Final Word on All Manners of Travel

In this post I will discuss how Einstein's relativity essentially destroys two sci-fi plot elements - time travel and teleportation. In doing this, I will also demonstrate how relativity makes teleportation a subset of time travel (or vice-versa!). Essentially, I propose Augy's First law:

"It would be impossible to tell a teleported person from a timeshifted person since the two phenomena are identical."

I recall having amusing discussions about time travel in high school during physics class. While being convinced that going in the past was impossible and no paradoxes can occur even then, I looked to the future full of optimism that sometime I would know enough about the world to have a clear, scientific basis for my intuitions. Now, it seems, I have that knowledge, but I do not feel even slightly more knowledgeable.

Beware of the man who works hard to learn something, learns it, and finds himself no wiser than before... He is full of murderous resentment of people who are ignorant without having come by their ignorance the hard way by Vonnegut is apt.

Time travel's a fun concept, but in our universe there can be no "time machines". Let me explain why the science ends up telling us that.

To fully grasp this and know that I am not making this up, you need only to familiarize yourself with two sources (or you can just take my word for it):

1) Krauss' lecture on our universe, a universe from nothing;

2) Brian Cox and Jeff Forshaw's book Why does E=mc²?

In those two is enough information to explain why time flow as we perceive it is stuck at going forward and why it is impossible to travel either back in time or into the future by means other than waiting.

First off, Einstein made the idea of time travel obsolete with his theory of special relativity back in 1905 by proposing that space and time made a compound, inseparable thingy - spacetime. Few caught on though. You can't travel just in time or just in space, because there is no time and space - there is only spacetime. When you get in a car and drive to work, you move from (relativistic) point A in spacetime (say, 8:15 AM, your home) to (relativistic) point B (8:45, work). You have traveled, inescapably, in both time and space.

Traveling in space without traveling in time a-la instantaneous teleportation would essentially mean that you have traveled at infinite speed (OK, I know the math actually renders it as undefined, but you can see it approaching infinity as the denominator approaches zero), certainly at a speed exceeding the speed of light c, the maximum speed possible in our universe, which is impossible. Another way to explain this is by saying: "Teleportation would mean going outside the light cone in Minkowsi spacetime, which is impossible."

"But what about standing still, yet traveling forward in time, Augy?" you may ask. Here is where E=mc² comes in. Turns out when you sit comfortably in a chair, supposedly unmoving (at least relative to the chair; relative to the Sun you and the chair are hurdling through curved spacetime along with Earth at at least 30 kilometers per second), you are in fact hurdling through spacetime at full throttle, c. In fact, the only way to move in time slower than c is to move in space, but even then the effect would be visible to observers that are slower relative to you (from your perspective, you might as well think yourself stationary and everything else moving slower in time than you) and it would still be 99% as fast as usual in speeds achievable for us. Only very close to the speed of light does time dilation become considerable).

So far so good. But what about traveling in the past? Here are two concerns - energy and causality, misinterpretation of which leads to the famed "what if I travel back in time and prevent myself from being born?" This is where Krauss comes in. Turns out we live in a universe that started from nothing, with a total energy of zero. That also means that, remembering the law of conservation of energy (or the more modern Noether's theorem  and Einstein's energy-momentum 4-vector), no matter how long the universe lasts, its total energy will remain zero. So zero energy and causality. Forget sending a whole time machine and a person back in time, be modest, start with a single atom. Sending even a single atom back in time would violate both energy conservation law and causality - it would have added the energy value of the atom to the universe and messed up causality by introducing the gravity from this atom in the universe - you would have made the atom pull itself away from where it was supposed to be in the future (albeit extremely slightly).

"Aha, perhaps there is a way out!" you might think. Two ideas might come to your mind - parallel universes and information travel, used in two (very enjoyable) movies - Deja Vu with Denzel Washington and Source Code with Jake Gyllenhaal respectively. And, no, they would not work, probably. I get speculative here, assuming universes besides ours share consistency of fundamental laws like ours does. The parallel universe-injection would not work, because, most likely, any universe that exists, is also a zero energy universe and same as you can't introduce duplicate atoms by traveling in the past, you may not inject matter and energy (and information for that matter, no pun intended) in another universe. Information injection, while a trickier and fresher perspective, is also untenable. In popular culture there seems to be an idea that information can exist separate from a medium (the quackery of torsion fields comes to mind). "We aren't gonna send atoms in time, but just information!" they would say. The fact is, that whatever is meant by information, will have to have physical consequences once it is sent (change in particle direction, spin, etc.) and the physical influence of this information would, again, face the same critique as traveling in back in time would.

Here is the ultimate possibility and why it will not work. One might send just information to a parallel universe that already is the way one might want. An example - you want to travel back in time and kill Hitler. Chances are there is a universe in which, naturally, a person popped into existence from seemingly nothing, (delusionally) believing to be a time traveler from another dimension and killed Hitler. You'd have no way to prove otherwise.

There is also no way to prove that there has ever been a past. The universe might have come into existence just now, with all the galaxies, background radiation and memories in everyone's mind. Unlikely, but it's possible.

True story.