Time travel has long been a staple of science fiction books and movies. But will we ever be able to build a time machine and beam ourselves backward and forward in time in real life?

In some sense, of course, we're all time travelers: We move forward in time from one minute to the next. But going back in time, whether to avoid some mistake or perhaps to repeat it, is something far more elusive. And for those who yearn to see what the world will look like a century or a millennium from now, the slow tick of your watch as time passes just doesn't cut it.

The theoretical underpinnings of time travel date back to 1905, when Albert Einstein wrote down his special theory of relativity that showed space and time are intimately linked, and to 1916, when Einstein's general theory of relativity showed that space and time are malleable — that is, they respond to the presence of matter or energy by warping, bending, expanding, and contracting. By extension, this means if one can imagine space being filled with some exotic form of energy, then space and time could warp in a way so that time, as well as space, could bend back upon themselves like circles, allowing one to move forward in a straight line and still return to one's starting point in both space and time.

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But even after thinking about the time travel problem for more than a century, physicists haven't advanced the ball very far. We recognize that while Einstein's equations do allow for round-trip time travel (at least in principle), other physics considerations probably rule out creating the exotic forms of energy that would make it possible.

The first person to write down a mathematical solution of the general relativity equations that described an exotic type of space-time that might permit time travel was mathematician Kurt Gödel, a close colleague of Einstein's at the Institute for Advanced Study in Princeton, N.J. He presented his result in a scientific paper that he gave Einstein as a birthday present on his 70th birthday in 1949.

But just being able to determine that general relativity allows configurations of space and time in which forward or backward time travel is possible doesn't mean those configurations can actually be created. Since general relativity implies that the configuration of space-time is determined by the nature of the matter and energy within it, one needs to determine whether it's possible to create the appropriate type of matter and energy in the laboratory.

Is it possible? Probably not, but we don't know for sure.

Perhaps the easiest way to understand the problem is to examine the simplest example of a theoretical time machine: a "wormhole" — essentially a shortcut through a curved space, like a tunnel under a mountain (or for our purposes here a tunnel connecting two distant points in space). That's why I chose this example when discussing time travel two decades ago in my book "The Physics of Star Trek."

So imagine a wormhole one of its mouths moving through space in a big circle, at, say, 95 percent of the speed of light. Now, special relativity tells us that observers in relative motion experience time differently, such that — to a ground-based observer — clocks on a fast-moving rocket ship would tick more slowly than clocks on the ground.

Thus an observer riding on the wormhole's mouth as it zooms through space might determine from his or her clock that the round-trip took a week. But an observer at the other end of the wormhole, at rest in the background space, would look at his clock and determine that the trip took, say, three years. If the second observer then moves through the wormhole and comes out the other end, he or she will arrive to meet his or her colleague at the other end — and discover that the time is now three years before he entered the wormhole in the first place!

Are wormholes possible in real life? The answer is … we don't know! We do know that no stable wormholes can exist if the only forms of matter and energy are the ones we've been able to create in laboratories: In that case, each mouth of the wormhole would collapse to form a black hole in a time shorter than it would take to traverse the wormhole. But if it were possible to create some material with very peculiar characteristics — namely a material that was gravitationally repulsive — it might be able to hold a wormhole open against gravitational collapse. And that would bring time travel a step closer to reality.

All the evidence so far suggests that it is probably impossible for us to create such a material. Yet ultimately, it is the limitation of our understanding of general relativity, especially in the domain where quantum mechanical effects might be significant, that currently prohibits us from proving the impossibility of building a time machine.

In other words, time travel is not yet ruled out in our universe.

Even so, most physicists now working would bet against the possibility of time travel, not merely because of the practical difficulties of generating the necessary conditions to allow it but also because of the implications of time travel if it becomes possible.

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For example, if were to go back in time and change the past, we would also change the future. This situation is a common plot twist of modern science fiction (including "Star Trek" and the film series "Back to the Future"). And it leads to a host of possible paradoxes, including what would happen if you went back in time and killed your grandmother before she gave birth to your mother. If your mother was never born, of course, then you would never have been born. But in that case, how did you go back in time and kill your grandmother in the first place?

One possible resolution of this paradox would be that the only kind of time travel allowed by the laws of physics is travel in which you are doomed to repeat the same sequence of events again and again, no matter how much you'd like to alter things. Time would travel in a circle instead of a straight line. As in the movie "Groundhog Day," you would be doomed to repeat events for all eternity.

Traveling forward in time and returning also produces problems of the sort seen in "Back to the Future." As I have pointed out previously, the fact that Bill Gates remains the richest person in the world argues against the existence of a forward-and-back time machine. For if you could jump forward even a single day and then return to the present time, within a year you could make investments in the stock market that would turn even a small sum into an astronomically large one. Gates's mere $80 billion fortune would seem minuscule.

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My colleague Stephen Hawking once presented another interesting argument against the possibility of time travel. He said that if it were possible, then we would forever be inundated with tourists from the future. (I countered by saying maybe they all went back to the 1960s and no one noticed!)

As much as the idea of time travel runs counter to our common sense understanding of reality, the universe is the way it is, whether we like it or not. Even if time travel appears to present difficulties for such notions as causality, the history of physics has taught us that new discoveries force us to periodically modify our understanding of cherished notions.

The bottom line? While the possibility of time travel continues to tantalize physicists and laypeople alike, the odds are against it. And if time travel were shown to possible in principle, the amount of energy required to create the conditions for time travel would likely be greater than the total energy available on Earth. At least for the foreseeable future, time travel will remain the stuff of science fiction.

*Lawrence M. Krauss is Director of the Origins Project at Arizona State University. His most recent book is "The Greatest Story Ever Told. So Far: Why are We Here?" (Atria Books, 2017)*

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