Bill Nye takes on the sticky science of time travel as imagined by science-fiction writers. While we don’t have any solid leads on a time machine, Japanese physicists have shown that time travel is theoretically possible if you travel in a ship at the speed of light. There are two restrictions: (1) You could only travel back in time to the point at which the ship you’re traveling in was created and (2) the amount of energy required to power the machine, enough to get the craft to the speed of light, could stretch your body such that, well, it’s not a happy ending, says Nye.

Emre: Hi Bill Nye. My name is Emre. I thought of a time machine that basically ran on all the data based on your brain, so it basically calculated all the information on your brain and all the data in your life and it would create a vortex, which you would go back through and you would go back in time, so it's a time machine. Now I was thinking that if you went back once you would have the memory of going back so that would create another vortex in which you would go through again and once you got to the dimension, which was in the past, the other time machine, with your memory, would, because you're the person, it would also form another vortex, which you'd go through and you keep going through infinite vortexes until you were lost and come in infinite space and time. Now, my question is would it be possible to generate enough electricity to be able to create another vortex, which would accelerate time and bring you and generate enough power to bring you back, all the way back to your time and keep it stable with the time machine and you? So could you please answer that?

Bill Nye: Well, of course. It's just a very straightforward question. What? So one of the old problems, Emre? Is that right? Emre, one of the old problems, in all the science-fiction stories that involve time travel is can you go back in time and change things so that you didn't exist? Can you go back in time and change things so that by the time it's now, you're fabulously wealthy? Can you go back in time and change things so that the world is completely different and better? Or can you go back in time and accidentally screw everything up? These are old questions in science fiction. And I got to say so far no, you can't. However, recently, by recently in the last 10 years, some physicists, and the way I remember this — these guys were in Japan — showed that you could build a time machine if you are allowed to travel near the speed of light in this time machine in a big circle and you could only go back in time to when the machine was built; this is in theoretical physics. However, in order to go faster or as close to the speed of light you need enormous amounts of energy and it might stretch you where your head is being accelerated more than your feet and it would kill you maybe. These are great questions. And I noticed you use the word vortex a lot. A vortex is a whirlpool like; the center of a hurricane is a vortex and when the water goes down the drain in the bathtub, that's a vortex. And there is a fabulous thing I hope you learn about in a whole discipline called fluid mechanics, the mechanisms by which fluid flows, called vorticity.

Vorticity is a fabulous word. So I appreciate you throwing around the word vortex, but I'm not sure it's exactly what you mean when you're talking about time travel. But think about it, can you go back in time and change things so that you never existed? It's an old question in science fiction and it's cool, but as far as anybody knows, time only goes one way. And as far as anybody knows, nobody knows why that is. Going back in time is really hard and it's related to this thing that has to do with heat. Heat always spreads out. Heat never spontaneously concentrates on its own; by spontaneous I mean on its own without influence. In the example I'll give you, go all over the world, look at everything you're ever everything to look at; look at a glass of water; look at a lake; look at a pond; they never just freeze in the summertime. It never happens. They heat doesn't go down to the bottom of the pond or up into the atmosphere and then freezes on its own. Energy always spreads out and when something spreads out, it's moving over a distance over time. So time and the spreading out of energy are intimately connected or closely related and there's something about that that keeps us from building time machines. Carry on.