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5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

• Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
• Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

• Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
• Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

• Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
• Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

• Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
• Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

• Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

• The Solution: time travel is impossible because of the very paradox it creates.
• Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
• The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
• Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

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10 key destinations for the historical time traveler.

Two hundred years ago a traveler had to wait months to traverse oceans. We now have the means to wake up in New York and fall asleep in Sydney, all in the same day.

The trend: travel is becoming exponentially more accessible to the common man.

But the tradeoff is that culture and history are being lost. Remote islanders maintain their outdated tribal customs merely to get a buck from the nearest walking wallet with a camera. Cities that in ancient times were considered quaint and romantic have become nothing more than identical concrete jungles.

We’re losing the remnants of human history with each passing day – why not find a means to time travel for leisure? Where would you go if you had a weekend in any city in any century?

1. Rome, Height of the Empire

No one in living memory has ever really seen the Colosseum. Whatever your religious beliefs, there used to be gods in that city; watching over the empire from their marbled countenance, and ensuring trade on one of the first greatest centers of business in the western world: the Roman Forum.

Imagine being able to walk down the epitome of civilization; they didn’t call the period after Christianity spread and the empire fell The Dark Ages for no good reason. ..

2. Kyoto, 16th Century

In the 1500’s, Kyoto served as the national capital and home to the imperial family. Tokyo (then Edo) was little more than a fishing village at this time, not yet placed on the map by the empowering of the Tokugawa Shogunate.

In modern times, many travelers journey to Kyoto to discover only remnants of what was once one of the most beautiful and mystical places on the planet. Back in its heyday, this Japanese city would have been the richest and most populated next to Osaka.

The predecessors to geisha gently walking in their kimonos, made from imported Chinese silk; visions of the mountains to the north and east not yet lost in a sea of grey; everything under ten meters high.

The only downside? Not much fresh fish or sushi: transporting the latest catch from Osaka to Kyoto took a while to perfect, and sushi was still in its infancy.

3. United States, The Old West

Back to the Future had the right idea – many people at one point imagine themselves as a cowboy or cowgirl.

What would you give to be riding on horseback on a cool summer morning in the undeveloped expanse of the western territories? Nothing for hundreds of miles in any direction, except perhaps the whistle of a steam locomotive and wandering tribes of Native Americans.

Of course, if you go back far enough in history, any land can be considered unexplored or undiscovered, but there’s a certain romantic connotation that stirs up when thinking about the American movement to the west.

It’s the promise of the unknown – traveling towards the Pacific, having uprooted everything stable, everything civilized in the east, and seeing where the Oregon Trail took you.

4. Ancient Egypt, c. 2500 BC

Watch the building of the pyramids and learn more about archaeoastronomy – skylights in the pyramids were carved so that certain constellations could be viewed at a set time of year.

Even the great structures themselves were arranged on the sand corresponding to the placement of three stars overhead. Discover the meaning of the Great Sphinx – who knows why it was built? Maybe some pharaoh just had a mutant pet.

Cairo will surely be hot and dry during this period in history, so remember to pack light loose-fitting clothing and plenty of sunscreen. If you wait around for another five hundred years, you might catch the finishing touches on Deir el Bahri .

5. London, 14th Century

“…the truth was that the modern world was invented in the Middle Ages. Everything from the legal system, to nation-states, to reliance on technology, to the concept of romantic love had first been established in medieval times.” – Timeline, Michael Crichton

Everyone wants to be a knight in shining armor or a princess fair and true. Chivalry isn’t dead. In fact, if you choose to travel to London roughly seven hundred years ago, you’ll find it quite alive and well.

A walking tour of this city will let you face the first real London Bridge, providing the only access across the Thames. Canterbury Tales by Chaucer paints a rather vivid picture of this era. Many of the buildings in London we associate with medieval times were already in place: The Tower of London, Westminster Hall, Westminster Abbey.

Remember to apply insect repellant liberally, as the Black Death was known to pass through Britain and France in this century.

6. Chang’an, Han Dynasty

The origin of The Silk Road and a golden age in Chinese history, when Confucian principles laid down the foundation for society and Buddhism was just beginning to spread. Travel west along this trade route and in a matter of months, you’ll reach the Roman Empire.

7. Chichen Itza, 5th century

Although you may not be in a temple of doom, it’s wise to heed the words of Indiana Jones and “protect your heart!” As one of the largest Mayan cities on the Yucatan Peninsula, Chichen Itza was the site of human sacrifices.

It’s true that quite a few of the largest temples are very well preserved in modern Mexico, but I challenge you to find another time or place in which ancient games that could rival basketball were played. Best to arrive before the Toltec siege.

8. India, c. 600 BC

The Buddha had about forty-five good years of teaching from the time of his reaching enlightenment under the Bodhi Tree to his death. Don’t waste them. Meeting the Awakened One and learning the dhamma firsthand would be an experience for which almost anyone in Asia would trade his or her life. Try and eliminate the suffering in your heart before your departure…

9. New York City, Roaring 20’s

By the time the 1920’s dawned in New York City, the modern version of a cityscape was already formed: Macy’s department stores, the public library, Grand Central Terminal, and the then world’s tallest Woolworth Building.

Unfortunately construction on the Empire State Building won’t commence until after the crash of ’29, but take advantage of this period in history with your choice of taxicabs or horse drawn carriages. Watch Lindbergh start his journey across the Atlantic. Gaze at the audience of women in hoop skirts and men in all too stiff and uncomfortable suits.

10. Babylon, c. 600 BC

One of the seven wonders of the ancient world: the hanging gardens of Babylon. As one of the first empires in human history, Babylon was built in the shadow of ancient Sumeria near the Euphrates River, and may have even been the source of the legendary Tower of Babel with its own Temple of Marduk.

I’m sure there are many asking “Why not see some dinosaurs?” Think a little practically in this impractical form of travel and question if you’d prefer camping in the late Cretaceous (and being trampled to death), or blending with the masses and observing the election of a Roman consul firsthand.

Besides, if a Tyrannosaur doesn’t get you, the meteor will later on.

Community Connection

You’ve been to the past, now meet travelers that we still remember tale. Read 10 Travelers and Why Their Tales Made History . Also, what other trends might we see in the future of travel? Check out 6 Predictions For the Future Of Travel .

Where and when would you go if you had a ticket guaranteeing a weekend of fun in any place at any time?

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• time travel
• grandfather paradox

Time travel is theoretically possible, calculations show. But that doesn't mean you could change the past.

• Time travel is possible based on the laws of physics, according to researchers.
• But time-travelers wouldn't be able to alter the past in a measurable way, they say. 
• And the future would essentially stay the same, according to the reseachers. 

Imagine you could hop into a time machine, press a button, and journey back to 2019, before the novel coronavirus made the leap from animals to humans.  

What if you could find and isolate patient zero? Theoretically, the COVID-19 pandemic wouldn't happen, right? 

Not quite, because then future-you wouldn't have decided to time travel in the first place.

For decades, physicists have been studying and debating versions of this paradox: If we could travel back in time and change the past, what would happen to the future?

A 2020 study offered a potential answer: Nothing.

"Events readjust around anything that could cause a paradox, so the paradox does not happen," Germain Tobar, the study's author previously told IFLScience .

Tobar's work, published in the peer-reviewed journal Classical and Quantum Gravity in September 2020, suggests that according to the rules of theoretical physics, anything you tried to change in the past would be corrected by subsequent events.

Put simply: It's theoretically possible to go back in time, but you couldn't change history.

The grandfather paradox

Physicists have considered time travel to be theoretically possible since Albert Einstein came up with his theory of relativity. Einstein's calculations suggest it's possible for an object in our universe to travel through space and time in a circular direction, eventually ending up at a point on its journey where it's been before – a path called a closed time-like curve.

Related stories

Still, physicists continue to struggle with scenarios like the coronavirus example above, in which time-travelers alter events that already happened. The most famous example is known as the grandfather paradox: Say a time-traveler goes back to the past and kills a younger version of his or her grandfather. The grandfather then wouldn't have any children, erasing the time-traveler's parents and, of course, the time-traveler, too. But then who would kill Grandpa?

A take on this paradox appears in the movie "Back to the Future," when Marty McFly almost stops his parents from meeting in the past – potentially causing himself to disappear. 

To address the paradox, Tobar and his supervisor, Dr. Fabio Costa, used the "billiard-ball model," which imagines cause and effect as a series of colliding billiard balls, and a circular pool table as a closed time-like curve.

Imagine a bunch of billiard balls laid out across that circular table. If you push one ball from position X, it bangs around the table, hitting others in a particular pattern. 

The researchers calculated that even if you mess with the ball's pattern at some point in its journey, future interactions with other balls can correct its path, leading it to come back to the same position and speed that it would have had you not interfered.

"Regardless of the choice, the ball will fall into the same place," Dr Yasunori Nomura, a theoretical physicist at UC Berkeley, previously told Insider.

Tobar's model, in other words, says you could travel back in time, but you couldn't change how events unfolded significantly enough to alter the future, Nomura said. Applied to the grandfather paradox, then, this would mean that something would always get in the way of your attempt to kill your grandfather. Or at least by the time he did die, your grandmother would already be pregnant with your mother. 

Back to the coronavirus example. Let's say you were to travel back to 2019 and intervene in patient zero's life. According to Tobar's line of thinking, the pandemic would still happen somehow.

"You might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would," Tobar said, according to Australia's University of Queensland , where Tobar graduated from. 

Nomura said that although the model is too simple to represent the full range of cause and effect in our universe, it's a good starting point for future physicists.  

Watch: There are 2 types of time travel and physicists agree that one of them is possible

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MUMENTOUS: Football, Glue Guns, Moms, and a Super-Sized Tradition Born Deep in the Heart of Texas

200 North 5th Street Garland, TX 75040 (888) 879-0264 Website

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Adulti-Verse (21+) at Meow Wolf Grapevine

3000 Grapevine Mills Pkwy Suite 253 Grapevine, TX 76051 (682) 345-8832 Website

Adulti-Verse (21+) at Meow Wolf Grapevine 3000 Grapevine Mills Pkwy Suite 253 Grapevine, TX 76051

Vereins Quilt Guild Quilt Show

1407 E. Main St. Fredericksburg , TX 78624 (361) 676-2293

Vereins Quilt Guild Quilt Show 1407 E. Main St. Fredericksburg , TX 78624

Grapevine Main LIVE!

815 S. Main St. Grapevine, TX 76051 (817) 410-3185 Website

Grapevine Main LIVE! 815 S. Main St. Grapevine, TX 76051

Water Lantern Festival

Haggard Park W Chaparral St Amarillo, TX 79107 (435) 554-0134 Website

Water Lantern Festival Haggard Park W Chaparral St Amarillo, TX 79107

Dinosaurs Live! Life-size Animatronic Dinosaurs Returns

Dinosaurs Live! Life-size Animatronic Dinosaurs Returns 1 Nature Place McKinney, TX 75069

Blues on Main

101 North West Main Street Ennis, TX 75119 (214) 475-0042 Website

Blues on Main 101 North West Main Street Ennis, TX 75119

Ennis Pumpkin Patch & Hay Maze

111 NW Main St Ennis, TX 75119 (972) 878-4748 Website

Ennis Pumpkin Patch & Hay Maze 111 NW Main St Ennis, TX 75119

Ennis Autumn Daze

Ennis Autumn Daze 111 NW Main St Ennis, TX 75119

2nd Annual Texas Banana Pudding Festival

Slaton Bakery 109 S. 9th St. Slaton, Texas 79364 (806) 828-3253 Website

2nd Annual Texas Banana Pudding Festival Slaton Bakery 109 S. 9th St. Slaton, Texas 79364

Texas Forts Trail Caravan Part 3

Brownwood Event Center 601 E Baker Street Brownwood, Texas 76801 (800) 484-1462 Website

Texas Forts Trail Caravan Part 3 Brownwood Event Center Complex 601 E Baker Street Brownwood, Texas 76801

Lubbock’s First Friday Art Trail

Louise Hopkins Underwood Center for the Arts 511 Ave. K Lubbock, Texas 79401 (806) 762-8606 Website

Lubbock's First Friday Art Trail Louise Hopkins Underwood Center for the Arts 511 Ave. K Lubbock, Texas 79401

ItalianCarFest

626 Ball St. Grapevine, TX 76051 (817) 410-3185 Website

ItalianCarFest 626 Ball St. Grapevine, TX 76051

Vinyl Radio

112 West 6th Ave Corsicana, TX 75110 (903) 874-7792 Website

Vinyl Radio 112 West 6th Ave Corsicana, TX 75110

60th Anniversary of the Gilligan’s Island Fundraiser

209 N Broadway St, Brownwood, TX 76801 Brownwood, TX 76801 (325) 641-1926 Website

60th Anniversary of the Gilligan’s Island Fundraiser 209 N Broadway St, Brownwood, TX 76801 Brownwood, TX 76801

Guided Trails

Heard Natural Science Museum and Wildlife Sanctuary 1 Nature Place McKinney, TX 75069 (972) 562-5566 Website

Guided Trails Heard Natural Science Museum and Wildlife Sanctuary 1 Nature Place McKinney, TX 75069

All the Bells and Whistles: The History and Heritage of Homecoming Mums in Texas

Garland City Hall, 200 N. Fifth Street Garland, TX 75040 (972) 205-2749 Website

All the Bells and Whistles: The History and Heritage of Homecoming Mums in Texas Garland City Hall, 200 N. Fifth Street Garland, TX 75040

Jurassic Night Out

Jurassic Night Out Heard Natural Science Museum and Wildlife Sanctuary 1 Nature Place McKinney, TX 75069

A Wrinkle in Time

119 W. 6th Ave Corsicana, TX 75110 ​​903-872-5421 Website

A Wrinkle in Time 119 W. 6th Ave Corsicana, TX 75110

38th Annual GrapeFest — A Texas Wine Experience

636 S. Main St Grapevine, TX 76051 (817) 410-3185 Website

38th Annual GrapeFest - A Texas Wine Experience 636 S. Main St Grapevine, TX 76051

38th Annual GrapeFest

38th Annual GrapeFest 636 S. Main St Grapevine, TX 76051

Exhibition Opening; Target Texas: Studio Practice

1902 North Shoreline Boulevard Corpus Christi, TX 78401 (361) 825-3500 Website

Exhibition Opening; Target Texas: Studio Practice 1902 North Shoreline Boulevard Corpus Christi, TX 78401

Quanah Parker Day 2024

TAKE A STAND WITH AN ARROW TO HONOR QUANAH PARKER WHAT: Quanah Parker Day in the Texas Plains Trail Region WHEN: Sat., Sept 14, 2024 WHERE: Communities with Quanah Parker Trail giant arrow markers TEXAS PLAINS TRAIL REGION — Quanah Parker Trail arrows will stand a little taller on Saturday, Sept. 14 - Quanah Parker Day in Texas. The statewide observance, which Texas Governor Greg Abbott decreed for the second Saturday of each September, commemorates the life of Comanche Chief Quanah Parker. In the Texas Plains Trail Region, one of the ten regions designated as part of the Texas Heritage Trails Program of the Texas Historical Commission, volunteer leaders launched a Quanah Parker Trail project in 2010, which has organized some regional events and avenues for participating in the statewide recognition. About the Quanah Parker Trail Every Texas student passing through 4th and 7th grade by now has learned the story of Quanah Parker; his mother Cynthia Ann Parker, who was kidnapped as a child and raised among the Comanches; and their lives as Texas Indians. The Comanches were called “Lords of the Southern Plains” owing to their superb horsemanship and prowess in hunting bison from horseback. In the Texas Plains Trail Region (TPTR), we encourage you to honor Quanah Parker Day by paying a visit to a nearby steel arrow sculpture that marks a county’s inclusion on the Quanah Parker Trail (QPT). Take a stand with Quanah Parker and the arrow. Share your photos with us on social media and be sure to tell your family and friends about your adventure. The QPT placed within the TPTR is conceptual in nature. Counties of the TPTR installed 88 steel arrows created by sculptor welder Charles A. Smith (1943–2018), to commemorate their inclusion in the Comanchería, the territorial range of the Comanches in the 19th century. The trail is named for Quanah Parker, as he is considered as the most renowned Indian leader who frequented this area. The heart of the story is this: Every county’s land within TPTR was once part of the territory of the Comanches, known as the Comanchería. What local communities can do to honor Quanah’s legacy The site where QPT Arrows are installed can be cleared of tall growth that may obscure them from view. Weathered arrows can be touched up with paint made for metal tractors and available at local farm and feed stores, or hardware stores. Streamers of red, yellow and blue, the colors of the shield for the logo of the Comanche Nation, can be tied onto the arrows to flutter in the wind and draw the attention of travelers to the history of the region. More about Quanah Parker and the Comanches And the other part of the story this: At one time or another, Quanah crossed the lands where we live now. As a warrior, Quanah rode with the Kwahada division of the Comanche tribe. The Kwahada made their home on the southern High Plains of the Llano Estacado and as well traversed the Rolling Plains of Texas in the late 19th century. Under Quanah’s leadership, the Kwahada remained the last Comanche holdouts who resisted the U.S. military's effort to force all Indians to move onto reservations. After the Comanches moved to the reservation in Indian Territory by the end of 1875, Quanah continued as a leader, helping them adjust to a different way of life. He was appointed as a Comanche chief by Indian agents of the federal government. Quanah coordinated cultural and political activities felt to be necessary to aid the Comanche people in adapting to the challenges of modernism in the late 19th and early 20th centuries. In this capacity, he continued to travel throughout our region, by mule-drawn wagon, touring car, and train. He became a celebrity, and as a much sought-after representative of frontier history, in the TPTR alone he attended a funeral in Dalhart, addressed a crowd in Matador, attended a celebration in the town of Quanah named for him, and visited ranchers Samuel Burk Burnett in Guthrie and Charles Goodnight near Claude. For more information • About the Quanah Parker Trail in the TPTR, visit: www.quanahparkertrail.com Biographies of Quanah • Bill Neeley, “The Last Comanche Chief: The Life and Times of Quanah Parker” (J. Wiley, 1996). • William T. Hagan, “Quanah Parker, Comanche Chief” (University of Oklahoma Press, 1993). History about his mother, Cynthia Ann Parker • Paul Carlson and Tom Crum, “ Myth, Memory, and Massacre: The Pease River Capture of Cynthia Ann Parker ” (Texas Tech University Press, 2010). • Jack K. Selden, “ Return: The Parker Story ” (Clacton Press, 2006). History of the Comanches • Pekka Hämäläinen, “The Comanche Empire” (Yale University Press, 2009). • Ernest Wallace and E. Adamson Hoebel, “Comanches, Lords of the South Plains”(University of Oklahoma Press, 1952).

Quanah Parker Day 2024

Living History

7200 Paredes Line Road Brownsville, TX 78526 (956) 541-2785 Website

Living History 7200 Paredes Line Road Brownsville, TX 78526

Mimosas at the Market

Beaton St Corsicana, TX 75110 (903) 654-4852 Website

Mimosas at the Market Beaton St Corsicana, TX 75110

39th Annual Kolache Festival

Downtown on the Square in Caldwell, Texas Caldwell, TX 77836 (979) 567-0000 Website

39th Annual Kolache Festival Downtown on the Square in Caldwell, Texas Caldwell, TX 77836

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Is Time Travel Possible?

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

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

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

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

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

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

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

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

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

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

In Summary:

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

If you liked this, you may like:

Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

Disclosure statement

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

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

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

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

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

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

Laws of physics

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

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

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

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

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

Resolving paradoxes

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

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

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

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

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

Time is a river

Roman emperor Marcus Aurelius wrote that:

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

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

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

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

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

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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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Time travel: five ways that we could do it

Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later (No guests showed up). Time travel is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

But travel to the future? That’s a different story.

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.

But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

1. Time travel via speed

This is the easiest and most practical way to time travel into the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

This is not a just a conjecture or thought experiment – it’s been measured. Using twin atomic clocks (one flown in a jet aircraft, the other stationary on Earth) physicists have shown that a flying clock ticks slower, because of its speed.

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

And the closer you get to the speed of light, the more extreme the time-travel.

Computer solves a major time travel problem

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months , for us.

Amazingly, particle physicists have to take this time dilation into account when they are dealing with particles that decay. In the lab, muon particles typically decay in 2.2 microseconds. But fast moving muons, such as those created when cosmic rays strike the upper atmosphere, take 10 times longer to disintegrate.

2. Time travel via gravity

The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.

As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.

And anyway, the effect is not that strong so it’s probably not worth the trip.

Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

(The situation described in the movie Interstellar , where one hour on a planet near a black hole is the equivalent of seven years back on Earth, is so extreme as to be impossible in our Universe, according to Kip Thorne, the movie’s scientific advisor.)

The most mindblowing thing, perhaps, is that GPS systems have to account for time dilation effects (due to both the speed of the satellites and gravity they feel) in order to work. Without these corrections, your phones GPS capability wouldn’t be able to pinpoint your location on Earth to within even a few kilometres.

3. Time travel via suspended animation

Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

Could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards achieving inducing a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13 °C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects.

Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces a hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh.

4. Time travel via wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

5. Time travel using light

Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

According to Mallet, the right geometry could lead to time travel into either the past and the future.

Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

His ideas have not grabbed the rest of the physics community however, with others arguing that one of the assumptions of his basic model is plagued by a singularity, which is physics-speak for “it’s impossible”.

The Royal Institution of Australia has an Education resource based on this article. You can access it here .

Related Reading: Computer solves a major time travel problem

Originally published by Cosmos as Time travel: five ways that we could do it

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Time Travel

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

1.1 Time Discrepancy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2. The Grandfather Paradox

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

We can set out Horwich’s argument this way:

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

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

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

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

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

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

3. Causation

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

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

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

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

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

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

4. Time and Change

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

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

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

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

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

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

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

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

Earlier we posed two questions:

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

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

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

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

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

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

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

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

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Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

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Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

• If There Were a Time Warp, How Would Physicists Find It?
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• Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

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Reid J. Epstein and Katie Rogers cover the Kamala Harris presidential campaign. They reported from Chicago.

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The officials said Ms. Harris will return to the campaign trail next week with events in multiple battleground states, where she will aim to extend the enthusiasm generated at this week’s convention in Chicago.

Gov. Tim Walz of Minnesota, her running mate who was plucked from relative political obscurity 17 days ago, will return to his state before heading back on the campaign trail next week.

Presidential candidates typically come out of their conventions trying to build momentum with bus tours or rallies. Ms. Harris’s choice to delay the traditional barnstorming reflects the supercharged pace of the last few weeks and the changed nature of modern campaigns, where social media posts can be as important as speeches. It also is evidence of a shift from a sprint to more of a middle-distance effort that requires endurance and speed in equal measures.

“Campaigns often do barnstorm, but this has been such a compressed schedule for her and she has a debate in 17 days,” said David Axelrod, the veteran Democratic strategist who worked on Barack Obama’s campaigns. “A little downtime and maybe even prep would make sense. As we just saw they have a plethora of first-rate surrogates who can and should be deployed when she is not.”

While Mr. Obama in 2012 left his re-election convention in Charlotte, N.C., for a swing through battleground states including Iowa and Florida — where he was memorably lifted into the air by a pizza proprietor — campaigns today rely less on the local media coverage generated by stumping across the country.

Campaigns now seek to reach voters as much on social media apps as in person, a reality that was evident in how the Democratic National Convention gave status to influencers willing to cheer for Ms. Harris and spread the campaign’s message that was equal to or better than what it gave the traditional press corps at the United Center.

Ms. Harris’s self-imposed deadline for a national interview, next Saturday, presents the next hurdle for Ms. Harris. It will be a break from the rhythm of stadium rallies she has hosted since she named Mr. Walz her running mate.

Mr. Trump does not have a robust campaign schedule in the coming days either.

The Republican nominee sought to divert attention to himself during the Democratic National Convention, holding an event each day in a different battleground state. Mr. Trump plans on Friday to cap the week’s events in Las Vegas and in Arizona, where his campaign has teased a special guest that many expect to be Robert F. Kennedy Jr., who suspended his independent campaign for president on Friday and endorsed Mr. Trump.

Mr. Trump is then expected to spend the weekend at his golf club in Sterling, Va., according to two people familiar with his plans. But he does not have any public events scheduled until Monday, when he will address the National Guard Association of the United States in Detroit.

“President Trump will be working nonstop like he always does and doing an interview this weekend, which is one more interview than Kamala has done in 33 days,” said Karoline Leavitt, a spokeswoman for the Trump campaign.

Mr. Trump did not remain quiet after his convention. Two days after it ended in Milwaukee, he traveled to Michigan for his first joint rally with his newly crowned running mate, Senator JD Vance of Ohio.

Michael Gold contributed reporting.

Reid J. Epstein covers campaigns and elections from Washington. Before joining The Times in 2019, he worked at The Wall Street Journal, Politico, Newsday and The Milwaukee Journal Sentinel. More about Reid J. Epstein

Katie Rogers is a White House correspondent. For much of the past decade, she has focused on features about the presidency, the first family, and life in Washington, in addition to covering a range of domestic and foreign policy issues. She is the author of a book on first ladies. More about Katie Rogers

20 Best Time-Travel Shows Ranked

If you could travel back and forth through time, where would you go? What would you do? Who would you talk to? Even better, if you were writing a book, making a movie, or working on a television show about time travel, what would you include? The best TV shows about time travel all feature characters who visit other eras for various compelling (or even life-threatening) reasons. Maybe it's to prevent a coming apocalypse, maybe it's just to save one person's life — but as many of these shows teach, small changes can have big effects, and many of these characters learn that their time-traveling can change the world.  

Now, there are some great time travel-adjacent shows that don't quite fit this list. A fun romp like "Early Edition," for example, utilizes a time-traveling newspaper and potentially a time-traveling cat, but doesn't in and of itself feature a lot of time travel. Likewise, something like "Terminator: The Sarah Connor Chronicles" is rooted in a time travel premise, but stays mostly in one time. With all that said, here's a look at our choices for the 20 best time travel shows on TV.

Save the cheerleader, save the world. That's what future Hiro Nakamura (Masi Oka) tells present-day Hiro when he appears to him from the future, and that's what establishes "Heroes" as way more than just a superhero show.

The NBC series follows a group of regular people who develop special powers, not unlike mutants in the "X-Men" series, after a mysterious worldwide eclipse. Each character gains their own individual abilities. Claire Bennet (Hayden Panettiere) develops the ability to heal from any injury. Senator Nathan Petrelli (Adrian Pasdar) gains the ability to fly, while his brother Peter (Milo Ventimiglia) can temporarily absorb others' powers. Still, few of these characters have cooler abilities than Hiro, who can influence the space-time continuum. This means he can teleport, slow down time — and, of course, time travel.   

Understandably, Hiro's power set becomes a serious asset throughout the series, and his path to perfect his abilities is one of "Heroes'" strongest story arcs. The first few times he travels through time don't go as planned, and throughout the series, things can get in the way of him ending up where he wants to go or when he wants to be. While Hiro's time-traveling is just one part of the larger story, it's definitely one of the show's highlights – especially since Oka is so darn charming as the character.

19. 11.22.63

One of the best Stephen King TV series out there, the eight-episode "11.22.63" follows a man named Jake Epping (James Franco). He's a relatively normal guy who receives a chance to change history when his friend Al (Chris Cooper) tells him he's found a way to travel back in time. Al tells Jake that the portal he's discovered goes back to the year 1960 and that he's been working on a plan to stop the assassination of President John F. Kennedy.

Al's age and advancing cancer diagnosis prevent him from following through on the plan, however, and he asks Jake to take over for him. Jake agrees, but soon his quest is met with pushback from a mysterious source. As it turns out, the past doesn't want to be changed, and every step Jake takes toward preventing JFK's assassination leads to more cracks in the timeline. 

A charming and exciting time travel drama, "11.22.63" is a well-executed, twisty tale that only ranks so low on this list because it's in such great company. If you're looking for a quick, self-contained time travel miniseries that revolves around one of modern America's most notable events, this show is well worth a watch. 

When Oceanic Airlines Flight 815 crash lands on a deserted island, wacky and scary things start happening to the survivors. ABC's "Lost" deals with flashbacks, flash-forwards, mysterious groups that already have a presence on the island, a black smoke monster — and, as it turns out, an ancient battle between good and evil. One of the great appointment television shows before streaming broke through, "Lost" had fans talking about it and theorizing about its mysteries on a weekly basis.

The sci-fi drama captivated viewers for six seasons, and though time travel is referenced throughout the entire series run, it plays the biggest role in Season 4. As the island itself leaps from place to place and from time to time, the main group of characters jumps with it, encountering previous versions of themselves and island events that occurred in the past, and suffering from the effects of temporal displacement. The most beloved episode dealing with time travel is undoubtedly "The Constant," in which fan-favorite Desmond Hume (Henry Ian Cusick) figures out a way to stop his consciousness from jumping through time by finding his constant — his true love, Penny (Sonya Walger).

Of course, "Lost" is not just a time travel show, and famously covers such a wide variety of mysteries and sci-fi concepts that viewers might find it hard to keep up. As such, it ends up with this relatively low ranking. 

Like "Lost", "Fringe" is considered one of the most binge-worthy sci-fi shows of all time  but the fact that it isn't exclusively about time travel means it lands near the tail end of this particular list. The ABC show revolves around a science-fiction conglomerate that dabbles with interdimensional travel, wormholes, and alternate realities. Anna Torv stars as FBI Agent Olivia Dunham, who heads up the bureau's Fringe Division. With the help of "mad scientist" Dr. Walter Bishop (John Noble), his estranged son Peter (Joshua Jackson), and their lab assistant Astrid Farnsworth (Jakisa Nicole), Dunham explores cases involving fringe science — be they about time travel, mind control, experiments gone wrong or any other strange and obscure criminal activity.

Time travel is more of a looming presence early in "Fringe," particularly present in the character of the Observer (Michael Cerveris), a bald, pale, genetically advanced human from the future. While Season 1 and Season 2 deal with the battle between two dimensions and realities, time travel really becomes an element in Season 3. Seasons 4 and 5 then deal with alternate timelines and the Observers that infiltrate the world from the future, intent on wiping out humanity. As you might expect, things can get a bit confusing, but the show sure is fun.

16. The Umbrella Academy

You have to respect a show that's so high-concept that time travel doesn't even get top billing. "The Umbrella Academy" boasts mysterious events, family drama, dance numbers, a talking chimpanzee, some of the cleverest superpowers in superhero shows, and a robot mom — and that's just scratching the surface. Based on "The Umbrella Academy" comics created by Gerard Way of My Chemical Romance fame, the Netflix show is a saga that exploits everything from the butterfly effect to the grandfather paradox for emotional and comedic impact.

The central Hargreeves family consists of a group of kids all born on the same day, adopted by the same eccentric billionaire (Colm Feore). He has trained them to protect the world with their various superpowers, but they aren't particularly great at it, and their strict upbringing has left them with a wide array of issues and deep rifts between them. The dysfunctional bunch starts out fairly estranged, but slowly bonds to save humanity from an apocalyptic event ... only to cause another potential apocalyptic event by sprinkling themselves across time.

In between the tears in the space-time continuum, "The Umbrella Academy" is ultimately an ensemble story about found (and re-found) family, as well as a truly unique superhero show where personal failure and the side-effects of costumed crimefighter life play a huge role. However, since Season 1 largely approaches time travel through Number Five (Aidan Gallagher) and the Temps Aeternalis agency, and much of Season 3 focuses on a present-day alternate reality, only the 1960s-themed Season 2 goes truly all in on the concept of sending all main characters to a different era. 

15. Sliders

"Sliders" is a 1990s sci-fi adventure series that features Jerry O'Connell and friends getting lost across the multiverse. O'Connell ("Stand By Me") plays boy genius Quinn Mallory, inventor of the Timer — a device that lets him and his friends "slide" through a wormhole vortex into different versions of Earth. The thing about wormhole vortexes, though, is that they like to misbehave, meaning Quinn and his buds never know where they're headed next on their adventures. This makes their quest to get back home to their own Earth a tricky one.

"Sliders" starts off fun and strong, and is at its best when having bonkers fun — like when Rembrandt (Cleavant Derricks) discovers a world where he could have been Elvis-level famous — and when it's exploring real-world issues in a high-concept dimension, like when the crew visits an Earth that treats men worse than women. Even if you've seen it before, it's definitely worth a re-watch, because "Sliders" is one  TV show that's better than you remember.

14. Continuum

On "Continuum," Kiera Cameron (Rachel Nichols) is a Protector – think futuristic government agent from even more futuristic equipment — from the year 2077. She gets transported to the year 2012 along with a group of murderous terrorists, forcing Kiera to remain in the past as she chases them down. Fortunately, her gadgets and knowledge of the past soon come in handy and she finds loyal allies. Unfortunately, her enemies also know their history and plan on altering it for their own gain. 

"Continuum" milks the premise for all it's worth, while avoiding the pitfall of becoming a run-in-the-mill procedural with an unchanging status quo. While Kiera does handle her share of case-of-the-week story arcs, they're often connected to the group she pursues, and she never lets go of her primary target of stopping the terrorists. In order to avoid disrupting the timeline, she also has to go to great lengths to avoid revealing that either she or her targets are time travelers — and when their actions inevitably end up changing the future, she has to deal with the consequences. 

13. Timeless

If ever there was a time travel show that was canceled too soon, it's Eric Kripke and Shawn Ryan's "Timeless." The NBC sci-fi series stars Abigail Spencer as the historian Lucy, Matt Lanter as the soldier Wyatt, and Malcolm Barrett as Rufus, a scientist who makes up a team trying to prevent a mysterious organization from altering the courses of history through time travel. They're up againsts Garcia Flynn (Goran Višnjić), who travels throughout history intending to influence major events like the Hindenburg disaster. However, the team soon realizes that the villain they thought they were fighting is much larger and infiltrates the historical timeline in ways they never imagined. 

Instead of focusing on the usual historical suspects, "Timeless" often highlights forgotten people of color, women, and lesser-known historical figures, giving them their due and celebrating their contributions to society. This element of the show can be seen in the way Rufus, for instance, is reluctant to join the team because he knows how Black people are treated in the eras they visit. 

Despite its intriguing concept, the show was canceled after Season 1, but fans caused such an uproar that NBC reversed the decision of canceling "Timeless"  and renewed it for another season. After Season 2, NBC pulled the plug once more, and again, the fans cried foul. In a kind of compromise, NBC greenlit a special two-hour series finale that ties up loose ends and gives much-needed closure to the story. 

12. 12 Monkeys

The "12 Monkeys" SyFy series is based on the 1995 film of the same name that stars Bruce Willis and Brad Pitt — though the series makes a fair few changes to stretch the plot into a four-season sci-fi drama. The series stars Aaron Sanford as James Cole, a scavenger from the year 2024 who's tasked with traveling to 2015 in order to stop the release of a biological weapon. In the movie, James is helped by a psychologist named Kathryn Railly played by Madeleine Stowe, but here, he befriends a virologist named Dr. Cassandra "Cassie" Railly (Amanda Schull). Pitt's character, Jeffrey Goines, is also gender-swapped here, with Emily Hampshire playing Jennifer Goines.

Like the movie, the series deals with the Cassandra Complex, the idea that we have a hard time believing concerns about the future, no matter how likely and provable they are. It also deals with circular time and the idea that past events can be affected by future ones. If those aspects of the film lift your time travel antennae, the four-season show dives even deeper.

11. Paper Girls

"Paper Girls" is a brilliant time travel show that was canceled way ahead of its time. Based on the comics by Brian K. Vaughan and Cliff Chiang, this Amazon series tells the story of a group of 1990s tween girls who get attacked by futuristic invaders. They manage to escape into the future, where one of the girls, Erin (Riley Lai Nelet ), meets her adult self (Ali Wong).

The show dispenses with grandfather paradox hand-wringing and instead uses the concept of the girls confronting their past and future selves, to brutally honest and hilarious effect. Young Erin is horrified to find out how much of herself she's abandoned by the time she turns into Old Erin, and refuses to let life work out that way. It motivates Erin to want to return to her home time even more — this kid has a clock to beat. However, there are two sides to the coin, and Old Erin is also able to care for her young self in ways she never felt able to when she was younger. It's a beautiful and potent visual metaphor that other characters also make good on. 

All in all, "Paper Girls" is a feast for the eyes as much as its ensemble cast is a feast for the soul. Plus, Jason Mantzoukas playfully chewing scenery as the ominous Grand Father? This show could have lasted until the end of time — or at least until Season 2.

10. Timewasters

"Timewasters" is a time travel comedy about a Black British jazz band that accidentally time-slips back to 1920s London, among other timelines. The quartet stumbles into an earlier time perod via a disgusting elevator that, yes, doubles as a time machine. Once the crew shows up in the past, they're treated like freaks, but they gain some measure of success as musicians. While the crew eventually tries to return to the present, they also have a "Back to the Future" moment when they seemingly get stuck in the 1950s.

"Timewasters" is full of funny jokes and great music, and it's a groundbreaking show in a number of ways. "People like us never get to time travel — it's what white people do, like skiing or brunch," creator Daniel Lawrence Taylor told the Royal Television Society . "For me, race is so important." Taylor also stars in "Timewasters," along with Kadiff Kirwan ("Slow Horses"), Adelayo Adedayo ("Some Girls"), and Samson Kayo ("Our Flag Means Death"). The show is also an excellent destination if you're into spotting a variety of British actors and comedians ... including Joseph Quinn, who went on to rise to fame as Eddie Munson on "Stranger Things."

9. Outlander

Based on the series of novels by Diana Gabaldon, Starz's "Outlander" follows the story of a World War II nurse named Claire (Caitriona Balfe) who finds herself thrown back in time after visiting a circle of mysterious Druid stones. She arrives in 18th Century Scotland and, after being taken in by a band of gruff Scots, she marries the dashing young Jamie Fraser (Sam Heughan) in order to avoid being taken prisoner by her real husband's (Tobias Menzies) apparent evil ancestor, Black Jack Randall (Menzies). Claire lives through a time of great upheaval in Scotland when tensions with British control are rising and history-making battles loom in the near future. Despite being initially reluctant to stay, she and Jamie fall deeply in love, and their romance remains the backbone of the series.

The entire "Outlander" timeline  takes some time to explain, what with several 20th-century characters taking the trip to the 18th century and the show covering versions of notable real-world historical events. Without further spoilers, all there is to say is that if you enjoy time travel shows that lean heavily toward historical drama, "Outlander" is where it's at. Also, if you view Tobias Menzies as an incorrigible dweeb due to his performance as Edmure Tully on "Game of Thrones," his monstrous "Outlander" villain is guaranteed to erase that image.

8. Quantum Leap

"Quantum Leap" stars Scott Bakula as Dr. Sam Beckett, a physicist who invents a way to travel through time. When the corporation funding his project threatens to shut it down, Sam uses himself as a guinea pig to test out the method. He finds himself thrown back in time, but in another person's body. The only other entity aware of his 'leap" is a hologram of his colleague and best friend, Admiral Al Calavicci (Dean Stockwell). Al tells Sam that he must correct things that went wrong in the past before being allowed to leap back to his own time and body, and can only use the resources of the project's supercomputer, Ziggy.

With Sam leaping back and forth into different bodies at different times, the show uses a variant of the traditional procedural set up. New characters turn up to guest star and Sam gets to save the day, have a fling, and learn something new before leaping to the next destination, which just might be home one of these days.  

The series ran on NBC from 1989 to 1993, but its combination of time travel and case-of-the-week antics has proved enduring enough that "Quantum Leap" even gets a shout-out in "Avengers: Endgame." Despite being over three decades old, it remains a cool time travel series worth checking out.

7. The 4400

In the opening scenes of "The 4400," an enormous ball of light drops 4,400 people at the foot of Mount Rainier in Washington. They soon realize that they were all taken from some other point in time and deposited into the year 2004, unaged and without any memories of where they'd been. At first, everyone assumes that these people have been abducted by aliens. However, it soon turns out that the truth is far more time travel-related.

The returned people soon start developing "Heroes"-style powers that range from telekinesis to telepathy and super-strength, which people from the future have entrusted with to prevent various catastrophic events that they want to avoid in their timeline. Unfortunately, the 2004 government considers the powered folks a threat, and inhibits their powers with a neurological drug. 

The stories that unfold from this setup are exactly as complex and entertaining as you'd imagine, with various members of the titular group treating their powers in different ways and society having a hard time dealing with them. Unfortunately, "The 4400" ended abruptly after four seasons on a somewhat ambiguous note, but even so, it's a fun show to revisit.

6. Travelers

In Netflix's "Travelers," time-traveling operatives from a post-apocalyptic future are tasked with preventing certain events that have led to the downfall of society in their own present day of 2018. The travelers' consciousness takes over a person in the desired time who's just about to die, and the operative then lives out the rest of that person's days though with the mission in mind ... and a strict set of rules they must follow. Apart from a list of ways they're not allowed to interact with the past, they're also strictly forbidden from communicating with other known travelers outside their team, save for special circumstances dictated by the Director, who communicates by temporarily taking over children. 

It's a unique and complex premise, and the way the travelers scope out potential targets for takeover and learn to live as them is as timely as it comes — they use social media, GPS locations, and other readily available online information for their time-travel tricks. This adds a layer of present-day dread to the show's fascinating take on time travel. 

Loki Laufeyson (Tom Hiddleston) meets his match when he comes up against the Time Variance Authority in one of the Marvel Cinematic Universe's most ambitious Disney+ shows, "Loki." The TVA is so dedicated to maintaining a particular sacred timeline that they purge all alternate realities where someone made a choice they deem wrong, which might not always make sense, but precision isn't the point here. It's the idea of playfulness versus control. 

The Loki we see here is an alternate-timeline variant of the one the audiences are familiar with, and thus starts the show in full "The Avengers" villain mode before life — and time — starts grinding him down. Working with TVA agent Mobius M. Mobius (Owen Wilson), he starts redeeming himself by tracking down an apparently evil version of himself, Sylvie (Sophia Di Martino) ... and ultimately tackling the biggest challenges time can offer.  

The God of Mischief's surprisingly human path of reckoning is the heart of a show that's deliciously stylish, silly, and sometimes scary. "Loki" takes a cops-and-robbers crime caper into time travel territory and explores hefty themes with a light touch, from mindless compliance to self-serving overseers to criminalizing anyone deemed different. "Loki" isn't just a time travel show — it's a show about everything time can offer and more, with characters dancing between eras as you might step from room to room. Also, it has Alligator Loki, who's objectively the best Loki of all. 

If "Loki" is too light-hearted for you, Netflix's "Dark" might be your jam ... provided you can make sense of its incredibly convoluted time travel storyline. Four families weave a tangled web of time travel in this German-language psychological thriller about missing kids, a rotten town, and how almost all of our secrets come out in time. In other words, it's a good time travel show, but it's definitely not a feel-good time travel show. 

"Dark" follows its many characters over the course of their lifetimes and, at one point, has three timelines going at once. Part of the intrigue and challenge of watching the show is trying to understand how (and when) each timeline threads into the other. If you decide to watch it, it's best to have an evidence board and plenty of red yarn ready to chart the relationships and betrayals the town of Winden sees over the years.

While "Dark" is as much a show about human connection and how frayed it can become as it is about time travel, it's also the MVP of using as many time travel paradoxes as possible during its three-season run. "Dark" is also an innovator in the field of wormhole placement. Wormholes are already not to be trusted, but a wormhole underneath a nuclear power plant? No, thank you.

3. Beforeigners

What happens when a bunch of Viking-era warriors, 19th-century figures, and Stone Age people pop up in modern-day Oslo? "Beforeigners" attempts to answer that question while navigating twisty murder mysteries with such efficiency that the Norwegian series may be best described as "crime travel." Adding to the intrigue is the way it focuses more on the present-day relationship between the time refugees and their modern counterparts than on how they showed up in the first place.

"Beforeigners" centers around the odd-couple partnership between hardened police detective Lars Haaland (Nicolai Cleve Broch) and eager new Viking police recruit Alfhildr Enginnsdóttir (Krista Kosonen), who investigate things like the murder of a Stone Age victim and even look into crimes with possible ties to Jack the Ripper.

The metaphor of time migration is an apt one for immigration, and this sci-fi show explores tricky real-life issues with plenty of scope. Creators Anne Bjørnstad and Eilif Skodvin got their start in comedy writing, and their commitment to the bit is evident in the show, including the language used. "Early on, I contacted researchers, professors who helped us. We also constructed the language that Stone Age people spoke, and even with the language from the 19th century: We worked on it to make it sound right," Bjørnstad told Variety . "Why not invest in language, which is such a big part of a person's identity?"

2. Russian Doll

"Russian Doll" could be pitched as "Natasha Lyonne's 'Groundhog Day,'" but that still wouldn't hint at half of the show's charm and emotion. This Netflix offering is a mind-bending time loop dramedy that's a stylish and surreal exploration of life, death, and all the trauma in between. Season 1 of "Russian Doll" features Nadia (Lyonne) stuck reliving her 36th birthday until she inevitably dies and resets back to her friend's bathroom. Later in the season, she discovers a fellow time traveler (Charlie Barnett). They quickly realize that the way out of their dead ends and into a new life is through helping each other.

Season 2 takes some departures from the recursive reality set up in the first season, bending viewers' minds even more thoroughly. "Russian Doll" goes deep, but keeps a sense of humor even as it twists the knife in its characters' hearts — and their timelines. The show keeps audiences just oriented enough by linking its time loops to recognizable spaces and sound cues. You will never look at the subway the same way again, and you will probably never get Harry Nilsson's "Gotta Get Up" out of your head.

1. Doctor Who

Really, could any other show top a list like this?  The untold history of "Doctor Who"  goes all the way back to 1963, when the show premiered on the BBC. The series follows the adventures of a Time Lord who calls themselves the Doctor — an alien being from the planet Gallifrey who travels through space and time on a craft called the TARDIS, which is charmingly disguised as an old-fashioned British police call box and is famously bigger on the inside.  Every Doctor has their own companions  – humans who follow the Doctor throughout space and time, helping people, battling new and recurring villains, and dealing with the assorted wibbly-wobbly stuff on the Doctor's timeline .

The original series ran from 1963 through 1989 and established the neat trick of recasting the Doctor every few years or so, thanks to the premise that the character has multiple lives and can reincarnate himself into different physical bodies. The modern series was revived in 2005 with Christopher Eccleston as the Doctor, and talented actors like David Tennant (twice), Matt Smith, Peter Capaldi, Jodie Whitaker, and Ncuti Gatwa have followed in his footsteps. Even without the fact that no other show has time travel quite as integrated into its very premise as "Doctor Who," the show's sheer longevity and cultural impact are more than enough to make it the king of the time travel hill. 

Harris and Walz heading to Georgia as campaign seeks to build on convention momentum

Vice President Kamala Harris and Minnesota Gov. Tim Walz will launch a bus tour of southern Georgia next week, the duo’s first time campaigning in the state together and, as of now, their first public event after the Democratic convention in Chicago.

The pair will be using the momentum from the party’s convention to drive them into the last couple months of the general election. In addition to the bus tour, Harris and Walz are expected to tape their first joint interview next week and attend multiple fundraisers, most likely to take place in New York, California, Florida and Georgia, according to two sources familiar with the planning.

Following the tour, Harris will headline a solo rally in Savannah, Georgia. The trip will mark Harris’ seventh visit to the state this year and her second since launching her presidential campaign last month.

“Campaigning in this part of the Peach State is critical as it represents a diverse coalition of voters, including rural, suburban, and urban Georgians — with a large population of Black voters and working class families,” the Harris-Walz campaign said in a press release announcing the bus tour.

Harris and Walz’s visit will come as the Republican ticket ramps up its campaigning in the state. Vice presidential nominee Sen. JD Vance of Ohio held a campaign event in Valdosta on Thursday, following a rally by former President Donald Trump in Atlanta earlier this month. Republicans, too, have sought to capitalize on polling that suggests the party could pick up a larger share of Black and Latino voters this election cycle.

The southern Georgia bus tour is expected to echo the campaign’s bus tour of western Pennsylvania earlier this month, which included stops at a local campaign field office, a firehouse and a high school football practice.

Harris and Walz initially planned to visit Savannah during the campaign’s battleground state tour earlier this month, but they had to postpone the event due to Tropical Storm Debby.

Though it’s unclear exactly where Harris and Walz will go, southern Georgia is home to some of the state’s largest Black populations, including Dougherty County, which has the second-highest proportion of Black residents in the state. The campaign has opened field offices in the predominantly Black cities of Albany and Valdosta.

“The south Georgia region is a priority for the campaign: We have nearly 50 full time staff across seven offices in the area, including Valdosta. We have hosted more than 500 events in the region since May 31,” Harris-Walz Georgia campaign spokesperson Adelaide Bullock said.

Ranada Robinson, research director for the New Georgia Project Fund, said appealing to Black voters in both rural and urban areas will be critical to Harris’ success in the state, as it was to Biden’s victory in 2020.

“Black voters are the key to winning Georgia. Of course Black Georgians can’t do it alone, but we are absolutely the reason 2020 turned out the way it did,” she said. “Black voters had historical turnout, and it has to happen again for there to be victory in Georgia.”

Earlier this month, Harris held the second rally of her presidential campaign in Atlanta, an event that featured Megan Thee Stallion; the campaign said it attracted more than 10,000 people.

It then launched a mobilization effort in the state and now touts nearly 400,000 volunteers, 174 staffers and 24 coordinated campaign offices sprawled across Georgia. The campaign refers to its ground game there as “the largest in-state operation of any democratic presidential campaign cycle ever in Georgia.”

Harris campaign chair Jen O’Malley Dillon identified Georgia as one of the campaign’s top targets, noting shifting demographics that could aid the vice president in expanding support from 2020.

“The Vice President’s advantages with young voters, Black voters, and Latino voters will be important to our multiple pathways to 270 electoral votes,” Dillon wrote in a recent memo.

Harris and Walz are also expected to barnstorm battleground states around Labor Day, before Harris focuses more of her time on debate prep ahead of her September matchup with Trump.

A Trump adviser said the campaign expects Harris will get a “bump” coming off the convention but compared it to a “sugar high,” saying they didn’t believe it will change the overall state of the race.

Nnamdi Egwuonwu is a 2024 NBC News campaign embed.

Recap: Pile-up on M6 leaves lanes 'blocked' and long delays

There's congestion on the A500 as well

• Updated 18:26, 24 AUG 2024

A three-car crash on the M6 has triggered long delays. The collision has occurred on the northbound carriageway and has reportedly left two lanes shut off.

Traffic monitoring system Inrix issued an alert over the incident at 3.15pm. It states: "Queueing traffic and two lanes closed due to accident, three cars involved on M6 Northbound after Junction 16 A500 (Stoke-on-Trent / Crewe). Congestion to Keele Services."

It represents another issue on the motorway through North Staffordshire today. Earlier we reported in this blog how all traffic had to be held following a collision around the same junction.

That sparked delays on the A500 at the time. And the D-road is again heavily congested - especially at Junction 16 with traffic at a stand-still through Audley.

We will provide further updates in our dedicated blog below.

• Three-car smash shuts two lanes 13:28

All lanes back open

All lanes are open.

End of blog.

Current state of play on M6 and A500

The latest collision has unsurprisingly caused delays on both the M6 and A500. A live map from National Highways shows congestion on the northbound stretch of the motorway from Junction 15 to the outskirts of Junction 17.

Meanwhile the A500 is very busy near to Junction 16 through Audley with long queues there too.

Three-car smash shuts two lanes

While the other incident has been cleared - there has been another crash. Inrix is reporting two lanes have been closed off just after Junction 16 on the northbound side. It is beleievd three cars have been involved.

Delays on D-road through Audley

Meanwhile at Junction 16 and the delays are even worse - with a travel time of 20 minutes.

Travel time is 10 minutes on A500

Inrix reporting 'slow traffic' on the A500 near to Junction 15 where travel time is said to be ten minutes.

One lane shut

National Highways has confirmed that after all traffic had been stopped - one lane has remained closed. Traffic officers are on scene with motorists warned of around three miles of congestion.

National Highways map

A live map from National Highways shows traffic has come to a stop on the M6 - along with delays across the A500.

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