I am a time traveller. It’s fun, I like it, and I can’t imagine life without it. Unfortunately I don’t have much control over the travelling, so it only goes in one direction and at one rate. This is probably a good thing for world stability.
See, time travel isn’t just another ability. You thought Superman was overpowered? If you have unrestricted access to time travel, you won. The limiting factor on what you can do isn’t how much you’re capable of; it’s how long before you get bored.
Disclaimer: It’s true that closed timelike curves are not forbidden by relativity, but that’s a far cry from saying that they’re actually possible, let alone that they actually exist. Invisible flying unicorns are also not forbidden by relativity. However, this does mean that we can say some things about how the loops work if they do exist.
The basic rule is that paradoxes don’t happen. It’s a rule imported from mathematics: contradictions have probability zero. (Applied to time travel, this is called the Novikov Self-Consistency Principle and is one of my favorite things.) In practice, this means you can’t go back in time and kill your grandfather to prevent your own birth. Neither can you prevent yourself from gaining access to time travel, or cause any other contradiction. This sounds like it’s a pretty restrictive restriction, but actually it just means you have to be clever. Let’s start with the basic-level stuff. You have a problem and a time machine and you want a solution. Maybe you locked your keys in your car. How do you use your time machine to get them out?
The Novikov Principle says that you can’t just remind yourself not to lock the car, but there are still a bunch of ways in. One obvious one is to jump back to before you parked your car, then move to where it is going to be, then jump forward and end up inside it. Done. A slightly more interesting way is to simply decide that when you get your keys you will drop them off with your past self. Then do that. From your point of view, here’s what you see: Someone who looks exactly like you appears, gives you the keys, disappears. You go into the car, find an identical set of keys, and leave. Then you go back in time and hand the first set to someone who looks exactly like you. If you’re uncomfortable dealing with your past self, you can always just pick a location to hide the keys, and then look there. You can then leave them there earlier without having to meet yourself.
This is not that hard. If you decide in advance what you’re going to do, all you have to do is watch your future self do it and then go do it afterward while your past self watches. Actually you don’t even have to watch it; you can just take advantage of the preparation and then go prepare later. It’s a pretty powerful ability.
This can singlehandedly solve the vast majority of problems, and people in fiction really should do it more often. Think of a case where the Doctor is watching helplessly while Rose is cornered by a Dalek. The thing that happens next is inexplicably not a slightly older Doctor materializing his TARDIS around her. Realistically, fiction writers can’t do that because it would neutralize pretty much all the plots. That’s right; it’s banned for being too effective. Which is an excellent reason why you should use it all the time if you ever stumble upon a closed timelike curve.
Some tricks are more complicated. You can use the Novikov Self-Consistency Principle to solve your problem even if you can’t just plan what your future self is going to do.
Let’s say I’ve written down an integer between one and one thousand, and you want to guess what it is. You receive from the future a piece of paper containing a random number, and if they don’t match then you increase the number you received by one and send that back. That is of course a contradiction: You can’t receive a different message then you sent. And since they can’t be different numbers, you must have been right the first time. Lucky you.
Did you follow that? You’re basically blackmailing the universe: Do what I want or I cause a paradox. Since paradoxes are impossible, the thing you want happens.
Unfortunately, if you try this it won’t actually work. It might, or it might not, depending on how unlikely the event you want is. Here’s why: Let’s say you get a number from the Time Travel Express (Time Travel Express: for when you need it there yesterday!). And let’s say it’s not the right number. Are you really going to send a wrong number back, intentionally trying to cause a contradiction? The reason it won’t necessarily work is because all we know is that the same number gets sent and received. That could be because you read or wrote it wrong, it could be because it got changed in transit somehow, or it could be because you decided not to go through with the contradiction. Chicken. If you’re trying to guess one number out of a thousand, then any one of those possibilities could be more likely than guessing right the first time.
The standard solution here is to use a computer program, telling it to use the number that comes from the future as the input and send the output to the past. That gets rid of the risk of you backing out of causing a contradiction, since the computer can’t decide against it. Then it’s just a case of which is more likely between computer error and guessing right. Get a good enough computer and you’re good. (Note: I don’t actually know anything about computers. Consult someone who does before trying this.) There aren’t many uses for this where you couldn’t just watch your future self do it, but if you get this setup to work, it can basically solve any computational problem right away. You’re welcome.
Between these methods, you pretty much win at everything. You can always see everything coming. No matter how specialized, you can always just happen to have exactly the right knowledge prepared or item hidden right when and where you want it. Anything that is remotely possible to accomplish if handled right becomes trivially easy. And if you’re clever, you can do even better.
Let’s change history. Suppose that your cat disappeared last week, and you want him back. This is tricky because if you go back in time and save your cat, then you never had any reason to make the trip. This is the standard “grandfather paradox,” except instead of killing grandfathers I’m rescuing kittens. Aren’t I nice? Anyway, this is exactly the kind of thing that the universe doesn’t allow. But that’s fine, because you’re smarter than the universe and you can do this without a paradox. The simplest way is to rescue the animal and bring him to the future. You spent a bit of time catless between his assumed demise and his sudden reappearance, but that cat is definitely alive and not dead.
It’s harder if you have more information about the event you’re faking. Unexplained disappearance is easy, but maybe you heard from a friend that your beloved pet is not only merely dead but really most sincerely dead. That’s still easy; you just have to go back and explain the situation to the friend or otherwise convince them to tell you (past you) that your cat died. And of course you should know exactly what to say because you’ve hopefully already asked your future self. If you saw the body yourself, it gets even harder. The general theme is that whatever you remember seeing, you have to find some way to fake it. And it has to be good enough to fool your past self. Remember that all of history is a fixed point. You can’t just change something; you have to fake the old version well enough to convince all the witnesses (especially you) or you risk a paradox.
After a bit of time with a time machine, you’ll get in the habit of consciously avoiding information about anything bad. Ordinarily this doesn’t help, but if you have a time machine it’ll make it easier to fix afterward. If the thing to be fixed is extreme enough, you might even want to go to the effort of setting up an enormous conspiracy whereby your younger self can be raised to believe that history was a lot worse than it really was. Then when you get a time machine, figure out about the conspiracy you formed, and learn that everything you know about history was wrong, all of history is fair game to be changed for the better. This is pretty much the only way I can think of to change major well-documented events. It’s also extremely difficult.
Of course, you don’t get credit for having changed anything, because it has always been this way. Imagine if someone in real life claimed to have fixed English history by capturing and removing the terrible tyrant King Edward V when he was twelve and hadn’t yet become a tyrant. You’d think this person was crazy, because you know perfectly well that there never was a tyrannical King Edward V. Even if time machines are easily available, you know perfectly well that history can’t be changed. But this Edward removal is exactly the kind of thing you’d be doing. If your goal is to avoid looking like a crackpot then you’re doing it wrong.
Time travel can be a dangerous business, especially if you’re going around using it to stop wars or dethrone kings. But with the proper precautions, you’ve got tools at your disposal to make sure everything works out.
Try going back and meeting yourself at regular intervals. Say every month. Aside from giving you a chance to pass back useful information, this verifies that you will be alive and time-travel capable for the next month.
If your future self ever misses an appointment, panic. This tells you that something interferes with your schedule, and that’s bad. You should have some contingency plan set up where if you miss one of the monthly checks, you go back daily for the rest of the month, or as long as you can. On one of those days your future self will fail to show up, and that will tell you that the interference happened within 24 hours of that. Spend that day in a hidden bunker somewhere with your machine safely locked in with you. It will then turn out that the reason you missed that appointment was…because you locked yourself in a bunker and didn’t time travel. This is kind of annoying and paranoid, but if you’ve been given advance warning that you’re in danger, it’s probably justified.
But be careful of how much you use it. Not because you’ll age faster (hopefully people in The Future come up with some way of fighting aging at some point, and you can borrow that). Just because it’s an easy addiction to develop. I’m a time traveller, and I can’t even get through the day without it.