As I am prone to do, I spent an inordinate amount of time over the weekend considering the concept of Time Travel. Central to any and all discussions of time travel, once you’ve accepted the possibility of it, is the paradox.
The most common of which can be described simply: you build a time machine, go back in time to before you built it and stop yourself from building it.
If you stop yourself from building a time machine then you can’t have stopped yourself from building it, so you will build it and can then go stop yourself… It’s confusing to even talk about because impossible logical loops make most people’s brains hurt. You’ve heard this before, though probably more in the “go back and kill your grandfather” mode where you actually prevent yourself from being born and then wouldn’t be alive to travel back and prevent yourself from being born which means you’d be alive, and so on…
So, taking that into account, there exists only two kinds of time travel that are logically possible.
The Closed Loop
In the closed loop, you can’t change anything. If you were to go back in time to try to prevent yourself from building the time machine, you will, in some fashion, fail at your task. In fact, unless you recall someone trying to stop you when you were building your time machine, not only will you fail, you won’t even get to try.
Going with the grandfather example. Your grandfather tells you a story about this one time he was almost hit by a car, but he jumped clear, the car swerved, drove off an embankment and the driver died in a fiery wreck. When you are older, you build a time machine and on your journey you decide to visit your grandfather as a young man, you see him on the street, lose control of the car you are driving, almost hit him but he dives clear, the car swerves and drives over an embankment and you die in a fiery wreck.
What makes the closed loop interesting as a story telling device is that no matter how much evidence you provide that things cannot be changed, the reader, along with your protagonist, will fight you and insist that it can be changed. The challenge of using a closed loop is craft the story in such a fashion that even though the inevitable inevitably happens and nothing changes, nothing changing turns out to be what needed to happen to get the resolution the story demands. The book and movie The Time Traveler’s Wife actually handles the closed loop very well. It manages to tell a compelling love story while both characters experience it in different orders, and even with every event being unchangeable the expectations of the audience is twisted to keep elements of it surprising. In fact, the only real sticking point it has (the lottery ticket) is handle well enough that it still fits within a closed loop design.
The Parallel Reality
In this kind of time travel, you can change things, but by doing so you create a separate reality. You build a time machine, you travel back and then you successfully prevent yourself from building a time machine… but you still have the one you built, so you hop back in and return to your own time, two weeks from this moment, only to discover you are now at the correct time, but in a world where you didn’t invent the time machine. You are an anomaly, because the other you, the one without a time machine, is still hanging around – he doesn’t have a time machine to get into a vanish with unless he steals yours.
Meanwhile, back in the world that you left after building your time machine, you’ve gone missing. Once people notice, police reports are filed and searches are made, and eventually you become a segment of a TV show like Unsolved Mysteries about a man who vanished without a trace.
Here is where it gets complicated. You decide you want to go home, so you travel back in time again and prevent yourself from preventing yourself from building the time machine. Assuming that in your original timeline there were no attempts at all by other versions of yourself to stop you, that you are the “prime” reality, the world you are in right now still isn’t home. While the you in this reality has just built a time machine and sailed off into history to stop himself from building a time machine, you are standing in a world where not only do you (you prime) exist, but there is also an unconscious you (only because we assume that you didn’t kill the other you to stop him from stopping you) sitting on the ground. There are two anomaly yous (and third you who just vanished into history is about to go create more yous). The end result of these actions is that there will exist worlds without you and worlds with two yous (unless you do actually kill you to stop you, you bastard), on into infinity until you decide to stop trying to stop yourself.
This form of time travel intrigues me because I like the idea that you can’t change your own past, but you can change the past of another version of you. Imagine if you were building your time machine so you could prevent the death of a loved one. When you leave your own time, the world continues on with the loved one still dead and now with you missing, but in a parallel reality you save the loved one, everything is different, except you, who knows the loss parallel you will never know, and he’s knowing non-loss that you can only observe (unless you kill parallel you and take his place).
Of course, there are more theories, but most of those require mental gymnastics or forgiveness of giant flaws that make them feel of much lesser quality that the two I’ve illustrated. Most commonly used is the “reality fixes itself” idea, that the world changes around you when you alter the past, but it only does this by pretending that paradoxes don’t exists. You stop yourself from building a time machine and your machine vanishes and you are suddenly you again, only now you didn’t build a time machine… but do you remember doing it? If you don’t, why wouldn’t you just do it again? Or are we accepting that whatever you did to stop yourself was enough to convince you to never even attempt it? If you do remember it, are you just going to sit on that knowledge? When things go wrong in the future, why not just build that time machine and go fix it?
As much as I love the Back To The Future trilogy, if I spend too much time thinking about them, I get irritated. By the end, if Doc Brown is a commended scientist, respected in his work, then would he have been building a time machine using plutonium he has to swindle from Libyan terrorists? If Marty’s family is well off, he has the girl and the truck and his band is doing well, would he be hanging around Doc Brown at all? They even go to the extent of explaining in the second film that since the future has been altered they can’t go forward to fix time, they need to go back to prevent the divergence. Only before they go back and fix the divergence, wouldn’t there have a been a Marty that went to 1955 from the first film who returned to 1985 AFTER old Biff from the future had changed things but BEFORE having gone to the future himself? There would have to be, since old Biff gives young Biff the book and alters time BEFORE Marty from the first film leaves 1955. That said, in the future, after old Biff returns… wait… how did he return to a future that doesn’t happen anymore? He changed it by going back and giving himself the book. Old Biff should have gone to the future of alternate 1985, not the future of original 1985. And how did Marty get to alternate 1985 from the future anyway? Wasn’t he in the original timeline? They leave Jennifer on a porch in alternate 1985 because “reality will change around her”, but then why didn’t reality change around them in the future after old Biff dropped the book off to young Biff? I’m going to stop now. BTTF is great fun. I’m going to go back to enjoying it now and not analyzing it.
Most times it’s best just to avoid time travel altogether. And yet, I am fascinated by the concept and keep trying, unsuccessfully thus far, to craft a time travel tale. Someday…