but with the awareness of their previous life. This time, it’s someone who is a hardworking employee at a chaebol - a sprawling family-owned business, the kind that truly runs the economy in South Korea. When he sees something wrong and tries to do the right thing, he is killed for it. He ends up returning to life as the youngest member of a chaebol family in an earlier time and starts putting his knowledge of the future to use.

While the bare bones of the two stories are similar as both characters want revenge on those who took their lives, the worlds they explore are very different. Reborn Rich was nominated for a slew of awards in South Korea and ended up winning for Best Miniseries at the Seoul International Drama Awards.

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20 My Perfect Stranger

My perfect stranger (2023).

My Perfect Stranger is a 2023 TV series that follows the story of a woman who discovers a mysterious stranger may be the key to solving a tragedy from her past. As she uncovers the truth, she must navigate a complex web of secrets and lies to uncover the truth about her own life.

When the two cross paths, they find that maybe their time traveling, and their lives, are actually linked to one another.

While most Korean dramas involving time traveling only see one character do the actual time traveling, My Perfect Stranger takes a risk in making both of its main characters time travelers . One does so on purpose though, while the other does not, which is where much of the intrigue comes from.

Hae Jun (Kim Dong Wook) is a journalist investigating a serial murder case who discovers a time machine and uses it to follow his own investigation. Yoon Young (Jin Ki Joo) discovers her mother dead and is then struck by a car, waking up in the same time that Hae Jun traveled to. As is true for many time travelers, they become trapped instead of able to travel between times. When the two cross paths, they find that maybe their time traveling, and their lives, are actually linked to one another.

Released In 2017

Tunnel (2017).

Tunnel is a South Korean television series that premiered in 2017. The show follows a detective, Kwang-ho, who travels back in time from 1986 to 2016 and teams up with a detective in the present day to solve a series of cold cases. As they work together, they uncover hidden secrets and unravel the mysteries of the past.

Instead of traveling to the past, Tunnel is a time travel K-drama that takes the audience to the future - in a manner of speaking. The present-day of the story is actually 1985 in which a string of murders are being investigated. When the detective (Choi Jin Hyuk) pursuing a lead runs after a suspect through a tunnel, he’s hit in the head and ends up 30 years in the future.

The murder mystery element of the series is inspired by a real string of serial killings in Korea, but the story is very much fictional. While the show presents itself as a murder mystery with a time-travel element, there’s also a romantic subplot and a family at the center of the story that makes it a drama that really shines. Incredibly popular outside of South Korea as well, Tunnel has been adapted for Thai television and an Indonesian remake is in development.

Want to enjoy some new TV series from Korea but without all the love and sappy sentimentality of some K-dramas? Check out these 10 great options.

18 Familiar Wife

Released in 2018, familiar wife (2018).

Familiar Wife is a South Korean television series that aired in 2018. The show revolves around Cha Joo-hyuk, a bank employee who travels back in time to when he was in his college days and alters the course of his life, resulting in a different reality where he is married to his first love, instead of his current wife.

A married man (Ji-Sung) with two kids is unhappy in his life. He’s depressed, hates his wife, and is unsatisfied at work. If he had only made different choices in his youth, he might have had a happier life. After traveling through a toll booth and losing control of his car, he winds up in the past and is able to change some of those decisions.

This is one time travel series that doesn't confine the protagonist to one time. Able to travel back and forth between the past and the present, he’s able to see how his different decisions have big outcomes later in life. It’s a similar concept to something like Sliding Doors but with many small changes to help someone understand their path in life. The music truly helps set the stage in the series as well with the soundtrack being nominated at the Korea Drama Awards.

Released In 2021

Times (2021).

Times is a South Korean television series that premiered in 2021, following the story of Seo Jin-Woo, a man who discovers a way to communicate with his past self through a mysterious walkie-talkie. As he navigates this new power, he attempts to prevent a tragic event that occurred in his past.

Times isn’t a traditional time travel K-drama in which one character is transported to another time period completely. Instead, the time travel is a result of a phone line that is able to traverse timelines. Several series have used similar ideas, often with the use of radios instead of phones, with varying degrees of success.

Two journalists, one in 2015 and one in 2020, connect over the use of the same phone line. Initially, they work together to prevent the death of one of their fathers, but while investigating, they uncover a conspiracy that involves time travel and corrupt politicians. The series is definitely a unique take among the time travel shows created for Korean television, which usually focus on history and a romantic subplot instead of political intrigue. While the series didn't reach widespread acclaim, star Lee Joo-young was nominated for Best New Actress at the Baeksang Arts Awards.

16 Tomorrow With You

Tomorrow with you (2017).

Tomorrow With You is a fantasy romance television series that follows Yoo So-joon, a successful real estate CEO who has the ability to time-travel through a subway. He meets Song Ma-rin, a photographer, and falls in love with her, but their relationship is threatened when he discovers his ability to travel through time is linked to his fate with her.

While Tomorrow With You does use romance like most time travel K-dramas do, it also has a unique idea for the time travel itself. While most Korean time travel shows involve time traveling by accident, the main character in Tomorrow With You can decide to travel in time. It’s also done so while traveling the subway instead of falling through a portal or suffering a life-threatening injury in order to do it.

Yoo So-Joon (Lee Je-Hoon) takes on time-traveling adventures when he discovers that Ma-Rin (Shin Min-A) and he are linked by a tragic accident in the future. Intrigued by her, he decides to save both her and himself. He believes that he can change their fates thanks to his ability, but he has to figure out just how they are connected. He resolves to marry her in order to make sure, and they become truly linked as he falls in love.

15 The Best Hit

The best hit (2017).

The Best Hit is a 2017 South Korean television series. The show revolves around the life of a K-pop idol, Lee Ji-hoon, who travels back to 1994 and meets his younger self and a young woman named Kim Hyun-jae. As Lee Ji-hoon navigates his past, he must confront his mistakes and learn valuable lessons about life, love, and friendship.

In The Best Hit, 1993 K-pop idol Yoo Hyun-Jae (Yoon Shi-yoon) gets accidentally transported to 2017, discovers that he is presumed dead after he disappeared in 1994, and investigates why he traveled through time. He does all this while struggling with living in the future, of course. Meanwhile, Hyun-Jae's biological son, Lee Ji-Hoon (Kim Min-jae), is secretly training to become an idol himself and is even enrolled in Star Punch Entertainment's program - while making his parents believe that he's studying for the civil service exams.

Also known as Hit the Top , and Best Punch , The Best Hit is an underrated gem in the world of the best time travel K-dramas. It's also an interesting way for the audience to get a look at what it takes to become a pop idol. Some of the stars of the series are also popular musicians. Kim Min-jae, for example, is also a rap artist and was nominated for the Best New Actor award at the Seoul Awards following the debut of the show.

14 Sisyphus: The Myth

Sisyphus: the myth (2021).

Sisyphus: The Myth is a South Korean television series that follows the story of Han Tae-sul, a genius engineer who tries to uncover the truth behind his brother's death. As he delves deeper into the mystery, he discovers a conspiracy that threatens the entire nation.

The storyline introduces a character who travels from the future to his current time.

This 2021 Netflix K-drama has some serious Terminator vibes to it, but it's full of good laughs, color, and drama. Han Tae-Sul (Cho Seung-woo) is a talented engineer who is determined to uncover the real reason behind his older brother's murder. If that wasn't enough, the storyline introduces a character who travels from the future to his current time. In the future, the world is dominated by gangs and military cliques. Survival entails having serious combat skills, and Gang Seo-Hae (Park Shin-Hye) is just the right warrior.

She travels back in time to help Tae-Sul on his dangerous journey after learning key evidence about his brother's death. What's interesting is that the series doesn't focus solely on the one-way time travel trip, but insists that the two main characters are responsible for breaking a time loop. Some critics disliked that Sisyphus: The Myth breaks its own established rules about time traveling, but it certainly keeps the audience on the edge of their seats.

13 Splash Splash Love

Released in 2015, splash splash love (2015).

Splash Splash Love is a South Korean television series that premiered in 2015. The fantasy romance drama tells the story of Lee Do, a high school student who travels back in time to the Goryeo Dynasty and meets Kim Mu, a prince who becomes his ally and friend. As Do navigates the complexities of court politics, he finds himself falling for the prince's sister, Princess Wonseong.

Splash Splash Love is one of those K-dramas that combine time travel with romance . In the television series, a high-school student gets transported to 15th-century Korea. She soon meets the young king of the kingdom who wants to learn more about mathematics. Thankfully, the heroine of the show is a patient teacher who is more than willing to tutor the King.

Splash Splash Love is a quick and easy watch as it is one of the shortest K-dramas out there, with only two episodes. This is part of why the show has near-perfect pacing and ranks among the best time travel K-dramas. It was originally broadcast as a 10-part webseries on Naver TV Cast. Its popularity led to it being edited into two hour-long episodes, so binging it is really like watching a movie.

12 Rooftop Prince

Released in 2012, rooftop prince (2012).

Rooftop Prince is a South Korean television series that tells the story of Crown Prince Lee Gak, who travels from the Joseon Dynasty to modern-day Seoul in search of his missing bride. With the help of Park-ha, a young woman who resembles his lost love, he navigates the complexities of contemporary life while trying to uncover the truth behind his bride's disappearance.

Rooftop Prince has a lot of storylines to process, but they're well worth it. The titular protagonist is a prince who wakes up in present-day 2012, even though he's actually from the Korean Joseon dynasty/era . The crown prince and his two confidants find themselves on the rooftop of Park-ha's (Han Ji-min) home. After the prince realizes he's time-traveled 300 years into the future, he also sees the long-lost sister of Park-ha - who is the spitting image of his late wife who drowned.

The prince is convinced he will find the answers to his past in 2012 and poses as the grandson of a powerful CEO, who in reality was killed by another family member. Both the characters of the past and the characters of the present have intense backstories and the series leans into the soap-opera-like aspects of those. It works in the show's favor, as the series has become even more popular outside of Korea than in its home. It is most popular in Japan and China.

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11 The King: Eternal Monarch

Released in 2020, the king: eternal monarch (2020).

The King: Eternal Monarch is a fantasy romance drama that tells the story of Emperor Lee Gon, who discovers a mysterious portal that leads him to a parallel universe. He meets a detective named Jung Tae-eul, and together, they navigate between their two worlds, uncovering secrets and conspiracies that threaten the stability of both kingdoms.

Not only does The King: Eternal Monarch have a phenomenal female lead in Jeong Tae-eul (Kim Go-eun), but it's also full of riveting action, features a unique story of parallel worlds, and is also one of the unique time travel K-Dramas available on Netflix. In the show, there are two co-existing realities: present-day South Korea and the Kingdom of Corea.

The current king of Corea witnessed his father's murder as a child at the hands of his uncle. When he discovers a doorway to the parallel world and present-day South Korea, he also discovers that his uncle has been traveling between the two worlds and trying to raise his own army. The king must stop his uncle, but along the way, he learns the truth about the mysterious figure who once saved his life and finds he must travel to a moment in the past. The series explores the idea of a single moment in time altering someone's whole reality.

10 Queen In-Hyun's Man

Queen in-hyun's man (2012).

Queen In-Hyun's Man is a 2012 South Korean television series that tells the story of Kim Boong-do, a dignified and charismatic nobleman, who travels through time from the 17th century to the present day, where he meets and falls in love with actress Choi Hee-jin.

A Joseon Dynasty scholar meets an aspiring actress in the year 2012 in the political and romantic Korean drama , Queen In-Hyun's Man, one of the best time travel K-dramas. In 1694, a noble-born scholar is the only survivor after his family was murdered, but there's a bigger story at play while he supports the reinstatement of the deposed queen and there's a conspiracy involving a royal concubine. Due to a magical talisman, he time travels 300 years into the future and meets an actress who he has a connection with.

What makes the rom-com elements of the scholar from the past meeting the actress from the future is that the actress, so far, is not a household name. instead, her big break is about to be playing Queen In-Hyun, the very woman the scholar is a supporter of.

Faith (2012)

Faith is a South Korean television drama that aired in 2012, starring Lee Min-ho as Choi Young, a skilled warrior, and Kim Hee-sun as Yoo Eun-soo, a plastic surgeon who travels back in time to the Goryeo era. The story follows their love story amidst the political turmoil of the time.

The time travel K-drama Faith follows the story of a plastic surgeon who gets transported to 14th-century Goryeo. She soon learns that a royal guard warrior was the reason why she was brought back more than 600 years into the past. The royal guard begs the plastic surgeon to help their injured queen. However, the plastic surgeon has actually caught feelings for her kidnapper.

Faith is considered one of the better time travel and fantasy-themed romance K-dramas thanks to the stellar performances of the actors, as well as the detailed story that shines a light on Korean history and mythology. It's also got a multi-layered title. While the literal translation of the title is "faith" as in "belief in justice," the Hanja lettering (Chinese letters used to write Korean) actually means "divine doctor," allowing the audience to understand that both terms are important to the long-term storyline of the show.

8 Nine: Nine Time Travels

Released in 2013, nine: nine time travels (2013).

Nine: Nine Time Travels is a South Korean television series that follows the story of Park Sun-woo, a man who discovers a way to travel through time and sets out to prevent a family tragedy from occurring.

In the time travel K-drama Nine: Nine Time Travels , Park Sun-woo (Lee Jin-wook) discovers nine incense sticks that could take him back 20 years in time. He uses his time in the past to save his family, who have been victims of a terrible tragedy. However, he needs to be extra careful when he goes back in time, as he can only do it nine times - the twist that makes Nine: Nine one of the best time travel K-dramas. Park soon finds out that whatever he changes in the past also resonates and affects his present life.

The K-drama was nominated for several awards across Korea for its great cast and romantic storyline, including snagging nominations at the Korean Drama Awards and the Baeksang Arts Awards. It also became popular outside of Korea, broadcast on television in China, Taiwan, Malaysia, and Thailand.

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7 Go Back Couple

Go back couple (2017).

Go Back Couple is a South Korean television series that explores the lives of a married couple, Choi Ban-do and Ma Jin-joo, who are struggling to balance their careers and relationship. After 18 years of marriage, they are suddenly transported back in time to their college days, giving them a chance to re-evaluate their life choices and relationships.

Inspired by a webtoon called Do It One More Time , Go Back Couple (also simply called Go Back in some regions) is a Korean miniseries where a couple wishes that they never met each other. They have both become unhappy and exhausted with their marriage and wish they could have changed their unhappiness at the seeming source of all of their problems - meeting each other. The couple soon finds that they've reverted to their 20-year-old selves, faced with trying to make decisions that will make their future selves happy and satisfied.

The show features plenty of funny and heartwarming moments that will make viewers swoon — and an ending that will surely tug at the heartstrings of every long-time K-drama viewer. After its broadcast, the series was nominated for a slew of KBS Drama Awards, picking up two, one for Best Couple for the lead actors in Son Ho-jun and Jang Na-ra, and Excellence Award for Actress in a Miniseries for Jan Na-ra.

6 Chicago Typewriter

Chicago typewriter (2017).

Chicago Typewriter follows the lives of three individuals: Han Se-joo, a talented writer; Jeon Seol, a passionate fan; and Yoo Jin-oh, a mysterious ghostwriter, who become entwined as they navigate their complex relationships and uncover the mysteries of their pasts.

Out of the most compelling and best time travel K-dramas (as well as general fantasy K-Dramas ), Chicago Typewriter is one of the few that's about reincarnation rather than strictly about traveling through time. Three resistance fighters from the 1930s Japanese occupation of Korea find themselves in a new time period, but not because of mysterious scientific machines or magical objects. Instead, the three are reincarnated as a writer, a fan, and a ghostwriter.

Switching between the 1930s and the 21st century, these three characters start to see parallels between both time periods. As they start to uncover the truth behind their pasts, they worry about how it might affect their present. K-drama fans tired of the same old romantic comedy fare will love the truly unique story elements of Chicago Typewriter. The series was also filmed on location around Seoul, South Korea, so fans will enjoy picking out the real sites.