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.