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The trouble with time travel...

In Back to the Future, Marty McFly faces some mind-warping time travel paradoxes. John Gribbin explains the science behind time travel in the movies and whether travelling through time is possible.

John Gribbin

TheGrandfatherParadox

Time travel theory

A time traveller goes back in time and either accidentally or deliberately (if you are in a macabre mood) kills his own grandfather, before the time traveller’s father has been conceived. So the time traveller is never born, so he never goes back in time, so his grandad never dies, so the time traveller is born. And so on.

In the movies

If Marty McFly accidentally prevents his own parents falling in love and marrying, he will not exist. But as the science fiction writer Robert Heinlein put it, “a paradox can be paradoctored.” Marty does change the past, but in a positive way. He comes home to a different, better future than the one he left.

Is it possible?

An idea loved by sci-fi writers, and with some basis in scientific fact, is that all possible realities exist in some sense side by side. These ‘many worlds’ are like the branches of a tree, each with its own version of history. If you went back in time, into the trunk of the tree, and then forward again, you might go ‘back’ up a different branch from the one you came down. You go back in time and stop your parents meeting, then forward in time up the branch in which they never met and you were never born.

There would, though, be one timeline in which the traveller had vanished, never to return (as in HG Wells’s original time travel story, The Time Machine) and another with a person who has no parents. Complicated, but not paradoxical.

The Bootstrap Paradox

An item or a piece of information is passed from the future to the past, becoming the same item that is passed from the past to the future.

In Back To The Future, at the dance in 1955, Marty McFly gets on stage and sings “an oldie” where he comes from: Chuck Berry’s Johnny B. Goode. Chuck’s cousin, Marvin, is present at the dance and holds up a phone so Chuck can listen in. Inspired by the sound, Chuck later releases the song and Marty would then hear it in the future. But who wrote it?

Where did this idea come from?

This theory was popularised in Robert Heinlein’s 1941 short story, By His Bootstraps (hence the paradox's name), where a notebook is found by the main character in the far future. He takes it, then travels to an earlier point in the future and uses the useful translations within the book to help establish himself as a benevolent dictator. When the notebook becomes worn and dog-eared, he copies the information into a fresh notebook and discards the original. Towards the end of the novel, he muses that there were never two notebooks – the new one is the one that was found by him when he arrived. Mindbending!

Polchinski’s Paradox

If a billiard ball is rolled into the mouth of a wormhole at just the right speed and angle, it will come out the other end just in time to cannon into its younger self and prevent the younger self going into the tunnel.

This is one theory that is taken very seriously by scientists, and has been discussed in research papers published in respectable journals. Their interest is fired by the fact that there is nothing in General Relativity that forbids time travel, and that closed time-like curves (CTCs), or a path through space-time that ends up back where it started, at the same point in space and time, are allowed.

Is that the full story?

Not exactly, there are also self-consistent CTCs. If the billiard ball is approaching the mouth of the tunnel, its older self can emerge from the second mouth and give its younger self a glancing blow exactly right to send it through the wormhole in such a way that it will emerge to give itself a glancing blow that does the same thing. There are many possible consistent trajectories of this kind. In the most extreme case, the ‘second’ ball emerges from the tunnel and knocks the ‘first’ ball completely away, but in turn it gets deflected into the wormhole, whereit then takes the place of the ‘first’ ball.

A bunch of rather hairy calculations showed that there is never a system like this where there are no self-consistent trajectories, and that in some cases there can be an infinite number of self-consistent solutions to the equations.

All of this lends weight to Russian physicist Igor Novikov’s conjecture, sometimes more grandly referred to as Novikov’s Self-Consistency Principle, which states that time travel is allowed, but paradoxes are forbidden. This could resolve the Grandfather Paradox – if you go back and try to kill your grandfather, something will always go wrong. Take a gun to shoot him, and the bullet will misfire; poison his wine, and someone else will drink it, and so on.

TheTwin Paradox

If one member of a pair of twins goes on a journey at a sizeable fraction of the speed of light, he or she will age more slowly than the twin who stayed at home. And when the travelling twin returns home, he or she will be younger than the twin who stayed behind. Literally younger, in biological terms.

In Planet of the Apes, astronaut George Taylor, played by Charlton Heston, and his crew spend 18 months in hibernation at near-light speed before landing back on Earth some two thousand years into the future.

This is the most scientifically sound and least paradoxical of all time travel theories. Special Relativity tells us that moving clocks (including biological clocks) run slow. This has been tested by experiments in accelerators like the Large Hadron Collider. Particles with a known lifetime when stationary in the lab ‘live’ longer when they are moving close to the speed of light.

But hang on ...

Special Relativity says that all observers are equivalent, doesn’t it? Why can’t the travelling twin say that they are at rest, while the Earth and the other twin go on a journey into the future? All inertial observers, those in straight lines at constant speed relative to each other, are equivalent. Acceleration changes the rules of the game, and in order to get home the traveller has to decelerate and then accelerate back in the opposite direction.

In the extreme example often used by paradoxers, this turnaround is instantaneous. Cosmologist Hermann Bondi says that if you give each twin a paper bag full of eggs to hold, at the end of the experiment you will find the traveller covered in egg, while the stay at home twin is clean. They are not ‘equivalent’. All of this can be described accurately using equations, and the result is the same.

This kind of time travel is solid science, and it works. One snag: unless you can find a handy wormhole, there is no way to get back from the future. But that is a great idea for a film title…

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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

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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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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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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Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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• GENERAL TRAVEL

10 Annoying Travel Problems and Their Solutions

Raised in a multicultural family in the colorful Chicago suburbs, Raquel’s greatest joy is ...

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It’s been a glorious day exploring Thailand’s ancient temple ruins. You arrive at your hotel room with a phone full of pictures and rich stories that will live on forever. But when you finish emptying your day bag, something isn’t there.

Your passport is gone and in its place is one of a traveler’s worst nightmares and biggest travel problems.

Don’t lose your passport. We repeat: DON’T LOSE YOUR PASSPORT.

With just a bag or two in tow and (sometimes) cultural and linguistic barriers, issues that pop up while traveling overseas can often seem magnitudes worse than if they were to happen at home. And when you travel to unwind, grow, learn, or tackle new adventures, these little snafus are the last things we want to get in our way.

As Albert Einstein once said, “A clever person solves a problem. A wise person avoids it.” So in the spirit of a literal Einstein, let’s get familiar with some of the most common travel problems and how to solve them (or better yet, avoid them).

Is there a way to avoid annoying travel problems?

The unpredictability of travel lends to its charm and feeling of adventure. If a sprinkle of risk is an absolute no-go, we’d all be taking ultra-planned river cruises with our grandparents. Yet however thrilling unpredictability is, annoying travel problems are never welcome.

The best way to avoid most inconvenient headaches when out and about around the world is to overplan and prep before you even leave home. While you can’t foresee what will happen, you can at least have a backup plan or course of action lined up if you do find yourself in a pickle.

Familiarize yourself with a destination’s transportation system, the area around your accommodations, and what you’re able to buy in stores abroad—for example—all before arriving. Triple checking the ol’ packing list doesn’t hurt either .

10 common travel problems and how to deal with it

So, what kinds of bumps may pop up unexpectedly throughout your travels? Let’s dive in.

Is there a worse headache than the one caused by lost luggage?

1. Canceled, delayed, or unreliable transportation

No matter where you travel in the world, transportation will never be perfect—except maybe in Japan where trains’ annual average delay is mere seconds. From missed transfers to routes that have temporarily changed with a note posted in a language you don’t understand, there are tons of ways transportation issues can botch your journey.

• How to solve it: Transportation snags are largely out of your own hands, so avoiding them may not always be possible. Instead, the best way to minimize the impact is to seek out alternative routes to arrive at a destination just in case. Familiarize yourself with all possible options like a bus vs train, and what to do if your plans get altered. Knowing who to contact about a canceled train beforehand, for example, will help you act quicker and be less stressed if faced with that situation.

2. Forgotten can’t-live-without items

You’re settling into a hotel room after a whole day of flying and fighting jet lag only to find that your retainer didn’t make it into your suitcase. Now that you think of it, it’s still lying at home on the bathroom counter.

Universal items like toothbrushes or clothes are easy to replace wherever you go at the drop of a hat. However, personalized life companions like prescription meds, eyewear, or a phone charger with a specific voltage may not be readily available in your destination.

• How to solve it: This may go without saying, yet it’s not always a rule of thumb that travelers follow: Create a packing list and double check before leaving! Yes, that means start writing down your absolute essentials days or even weeks before your departure to make sure you account for everything. Leaving it all to your memory in a last minute packing spree won’t do you any favors.

3. Currency exchange

Do yourself a favor and have a bit of the local currency with you before you arrive abroad.

Exchanging currency has gotten so much easier over the past few years (good riddance, traveler’s cheques!). However, each country and region still has its own process for exchanging currency, so bringing a wad of cash with you and crossing fingers that it will all work out may not be your best bet. In some areas, traveling with that much cash on hand may raise safety concerns as well.

• How to solve it: Check in with your home bank to see what your options are for your target destination. In some cases, you can easily access an ATM with a debit card abroad for minimal fees. Some banks will also exchange money for you before you leave if you request it ahead of time, so you have a bit of cash on hand to navigate the first few days on a trip.

READ MORE: Is it Better to Travel with Cash or Card?

4. figuring out where and when to get food and water.

To be clear, you should definitely have access to drinking water and food anywhere you go! This common travel problem refers specifically to whether you can drink the tap water and what food sources you’re advised to avoid. Free drinking water also isn’t as widespread around the world as it is in the United States, especially in water-scarce regions.

• How to solve it: If you’re participating in an organized travel program like study abroad or a language school abroad , you’ll have great resources available already to ask about safe food and water sources. Your accommodations, such as a hotel or host family, can also give the skinny on whether street food is a yay or nay and if the tap is trusted. When in doubt (say you’re super off the grid), just stick to prepackaged food and beverages.

5. Knowing local emergency protocol

Knowing to dial 911 in an emergency is a no-brainer home, but what if you find yourself in need of help abroad ? Also, who do you contact if you are victim to a crime, lost valuables, or are hurt? While traveling is usually a positive adventure for most, these annoying travel problems can happen. And the last thing you need in an emergency is to feel lost and alone.

• How to solve it: Of course, you should definitely acquaint yourself with the local emergency phone numbers. If there is a language barrier and no one that can help translate, another great option is to know the contact for your embassy or consulate in your country of stay. Your embassy can act as a liaison between you and local authorities, as well as help you access medical care among other services.

6. Luggage weight limits

Don’t overpack, otherwise you might be stuck paying extra baggage fees.

Weight restrictions are a big limiting factor when traveling by air. But honestly, who can blame you for wanting to bring back an entire new wardrobe from Italy? If you’re hopping around to multiple destinations, e.g. adventure travel or a gap year , schlepping 100 lbs. of stuff around with you isn’t exactly ideal either.

• How to solve it: Ah, the internet. It provides so many wonderfully helpful free resources for how to pack light. Versatility is the way to go with clothes, but also think of multipurpose shoes, as multiple pairs of footwear can really add on the pounds. Another great packing hack is to bring a single suitcase with an empty duffle bag inside so you are armed and ready to bring back all the new goodies you buy abroad.

7. Gaps between check-out and check-in times

You’ve got a train to catch to your next location in the afternoon, where the check-in time is hours later, but you need to check out of a short term rental by 11:00 a.m. That leaves a few hours during which you’re on your own with bulky luggage and nowhere to go. Frequent travelers are likely quite familiar with these awkward gaps between check-out and check-in times.

• How to solve it: Accommodation hosts can be quite flexible when there isn’t another guest immediately arriving the same day, so see if you can get a check-out extension. Other great options are to ask about luggage storage at your accommodation (usually available at no additional cost) or even at bus and train stations.

READ MORE: Choosing the Right Travel Accommodations

8. packing for multiple climates.

Whether you’re staying put for a semester or year, or have multiple locations bundled up into one trip, you’ll likely face a range of climates. While keeping clothes for more than one season at a time in a single closet is already a struggle back home, this only gets all the trickier when you have to whittle that down to a few tops, bottoms, and shoes.

• How to solve it: Start a packing list with your bare essentials that you’ll need in any season or climate zone. Then, think about how you can get the most functionality out of the items remaining. Packing gurus typically advise travelers to lean on layers, so you can get full use out of each and every item.

9. Language barriers

Nowadays, it can be a little easier to find your way around language barriers.

Even though technology has evolved to the point of instant translation on a phone (admittedly, the translation isn’t always perfect), language barriers are still prevalent. Walk into a restaurant and sit down to order, then WHOOSH—you’ve never wanted pictures on a menu more badly in your whole life. The server comes over to ask something and BAM—maybe you aren’t so hungry after all...

• How to solve it: With simple Google Translate and other apps , you can overcome some of the most common travel problems with understanding signs and written text. When it comes to spoken language, having a travel program, host family, local friend, or even buddy who’s studied the local language more than you will certainly be helpful.

10. Locals who only want to speak English

On the flip side of traveling on a lonely English-speaking island, there are the language enthusiasts who have diligently studied the language of their destination for years. But, uh-oh, someone you proudly ask a question to senses an accent or maybe a grammar mistake. That’s it, they answer in English and now there’s no going back.

• How to solve it: Practice makes perfect with learning new languages, but every so often you’ll come across the stubborn English-enthusiast who just wants to save you the trouble of speaking their native tongue. They mean well, but it’s not doing your fluency any favors by switching to English. If you do get a reply in English, stay firm and keep going in the original language. That way, the person will understand that you know more than they thought and you’re comfortable sans English.

If you’re ready to go abroad, our FREE Online Advisor will send you 5 personalized travel program matches

Don’t worry—you can learn how to handle the biggest travel problems.

For first time travelers , the possibility of meeting with common travel problems can be overwhelming. But once you become a seasoned globetrotter, you’ll know how to dodge issues and solve them like a pro. And who said you have to go at it alone?

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April 26, 2023

Is Time Travel Possible?

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

By Sarah Scoles

yuanyuan yan/Getty Images

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ONE CHEL OF AN ADVENTURE

10 Common Travel Problems (and How to Solve/Prevent them)

Traveling is undoubtedly exhilarating, but let’s be real—it’s not always smooth sailing. From unexpected mishaps to downright frustrating situations, we’ve all encountered our fair share of travel woes. With that in mind, here are some tried-and-true tips to help you navigate through 10 common travel problems and their solutions!

Most Common Travel Issues + Problems

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1. Getting Lost While Traveling

Whether it’s wandering aimlessly in a maze-like city or taking the wrong turn on a remote hiking trail, getting lost is practically a rite of passage for travelers. 

Sometimes, getting lost can lead to some of the most memorable experiences! Embrace the moment, ask locals for directions, use maps (both digital and paper), and consider downloading offline maps  to your phone before setting off.

2. Getting Mugged While Traveling

Safety should always be a top priority while traveling. Unfortunately, muggings can happen, especially in tourist-heavy areas. Stay vigilant, avoid flashing valuables, use discreet/theft-proof bags , and trust your instincts. It’s also wise to split your cash and keep copies of important documents in a separate location. 

Men: You should definitely think twice about wearing a nice watch if traveling to Europe (and in general) — there has been a rise in luxury watch theft lately !

In case the worst happens and you do find yourself in this situation, cooperate and prioritize your safety above all else .

Related Post:   How to Stay Safe While Traveling

3. Losing Your Phone

In today’s digital age, losing your phone can feel like losing a limb. But fret not, there’s hope! Before departing, install tracking apps and enable remote wiping features on your device, especially if you store banking info and other sensitive info on your device (as most of us do). 

Additionally, keep a physical backup of important information such as emergency contacts and reservation details. I typically bring one of my old phones with me as a back up just in case. 

4. Getting Sick in an Unfamiliar Place

Nothing puts a damper on travel plans like falling ill. To prevent sickness , stay hydrated, eat well-balanced meals, and get plenty of rest. Pack a small first-aid kit with essential medications, and consider purchasing travel insurance for added peace of mind. 

If you do fall ill, don’t hesitate to seek medical assistance or rest until you’re feeling better. Don’t make the same mistake I did in  Southeast Asia , it could have been really bad! 

5. Language Barrier

Ah, the beauty of language diversity! While it enriches our travel experiences, it can also pose challenges. To avoid issues, try learning a few basic phrases in the local language, utilize translation apps , and embrace non-verbal cues like gestures and smiles. Remember, a genuine effort to connect goes a long way!

6. Feeling Lonely

Solo travel can be incredibly rewarding, but it’s not uncommon to feel lonely at times. Combat loneliness by staying in social accommodations like hostels or joining group tours and activities. Embrace opportunities to meet fellow travelers, strike up conversations with locals, and stay connected with loved ones back home. 

7. Running Out of Money

Budgeting woes can put a damper on even the most meticulously planned trips. To avoid running out of funds, create a realistic budget before departure and track your expenses along the way. Look for ways to save money , such as cooking your meals or opting for budget accommodations.

And always have a backup plan, whether it’s a stash of emergency cash or access to financial assistance.

8. Missing a Flight

Missed flights are every traveler’s nightmare, but they’re not the end of the world. Stay calm, contact your airline immediately, and inquire about alternative options.

  Travel insurance  can often cover additional expenses incurred due to missed flights, so be sure to review your policy. 

9. Travel Sickness

Motion sickness can turn even the most scenic journey into a nauseating ordeal. To combat travel sickness, sit in the front or middle of vehicles, focus on the horizon, and avoid heavy meals before travel. 

Over-the-counter medications like Dramamine can also provide relief for mild cases. And don’t forget to take breaks and get some fresh air whenever possible.

10. Losing Luggage

Arriving at your destination only to find your luggage missing is undoubtedly frustrating. To minimize the risk, pack essentials in your carry-on and use luggage tags with your contact information. And I always, always, ALWAYS have an Apple Airtag in my luggage so I can see exactly where it is at all times! 

If your luggage goes astray, file a report with the airline immediately and keep all relevant documentation. Most airlines have protocols in place to track and reunite lost luggage with its owner.

Traveling is a rollercoaster ride filled with highs and lows, but it’s the challenges that make the journey worthwhile. By arming yourself with knowledge, preparation, and a positive attitude, you can overcome these Common Travel Problems. 

So, embrace the adventure, stay curious, and remember that the best stories often arise from the most unexpected moments. Safe travels, fellow adventurers!

If you have a question, leave a comment below or send me a DM on Instagram ! 

Related Posts:

• How to Stay Safe While Traveling
• 12 Tips for Overcoming Anxiety While Traveling
• How to Stay Healthy While Traveling

You Might Also Like:

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Travel is back, but that doesn't mean it's easy (or cheap).

Airport chaos has made headlines more than a few times over the past couple of years, but if you opt to drive, you'll have to contend with soaring gas prices. Plus, hotel prices have hit all-time highs , even though many properties cut some services like daily housekeeping.

Despite all those deterrents, traveling might be non-negotiable. You might have five weddings to attend this year. Then there’s the work offsite to finally meet the coworkers you’ve never met face to face. And now that your youngest kids are vaccine-eligible, you owe them that (expensive) Disney trip they’ve been begging for.

Ah, suddenly the wanderlust you felt when envisioning your dream “revenge trip” back in 2020 has been zapped away, instead replaced by anxiety and plenty of unwanted expenses.

But it doesn’t have to be that way. Combine solid planing with a healthy mindset, and you might skip the common problems with traveling. Who knows? This summer vacation might become the best trip ever.

1. Acknowledge your trip won’t be perfect

Go into your trip with realistic expectations. Delays are all but inevitable, and things might be more expensive than you budgeted for. If you go in knowing the caveats, you’ve already set yourself up better to avoid disappointment.

With that, focus on these next tactics that you can control.

2. Book flights with lower odds of a delay

As if traveling wasn't stressful enough right now, it’s impossible to guarantee your flight won’t be delayed, as even a private jet could still be subject to air traffic or weather delays. But certain booking strategies can at least increase the odds of on-time arrival:

Fly earlier in the day before a previous flight can delay your plans.

Avoid layovers if your budget and route allow.

Book with airlines with strong histories of on-time arrivals (Delta Air Lines, Hawaiian Airlines, American Airlines and United Airlines rank highest, according to data from the U.S. Bureau of Transportation Statistics).

3. Apply for TSA PreCheck

One of the easiest airport lines to avoid: the traditional security line. With TSA PreCheck membership, you can access dedicated, often-shorter lines that enable you to flow through faster. Plus, you won’t need to remove your shoes or laptops.

Applying for membership takes time, and perhaps money, too. But carving out time to apply now is likely better than cutting into your precious vacation time by standing in an annoying security line. The application fee is $78, but many travel credit card benefits include TSA PreCheck reimbursement .

4. Don’t check bags

Another line to skip? Bag check. While you can sometimes skip this line by holding airline elite status , the easiest way to skip it and avoid unnecessary travel obstacles is by not checking bags, period.

There are plenty of other reasons to pack light. There’s no risk of checked luggage getting lost if you fly carry-on only, and you won’t have to stand at the baggage carousel on the other end of your flight. And should you need to make a last-minute switch to another flight, you won’t be held back because your stuff is stuck on another aircraft.

5. Do pack a 'delay emergency' kit

While packing light is essential, make space for some essentials to account for one of the all-too-common travel problems: delays or other inevitable travel snafus. Things to include:

Packable snacks: “Hangry” travel can turn a bad trip worse. Jerkies and protein bars are filling, without filling up too much space in your bag. Plus, you’ll avoid long lines at airport cafes.

Portable chargers: If flights are delayed and the airport doesn’t have power outlets, you’ll stay plugged in.

Copies of your passport, COVID-19 vaccine proof and other important records: You might not necessarily need physical copies, but digital copies don’t take any space and might come in handy.

Entertainment: Bring a book or laptop so you’ve got entertainment if you get delayed.

6. Gift yourself lounge access

Speaking of what to do during a delay, the airport lounge might be your oasis. Airport lounges, which can typically be accessed via programs like Priority Pass (membership is sometimes included with certain credit cards ), can sometimes make a delay not merely tolerable, but actually enjoyable.

Lounges vary in quality, but the best ones have luxuries like nap suites, Peloton bikes, showers and buffets. Consider travel hassles like delays simply as opportunities to treat yourself to another complimentary cappuccino.

7. Let your phone assist you

Smartphone apps can simplify travel. Most airlines and hotels now offer online or in-app check-in, upon which you’ll receive a mobile boarding pass or virtual room key to bypass the physical counter.

Just this year, Starbucks rolled out the ability to order ahead from many airport locations, removing yet another irritating line you might otherwise stand in.

Apps can also notify you of a flight delay, help navigate new routes due to traffic, and find cheap gas stations .

8. Reserve the 'pay later' rate, even if it’s more expensive

Many rental car companies and hotels allow you to reserve now, but don’t require payment until you arrive. Sometimes they offer a discount for paying upfront, which can be worth it if you’re certain you’ll make the trip.

But given the uncertainty of travel issues these days, it might be worth paying what’s likely only a small percent more. The trade-off — no headache of trying to get your money back — can be worth it.

Plus, if prices drop between booking and check in, you can rebook the same reservation at a lower rate. In fact, such occurrences are surprisingly common. Hotel room rates were cheaper 73% of the time when booked 15 days out versus four months out — with an average savings of 13% over the past three years, according to a 2021 NerdWallet study .

9. Consider an all-inclusive or group tour

Group tours and all-inclusives can sometimes be more expensive upfront, but they may entail less headache given there are fewer reservations to book (and worry about going awry). When you book guided group vacations, the trip is in your guide’s hands once you arrive.

Trip challenges — whether a vehicle breakdown, unanticipated closure or something else — are almost inevitable these days. But if the trip is up to the guide you hired, then problem-solving is largely up to them, too.

10. Have travel insurance

Travel insurance can help you get money back for canceled or interrupted trips. It can also fund expenses like additional clothing if your luggage gets lost or an extra hotel room if you need to stay overnight due to a flight delay.

Some credit cards offer travel insurance on trips purchased with that card.

Read the policy, though, as many plans come to your rescue only if you experience a covered reason, like an injury or jury duty. You generally can’t expect a refund if you cancel “just because,” unless you purchase the more expensive “ Cancel For Any Reason ” coverage. And even still, this more-flexible coverage typically only refunds about 50% to 75% of the total cost, according to the National Association of Insurance Commissioners.

11. Tip where appropriate

Travel is already expensive, and tipping can sometimes feel like something you do begrudgingly, especially when you’ve already been hit by rapid inflation, resort fees and maybe even a COVID-19 surcharge.

Tipping can help a likely overworked employee (as of May 2022, leisure and hospitality employment still remains roughly 8% below pre-pandemic levels, according to the U.S. Travel Association). Likewise, your generosity might literally pay off for you, too.

Tipping the cleaning staff at the hotel breakfast buffet might net you a complimentary premium drink from the coffee bar. Some cash for the hotel housekeeper might mean chocolates and towel animals on your bed.

12. Be kind to workers and fellow travelers

You don’t necessarily have to tip to get superior service — kindness is free. Berating the gate agent because your flight was delayed won’t get you there any faster (not to mention it’s likely not their fault the flight is late). But, being nice means they’re more likely to pull some strings to get you on another flight.

And try to be patient with and considerate of other travelers. For a lot of rusty travelers, it’s their first trip in years. And for some other travelers, they’re flying to their fifth wedding of the month. They deserve your sympathy, too.

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Travel Problems: Solutions That Can Save Your Trip

These days, travel problems aren’t just a possibility, they’re almost an expectation. Learn ways to prevent common issues and deal with any that do occur.

Travel affords fantastic benefits. It allows us to meet new people, experience new cultures, and grow as a person. However, travel does not come without difficulties. There are problems with language barriers, culture shock, and bad weather. Major travel problems aren’t just a possibility at this point; they are almost an expectation. Air travel, after the pandemic, is struggling to keep up with the surge in travelers in 2022, and more complex problems like pilot shortages, poor staffing, fuel problems, and a little bit of rustiness are adding fuel to the fire. So, how do you deal with these almost inevitable disruptions? Is there a way to salvage your trip?

In this guide, we will cover some of the most common travel problems as well as tried-and-true ways to overcome them. We’ll talk about post-pandemic madness, but we’ll also hit on travel concerns that pop up all the time. No matter what travel hardship you come upon, the most important challenge is to keep your head and quickly weigh the options. There are always options! They might not be what you originally intended. They might result in a completely different summer travel experience than you planned, but you know what? That doesn’t have to be devastating. It can actually be really exciting! Let’s dive in.

Plan a sightseeing scavenger hunt!

Looking for just the right tour to start off your trip? Consider an app-guided scavenger hunt from Let’s Roam . We have hundreds of adventures all over the world! Each unique experience includes trivia, photo challenges, and more. Engage in some friendly competition as you and your travel companions learn about landmarks and make new memories!

10 Tips On Being a Prepared Traveler

To be perfectly honest, you can prevent a lot of heartache just by doing things right from the get-go. Before we get to solutions for when problems occur, let’s chat a minute about some travel tips to keep them from happening in the first place!

1. Stay away from the masses.

Flying in and out of New York City, or London Heathrow is a bad idea right now. Large international airports do have the best prices, generally. However, they also have the most flights coming in and out. If one or two get delayed, guess what, they are affected. If you have the ability, book a flight from a smaller airport.

This same strategy goes for choosing your destination. Right now is not a good time to tour London, Greece, or Paris. Check out the Albanian Riviera instead!

2. Get good travel insurance.

Whether you invest in a great travel credit card or you buy extra travel insurance through a company like Safety Wing, make sure that your trip is covered. You may be eligible for lost baggage reimbursement, coverage for a hotel if your flight is delayed overnight, or even full reimbursement if your trip is canceled.

It is important to be familiar with your particular travel insurance company and how it works. Make sure you have read your policy. Print it out and carry it with you so that you have the customer service number and your rights handy. The same thing goes for the airline you are flying.

3. Don’t overbook.

Normally, it is a great practice to book things early, at least from a budget standpoint. That is not the case in today’s travel universe. The more you book in advance, the more you are going to have to fight to get back if you have a missed flight or other disruption.

4. Make sure your trip is refundable.

If you do choose to book ahead, make sure that everything you book is refundable up until the day of service. That means that you only book fully-refundable hotels, activities, etc. Do not book anything expensive for the first two days of your vacation. If your flights are delayed, and you just had free days planned anyway, then you didn’t miss much.

5. Pack light.

Seriously, do not check bags, unless you absolutely have to! For almost any trip in the world, you can manage with a carry-on and personal item. I realize this seems insane to some of you, but I promise it is possible. You can always purchase larger items when you get there. There is probably a Walmart or similar store. Don’t stock up at home on new clothes, shop when you arrive!

6. Pick a straight path.

Direct flights are the most expensive flights. However, they are also the least likely to have problems. If your direct flight is delayed a few hours, so what? You don’t miss your next flight because of it. You will still end up at your destination—just a little later than you had hoped for.

This may not even mean choosing a more expensive flight. It just might mean picking the correct airport. You may need to drive a couple of hours to a different one instead of choosing your nearest and usual. You may need to choose a vacation destination that is pretty close. Most Caribbean, Central American, and even Northern South American destinations can be reached with a non-stop flight from the US.

7. Check in early, and position yourself appropriately.

While I usually don’t recommend this, for travel in 2022, you need to be at the airport at least three hours early. Poor customer service, long waits to check bags, and frequent gate changes mean that there is a lot of opportunity for error. Give yourself time to adjust.

Once you have made it through TSA and to your gate, position yourself close to the flight attendant desk. The desk clerks are the fastest way to get a new flight if you are one of the first ones in line. Otherwise, there are better options we will discuss later.

8. Download all the apps.

The best travel apps aren’t just money savers—they can also help you keep your trip on track. If there is a smartphone app for your trip, download it. That includes the airport, airlines, hotels, etc. Turn on the notifications and set it to vibrate so you will feel when you get an alert. Some airlines will even put flight delays up hours in advance. This can save you from sitting in the airport for half your day when your early morning flight has been delayed until the afternoon.

9. Do your research!

When you leave for a trip, you should have some key information handy. Will your cell data work when you get there? If not, you should have the directions and map to your hotel downloaded. If you land in the morning, and you can’t check in until 3 pm, you should have a plan. Where are you going to get local money? Do they have a certain rideshare app that is prevalent? What are the major scams in the area? Know what to look for as soon as you step off the plane. You can’t research everything, but you can be prepared for 99%.

It is vastly important, that you have emergency services saved on your phone or written down (or both). You should have the address and phone number of the closest hospitals, the emergency service number, your roadside assistance, and the location of the US Embassy handy at all times.

10. Be flexible.

The key to surviving travel (and making the most of a bad situation) is keeping your mind open. If you travel enough, you will run into problems. It is inevitable. The kicker is…how do you react to them? Can you salvage the trip, or are you going to sit in it and be miserable?

Common Travel Problems and Last-Minute Solutions

Some travel-related problems are due to poor planning. Some are due to faulty systems and industry issues. Some are cultural. Some are situational, and some are just downright bad luck! We will cover them all in this section and give you a few tested solutions to make the most of a rotten situation.

Flight Issues

Beforehand: Make sure your flight is insured through credit card or travel insurance. Purchase refundable flights. Have a backup plan and a secondary destination, in case you need to change your entire trip.

In the moment: If you have to deal with a flight cancellation, don’t freak out! Keep your head and get to the customer service desk as soon as possible. If you are in a long line, then begin to contact customer service via social media or the phone number. Believe it or not, it is often quicker to use Twitter than wait for a person.

You will need to decide if you want to cancel your trip, get on the next available flight, or change your destination. Normally, if your flight is canceled, you will automatically be booked on the next available flight. However, you have the right to turn this down. This is where knowing those policies comes in handy. Don’t cancel a flight that is nonrefundable without knowing that you can get some reimbursement or a new flight.

For more information on this common travel problem, we’ve got expert advice on “ How to Make the Most of a Flight Delay “!

Language Barrier

One of the most frustrating things on the planet is to be stuck in a pickle and you can’t speak the local language. While a good chunk of the world speaks some English nowadays, it seems you’re always in a tiny village or rural area when you get in a jam, and there isn’t an English speaker in sight.

Beforehand: Before you travel, make sure to download the destination language on Google Translate on your phone. This way it will work even if you don’t have data service.

In the Moment: There are a few things you can do if you find yourself in an area where you can’t communicate. First, head for the most touristy thing you can find. Generally, hotels, tourist attractions, and popular restaurants are going to have the highest chance of finding someone who speaks your language.

Secondly, make friends! Talk to people on the bus. Look for ex-pats or digital nomad hangouts . I don’t know how many times our travel has been made 100x easier because one of our travel companions spoke a bit of the language. Friends are invaluable!

Don’t be afraid to ask for help. It is likely that a local speaks some English, and if you are brave enough to ask for help, you will find someone who is willing to help you. (Looking really pathetic and lost is also helpful)!

Lost Luggage

Beforehand: Make sure your bags are properly labeled with your name and home address. Have those policies printed out! Keep your claim tag information.

In the Moment: Consult the lost baggage service desk for your airline. It is usually located near baggage claim. You ask for the appropriate amount of money, per their policy, for delayed baggage.

If the airline refuses to give you a stipend, you can still rely on your travel credit card or insurance. Generally, this will be a reimbursement kind of thing, so stick to the budget in your policy and keep all your receipts.

Beforehand: Hydrate, hydrate, hydrate, both before and during your flight. You can also try to book flights that will have you arriving in the later afternoon or evening. Check out our tips for conquering jet lag !

In the Moment: Try not to nap when you land at your destination. You want to get on the local time as soon as possible. Try to get a good night’s sleep on the first night. This may involve a sound app on your phone, a sleeping mask, or adjusting the temperature. Do what you need to do to get a good night’s sleep.

Schedule Gaps

One of the most common frustrations is an early check-out time and a late check-in. You now find yourself with hours to waste, all your heavy bags, and nowhere to go.

What to do: The first solution is to contact your next hotel. They will likely allow you to check in early or store your bags in a safe luggage closet until check-in. If not, check into local locker storage. You can use Google or a specialized app like Bounce, Nannybag, or Stasher to find lockers near you.

When in doubt, find a Let’s Roam scavenger hunt nearby! That will kill 1.5-2 hours and you’ll have a great experience.

Bad Weather

Beforehand: Normal weather patterns should be part of your pre-trip research. You want to choose the season with the least amount of rain, no hurricanes, etc. However, the weather doesn’t always play along. If the weather is questionable, only book a night or two in your preferred hotel. You might need to relocate.

In the Moment: If you wait it out a day, and the weather still stinks, talk to your resort first. They will often be willing to relocate you to a sister hotel in another region. In some countries, just moving a couple of hours can put you in a completely different climate. In Costa Rica, for instance, the east coast has the opposite rainy season as the west coast, and they are only a couple of hours from each other.

If moving is not a possibility, then look into what you can do to have some fun indoors. We can help you with that! At Let’s Roam we have a series of amazing indoor activities for every kind of adventurer. Try one of our indoor scavenger hunts, a virtual murder mystery, or our home date nights. We know it isn’t a substitution for the vaca you planned, but it’s better than moping!

Consult the locals! The local people obviously live in this kind of weather. They know how to entertain themselves during monsoons, keep safe during major storms, and they will be more than willing to share their tips and tricks.

Tummy Troubles

Heard of “Delhi belly”, “the Haitian Sensation”, and “Montezuma’s Revenge”? Travel-related illness is all real, and they are brutal!

Beforehand: Ask your physician for the best antibiotic for food poisoning and traveler’s diarrhea for your destination. They will almost always write you a prescription to take with you. Make sure you research foods to stay away from and how to get clean water to your destination. Carry a filter water bottle.

In the Moment: If you get the dreaded sickness, hydration is key. You will have to force yourself to intake clean water, herbal teas, or whatever you can keep down. In many countries, the pharmacist has much more autonomy than in the United States. They always know the best medications for common ailments, and they can usually give you the correct medications without the need to see a doctor. If you are not better in a couple of days, you may need to visit a clinic or hospital for IV fluids.

Forgot Something

Beforehand: Carry a paper or digital copy of any important prescriptions.

In the Moment: If you have forgotten something you cannot do without. Consider having a family member overnight it by FedEx to your accommodation. If this isn’t possible, you can check with local facilities to get a replacement. Many eye doctors, pharmacists, etc. will fill your valid American prescription.

Getting Lost

Beforehand: Download your directions from Google Maps, or take a screenshot of the detailed directions. Take a screenshot of your hotel address.

In the Moment: Your first concern is to find safety. Drive, or walk, until you find a well-lit and populated area where you can think. Once you reach a store or gas station, ask a local for help. If you are in a rural area, call roadside assistance. When phone maps don’t work, you are best to just get a local taxi to take you safely to your destination.

Being Robbed

You’ve probably been saving for a while to cover your travel costs. Most people work very hard for the right to travel, so what happens if you get robbed? The most common assault is going to be a quick grab and run. Generally, they don’t want to hurt you, they just want your stuff. You are probably better off just to give it to them, and get to safety as soon as possible.

Beforehand: The key to not getting robbed is to pay attention and try not to look like a lost tourist. Do not flash your money or jewelry. Do not discuss money with strangers. Do not carry expensive electronics into crowded areas. Carry a moneybelt or slash-proof purse, and carry them crossbody. Men, put your wallet and documents in your front pockets. Do not keep all your cash, credit cards, and valuables in one place. Split them between your room safe, your travel partner, and a few places on your body. Be extra vigilant outside airports, train stations, and popular tourist attractions.

In the Moment: Get to safety quickly! Notify proper authorities immediately. If you still have your phone, contact your credit card companies, and your bank, and have your cards put on hold. If your phone has been stolen, get online and locate your phone (using the Find My Device App or something similar). This may help the police find the thief. You can also lock or erase your phone so that the thief does not have access to your accounts.

Getting Scammed

Beforehand: Every region has its favorite scam! Do some research on your destination and know what to look for before you travel. Be sure to read our advice on avoiding travel scams around the world !

In the moment: There really isn’t much you can do. If a prosecutable crime has been committed, you need to contact the police. If not, your primary goal is to get to safety. Do not engage with the person or persons, as causing a scene as a foreigner is usually not going to go in your favor.

Relationship Issues

If you travel long enough, you will eventually find travel buds, and they can be amazing! However, good things don’t last forever. It’s pretty common for travel relationships to go bad.

Beforehand: Keep it light. Don’t get into a long rental car or accommodation contract together. Make very clear expectations when you agree to travel with someone. It might sound formal and silly in the moment, but making sure everyone is on the same page about responsibilities, money, and expectations is really important.

In the Moment: When you feel the vibe changing, get out before it gets ugly. You can make up an excuse for why you need to leave, or just say that you need some time alone. Offer to pay your share of the expenses. Have a final drink together, and then part ways. If you don’t, it is going to blow up in your face, and you risk losing a friend totally, or worse.Exhausting Itinerary

This is a common one. If you are an American, you have limited vacation days, and you need to see it all! We get it.

Beforehand: Get a great guidebook! Rick Steves and Lonely Planet will both give you suggested itineraries and help you plan a schedule that is doable and enjoyable.

In the Moment: What do you already have tickets for? What are your top remaining must-sees? Go see these, and get rid of the rest. Ask a few locals (not in the tourist industry) which sites you should keep and which ones can go.

Lost Passport

Losing your travel documents is terrifying, but it happens to the best of us!

Beforehand: You should carry a photocopy of all your important documents with you. Use a secure documents app, like Last Pass . This will make the replacement of your documents much faster and easier!

In the Moment: Contact the local US embassy immediately. They will schedule an appointment to go over your case and get you new documents.

Culture Shock

If you’re heading to a destination for the first time, don’t be surprised if you feel awkward or uncomfortable at first. Culture shock is a real thing, and it can ruin your trip if you don’t know how to deal with it.

We’ve got tips to make you more culture conscious , and they would certainly be worth a read before you travel. Changing your mindset is a great way to ward off culture shock!

Beforehand: Research your destination culture and familiarize yourself with the stages of culture shock. This way you will know the symptoms and how to migrate through them.

In the Moment: If you find that you are judging your destination culture for the “weird” things they do, it’s time to respectfully ask some questions. The key to getting to a healthy place with a new culture is to understand why people do “weird” things, and why things are the way they are. Find a trusted local or traveler that is familiar with the culture. With a little background information, you will find that you can usually sympathize and move forward to a place of acceptance. Understanding and grace should always be your motive. Your way is not the only way or even the best way. They likely know something you don’t.

Terrible WiFi

If you are on a short trip, this isn’t such a big deal, but if you are traveling long-term, the lack of stable WiFi can make you insane. You do need to work, pay bills, and of course, binge Netflix, after all.

Beforehand: Carry a hotspot of your own, with a subscription option, like Skyroam . Always carry an unlocked phone with you.

In the Moment: Pop into the local phone carrier and grab a sim card with data. They usually have large amounts of data, free social media, and a few texting and call minutes for super cheap prices.

You’ve got this!

The best thing you can do to ensure that your trips are enjoyable is to do as much research and planning as you can from home. This will help you form realistic expectations, prepare for common scams, and have all the safety information you need.

You can’t plan for everything though, and problems will happen. The most important in-the-moment advice is to keep your head! If you freak out, you will not be able to make appropriate decisions. Don’t be afraid to ask for help, and remember that a bad vacation is usually salvageable if you are flexible!

In this guide, we have hit on a few of the common problems in travel, but a life of travel presents different problems than a short-term trip. For a good look at the reality of full-time travel, check out “ Essential Considerations for Long-term Travel .”

Frequently Asked Questions

The most common travel problems are lost passports, delayed or canceled flights, and lost baggage. Always carry photocopies of your documents and invest in travel insurance.

Long trips are rife with possible problems. The most common travel problems are relationship issues , canceled or delayed flights , bad Wifi connectivity , and culture shock .

Inclement weather is a common travel problem . If you find yourself in it, look for adventurous indoor activities , and consult locals on what they do when the weather is crummy!

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• Entertainment

My Old Ass dodges the usual time-travel movie problems, with good reason

Aubrey Plaza and Maisy Stella star in this coming-of-age comedy

by Petrana Radulovic

The gimmick at the center of Megan Park’s dramedy My Old Ass is grabby: On the night of her 18th birthday, a teenager blows off her family birthday celebration to go do drugs in the woods. There, she encounters an adult version of herself, and promptly proceeds to rag on herself for being so old. (The other version of her is just 39.) Seeing a confident teenager and her older self playfully butt heads over future job prospects and appearances is already funny. But woven through the Oh my God, is this what I’m going to be like when I’m OLD?! back-and-forth is a powerful movie about growing up.

Writer-director Megan Park ( The Fallout ) uses a high-concept plot to drill in on specific emotions and experiences, while dodging genre trappings and specifics about time travel. Skimping on the sci-fi mechanics while leaning into the emotions created by the situation lets her craft My Old Ass into a contemplative coming-of-age story — one that perfectly encapsulates the feeling of that one last carefree adolescent summer before everything changes.

[ Ed. note: This piece contains light setup spoilers for My Old Ass. ]

Maisy Stella (ABC’s musical drama Nashville ) plays Elliott, a confident young woman with big dreams of leaving her family’s cranberry farm behind when she goes to University of Toronto in the fall. While tripping on hallucinogenic mushrooms, she’s visited by an older version of herself (played by Parks and Recreation ’s Aubrey Plaza ), who offers her some advice about this very transitory time in their life. The two manage to keep up a correspondence via cell phone, with older Elliott trying to guide younger Elliott without giving away too much about the future. Her biggest warning: Stay away from Chad (Percy Hynes White), the charming boy who’s working at her family’s farm over the summer.

Throughout the movie, it’s a little ambiguous about whether actual time travel is occurring, or Elliott is just experiencing a side effect of her psychedelic trip. But that blurry line means Park doesn’t have to sweat over paradoxes or otherwise waste time laying out the rules of time travel. To young Elliott, chatting up her 20-years-older self is just a random, weird thing that’s happening, and she’s rolling with the punches. They never really interrogate the larger space-time consequences of the experience, or worry that they might break reality by touching. Which is a good thing, because Park uses the time-travel element as a tool to really hammer home the bittersweetness of growing up.

Young Elliott is confident about her place in the world, and Stella imbues the character with a particular brash brightness. When she starts to question what she’s taken for granted about herself and her family, and what that means for the future, her performance comes with a tangible vulnerability. Meanwhile, Plaza nails the older, more world-weary version of the character — but one who’s never too jaded or cynical. The two of them share some wonderful banter: It’s a testament to their chemistry that a lot of their interaction happens via phone, and yet it never feels stilted or shortchanged.

Older Elliott doesn’t share specific details about the future with her younger self, with good reason: She wants to let younger Elliott experience surprise at everything life has to offer. Her vague words of advice — effectively, slow down and spend more time with her family while she can — could be overused adages. But because they’re so universal, they can also apply to Elliott’s specific situation, like how she feels like she’s too good for her family’s cranberry farm. And the nebulous warning older Elliott delivers about that cute boy also could be a cliché — except that until she meets Chad, Elliott has only been attracted to girls. It’s a refreshing take on a coming-out story, and it makes older Elliott’s warnings even more intriguing, especially as younger Elliott and Chad clearly hit it off.

Beyond the time-travel setup, My Old Ass ’s most immediate hook is the leads and their easy rapport. This movie could have just been a collection of hijinks and jokes about touching your older self’s butt. But Park uses the timey-wimey elements to craft a story about those unheralded last moments, the ones we don’t realize will be watersheds on the way to growing up. Younger Elliott is eager to leave everything behind and move on to her next great adventure, but older Elliott is able to offer some perspective. At the same time, older Elliott gets to savor her bygone youth and tap into the days of being a fearless teenager who could conquer the world. My Old Ass is about growing up — the joy, the pain, and those little moments that resonate with us far longer than we think they will — and Park smartly pulls it off by drawing on Elliott’s perspectives of both the past and the present.

My Old Ass is out in select theaters starting Sept. 13, and everywhere on Sept. 27.

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• 2024 Elections

Election Officials Warn Problems With U.S. Mail System Could Disrupt Voting

(TOPEKA, Kan.) — State and local election officials from across the country on Wednesday warned that problems with the nation's mail delivery system threaten to disenfranchise voters in the upcoming presidential election, telling the head of the U.S. Postal Service that it hasn't fixed persistent deficiencies.

In an alarming letter, the officials said that over the past year, including the just-concluded primary season, mailed ballots that were postmarked on time were received by local election offices days after the deadline to be counted. They also noted that properly addressed election mail was being returned to them as undeliverable, a problem that could automatically send voters to inactive status through no fault of their own, potentially creating chaos when those voters show up to cast a ballot.

The officials also said that repeated outreach to the Postal Service to resolve the issues had failed and that the widespread nature of the problems made it clear these were “not one-off mistakes or a problem with specific facilities. Instead, it demonstrates a pervasive lack of understanding and enforcement of USPS policies among its employees.”

Read More: Your Questions About Early Voting, Answered

The letter to U.S. Postmaster General Louis DeJoy came from two groups that represent top election administrators in all 50 states. They told DeJoy, “We have not seen improvement or concerted efforts to remediate our concerns.”

“We implore you to take immediate and tangible corrective action to address the ongoing performance issues with USPS election mail service,” they added. "Failure to do so will risk limiting voter participation and trust in the election process."

A message seeking a response from the U.S. Postal Service was not immediately returned.

The two groups, the National Association of Secretaries of State and the National Association of State Election Directors, said local election officials “in nearly every state” are receiving timely postmarked ballots after Election Day and outside the three to five business days USPS claims as the standard for first-class mail.

The letter comes less than two weeks after DeJoy said in an interview that the Postal Service was ready to handle a flood of mail ballots expected as part of this November's presidential election and as former President Donald Trump continues to sow doubts about U.S. elections by falsely claiming he won in 2020.

That year, amid the global pandemic, election officials reported sending just over 69 million ballots in the mail, a substantial increase from four years earlier.

While it's likely that number will be smaller now, many voters have embraced mail voting and come to rely on it. And both Democrats and Republicans have launched efforts to push supporters to vote early, either in person or by mail to “bank” their votes before Election Day on Nov. 5.

The letter went out on the day the first mailed ballots of this year's general election were being sent, to absentee voters in Alabama.

Postal Service officials told reporters last month that almost 98% of ballots were returned to election officials within three days in 2020, and in 2022, the figure was nearly 99%. DeJoy said he would like to inch closer to 100% this election cycle and that the Postal Service is better positioned to handle ballots than four years ago.

But officials in rural states have been critical of the Postal Service for years as it has consolidated mail-processing centers to cut its costs and financial losses.

Read More: Breaking Down the 2024 Election Calendar

In addition to being signed by the current and incoming presidents for both groups of election officials, the leaders of groups that represent local election officials in 25 states were listed.

The election officials warned that any election mail returned to an election office as undeliverable could trigger a process outlined in federal law for maintaining accurate lists of registered voters. That means a voter could be moved to “inactive” status and be required to take additional action to verify their address to participate in the election, the officials said in the letter.

Kansas Secretary of State Scott Schwab, the recent past president of the National Association of Secretaries of State, sent his own letter in recent days to DeJoy. He said nearly 1,000 ballots from his state's Aug. 6 primary election couldn't be counted because they arrived too late or without postmarks — and more continue to come in.

“The Pony Express is more efficient at this point," Schwab posted on the social media platform X in late August.

Schwab and other Kansas election officials also have said some ballots arrive on time but without postmarks, which keeps them from being counted under Kansas law. What's more, Schwab told DeJoy, local postal clerks have told election officials that they can't add postmarks later even if it's clear that the Postal Service handled the ballot ahead of the mail-in deadline.

Kansas will count ballots postmarked on or before Election Day if they arrive within three days. The Republican-controlled Legislature created that grace period in 2017 over concerns that mail delivery had slowed after the Postal Service shut down seven mail-processing centers in the state. That left much of the state's mail handled through larger centers in Denver, Amarillo, Texas, and Kansas City, Missouri.

Schwab has promoted the use of local ballot drop boxes for voting in advance, breaking with other Republicans who have suggested without evidence that they can be sources of fraud. Schwab has long said the boxes are more secure than the U.S. mail.

"Keep your ballot out of the hands of the federal government!” he advised voters  in a post on X  after the August primary.

In their letter Wednesday, election officials said colleagues across the U.S. have reported that Postal Service staff, from managers to mail carriers, are uninformed about the service's policies for handling election-related mail, give them inconsistent guidance and misdeliver ballots.

“There is no amount of proactive communication election officials can do to account for USPS’s inability to meet their own service delivery timelines,” the officials wrote. “State and local election officials need a committed partner in USPS.”

____ Cassidy reported from Detroit.

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A cloud computing approach to superscale colored traveling salesman problems

• Published: 11 September 2024

Cite this article

• Zhicheng Lin 1 ,
• Jun Li 1 &
• Yongcui Li 2  

The colored traveling salesman problem (CTSP) generalizes the well-known multiple traveling salesman problem by utilizing colors to describe the accessibility of cities to individual salesmen. Many centralized algorithms have been developed to solve CTSP instances. This work presents a distributed solving framework and method for CTSP for the first time. The framework consists of multiple container-based computing nodes that rely on specific cloud infrastructures to perform distributed optimization in a pipeline style. In the framework, we develop a distributed Delaunay-triangulation-based variable neighborhood search (DDVNS) algorithm for solving a CTSP case decomposed into many traveling salesman problems. DDVNS exploits a two-stage initialization to generate an initial solution for all TSPs. After that, Delaunay-triangulation-based variable neighborhood search (DVNS) is employed to find local optima. Furthermore, the obtained solutions are improved by reallocating multicolor cities and iterating the search progress, ultimately leading to a group of CTSP solutions. Finally, extensive experiments show that DDVNS outperforms the state-of-the-art centralized VNS algorithms in terms of search efficiency and solution quality. Notably, we can achieve the best solution in a superscale case with 16 salesmen and 160,000 cities within 15 minutes, breaking the best record of CTSPs.

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The authors are supported by the Qingdao New Qianwan Container Terminal Co., Ltd. and the National Key Research and Development Program of China under Grant 2021YFF0500904.

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Conceptualization contributed by J.L., Z.L., and Y.L.; methodology contributed by Z.L. and J.L.; supervision contributed by J.L.; validation contributed by Z.L. and Y.L.; investigation, data curation, software, formal analysis, and writing—original draft preparation contributed by Z.L.; writing—review and editing contributed by J.L.; funding acquisition contributed by Y.L.

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The problem with pinning Donald Trump down: Americans' attention spans are too short

Doing the work of saving democracy will require the american people to develop a different relationship with time, by chauncey devega.

The Age of Trump has assaulted and undermined America’s governing institutions and the rule of law, civil society, and shared sense of normalcy. Beginning to heal these many great challenges, as President Obama and Michelle Obama implored last month in Chicago, will take much hard work. In total, it will be a project of democratic renewal that will last decades and not just one debate or election cycle. Doing the work of democratic and civic renewal will require that the American people develop a different relationship to time.

The attention economy with its endless and empty “content” and algorithms is how the late capitalist experience machine distracts and profits. This dynamic of constant stimulation in an age of spectacle has contributed to epidemic rates of loneliness , social atomization, depression and other mental and emotional unwellness. We've seen a decline in the type of social capital essential for a healthy democracy. How can the American people and their leaders do the work of reinvigorating democracy and defeating Trumpism and the larger neofascist threat if average attention spans have been greatly decreasing over the last two decades?

Doing the work of democratic and civic renewal will require that the American people develop a different relationship to time.

For example, research shows that the average American’s ability to focus on a computer screen  has decreased from 2.5 minutes in the early 2000s to 47 or so seconds today . Other research suggests that many Americans can only pay attention to one task for 8 seconds —  that is less than a goldfish .

For decades, the American news media with its endless 24/7 coverage, sensationalism, traffic-chasing, and an “if it bleeds it leads” ethos has contributed to this problem instead of intervening against it. The Age of Trump has been enabled and normalized by that behavior and how the corrupt felonious traitor sexual assaulter as confirmed by a court of law ex-president is worth billions of dollars in advertising revenue to the news media.

Anton Jäger describes this late capitalist media and political environment  with its rejection of substantive mass politics in favor of empty identity politics and performance, with its “brands” and virtue-signaling to get attention on social media and across the digital space as an example of  “hyper-politics” :

“hyper-politics” is what happens when post-politics ends — something like furiously stepping on the gas with an empty tank. Questions of what people own, and control are increasingly supplanted by questions of who or what people are, replacing clashes of classes with the collaging of identities and morals.

Social theorist Henry Giroux offers a complementary framewor k (what he calls “emergency time”) for understanding the crisis in American and global democracy:

The Republican Party is now mostly a vehicle for fascist politics. The United States has reached the endpoint of a cruel economic and political system that resembles a dead man walking–a zombie politics that thrives on the exploitation of the working class, immigrants, the poor, dispossessed, and helpless children dying under the bombed-out rubble of state terrorism. White Christian nationalism merges with the most extreme elements of capitalism to enforce cruel and heartless policies of dispossession, elimination, and a politics of disposability. Mouthfuls of blood saturate the language of authoritarianism, and policies of destruction, exploitation, and utter despair follow. Public time based on notions of equality, the common good, and justice fades into the dustbin of a white-washed history. As James Baldwin once noted, until the Nazis knock on their door, these “let’s be balanced” types refuse to have the courage to name fascism for what it is. In the face of emergency time, it is crucial to develop a great awakening of consciousness, a massive broad-based movement for the defense of public goods, and a mobilization of educators and youth who can both say no and fight for a socialist democracy.  The fight against fascism cannot take place without new ideas, vision, and the ability to translate them into action. Dangerous memories and the resuscitation of historical consciousness help. And are even more necessary as democracy is choking on the filth of demagogues, white nationalism, class warfare, militarism, and Christian nationalism.  Those Americans who believe in democracy and justice can no longer accept being reduced to a nation of spectators; they can no longer define democracy by reducing it to a voting machine controlled by the rich; nor can they equate it with the corpse of capitalism;  they can no longer allow the silence of the press to function as a disimagination machine that functions to largely depoliticize the public;  they can no longer allow education to be pushed as machinery of illiteracy, historical amnesia, and ignorance.

Ultimately, the American people and their leaders need to take a breath, slow down, and gain perspective as they try to step away from the spectacle.

We need your help to stay independent

As I often warn here at Salon and elsewhere, the American people are not well. Sick societies produce sick leaders. In the Age of Trump, it appears that we "the Americans" are even more afflicted with a 21st-century version of the malady known as “Americanitis."  In an essay in Smithsonian Magazine, Greg Daugherty applies this therapeutic language : “Too much stress, too little sleep, rushed meals, technology that seems to change faster than we can begin to keep up with. If those complaints sound familiar, chances are they’d have resonated with your great-great grandparents too":

More than a century ago, Americans had much the same concerns, and some leading thinkers and medical practitioners even took it a step further. They suggested that the country’s legendary work ethic and go-getter spirit might be a form of mental illness that they called “Americanitis.” The Journal of the American Medical Association acknowledged the condition as early as 1898, linking it in one article to the increased noise level in industrialized America. “Who shall say how far the high-strung, nervous, active temperament of the American people is due to the noise with which they choose to surround their daily lives?” the author asked. It wasn’t long before Americanitis had spread beyond the medical journals and into everyday vocabulary, shorthand for a deadly mix of hurry and worry. Orison Swett Marden, a self-help author and editor of Success magazine, and Elbert Hubbard, the flamboyant “Sage of East Aurora,” were two of the many popular writers to address the subject. Marden devoted a chapter to “The Cure for Americanitis” in his book, Cheerfulness as a Life Power. “How quickly we Americans exhaust life!” he wrote. “Hurry is stamped in the wrinkles of the national face.” The “cure,” as he saw it, was to stop worrying so much. “Instead of worrying about unforeseen misfortune,” he advised, “set out with all your soul to rejoice in the unforeseen blessings of all your coming days.” Hubbard attributed the disease to “an intense desire to ‘git thar’ and an awful feeling that you cannot.” He advised readers to “cut down your calling list, play tag with the children, and let the world slide. Remember that your real wants are not many—a few hours work a day will supply your needs—then you are safe from Americanitis and death at the top.” Theodore Dreiser raised the question “Americanitis—Can It Be Cured?” in The Delineator, a women’s fashion magazine he edited. “The morning paper gives us a daily list of deaths by suicide, apoplexy, and insanity,” he lamented, “men in the prime of life rushing into eternity, desperate because they are left behind in the race, or driven mad by the rush of the business world.” He recommended learning to relax the muscles.

Harris, Walz, the Democrats, and other pro-democracy leaders and organizers have a great challenge ahead of them. They need to continue to sound the alarm about the existential dangers of Trump and the MAGA movement and the larger neofascist tide without creating a sense of mass despair, futility, learned helplessness, and nihilism that fuels the vibe that “nothing really matters anymore.”

They must also continue to mine the energy of “joyful warriors” for democracy. But being joyful warriors must not mean being conflict-avoidant or retreating from a high-dominance leadership style against Trump and his MAGA forces and their agents. In all, Harris and Walz can be joyful warriors who still pack a great punch and are willing to do all that is necessary to defeat Trump and the other enemies of American democracy.

Want a daily wrap-up of all the news and commentary Salon has to offer? Subscribe to our morning newsletter , Crash Course.

Whatever the outcome on Election Day, the American people will need to take a breath and collect themselves as they prepare for the hard work of democratic renewal if Harris and Walz win or what will be the even more difficult work of resistance if the worst outcome of Dictator Trump and his running mate, Ohio Sen. JD Vance, taking the White House comes to pass. Either way, time is not (and will not be) on the side of the American people.

On this,  Rick Wilson, co-founder of the pro-democracy organization the Lincoln Project, recently counseled here at Salon : 

 That was my first big campaign,1988. At this point in the election contest, Dukakis was ahead by 17 points. Here are the lessons from that defeat. Don't ever take anything for granted. Run through the tape. Never take your foot off the pedal. It's a simple and what should be an inviolate rule. You have to run all the way through. You have to fight all the way down. You can never, ever, take a break from the campaign at this early point. You've got to drive through the tape. One of the things we have to watch for here is that Trump can beat Trump. Harris can beat Trump. Trump, I do not believe can beat Harris with the popular vote. But Trump could beat Harris because of the Electoral College….My worry tank is diminished meaningfully. My hope tank is on three-quarters, but I'm going to stop for gas again soon before Election Day in November.

about this topic

• Donald Trump's incoherence makes the media's double standard hard to hide
• The media's double standard comes for Kamala Harris — and misses
• Republicans for Harris: GOP gets ready to rescue MAGA from Trump

Chauncey DeVega is a senior politics writer for Salon. His essays can also be found at  Chaunceydevega.com . He also hosts a weekly podcast,  The Chauncey DeVega Show . Chauncey can be followed on  Twitter  and  Facebook .

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Elevated Lead Levels Found in One-Third of Cinnamon Samples

Consumer Reports found lead in a variety of products, at a time when federal regulators are seeking authority from Congress to combat the problem of heavy metals in foods.

By Christina Jewett

One dozen of 36 cinnamon products tested by a consumer group contained elevated levels of lead, according to a study released on Thursday that reinforced concerns about metals in foods after tainted cinnamon applesauce poisoned dozens of children last year.

The study, by Consumer Reports , documented levels that were far lower than the amounts discovered last year .

The Consumer Reports team tested the spice and found high levels in lead in 12 items sold at discount stores and ethnic markets, with lead levels reaching 3.5 parts per million. New York, the only state with tough lead standards in spices, recalls spices — among them curry powder, chili powder, cumin and five-spice powder — with more than one part per million of lead. Consumer Reports advised that people throw out items with that amount.

Badia, one common brand, sold cinnamon with one part per million of lead, according to the report. The company did not respond to a request for comment.

While the levels in the cinnamon applesauce recalled last year were “astronomical,” those in the new report were still 1,000 times as high as the levels that concern lead-exposure experts who focus on children’s health, said Tomás R. Guilarte, a neuroscience and environmental health professor at Florida International University.

“These are extremely high levels of lead,” he said. “Clearly they shouldn’t be used.”

Earlier this year, the Food and Drug Administration urged a series of recalls of cinnamon products. The discoveries provided the impetus for consumer advocacy groups to generate greater public awareness of the dangers of lead and other metals, and for the agency to push Congress for tough limits on those heavy metals in food.

An agency spokeswoman said the F.D.A. had no authority to require companies to test final products for heavy metals, including foods consumed by babies and toddlers, who are particularly sensitive tothe effects of lead. Their intestines absorb more lead, and their blood-brain barrier is still developing, Dr. Guilarte said.

The agency said it was asking Congress for the power to require food companies to do such testing and to review company records remotely whenever necessary.

“There has to be some level of enforcement and testing,” Dr. Guilarte said.

Earlier this year, the F.D.A. conducted tests and pressed for recalls of several cinnamon powders, including one by Supreme Tradition sold at Dollar Tree stores, which contained two to three parts per million of lead. At the agency’s prompting, Save A Lot stores also recalled cinnamon from the Marcum brand, which contained about three parts per million of lead. Neither company responded to requests for comment.

The F.D.A. said that consumers should dispose of those products and that its testing was continuing.

Consumer Reports found far lower levels of lead in cinnamon in other powders, including from some brands it said were safe to use: Whole Foods 365, McCormick, Penzeys and Morton & Bassett. Cinnamon mainly grows in Indonesia, Sri Lanka and Vietnam, the report said, and then is shipped around the world, including to the United States.

Food safety experts say lead could enter cinnamon when its trees grew in contaminated soil or when the spice was processed with aging machinery. In the case of the cinnamon applesauce recall in 2023, the F.D.A. has said it suspected that the cinnamon had been intentionally adulterated with lead chromate, a powder used to stretch the valuable commodity and increase profits.

In August of last year, local health departments alerted the Centers for Disease Control and Prevention to lead poisoning in children at levels far higher than those found in the water in Flint, Mich., years earlier.

The agency then identified more than 500 people, mostly toddlers, infants and children, who had elevated blood-lead levels and were believed or confirmed to have consumed cinnamon applesauce in pouches. The pouches, by a company called WanaBana, were sold at Dollar Tree stores and under private labels at Weis and Schnucks grocery stores.

The F.D.A. has said that the cinnamon in the applesauce had lead levels ranging from about 2,000 to 5,000 parts per million and that the pouches also contained chromium, another toxic metal.

A New York Times review of the matter found that the applesauce pouches had sailed through a food safety system that was meant to keep tainted food out of the hands of consumers. It also found the agency inspections were being done at a fraction of the level outlined in the law, with officials visiting about 1 percent of overseas food establishments last year.

Last month, the F.D.A. sent a warning letter to Austrofood , the U.S.-regulated supplier of the cinnamon applesauce, which is based in Ecuador. Warning letters can be a final penalty, if a company satisfies agency concerns after receiving one, or may lead to additional enforcement like fines or import bans.

In the letter, the F.D.A. said that lead powder could be intentionally added to spices for economic gain, and that Austrofood should have considered the possibility as part of its food-safety plan and tested the cinnamon it used in the applesauce, which had been provided by another supplier.

Christina Jewett covers the Food and Drug Administration, which means keeping a close eye on drugs, medical devices, food safety and tobacco policy. More about Christina Jewett

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