The film *Back To The Future *explored the idea, as did Stephen King’s moving novel *11/22/63*. H.G. Wells probably did the most to fire public imagination over the idea with his 1895 novel, *The Time Machine*. And there have been plenty of other efforts too.

The idea being, as you’ve no doubt guessed, time travel. Few other themes can and do inspire as much romance and speculation. And while going into the future has its attractions, the past arguably raises the really interesting questions. What would happen if you could travel back in time? To when in the past would you choose to go? How would you explain your presence to those you meet? How might you affect the course of history?

That’s not so grandiose a question as you might imagine, actually. Think of going back in time to that day in 1948, for example. To Birla House in Delhi, venue of Mohandas Gandhi’s last prayer meeting. You know what’s about to happen, of course—that comes with the privilege of actually having lived several years in the future. So let’s say you’re wandering through the crowd, looking for the young man you know is intent on killing an old man this evening. You find him just as he is about to shoot. You grab his arm, so the bullets arc harmlessly into the air instead of slamming point-blank into Gandhi’s chest. What have you done to the way modern Indian history plays out?

Fascinating what-ifs all round. What’s also interesting is that they assume that time travel is possible, and that we’ve figured out a mechanism to do it. Yet the truth is that we don’t know if it is possible; and if it is, how we’d do it. Yet too, answering those questions can be at least as fascinating as the what-ifs.

In a sense, we’ve known for about a century that time travel is at least theoretically possible—ever since Einstein, in his theory of relativity, introduced us to the concept of “spacetime”.

Now we all understand space—because we visibly and tangibly live it all the time (pun intended). For example, I see the home of a famous cricketer across the street. I can estimate how far it is—10m to his gate?—walk across, take my pouting selfie and then walk back. Similarly, you see ripe mangoes in the tree above and estimate how far they are from your grasping hands. Though this time, you quickly conclude that you cannot jump that high to grab one. Or if someone asks you the way to the library, you will easily direct her: Make a U-turn, go back the way you came and take the third right, pass the tennis courts and the library is the next building on the left.

In much the same way, we understand time. Five days since I last did any useful work; 10 years since I lost my father; an hour before I need to set out on my morning walk.

The point is, we are so familiar with these themes that we don’t even think about them. But notice that one of them is reversible, but the other one is not. I can walk the 10m across the street—and then walk the same 10m back. I can spend the next 10 minutes twiddling my thumbs—but whatever I do, I can never have those 10 minutes back. Space extends in every direction, but time is unidirectional. Inexorably, unstoppably, it marches forward.

This is why humans have always thought of space and time as fundamentally different ideas. It was Einstein who suggested that they are really the same—and if we treat them that way, we can explain all kinds of phenomena. That is, what we are visibly and tangibly living all the time is not simply space, but Einstein’s “spacetime”, in which time is just one more dimension. We measure it just like we measure space—only, the units are different (centimetres and metres, versus seconds and weeks). But it’s profounder still than that. In a previous column here (“The Waves Come Ashore”, 11 February 2016; *goo.gl/fuMPZ0*), I wrote these lines:

“Einstein suggested that space and time, considered together, constitute the fabric of our universe. That is, celestial objects don’t simply exist in some mysterious vacuum. They actually rest in the fabric, the continuum, of spacetime. [And thus cause depressions in that continuum.]”

“Einstein’s remarkable insight was that this is how gravity works: that in their paths through the universe, objects simply follow the curves and depressions of spacetime—curves that are caused by other objects.... Spacetime is an idea, a way of thinking—but very real, and with deep consequences.”

So the Moon orbits Earth because it is following the depression Earth makes in spacetime; just as Earth follows the much greater depression the Sun makes. It’s easy to understand that the Moon orbits Earth and returns to where it started; just like I can walk around my sleeping cat and return to where I started. But now consider this: If time is no different from space, the Moon’s orbit could be in time as well. That is, it should be possible for the Moon to return not just to where it started, but to when it started.

It’s this simple: Spacetime implies the possibility of time travel. How it would happen is a question for another time. Still, Ben Tippett, a mathematician at the University of British Columbia, was in the news a few weeks ago because he came up with a mathematical model for a time machine.

The mathematics is beyond me, of course. But Tippett really started with this notion of spacetime and went forward from there.

Went into the future from there, you might say.

*Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter Of Numbers explores the joy of mathematics, with occasional forays into other sciences.*

*Comments are welcome at dilip@livemint.com. Read Dilip’s* Mint *columns at *www.livemint.com/dilipdsouza