Hi, engineers!

Q) I’ve been thinking a lot about myself 10 years ago, and the myself of 10 years in the future. I’d really like for all of us to have a happy rendezvous. So… Time travel?

Well going forward in time is easy. You are currently going forward in time right now, although at a one-to-one ratio with everyone else. If you want to go forward faster, you’ve got a few options for how to do it.

You can fly very fast, as in near the speed of light fast. This will slow down time for you according to special relativity. (Thanks Einstein!) So if you flew to Alpha Centauri and back at like 95% the speed of light and the trip took you about 1.9 years, when you return to Earth everyone else would be 8.3 years older. You just traveled forward in time by 6.4 years!

You can get a really similar effect by hanging out near large gravity wells like a black holes, but this is probably more dangerous because if you got too close to the black hole you would super-die and get spaghettified (the official science term for what happens if you get too close to a black hole).

Going backwards in time is far, far more difficult. In fact it is probably impossible. Think about it, if time travel existed then there would be loads of time travelers everywhere right now because they would have all of the rest of the universe in time worth of mad scientists, time vacationers and killer Aryan robots who would probably love to visit the prehistoric times we live in now when human civilization was just getting its start. Not to mention that traveling backwards in time breaks causality (the idea that there is a cause for every effect).

Seeing back in time is easy. Look at the stars one night. They are all at least 4.2 light years away from us right now. This means that whatever you see them doing (or with a big enough telescope, what is happening on their planets), actually happened years ago and the light is just now reaching us. So if all of the stars blew up right now, we wouldn’t even know for another 4.2 years for the closest ones and hundreds of years for the more distant ones we can see. Even our sun is 8 light-minutes away so all that beautiful warmth and light from the sun is actually 8 minutes old.

Now that you have made it back in time, here is some stuff to watch out for: TIME PARADOXES! What if you tried to kill your grandfather? The instant you kill him, you would never have been born and therefore could not have gone back in time to kill him. However, then you would not have killed him, so you have been born again! Which means you can kill him again! Which means you were not born again! So that is something you want to avoid because that makes no sense. That is an extreme example, but there are plenty of things you could do that could create time paradoxes if you aren’t careful. Even doing something really minor could have a “Butterfly Effect” that could cause your parents to not meet, or you to not get your job at the time machine factory! It’s possible that causing one these paradoxes could cause the universe to explode or time to end or other terrible things.

There are a couple of theories about how the universe could handle these paradoxes, assuming it’s response is NOT to just blow up everything. One is the post-selected model of time, which says that the universe does not allow paradoxes to occur. In our example, your gun would jam or a bird would poop and you would slip or you would get hit by a car, whatever it would take to make sure that you do not kill your grandfather and thereby cause a paradox. Another option is that every time you go back in time and cause one like that, you simply split off a parallel dimension in which he died, so in your “home” universe he would be fine and you could return like normal, but you would have just created a whole universe in which the only difference is that your grandfather died!

Confused yet? Good, I hope this taught you to never, ever, ever go back in time. It’s probably not possible, but even if it is don’t do it! If you want more proof that you will never get a time machine in the universe, try making a promise to yourself that if you ever get a time machine you will come back and visit yourself at a specific time and place about five minutes from now. Memorize this location, write down the time in every calendar and leave sticky notes all inside your fridge (you know you won’t find those for a couple years). Probably nothing will happen and you will feel silly, and will have learned that time travel is impossible (or heavily regulated by the Time Police).