the science fiction genre in books and film have long written and shown about the possibilities of time travel and the scenarios that may or may not come with them.  they have nonsensical theories about how you could go murder your parents and be your own grandparents (i’m looking at you heinlein), or you could alter photographs and erase memories a la back to the future, or go see what your future holds in store for you, then go back in time and alter it to favour yourself and then get stuck in infinite time loops and paradoxes.

or not.

discover magazine gets into the nitty gritty of the actual facts behind it all and gets into what actually would be possible if and when we were able to travel back and forth in time. i’ll highlight a couple of points below, click on through to the article to read the rest of the descriptions.

0. There are no paradoxes.

This is the overarching rule, to which all other rules are subservient. It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won’t happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.

1. Traveling into the future is easy.

We travel into the future all the time, at a fixed rate: one second per second. Stick around, you’ll be in the future soon enough. You can even get there faster than usual, by decreasing the amount of time you experience elapsing with respect to the rest of the world — either by low-tech ways like freezing yourself, or by taking advantage of the laws of special relativity and zipping around near the speed of light. (Remember we’re talking about what is possible according to the laws of physics here, not what is plausible or technologically feasible.) It’s coming back that’s hard.

2. Traveling into the past is hard — but maybe not impossible.

3. Traveling through time is like traveling through space.

4. Things that travel together, age together.

5. Black holes are not time machines.

6. If something happened, it happened.

7. There is no meta-time.

8. You can’t travel back to before the time machine was built.

9. Unless you go to a parallel universe.

10. And even then, your old universe is still there.