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Is time travel possible? An astrophysicist explains

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

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

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

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

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

Time is relative

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

Related: The speed of light, explained

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

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

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

Time travel paradoxes and failed dinner parties

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

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

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

Telescopes are time machines

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

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

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

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

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No sense in nonsense —

Why the [expletive] can’t we travel back in time, if the inability to time travel were a fundamental part of our universe, you’d expect equally fundamental physics behind that rule..

Paul Sutter - Nov 30, 2021 12:30 pm UTC

Look, we’re not totally ignorant about time. We know that the dimension of time is woven together with the three dimensions of space, creating a four-dimensional fabric for the Universe. We know that the passage of time is relative; depending on your frame of reference, you can slip forward into the future as gently as you please. (You just need to either go close to the speed of light or get cozy with a black hole, but those are just minor problems of engineering, not physics.)

But as far as we can tell, we can’t reverse the flow of time. All evidence indicates that travel into the past is forbidden in our Universe. Every time we try to concoct a time machine, some random rule of the Universe comes in and slaps our hand away from the temporal cookie jar.

And yet, we have no idea why. The reasons really seem random; there is nothing fundamental we can point to, no law or equation or concept that definitively explains why thou shalt not travel into the past. And that’s pretty frustrating. It’s obvious that the Universe is telling us something important… we just don’t know what it’s saying.

Go ahead, kill your grandfather

There are all sorts of philosophical debates for and against the possibility of time travel. Take, for example, the famous “grandfather paradox.” Let’s say you build a time machine and travel back in time. You find your own grandfather and shoot him dead (I don’t know why, but roll with me here). But wait… if your grandfather is dead, it means he can’t father your father, which means you never exist. So how did you go back in time to do the awful deed?

Perhaps, however, time travel into the past is, indeed, allowed, but your actions are constrained. Maybe the past already exists and is completely set in stone. What has happened has simply happened. If you had the ability to travel back in time and monkey around with the past, then the past should already encode those acts—nothing is new, because it’s literally in the past. So you can’t kill your grandfather because you never have, but you could be the stranger that sets him up on a blind date with grandma.

Maybe, like, time doesn’t even exist, dude. Maybe it’s a construct of our human consciousness as a way to organize and synchronize our sensory inputs. Maybe we’re imposing some deep, fundamental preconceived notion on a Universe that doesn’t care, and so this whole discussion is moot.

This is all part of very legit discussions of philosophy. But let’s see if physics can take a crack at it. After all, if we could (even theoretically) build a time machine, then that would settle a lot of late-night bar bets.

Closed time-like curves

Physicists use a very particular language when trying to build time machines: the language of gravity, given to us by old Albert himself in the form of general relativity. That’s because the language of gravity as interpreted in GR is a story of the bending and warping of spacetime. GR is a theory of motion in our Universe and how that motion is tied to the underlying four-dimensional fabric of spacetime.

In GR, matter tells spacetime how to bend, and the bending of spacetime tells matter how to move.

To determine whether we can build a time machine, physicists want to know if it’s possible to construct a spacetime—to find a particular and peculiar arrangement of matter—that allows one to travel into the past.

The goal is to find “closed time-like curves,” or CTCs.

“Curve” means exactly what you think it does—a path through space and time. “Time like” means no cheating—at no point are you allowed to travel faster than light. “Closed” means that the curve meets up back with itself—imagine traveling in one direction, always moving forward, never exceeding light speed. Yet at the end of your journey, you find you’ve arrived in your own past.

That’s a time machine. That’s a CTC.

The weird thing is, CTCs exist! Over the decades we have managed to uncover many solutions of general relativity that allow for backward time travel:

• The (in)famous mathematician Kurt Gödel (yes, that Kurt Gödel) discovered if a universe is filled with uniform dust that was slowly rotating, you could find trajectories in that universe that wind up in their own past.
• You know wormholes, right? Those shortcuts through space? They can also act as time machines. The trick is to take one end of the wormhole and hold it still. Then take the other and accelerate it close to the speed of light. Keep it at that speed for however long you want. Now bring that end back to the original one. The two ends of the wormhole now no longer have synchronized clocks because of the time dilation effects of the near-lightspeed travel. Since one end is in the past of the other end, you can just hop on in and travel back in time.
• Let’s say you had an infinitely long cylinder (maybe you pick it up at your local home improvement store). Rotate that cylinder to nearly the speed of light. If you follow a careful, corkscrew path around the rotating cylinder, then, by golly, you’ll wind up in the past.
• The inside of a rotating black hole is a pretty interesting place, where the competing countercurrents of gravitational and centrifugal forces meet to open a throat in the center of a black hole, creating the possibility of CTCs.

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

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

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

Is time travel possible?

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

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

Learn more: Best travel insurance

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

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

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

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

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

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

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

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

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

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A beginner's guide to time travel

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

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

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

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

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

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

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

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

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

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

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

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

A party for time travelers 

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

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

The arrow of time 

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

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

Time travel paradox 

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

Killing Hitler

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

Killing your grandfather

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

A closed loop

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

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October 21, 1999

According to current physical theory, is it possible for a human being to travel through time?

As several respondents noted, we constantly travel through time--just forward, and all at the same rate. But seriously, time travel is more than mere fantasy, as noted by Gary T. Horowitz, a professor of physics at the University of California at Santa Barbara:

"Perhaps surprisingly, this turns out to be a subtle question. It is not obviously ruled out by our current laws of nature. Recent investigations into this question have provided some evidence that the answer is no, but it has not yet been proven to be impossible."

Even the slight possibility of time travel exerts such fascination that many physicists continue to study not only whether it may be possible but also how one might do it.

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One of the leading researchers in this area is William A. Hiscock, a professor of physics at Montana State University. Here are his thoughts on the matter:

"Is it possible to travel through time? To answer this question, we must be a bit more specific about what we mean by traveling through time. Discounting the everyday progression of time, the question can be divided into two parts: Is it possible, within a short time (less than a human life span), to travel into the distant future? And is it possible to travel into the past?

"Our current understanding of fundamental physics tells us that the answer to the first question is a definite yes, and to the second, maybe.

"The mechanism for traveling into the distant future is to use the time-dilation effect of Special Relativity, which states that a moving clock appears to tick more slowly the closer it approaches the speed of light. This effect, which has been overwhelmingly supported by experimental tests, applies to all types of clocks, including biological aging.

"If one were to depart from the earth in a spaceship that could accelerate continuously at a comfortable one g (an acceleration that would produce a force equal to the gravity at the earth's surface), one would begin to approach the speed of light relative to the earth within about a year. As the ship continued to accelerate, it would come ever closer to the speed of light, and its clocks would appear to run at an ever slower rate relative to the earth. Under such circumstances, a round trip to the center of our galaxy and back to the earth--a distance of some 60,000 light-years--could be completed in only a little more than 40 years of ship time. Upon arriving back at the earth, the astronaut would be only 40 years older, while 60,000 years would have passed on the earth. (Note that there is no 'twin paradox,' because it is unambiguous that the space traveler has felt the constant acceleration for 40 years, while a hypothetical twin left behind on a spaceship circling the earth has not.)

"Such a trip would pose formidable engineering problems: the amount of energy required, even assuming a perfect conversion of mass into energy, is greater than a planetary mass. But nothing in the known laws of physics would prevent such a trip from occurring.

"Time travel into the past, which is what people usually mean by time travel, is a much more uncertain proposition. There are many solutions to Einstein's equations of General Relativity that allow a person to follow a timeline that would result in her (or him) encountering herself--or her grandmother--at an earlier time. The problem is deciding whether these solutions represent situations that could occur in the real universe, or whether they are mere mathematical oddities incompatible with known physics. No experiment or observation has ever indicated that time travel is occurring in our universe. Much work has been done by theoretical physicists in the past decade to try to determine whether, in a universe that is initially without time travel, one can build a time machine--in other words, if it is possible to manipulate matter and the geometry of space-time in such a way as to create new paths that circle back in time.

"How could one build a time machine? The simplest way currently being discussed is to take a wormhole (a tunnel connecting spatially separated regions of space-time) and give one mouth of the wormhole a substantial velocity with respect to the other. Passage through the wormhole would then allow travel to the past.

"Easily said--but where does one obtain a wormhole? Although the theoretical properties of wormholes have been extensively studied over the past decade, little is known about how to form a macroscopic wormhole, large enough for a human or a spaceship to pass through. Some speculative theories of quantum gravity tell us that space-time has a complicated, foamlike structure of wormholes on the smallest scales--10^-33 centimeter, or a billion billion times smaller than an electron. Some physicists believe it may be possible to grab one of these truly microscopic wormholes and enlarge it to usable size, but at present these ideas are all very hypothetical.

"Even if we had a wormhole, would nature allow us to convert it into a time machine? Stephen Hawking has formulated a "Chronology Protection Conjecture," which states that the laws of nature prevent the creation of a time machine. At the moment, however, this is just a conjecture, not proven.

"Theoretical physicists have studied various aspects of physics to determine whether this law or that might protect chronology and forbid the building of a time machine. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In 1982, Deborah A. Konkowski of the U.S. Naval Academy and I showed that the energy in the vacuum state of a massless quantized field (such as the photon) would grow without bound as a time machine is being turned on, effectively preventing it from being used. Later studies by Hawking and Kip S. Thorne of Caltech have shown that it is unclear whether the growing energy would change the geometry of space-time rapidly enough to stop the operation of the time machine. Recent work by Tsunefumi Tanaka of Montana State University and myself, along with independent research by David Boulware of the University of Washington, has shown that the energy in the vacuum state of a field having mass (such as the electron) does not grow to unbounded levels; this finding indicates there may be a way to engineer the particle physics to allow a time machine to work.

"Perhaps the biggest surprise of the work of the past decade is that it is not obvious that the laws of physics forbid time travel. It is increasingly clear that the question may not be settled until scientists develop an adequate theory of quantum gravity."

John L. Friedman of the physics department at the University of Wisconsin at Milwaukee has also given this subject a great deal of consideration:

"Special relativity implies that people or clocks at rest (or not accelerating) age more quickly than partners traveling on round-trips in which one changes direction to return to one's partner. In the world's particle accelerators, this prediction is tested daily: Particles traveling in circles at nearly the speed of light decay more slowly than those at rest, and the decay time agrees with theory to the high precision of the measurements.

"Within the framework of Special Relativity, the fact that particles cannot move faster than light prevents one from returning after a high-speed trip to a time earlier than the time of departure. Once gravity is included, however, spacetime is curved, so there are solutions to the equations of General Relativity in which particles can travel in paths that take them back to earlier times. Other features of the geometries that solve the equations of General Relativity include gravitational lenses, gravitational waves and black holes; the dramatic explosion of discoveries in radio and X-ray astronomy during the past two decades has led to the observation of gravitational lenses and gravitational waves, as well as to compelling evidence for giant black holes in the centers of galaxies and stellar-sized black holes that arise from the collapse of dying stars. But there do not appear to be regions of spacetime that allow time travel, raising the fundamental question of what forbids them--or if they really are forbidden.

"A recent surprise is that one can circumvent the 'grandfather paradox,' the idea that it is logically inconsistent for particle paths to loop back to earlier times, because, for example, a granddaughter could go back in time to do away with her grandfather. For several simple physical systems, solutions to the equations of physics exist for any starting condition. In these model systems, something always intervenes to prevent inconsistency analogous to murdering one's grandfather.

"Then why do there seem to be no time machines? Two different answers are consistent with our knowledge. The first is simply that the classical theory has a much broader set of solutions than the correct theory of quantum gravity. It is not implausible that causal structure enters in a fundamental way in quantum gravity and that classical spacetimes with time loops are spurious--in other words, that they do not approximate any states of the complete theory. A second possible answer is provided by recent results that go by the name chronology protection: One supposes that quantum gravity allows microscopic structures that violate causality, and one shows that the character of macroscopic matter forbids the existence of regions with macroscopically large time loops. To create a time machine would require negative energy, and quantum mechanics appears to allow only extremely small regions of negative energy. And the forces needed to create an ordinary-sized region with time loops appear to be extremely large.

"To summarize: It is very likely that the laws of physics rule out macroscopic time machines, but possible that spacetime is filled with microscopic time loops.

What would happen if you moved at the speed of light?

The biggest issue you'd face is reaching that speed in the first place.

In science fiction, people often find a way to move at the speed of light. But you might find yourself asking, could your body survive going so fast? What would happen to it?

First, let's assume that it is possible — though it is not — for a human to move at the speed of light , which is 299,792,458 meters per second (983,571,056 feet per second), or about 186,000 miles per second. There's no issue, per se, with a person moving at a very fast constant speed. Humans can't feel constant velocity, so you wouldn't even necessarily notice you were moving that fast. 

Your biggest issue would be acceleration — actually reaching that speed. Too much acceleration force can hurt, and even kill, us. At high accelerations, "your blood will have a hard time pumping to your extremities," said Michael Pravica , a professor of physics at the University of Nevada, Las Vegas.

Related: Why is the speed of light the way it is?

Most humans can handle acceleration forces of about four to six times that of gravity (4 to 6 g) for a short period of time. As the g-force increases, your body's ability to circulate your blood from your feet to your head becomes limited. As your blood begins to pool, you will pass out, and if the force doesn't lessen or stop, you will eventually die as your body is starved of the oxygen your blood transports throughout your body.

Fighter pilots and other people who experience high levels of g-force are taught techniques to keep from passing out, such as tensing muscles in their extremities, and they use special suits to withstand up to 9 g for short periods of time. But if you were to accelerate to light speed in a few seconds — like in the "Star Wars" movies — you would quickly become a human pancake as the force of over 6,000 g slammed into you, according to Omni Calculator's g-force calculator .

If you wanted to accelerate to light speed more safely — say, at 2 g — it would take over five months to accelerate to light speed, assuming you were moving in a straight line and there was no air resistance. At 1 g, the acceleration of free fall, it would take over 11 months. 

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Unfortunately, reaching this lofty speed turns out to be impossible. "You cannot go at the speed of light, given that you have a finite mass," Pravica said.

Einstein's theory of special relativity shows that as an object with mass gets closer to the speed of light, the mass starts to increase as it nears the speed of light, Pravica said. If an object could reach the speed of light, it would become infinitely massive and would require infinite energy to maintain that speed.

— How long could you survive in space without a spacesuit?

— What would happen if Earth stopped spinning?

— How does light slow down?  

Still, humans have gotten some things to go very, very fast — if you can call subatomic particles "things." Particle accelerators can get particles like electrons to over 99.9% the speed of light, Pravica said. But there's a big difference between getting an electron to move that fast and launching a person at that speed, which would require so much energy as to be extremely improbable, even if it didn't break the laws of physics.

If you could move at near light speed, you would experience the effects of relativity on time, Pravica said. Time would move more slowly for you than for people moving at more everyday speeds, though your experience of time wouldn't change. If you could observe people moving at "normal" speed, Pravica said, they would appear to be moving in slow motion. 

There is one sense in which we could move at close to the speed of light. Our planet and everything in the universe are constantly moving. Earth is rotating and revolving around the sun , and even our galaxy is in motion. It's possible that if we were moving away from a galaxy very quickly — and that galaxy were also moving away from us — we would be moving, relative to that galaxy, at near the speed of light. It's possible that we already are.

"That's what Einstein showed," Pravica said. "Everything is relative." 

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Rebecca Sohn is a freelance science writer. She writes about a variety of science, health and environmental topics, and is particularly interested in how science impacts people's lives. She has been an intern at CalMatters and STAT, as well as a science fellow at Mashable. Rebecca, a native of the Boston area, studied English literature and minored in music at Skidmore College in Upstate New York and later studied science journalism at New York University. 

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• Omits Well, the question seems to be a non starter! Reply
• Classical Motion Even if you had the where with all and 1 year to achieve c, I don't think one could survive it without very heavy and firm shielding. Even though there are only a few particles in a cubic of space, at that speed those particles become a flux. It would shred your body structure.....and craft structure. Imagine the flux if going 100 or 1000 c. These would be c collisions. Producing a huge flux of disintegrating, ionizing charge fragments. Like standing in front of a CERN stream. Reply
• Atlan0001 What is explained is closed systematic! Open systemic, you at '0'kps never approach the speed of light 'c' ((+)300,000kps |0| (-)300,000kps) closer than (+/-)300,000kps. It is a "go with" constant. Also, according to the Heisenberg uncertainty principle if you know your velocity in the universe (in this case to exactitude) you can never know your position (in this case you'd be all over the map (in the dead center of Stephen Hawking's "Grand Central Station" of the universe, aka where Einstein landed in his mind's eye trip to the speed of light))! Reply
• Robert Lucien Howe This is a question where we get a reverse Dunning Kruger effect - it is beyond the region of physics that science really knows or understands and everything said ends up being essentially speculation. What everyone does know is that mass dilation increases the effective mass of objects near the speed of light until at light an object's mass becomes infinite. (making that impossible) Similarly time dilation may slow the speed of time at the speed of light to zero. From the objects perspective as it accelerates it becomes infinitely fast. A fairly obvious point is that the speed of light is an intersection point between the STL and FTL realms of speed. Another obvious point is that light itself does have mass though zero rest mass. If the mass of a massed object could somehow be balanced to zero then it could move at the speed of light. In fact it would immediately accelerate to the speed of light - for 'free'. There are two basic hypothetical approaches to balancing mass to zero - either adding something with negative mass or creating some kind of 'shield' that hides the mass. Curiously if we could create one a real Schrodinger box would be a possible way of achieving the second. So maybe not completely absolutely impossible after all. Reply

Robert Lucien Howe said: This is a question where we get a reverse Dunning Kruger effect - it is beyond the region of physics that science really knows or understands and everything said ends up being essentially speculation. What everyone does know is that mass dilation increases the effective mass of objects near the speed of light until at light itself this in theory becomes infinite. Similarly time dilation may slow the speed of time at the speed of light to zero. From the objects perspective as it accelerates it becomes infinitely fast. A fairly obvious point is that the speed of light is an intersection point between the STL and FTL realms of speed. Another obvious point is that light itself does have mass though zero rest mass. If the mass of a massed object could somehow be balanced to zero then it could move at the speed of light. In fact it would immediately accelerate to the speed of light - for 'free'. There are two basic hypothetical approaches to balancing mass to zero - either adding something with negative mass or creating some kind of 'shield' that hides the mass. Curiously if we could create one a real Schrodinger box would be a possible way of achieving the second. So maybe not completely absolutely impossible after all.

Atlan0001 said: Some (not me except in picturing) have already been there, done that, in the math. The total mass matter and energy of the universe equals zero. ..

• DC8Captain KillJoy Reply
• Questioner At the speed of light one's mass becomes infinite, but is that mass symmetric or only in the direction of travel? Otherwise one would turn into an actual black hole at the speed of light. All light would be photon sized black holes. Reply
• Preem What would happen if you moved at the speed of light? Well, you can't. :grinning: Reply
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Is There a Particle That Can Travel Back in Time?

A hypothetical particle could be the answer, but traveling in time would still be a complicated venture..

Yes, there is a hypothetical particle, called the tachyon , that could travel back in time. One catch: It almost certainly doesn’t exist.

Time and Speed of Light

Before we start talking about time travel, we first must talk about the speed of light . All objects in our universe are constrained to go no faster than the speed of light. The only particles capable of achieving light speed are massless particles, like light itself. Anything with even a tiny amount of mass will find it impossible to achieve light speed. That’s because the faster you go the more massive you become, and at light speed your mass becomes infinite, which would take an infinite amount of energy to accelerate.

But the speed of light isn’t just an expression of how fast objects can travel. It’s an expression of how fast objects can influence each other. Every single interaction in the universe, whether it’s your sibling hitting you or a supernova ’s shock wave blasting through a gas cloud, is limited to the speed of light. The speed of light is actually the speed of causality – the fastest possible way that one cause can create an effect, and the fastest possible way that events can influence each other.

Read More: A Major Time Travel Perk May Be Technically Impossible

Going faster than light means that you could go faster than causality. Said another way, going faster than light means going faster than time itself , meaning that faster-than-light travel automatically allows for time travel into the past.

Tachyon and Time Travel

There is a hypothetical class of particles that always travel faster than light. Einstein himself played around with the idea, calling them “ meta-particles ,” but today we call them tachyons , a word coined in 1967 by physicist Gerald Feinberg from the Greek word meaning “swift.”

Tachyons would be strange. Just as we massive objects could never ever exceed the speed of light, tachyons could never dip below light speed – they would be equally constrained on the other side of that invisible boundary. For tachyons, slowing down means increasing mass, and slowing down all the way to light speed would require an infinite amount of energy. To make this work, the mass of the tachyon has to be imaginary, but in the mathematical sense: Its mass would be multiplied by a factor of the square root of negative one.

At first glance, tachyons wouldn’t cause much trouble. You could fly out in a rocket ship, and on Earth, I could beam tachyon messages to you. If you were looking back at me through a telescope, those tachyons would reach you before the photons carrying the image of me sending the message arrived in your telescope. That’s a little weird but doesn’t necessarily violate anything about physics.

Read More: Black Holes Are Accelerating The Expansion Of The Universe, Say Cosmologists

Time-Travel Paradox

The problem is that with tachyons, you could start to construct some truly weird scenarios. For example, you could move in a certain direction with a certain speed and send a tachyon signal back to me. If you construct things just right in the rocket ship, that signal can arrive back to me before I sent the original one out.

Suppose that signal back to me contained instructions to destroy my transmitter. The only way to destroy the transmitter is through the reception of your signal, but the only way to get your message is for me to first send mine. If I get my signal out, then my transmitter was destroyed in the past. But if the transmitter was destroyed, I can’t get the signal out.

This is just one of many common time-travel paradoxes brought about by traveling faster than light. This doesn’t rule out the existence of tachyons explicitly, but it does signal that they likely don’t exist. It seems impossible for us to travel backwards into the past or send signals into our own past: Everything in our universe not only travels no faster than the speed light, but also always in the direction of the future.

Impossible, or Not Proven?

Physicists have proposed the “causality protection conjecture,” which says that faster than light travel (and travel into the past) is outright impossible. As of now this conjecture is merely a, well, conjecture, and not proven. We do not currently understand why travel into the past is forbidden, but we hope that someday we can construct a law of physics that tells us why.

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Why does time change when traveling close to the speed of light? A physicist explains

Assistant Professor of Physics and Astronomy, Rochester Institute of Technology

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

Rochester Institute of Technology provides funding as a member of The Conversation US.

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

Why does time change when traveling close to the speed of light? – Timothy, age 11, Shoreview, Minnesota

Imagine you’re in a car driving across the country watching the landscape. A tree in the distance gets closer to your car, passes right by you, then moves off again in the distance behind you.

Of course, you know that tree isn’t actually getting up and walking toward or away from you. It’s you in the car who’s moving toward the tree. The tree is moving only in comparison, or relative, to you – that’s what we physicists call relativity . If you had a friend standing by the tree, they would see you moving toward them at the same speed that you see them moving toward you.

In his 1632 book “ Dialogue Concerning the Two Chief World Systems ,” the astronomer Galileo Galilei first described the principle of relativity – the idea that the universe should behave the same way at all times, even if two people experience an event differently because one is moving in respect to the other.

If you are in a car and toss a ball up in the air, the physical laws acting on it, such as the force of gravity, should be the same as the ones acting on an observer watching from the side of the road. However, while you see the ball as moving up and back down, someone on the side of the road will see it moving toward or away from them as well as up and down.

Special relativity and the speed of light

Albert Einstein much later proposed the idea of what’s now known as special relativity to explain some confusing observations that didn’t have an intuitive explanation at the time. Einstein used the work of many physicists and astronomers in the late 1800s to put together his theory in 1905, starting with two key ingredients: the principle of relativity and the strange observation that the speed of light is the same for every observer and nothing can move faster. Everyone measuring the speed of light will get the same result, no matter where they are or how fast they are moving.

Let’s say you’re in the car driving at 60 miles per hour and your friend is standing by the tree. When they throw a ball toward you at a speed of what they perceive to be 60 miles per hour, you might logically think that you would observe your friend and the tree moving toward you at 60 miles per hour and the ball moving toward you at 120 miles per hour. While that’s really close to the correct value, it’s actually slightly wrong.

This discrepancy between what you might expect by adding the two numbers and the true answer grows as one or both of you move closer to the speed of light. If you were traveling in a rocket moving at 75% of the speed of light and your friend throws the ball at the same speed, you would not see the ball moving toward you at 150% of the speed of light. This is because nothing can move faster than light – the ball would still appear to be moving toward you at less than the speed of light. While this all may seem very strange, there is lots of experimental evidence to back up these observations.

Time dilation and the twin paradox

Speed is not the only factor that changes relative to who is making the observation. Another consequence of relativity is the concept of time dilation , whereby people measure different amounts of time passing depending on how fast they move relative to one another.

Each person experiences time normally relative to themselves. But the person moving faster experiences less time passing for them than the person moving slower. It’s only when they reconnect and compare their watches that they realize that one watch says less time has passed while the other says more.

This leads to one of the strangest results of relativity – the twin paradox , which says that if one of a pair of twins makes a trip into space on a high-speed rocket, they will return to Earth to find their twin has aged faster than they have. It’s important to note that time behaves “normally” as perceived by each twin (exactly as you are experiencing time now), even if their measurements disagree.

You might be wondering: If each twin sees themselves as stationary and the other as moving toward them, wouldn’t they each measure the other as aging faster? The answer is no, because they can’t both be older relative to the other twin.

The twin on the spaceship is not only moving at a particular speed where the frame of references stay the same but also accelerating compared with the twin on Earth. Unlike speeds that are relative to the observer, accelerations are absolute. If you step on a scale, the weight you are measuring is actually your acceleration due to gravity. This measurement stays the same regardless of the speed at which the Earth is moving through the solar system, or the solar system is moving through the galaxy or the galaxy through the universe.

Neither twin experiences any strangeness with their watches as one moves closer to the speed of light – they both experience time as normally as you or I do. It’s only when they meet up and compare their observations that they will see a difference – one that is perfectly defined by the mathematics of relativity.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

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Faster Than Speed of Light: Can Tachyon Really Travel Back in Time?

The concept of time travel has long been discussed by physicists, with the theory of general relativity suggesting that it is possible to travel back in time. One of the most intriguing entities in the theory of relativity is tachyons , the hypothetical particles that can beat the speed of light .

Time Travel and the Speed of Light

No object in the universe has demonstrated the ability to travel faster than the speed of light. Massless particles like light are the only entities capable of achieving this speed. This means that anything with even a tiny amount of mass will find it impossible to reach the speed of light. This is because the faster an object moves, the more massive it becomes. At light speed, an object's mass becomes infinite and would take infinite energy to accelerate.

The speed of light is not just an expression of how fast an object moves. It also demonstrates how fast things can influence each other. In other words, every interaction in the universe is limited to the speed of light.

Light is also the speed of causality, or the fastest way a cause can generate an effect. It also refers to the fastest way events can influence one another. It is going faster than light, which means going faster than causality. It also means that traveling faster than the speed of light means going faster than time itself. This can be interpreted as a faster-than-light travel, automatically allowing time travel into the past.

READ ALSO: Speed of Light Explained: How Fast Is It and Why Does It Matter?

Traveling in the Past With Tachyons

The universe comprises various types of particles that interact with each other and give off energies of different styles and spectrums. Among these particles, a tachyon is a quasiparticle capable of traveling faster than light. Although they are still not detected physically in any experiment, their existence has been mathematically hypothesized.

The idea of particles traveling faster than light was first conceptualized in 1904 by German physicist Arnold Sommerfeld. It was first called meta-particles by Sommerfeld until Gerald Feinberg coined the term tachyon in 1967 in his study about faster-than-light particles and their kinetics concerning special relativity.

Tachyons exhibit an unusual property  where the increase in speed results in a decrease in energy. Like other subatomic particles, the power increases with every rate increase. Therefore, it would require infinite energy to slow down a tachyon to the speed of light.

As tachyons move at speeds greater than the speed of light, observing them in real time would be impossible. After they pass through a point in space, the observers would see two images of them. This is because the observer would witness an appearing and a departing image in the opposite direction. This is called the double image effect, a phenomenon normally observed in a superluminal object's light field.

If tachyons can move faster than light, then they would violate causality as explained by the theory of relativity and can give rise to situations such as the Grandfather's paradox. There would be disagreement on the simultaneity of two special events at two points in space, a phenomenon that cannot occur in different inertial frames of reference.

RELATED ARTICLE: Ways on How Particles Travel Nearly the Speed of Light

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Here's What Actually Happens When You Travel at the Speed of Light, According to NASA

NASA created a fun video to answer all of our burning questions about near-light-speed travel.

Ever wish you could travel at the speed of light to your favorite destinations ? Once you see the reality of that speed, you may rethink everything.

"There are some important things you should probably know about approaching the speed of light," NASA's video, Guide to Near-light-speed Travel , explains. "First, a lot of weird things can happen, like time and space getting all bent out of shape."

According to the video, if you're traveling at nearly the speed of light, the clock inside your rocket would show it takes less time to travel to your destination than it would on Earth. But, since the clocks at home would be moving at a standard rate you'd return home to everyone else being quite a bit older.

"Also, because you're going so fast, what would otherwise be just a few hydrogen atoms that you'd run into quickly becomes a lot of dangerous particles. So you should probably have shields that keep them from frying your ship and also you."

Finally, the video tackles the fact that even if you were moving at the speed of light, the "universe is also a very big place, so you might be in for some surprises." For example, your rocket's clock will say it takes about nine months to get from Earth to the edge of the solar system. An Earth clock would say it took about a year and a half. Fortunately, NASA astronauts have a slew of tips for avoiding jet lag along the way.

"If you want to get to farther out vacation spots," the video explains, "you'll probably need more than a few extra snacks. A trip to the Andromeda Galaxy, our nearest large neighbor galaxy, can take over one million years. And a trip to the farthest known galaxy where it currently sits might take over 15 billion years, which is more vacation time than I think I'll ever have."

The video doesn't explain how your rocket will travel at the speed of light. Our technology just isn't there yet, but maybe the aliens will share that tech with us soon. Until then, you can track the first crew launch of Artemis II , a rocket that will fly around the moon in 2024 before making its first lunar landing in 2025.

NASA's Guide to Near-light-speed Travel

• Released Friday, August 14, 2020
• Produced by:
• Chris Smith
• Written by:
• Visualizations by:
• Krystofer Kim
• Scientific consulting by:
• Ryan DeRosa
• and Scott Noble

So, you've just put the finishing touches on upgrades to your spaceship, and now it can fly at almost the speed of light. We're not quite sure how you pulled it off, but congratulations! Before you fly off on your next vacation, however, watch this handy video to learn more about near-light-speed safety considerations, travel times, and distances between some popular destinations around the universe. You can also download shorter clips from the video and printable postcards to send to your friends.

Near-light-speed Travel GuideThis handy video will help acquaint you with the quirks of near-light-speed travel, expected travel times, and the distances to some popular (at least, we think so) destinations!Credit: NASA's Goddard Space Flight CenterMusic: "The Tiptoe Strut" from Universal Production MusicComplete transcript available.

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Complete transcript available.

Near Light Speed 101: Effects of Near-light-speed TravelTravel at near the speed of light offers a few quirks you should be aware of, from time and space weirdness to protecting yourself from dangerous cosmic particles. This video covers some of the important ones!Credit: NASA's Goddard Space Flight CenterMusic: "Dinner With the Vicar" from Universal Production MusicComplete transcript available.

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Near Light Speed 101: Near-light-speed Travel TimesEven if you've figured out how to travel at almost the speed of light, the universe is still a huge place! Watch this video to learn more about how long it takes to cruise around the cosmos.Credit: NASA's Goddard Space Flight CenterMusic: "Dinner With the Vicar" from Universal Production MusicComplete transcript available.

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*compared to friends that stayed behind.

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Visit sunny Glerbax-29, home of eight unique, beautiful planets! The locals call it the solar system.

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A vacation to Andromeda, our nearest spiral neighbor galaxy, is a long, long, long, long, long trip, but it's worth it! And this postcard will prove it to your friends.

• Astrophysics

Please give credit for this item to: NASA's Goddard Space Flight Center

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• Ryan DeRosa  (NASA/GSFC)
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This page was originally published on Friday, August 14, 2020. This page was last updated on Wednesday, May 3, 2023 at 1:44 PM EDT.

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Screen Rant

How the delorean time travelled without going 88mph in back to the future 2.

Doc Brown's DeLorean had to reach 88 MPH to travel through time, but at the end of Back to the Future Part 2, it looked as if the rules were changed.

Back to the Future Part 2 seemed to break its own rule, allowing the DeLorean to time travel without reaching 88 MPH, but it turns out this isn't a pothole after all. Naturally, the beloved science fiction franchise is full of such questions and mysteries, and it often takes a bend of the imagination to make everything make sense. Whenever time travel gets involved, there are sure to be some paradoxes. However, in the case of Doc's jump to 1885 at the end of Back to the Future Part 2 , there is a perfectly reasonable explanation.

By the end of the second Back to the Future movie , Marty and Doc had become pretty familiar with the ins and outs of time travel. They jumped back to 1955, returned to 1985, traveled to 2015, and repeated this cycle several times before a final trip back to 1955. From here, The pair were ready to finally head back to their true time, 1985, once and for all. However, just as Doc and his flying DeLorean were about to return to the ground and pick up Marty, the time-machine car was struck by lightning and sent to 1885 . The problem is, the DeLorean wasn't traveling at the requisite 88 MPH when it jumped.

Back To The Future: All 8 Timelines In The Movies Explained

The delorean reached 88 mph thanks to being struck by lightning in back to the future 2, the delorean traveled at 88 mph in a rapid loop-de-loop.

The problem regarding the stationary DeLorean being sent back in time was spotted relatively quickly after Back to the Future Part 2 was released in 1989, and it has plagued many ever since. It was clear that the time machine still needed to get up to 88 MPH since Marty and Doc had to use a steam engine to get it up to speed at the end of Back to the Future Part 3 . So, the change didn't come down to upgrades made to the DeLorean in 2015 (which eliminated the need for plutonium after the first Back to the Future ). As it turns out, the answer is pretty simple.

The DeLorean did get up to 88 MPH. In fact, it likely traveled much faster than that.

According to Back to the Future creator Bob Gale , the DeLorean did get up to 88 MPH. In fact, it likely traveled much faster than that. When the car was struck by lightning while flying in the air, the vehicle was sent spinning on its axis . It looped around so quickly that the required speed was achieved, and between this and the power of the bolt of lighting (or perhaps Mr. Fusion), the DeLorean's time-traveling capabilities were activated. Of course, it all happened so quickly that Marty (and the audience) could barely see it. Still, there was clear evidence of this sudden loop-de-loop.

The Backwards 99 At The End Of Back To The Future 2 Explained

The mysterious number in back to the future part 2 didn't have a secret meaning after all.

Gale's answer about how the DeLorean got up to 88 MPH makes perfect sense when remembering the mysterious backward 99 that appeared in the sky in Back to the Future Part 2 . This number became the inspiration for a wide variety of fan theories. Some believed the number referred to different points where Doc and Marty had already traveled in time. The number nine was visible on street signs in both 1955 and 2015, and the backward 99 might have referred to Doc going backward in time. However, the truth is much simpler.

As previously explained, the DeLorean was thrown into a rapid loo-de-loop after it was struck by lightning, and since the speed was beyond 88 MPH, the car went back in time. Whenever the time machine jumped before this, fiery tire marks were left in its wake. Of course, this wasn't quite possible at the end of Back to the Future Part 2 since the DeLorean was flying in the air. So, the fiery tread marks were left floating in the sky instead. The backward 99 was just the visible path the car had taken to get up to speed after being struck.

Why The DeLorean Sends Doc Brown Back To 1885

A malfunction was responsible for kicking off back to the future part 3.

Another mystery connected to the DeLorean's spontaneous jump is why Doc traveled back to 1885 when the car was set to go forward to 1985. This was subtly answered moments before lighting struck at the end of Back to the Future Part 2 when Doc gave the time circuits a good old-fashioned punch to set them right again. Since they had been malfunctioning only moments before, it's no great surprise that a jolt of lightning would cause the dial to change from 1985 to 1885. Of course, without the letter that Doc arranged to be delivered to Marty, no one would have known precisely where he ended up.

Back to the Future Part II

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Taking up where the first movie left off, Back to the Future Part II sees Marty McFly and Doc Brown travel to the year 2015, where their efforts to fix the future end up causing even bigger problems as Biff Tannen wreaks havoc across the timeline with the help of a stolen sports almanac. Martin J. Fox and Christopher Lloyd return in Robert Zemeckis and Bob Gale's second installment of their iconic trilogy.

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News Analysis

China Feels Boxed In by the U.S. but Has Few Ways to Push Back

China seeks to project military power in the seas around its coastline, yet also faces pressure to mend relations with neighbors for the good of its economy.

By David Pierson and Olivia Wang

Reporting from Hong Kong

President Biden’s effort to build American security alliances in China’s backyard is likely to reinforce the Chinese leader Xi Jinping’s view that Washington is leading an all-out campaign of “containment, encirclement and suppression” of his country. And there is not much Mr. Xi can do about it.

To China, Mr. Biden’s campaign looks nothing short of a reprise of the Cold War, when the world was split into opposing blocs. In this view, Beijing is being hemmed in by U.S. allies and partners, in a cordon stretching over the seas on China’s eastern coast from Japan to the Philippines, along its disputed Himalayan border with India , and even across the vast Pacific Ocean to a string of tiny, but strategic , island nations.

That pressure on China expanded Thursday when Mr. Biden hosted the leaders of Japan and the Philippines at the White House, marking the first-ever trilateral summit between the countries. American officials said the meeting was aimed at projecting a united front against China’s increasingly aggressive behavior against the Philippines in the South China Sea and against Japan in the East China Sea. Mr. Biden described America’s commitment to defense agreements with Japan and the Philippines as “ironclad.”

The summit ended with agreements to hold more naval and coast guard joint exercises, and pledges of new infrastructure investment and technology cooperation. It builds on a groundbreaking defense pact made at Camp David last August between Mr. Biden and the leaders of Japan and South Korea, as well as on plans unveiled last year to work with Australia and Britain to develop and deploy nuclear-powered attack submarines.

Mr. Biden has also sought to draw India, China’s chief rival for influence with poorer countries, closer to Washington’s orbit through a security grouping called the Quad. And a high-profile visit to Washington by the Indian leader last year has intensified Chinese suspicions about India.

“China is clearly alarmed by these developments,” said Jingdong Yuan, director of the China and Asia Security Program at the Stockholm International Peace Research Institute. “Chinese interpretations would be that the U.S. and its allies have clearly decided that China needs to be contained.”

In response, China has been bolstering its own ties with partners like Russia and North Korea. As recently as Tuesday, the Chinese and Russian foreign ministers, meeting in Beijing, warned the United States not to replicate the North Atlantic Treaty Organization in Asia. Zhao Leji, a senior Chinese leader, traveled to Pyongyang this week and pledged to “strengthen strategic coordination” between the countries.

The United States and its allies are “stoking confrontation in the name of cooperation, flexing muscles in the name of peace, and sowing chaos in the name of order,” the Global Times, a Communist Party newspaper, wrote in an editorial this week. On Friday, China’s coast guard patrolled the waters near the disputed islands in the East China Sea known in China as Diaoyu and in Japan as Senkaku.

But aside from pointed words and the perfunctory maritime patrol, Beijing’s options to push back against U.S. pressure appear limited, analysts said, especially as China contends with slowing economic growth and mounting trade frictions.

Its military, while rapidly modernizing, is untested and would be taking an immense risk by confronting a U.S.-led alliance. Beijing’s resolve is currently being challenged in the South China Sea, amid a standoff with Manila over disputed waters.

Tensions with the Philippines have been running high since President Ferdinand Marcos Jr., came into power in 2022 and adopted a more muscular foreign policy, which included resisting China’s vast claims to waters near its shores. Chinese boats have rammed and pointed lasers at Philippine ships, and last month a Chinese coast guard vessel injured three Philippine soldiers with a water cannon.

China has depicted the Philippines as another pawn of the United States and Japan, and sought to portray itself as a victim of U.S. aggression.

Analysts say that dismissive approach, coupled with China’s buildup of artificial islands in the South China Sea replete with military installations and airstrips, has changed the calculus of the Philippines and motivated it to embrace the United States.

China “should know better, as its own activities asserting very aggressively its territorial claims in the South China Sea would push the Philippines toward strengthening ties with the U.S.,” Mr. Yuan said.

Similarly, the Camp David summit last year underscored the depths of Tokyo’s and Seoul’s unease about China’s growing assertiveness, prompting the two Asian neighbors to set aside decades of lingering tension over colonial occupation and World War II.

Whether Mr. Biden’s strategy succeeds in deterring China in the long run remains to be seen. Nationalists in China view American alliances as fragile and subject to the whims of each U.S. presidential election. Then there’s Mr. Xi, who perceives the West to be in structural decline, and China’s ascendance as Asia’s dominant power to be inevitable.

“The Americans should not think so highly of themselves. They could not solve Afghanistan or Ukraine,” said Zheng Yongnian, an influential political scientist at the Chinese University of Hong Kong’s campus in Shenzhen. He said that China still hoped to resolve its disputes peacefully. “The reason we are not touching the Philippines is not that we are afraid of the United States.”

China has also launched a diplomatic blitz targeting nonaligned powers such as Indonesia, Saudi Arabia and South Africa. And tiny island nations in the Pacific, which hold great strategic value in the contest for naval supremacy, have also been beneficiaries of China’s charm offensive.

On Tuesday, Mr. Xi hosted President Wesley Simina of the Federated States of Micronesia, an archipelago nation of over 100,000 people that has long been part of the U.S. sphere of influence. Mr. Simina was treated to an honor guard and a red carpet en route to a meeting in the Great Hall of the People, where Mr. Xi promised more Chinese largess.

“China is ready to continue to provide support to the development of island countries to the best of its ability,” Mr. Xi said.

Days earlier, Beijing took the highly unusual step of welcoming Indonesia’s president-elect, Prabowo Subianto, and giving him a meeting with Mr. Xi. Such an honor is usually reserved for a leader after inauguration, and could reflect regret for not courting Mr. Marcos more aggressively after he took power.

Still, Beijing’s room to maneuver against Washington is limited by its struggling economy, which has been hit by a property crisis and a cratering of foreign investment. China has been increasing exports, but that has already caused friction with countries concerned about a flood of cheap Chinese goods.

The broader American pressure campaign may also be nudging China to avoid escalating tensions further. Despite its differences with the United States, China is engaging in talks between the countries’ leaders and senior officials . Relations with some neighbors, such as Australia , are slowly thawing. Analysts have noted that Beijing has also avoided escalating its military presence around Taiwan in recent months, despite the island’s election of a leader the Communist Party loathes .

“They are definitely being more cautious and demonstrating a willingness to engage,” Ja Ian Chong, an associate professor of political science at the National University of Singapore, said of Beijing. “They are realizing there are actual risks to letting frictions escalate. We just haven’t seen any substantive compromises yet.”

David Pierson covers Chinese foreign policy and China’s economic and cultural engagement with the world. He has been a journalist for more than two decades. More about David Pierson

Olivia Wang is a Hong Kong-based reporter and researcher who covers mainland China and Hong Kong. More about Olivia Wang

Trump's $175M bond in civil fraud case on the line with critical hearing

New York AG Letitia James argues that the bond should be rejected.

Attorneys Donald Trump are expected back in court on Monday to defend the $175 million bond in the former president's civil fraud case, days after New York Attorney General Letitia James urged the court to reject the bond and give Trump seven days to find a new one.

Judge Arthur Engoron ordered the hearing earlier this month after James took exception to the bond and asked the company behind the bond -- Knight Specialty Insurance Company -- to prove they are sufficiently collateralized to pay the bond if Trump's appeal of the $464 million judgment fails.

The bond hearing presents a legal double-header for the former president, who is required to attend the opening statements in his criminal hush money trial on Monday morning. Down the street from the criminal courthouse, Judge Engoron will hear arguments in civil court that could place the former president in financial dire straits if the bond is rejected.

MORE: New York AG asks court to reject Trump's $175M bond for civil judgment

Trump's bond saga began in February when Engoron ordered the former president and his co-defendants pay $464 million in disgorgement and prejudgment interest for engaging in what he found to be a decade of business fraud . Trump attempted to delay the fine, telling an appellate court that finding a surety willing to handle a half-billion-dollar bond was a "practical impossibility."

James vowed to begin seizing Trump's assets, including his namesake buildings, if he did not pay the judgment in time.

"If he does not have funds to pay off the judgment, then we will seek judgment enforcement mechanisms in court, and we will ask the judge to seize his assets," James said in an interview with ABC News.

At the deadline for Trump to pay the judgment, New York's Appellate Division First Department granted the former president's eleventh-hour request to reduce the size of his bond, permitting him to post a bond of $175 million.

Days later, Trump and his co-defendants posted a $175 million bond collateralized using $175,304,075 held in a Charles Schwab brokerage account controlled by the Donald J. Trump Revocable Trust.

Because the company behind the bond was not admitted in New York, James filed a notice that requires Knight Speciality Insurance to demonstrate they are capable of paying the bond if needed.

"KSIC is a respected, well-capitalized, Delaware-domiciled insurer that has long underwritten surety bonds and other types of insurance placed around the country," attorneys for Knight Speciality Insurance and Trump wrote in a filing last week.

The filing specified that the bond was secured by more than $175 billion held in a brokerage account controlled by Knight, which independently maintained more than $539 million in their own assets. The filing also stated that the company has access to more than $2 billion in assets through their parent company.

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"By any standard, KSIC has therefore provided assurance to the Plaintiff judgment creditor that she can collect the designated amount if the award is affirmed on appeal," the filing said.

In a filing on Friday, the New York Attorney General argued that the bond itself should be rejected because the defendants failed to prove that Knight could handle "this extraordinarily large undertaking" and that the bond was sufficiently collateralized.

MORE: In win for Trump, appeals court lowers his bond to $175M in civil fraud case

According to James' filing, Knight does not have the exclusive right to control the money in Trump's brokerage account, which could become problematic if the value of Trump's assets in the account dips below $175 million. James also raised issues with Knight's business practices, which she argued should make the company ineligible to do business in New York.

"KSIC is not qualified to act as the surety under this standard because its management has been found by federal authorities to have operated affiliated companies within KSIC's holding company structure in violation of federal law on multiple occasions within the past several years," the filing said.

Don Hankey -- the chairman of Knight Specialty's parent company -- declined to comment on the attorney general's recent filing on Friday.

In an interview with ABC News on April 4, he said he had "no concerns at all" about the bond.

"Seldom do our applications or our bonds get turned down. I imagine it is being scrutinized very carefully, and they're checking to make sure all the i's are dotted and the t's are crossed," Hankey said. "It's a large amount for anybody."

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What Is Daylight Saving Time, and When Does the Time Change in 2024?

Daylight saving time starts on the second Sunday of March and ends on the first Sunday of November. This means we’ll be “springing forward,” causing many of us to lose an hour of sleep during the transition (but able to experience later sunsets and more sunshine outdoors!). Then in the fall, we’ll be back to gaining an extra hour of sleep (excellent!), with the trade-off being earlier sunsets (bummer!). Though there is a movement to end daylight saving time , this twice-annual tradition is currently observed by more than 70 countries around the world, including the United States, save for two states .

But what is daylight saving time , exactly, and what is the point of it? Read on to find out.

Get Readers Digest s Read Up newsletter for more knowledge and insights, humor, cleaning, travel, tech and fun facts all week long.

What is daylight saving time?

To start off, its daylight saving, not savings, as it is commonly called. Daylight saving time, also known as DST, is a practice where we advance the clocks by one hour on the second Sunday of March and set them back by one hour on the first Sunday of November, at 2 a.m. The goal of DST is to make better use of the varying daylight hours caused by the Earth tilting at different points during its orbit.

Since the Earth tilts at different times of the year, this gives us our Northern Hemisphere seasons (and reverses them in the Southern Hemisphere). It also contributes to the shortening and lengthening of daylight hours. By moving the clocks forward an hour in sun-rich spring, shortly before the spring equinox , the intention is that the majority of the day will be lived under full daylight hours, until the time changes again in fall.

When does daylight saving time start in 2024?

Daylight saving time always starts and ends at 2 a.m. in the United States. This years daylight saving time starts on Sunday, March 10, and ends on Sunday, Nov. 3.

Here are the future start and end dates for 2025 and beyond:

• 2025: Sunday, March 9, to Sunday, Nov. 2
• 2026: Sunday, March 8, to Sunday, Nov. 1
• 2027: Sunday, March 13, to Sunday, Nov. 7
• 2028: Sunday, March 12, to Sunday, Nov. 5

Why do we have daylight saving time?

There are several stories about the origins of the DST concept. You might have heard that the idea stemmed from Benjamin Franklin, but that’s not strictly true. He did write a satirical letter to The Journal of Paris (where he was living in 1784) suggesting that the city would save 64 million pounds of candle wax if only its citizens would rise with the sun, but he also included a recommendation that they get the people on schedule by firing cannons in every street as a citywide alarm clock. We’re grateful for Franklin’s other inventions but glad this particular one did not catch on.

It wasnt until 1908 that Thunder Bay, Canada, became the first city to implement daylight saving time. Its purpose: to preserve daylight hours in the winter months. Then, in 1916, Germany and Austria became the first countries to implement DST, to save money on energy costs during World War I.

When did daylight saving time start in the United States?

While the World War Iera changes in Germany and Austria launched a daylight saving practice that was followed by most of Europe, the United States didnt follow suit until March 19, 1918, when the Standard Time Act was signed into law. (This law also established our five time zones.) But the story doesn’t end there. After World War I, the DST federal law was repealed, before being resurrected during World War II with the intent of saving money on energy costs. After the war, DST was made optional, which led to absolute chaos when traveling. A 35-mile bus journey from Moundsville, West Virginia, to Steubenville, Ohio, meant going through seven different time changes!

Finally, in 1966, Congress passed the Uniform Time Act, standardizing DST for the six months from April to October. It was extended to seven months in 1986, and finally to eight months in 2005, leaving us with the MarchNovember DST we have todayin 48 states, at least.

What U.S. states dont do daylight saving time?

There are only two U.S. states that dont observe daylight saving time: Hawaii and Arizona (except the Navajo Nation, in northeastern Arizona). Perhaps because both those states get plenty of sunshine year-round, they dont feel the need to hoard it.

Do other countries practice daylight saving?

Only about a third of the worlds countries practice daylight saving timemost in Europe, with Egypt being the only African nation that observes it.

In the U.K. and other European countries, where daylight saving is known as summer time, DST begins on the last Sunday of March and ends on the last Sunday of October. Another interesting fact ? The only European countries that dont currently follow the practice are Armenia, Azerbaijan, Georgia, Belarus, Iceland, Russia and Turkey.

Will daylight saving be eliminated in the United States?

As of now, no! However, Sen. Marco Rubio’s Sunshine Protection Act was reintroduced in March 2023. If enacted, the bill would end “falling back” in November, allowing us to keep a full year of DST. Progress has yet to be made, so as of now, daylight saving time is here to stay.

Why trust us

At Readers Digest , were committed to producing high-quality content by writers with expertise and experience in their field in consultation with relevant, qualified experts. We rely on reputable primary sources, including government and professional organizations and academic institutions as well as our writers personal experience where appropriate . We verify all facts and data, back them with credible sourcing, and revisit them over time to ensure they remain accurate and up to date. Read more about our team , our contributors and our editorial policies .

• Time and Date : “Daylight Saving Time Statistics”
• U.S. Department of Defense : Daylight Saving Time Once Known as ‘War Time'”
• Pew Research Center : Most Countries Dont Observe Daylight Saving Time
• Press release from Sen. Marco Rubio : Rubio Reintroduces Bill to Make Daylight Saving Time Permanent

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The post What Is Daylight Saving Time, and When Does the Time Change in 2024? appeared first on Reader's Digest .

'Family Guy' Season 22 Is Cancelling Christianity in Epic Time Travel Finale

Brian is using Stewie's time machine for nefarious purposes again.

The Big Picture

• Jesus returns in an epic time-travel finale for Family Guy Season 22.
• Brian falls for a religious woman and tries to go back in time to cancel Christianity so she'll sleep with him.
• The Family Guy finale promises the show's trademark offensive humor and some unexpected twists.

Our dear lord and savior returns to Family Guy in the Season 22 finale. At this point, Jesus has made so many appearances that he might just move in with The Griffins . It's been a while since he's appeared to clarify some things or respond when he's called upon, but tonight he returns in an epic time-travel episode. In an interview with TV Insider , showrunners Richard Appel and Alec Sulkin previewed the season finale, which airs tonight on Fox, and what Jesus has to do with it all.

This time around, he doesn't appear to the characters, and like when he was crucified all those millennia ago, he is seemingly innocent. His only crime is being associated with a religion that puts a damper on Brian's (voiced by Seth MacFarlane ) plans. In the "Faith No More" episode, the official episode synopsis (below) teases another one of Stewie (voiced by MacFarlane) and Brian's classic adventures.

"Brian becomes romantically interested in someone and it inspires him to use Stewie's time machine. Strange consequences result from their journey"

Brian Falls In Love With A Religious Woman

The dog is in love again. He falls in love with a devout vet technician (voiced by Mae Whitman ) and pretends to be religious himself to score with her. However, things don't play out in a neat rom-com manner, something Sulkin calls a mislead. "You go a little bit down a road with something that seems like it could be a Family Guy story, and then it turns into something else," he said.

What does a dog have to do to get frisky when his girlfriend has decided she's not having sex until marriage because she doesn't want to go to hell? For Brian, however, the answer is obvious. The dog will use Stewie's time travel tech . The goal? To cancel Christianity. How? By stopping Jesus before religion was invented and evolved to ruin his chances in the present. Stewie is not one to be left behind during a time-travel adventure, so the duo will jump back in time together.

They find themselves in medieval Israel circa A.D. 30, and they've already messed up. What can go wrong in a place and time they don't understand? And all this just so the dog can get lucky? The last time Jesus was on the show was in Episode 10 of Season 19, as Peter faced off with his mortal enemy: The Giant Chicken. It seems that we should be ready for another round of humor that will most definitely be deemed offensive , with Appel saying, “I never thought I’d hear myself say this as a writer, but the Standards and Practices department at Fox is easy to work with and pretty good with us.”

Catch the Family Guy Season 22 finale tonight on Fox. Stream past episodes on Hulu.

WATCH ON HULU