You likely don’t realize it, but when you’re done reading this article, you will have traveled perhaps 90 seconds into the future. The truth is that it is easier, theoretically speaking, to travel forward in time than it is to travel backward, and that’s partly because we’re all moving forward in time naturally.

The possibility of time travel stems from Albert Einstein’s theory of special relativity, which, loosely speaking, describes the relationship between space and time. An outgrowth is something known as “time dilation,” which suggests that time can move at different rates for different observers—and therefore at different rates in different places. This theory is borne out by the (rather freaky) fact that clocks on the space shuttle—whether internal clocks or atomic clocks placed aboard for experimental purposes—run more slowly than reference clocks on Earth. In this sense, astronauts on extended missions may already be considered time travelers, as they arrive home very slightly later than the elapsed time measured on their own instruments would suggest. Moreover, “Time beats faster on the moon than on Earth, and time beats slower on Jupiter,” says celebrated physicist Michio Kaku of the City College of New York. “So if you were to simply camp out on the moon or Jupiter, you’d be going backward and forward in time. Now, of course, these are for fractions of a second.”

A more useful implication of time dilation is the fact that the closer to the speed of light you’re moving, the slower your internal clock will be ticking relative to time on Earth. “If you reach 99 percent of the speed of light and spend like a year moving at that speed—around the solar system, say—and then come back to Earth, you will find that the Earth has moved on, 100 to 200 years into the future,” says Dr. Ulvi Yurtsever, coauthor of a seminal paper on time travel. So, in theory, if we could improve propulsion systems enough, we could skip ahead centuries. But we still couldn’t move backward.

Indeed, backward time travel, while theoretically possible, is far trickier and would involve black holes and “tunable wormholes” and more energy than a kindergarten class on a sugar binge. “You can write down solutions of the equations,” says Clifford V. Johnson, a professor of theoretical physics at USC, “and those equations tell you two things: how you twist up space and time, and what matter you need to do that. And every time you get those weird twists in space and time that look like a time machine, the matter and energy you need to do that is in a form that may not exist in this universe. So that’s just a fancy way of saying that the jury is out.”

So from a technical standpoint, it seems far more likely that we’d move forward in time first. But how about from an ethical one? “Scientists and physicists may say ‘You know what, it’s much safer for us to go to the future, because if it’s possible to alter the past and therefore have that reverberate into the present—create a paradox—that’s pretty dangerous,’ ” says Bob Gale, who has spent some time thinking about this stuff, given that he cowrote the 1985 time-travel blockbuster Back to the Future. “So they would say ‘Well, to preserve the sanctity of the space-time continuum, we better go into the future, because that provides the least amount of risk.’ ”

Ryan North, author of the recently published book , argues that by going backward, we could advance society hundreds of thousands of years. Even so, North muses that if we had a single shot at round-trip time travel, the better choice might be to plunge ahead. “Download the equivalent of Wikipedia onto a thumb drive and bring it back,” he says. “I’m all about stealing ideas from the future.”

So there you have it: 25th century, here we come.

Do you have unusual questions about how things work and why stuff happens? This is the place to ask them. Don’t be afraid. Nobody will laugh at you here. Email ­greatunknowns@popularmechanics.com.

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