Letter About Time Travel

image originally by Jaime Mendelbaum, jaimemandelbaum.com

I got this strange letter in the mail a week ago. I'm pretty sure it wasn't intended for me — or anyone who lives with me — but I can't be sure. The letter seems pretty sensible except for... well, a few things.

Hello Jonny,

This is D██! You wrote to me in March wanting to buy a Flux Capacitor. I really enjoyed reading your letter and I'm glad you liked my project so much. I only have one, but I'd love to tell you how you can make your own. In fact, I've already written instructions. Your parents can help you find them online. But I may have some bad news, too.

I'm afraid my Flux Capacitor might not be what you expect. You see, mine was made from an LED kit. It has flashing lights just like in the movie and looks a lot like that one, but it's pretty much only a toy. And (although we can never be 100% sure) I think even the one in the movies is a toy too. Toys can be great fun, I'm sure you agree, but they can't really take you back in time like you ask.

But wait! Don't give up hope! The fact that even grownups like me enjoy pretend time travel proves that it's an interesting idea. Lots of people have come up with ideas about how someone might time travel for real. No one has done it yet (that I know of), but that means that you could be the first! I'll give you a couple places to start. Feel free to find other ideas or even come up with your own!

People pretty much think time travel might happen in two places: either very small or very big. Einstein the dog was named after Albert Einstein the scientist. Being the smart guy that he was, Einstein figured out something called "General Relativity." Doc Brown talks about it in the movies. General Relativity has some good guesses for questions about how the world works, like: what happens when big things get really big, how fast can the fastest things go, and stuff like that. Some people think that if we were to go faster than light we would move backward instead of forward through time. But light is really, really fast, much faster than 88 mph, and no one knows yet how we might go that fast. One of the only possible ways people have come up with is to go near really big things in space, like black holes, and try to use their energy to swing around backwards... or even pop through them! If this sounds hard, that's because it is. But remember: hard doesn't mean impossible.

That's the very big, now for the very small. Anything you see in the world could be broken down into smaller pieces, and those pieces even smaller... so how small can they get? Scientists called "physicists" do experiments to figure out stuff like that, and the answer they found is: very VERY small... and things so small act very strange. They have crazy names like quarks, fermions, neutrinos, baryons, excitons, or positrons. Some (like anti-mesons and tachyons) might go faster than light. In some experiments, small things seem to get where they are going before they leave where they left from! Sounds like time travel to me. Other small things can move together the exact same way at the same time, totally disconnected, no matter how far apart they are. Weird! Maybe lots of things move together like this, but we don't notice because they move at different times. All this strange science of small is called "Quantum Mechanics" — very complicated stuff, but you can look it up if you'd like.

Even if we can get time travel to work, it might still be tricky. You saw an example in the first movie: if Marty stops his parents from meeting, they will never have him as their kid... but then he would never be able to go back in time and stop them from meeting! How confusing that must be. This is called a "causality paradox." Causality is when one thing makes another thing happen (I drop an apple, then it hits the ground... I caused it to hit the ground). A paradox is when something seems possible but does not make sense (I can say "I am telling a lie" but if I am actually lying, then what I told you was true, so I did not lie... Great Scott!). Some people think that the possibility of causality paradoxes proves that time travel is impossible. Other people think that causality paradoxes are impossible, and that if you went into the past you could not change things even if you tried (that's called the "Novikov Self-consistecy Principle"). Other people think that if you ever changed causality in the past, you would just make a new timeline, like what happened with Biff's Almanac. There might be many different timelines, also called parallel universes… people call that idea the "Many Worlds Hypothesis."

There is another possibility, which is that causality might go more than one-way. Could something in the future cause something in the present? It sounds crazy, but no one can prove it's impossible. Scientists call this idea "Retrocausality" and they are pretty sure that even if you could do it, you couldn't send back things like lottery numbers. If you could we'd all be millionaires by now! There are ways we might send some things back, but they can be very tricky. Even though not many people think about Retrocausality when they think about time travel, it seems a lot more practical than Quantum Mechanics or General Relativity. In fact, it's my personal favorite time-travelling method! I might not see any dinosaurs but I have a lot of fun anyways.

Well, that's about all I have to say. I hope I have given you lots of interesting things to think and read and dream about. Let your parents know which parts you liked best. Sorry about the Flux Capacitor... maybe it was the wrong time (ha! little time travel joke there for you). Even though that way can't work, now you know many other ways you can try. That's all scientists do, you know... we try things that don't work until we try something that does work.

Whatever it is you try to do, don't give up trying!

yours in friendship,

D██ M████
April 8, 2016