Informal Time Travel
What do we mean when we say time travel? We don’t generally mean just floating along on the stream of time; we do that normally. Time travel as depicted in pop culture is often a matter of somehow stepping outside the normal time stream and stepping in at a different point.
Sometimes when discussing time travel will be brought up time dilation as a viable means. The idea is built on something called the twin paradox, which is based in Einstein’s special relativity. The name comes from the thought experiment most often used as an illustration. In this we imagine a pair of twins. One stays on earth, the other travels at speeds close to the speed of light for a few years. Upon returning to earth, he finds his brother having aged much more than he has, perhaps even died from old age.
To me, this is not really time travel as we normally think of it. This effect of time dilation is not special for very high speeds, just more noticeable. Meaning that we experience this sort of “time travel” all the time.
Also, this form of time travel only allows for trips into the future, so that’s a bummer too.
As the run of given sci-fi/supernatural TV show grows longer, the probability of its having a time travel episode increases. And when this occurs, more often than not time loops come into play, and paradoxes are introduced but happily ignored and unresolved.
(I disagree with Wikipedia on the terminology on this one. What it calls “bootstrap paradox” I call “stable time loop” and what it calls “grandfather paradox” I call “unstable time loop.” But at least someone agrees with me.)
The most commonly known paradox when it comes to time travel is the grandfather paradox. As with the twins one, it comes from the most common metaphor associated with the general case.
Imagine you travelled back in time to shoot your grandfather dead before his son your father was born. If successful – what would happen? If your father was never born, then how could you have been born to travel back and kill him?
It’s like going back to kill Hitler when he was still a child. If successful, then you would have been born in a world where this act would be unneeded, which would mean you wouldn’t go back, which would mean he would become a dictator, so you would go back…
It’s easy to see why this is called a paradox.
A bootstrap paradox can be a bit sneakier, even if it’s pretty much the same thing, only backwards. It’s when things are causing themselves to happen.
One example would be the Terminator franchise where robots from the future travel back in time to murder the mother of the human resistance leader (which would introduce a grandfather paradox) before he was born.
The twist here is that it turns out that the precursors to the killer robots are made from studying a piece of the robot from the first film.
So why is this a paradox? Well, consider the question: where did the knowledge of building the robots come from? It came from an already built robot, which came from the knowledge. So they are both each other’s causes and each other’s results. But this also means that both events are their own causes.
Because if we have the causality loop ABABABAB… we can see that B always follows from A, which means that the mere act of A implies B. So we could put them together to form the superevent C and we would have the chain CCCCCCC… which suggests that C happens because of C.
The Ageless Coin
It perhaps becomes more obvious if we imagine a physical object instead of information being transported back. Let’s say you were visited by yourself from next week and this future self gave you a coin. Next week, you take the same coin, and travel back to give it to yourself.
How old is the coin? And where did it come from?
An error many people make here is to look for some first coin, special from the others; the original one. That the loop started with you taking a coin from your wallet and travelling back. But if we accept that there was some initial condition subsequently altered, then it’s altered, meaning it never happened. The first cause would be like your grandfather shot by your gun.
So to avoid one paradox we must accept another.
And the coin would be infinite in age, since when you get it, it has experienced at least a week’s subjective time. But since your future self is the same as you, the coin that person received was already aged one week. So your coin is two weeks then? No. Because if it’s two weeks, then it’s two plus two weeks, as per the logic that gave us two weeks from one week. Next step is 4+4, then 8+8 and so on for infinity.
But some atoms from the coin surely fell off while you handled it during the week before you went back to yourself? And this happened forever, from the coin’s perspective. So it would no longer exist. And even if no atoms fell off, the half-life of the common proton may be very, very long, but it is not infinitely long, meaning that even handled with infinite care, the coin would disolve into radiation on an infinite time scale.
Traveller, Teach Thyself
Let’s take it all to its logical extreme. Let’s say you’re sitting around doing nothing much when suddenly your future self walks in the door, presenting you with the plans (not the machine itself; remember the coin!) for the very same time machine in which they travelled back.
It seems more absurd than robot arms, but it’s really the same thing. Information being its own originator. This seems to be the predestination paradox, which is just a special case of bootstrap, from what I can tell.
So sending information back in time to change the outcome of something seems impossible. What does this mean for fortune tellers? It seems a pointless question, but of course there is a paradox about this too, even if people are not agreeing as to whether it actually is a paradox at all.
But let’s say you could predict the future with 100% accuracy. This would in practicality mean the exact same thing as if you sent information back through time. So you could predict what you would think tomorrow, which would include the memories of your predictions made today.
Could you change anything? If you do, the grandfather paradox slams down on you. Changing the future because of what you know will happen would make that knowledge never having been.
But if you then can’t change anything, is it really a paradox anymore? I mean, it would fit neatly into a deterministic universe; things happen because of what happened before, no matter when it happened before.
So to speak.
I’ve always had a much easier time accepting the bootstrap paradox as possible than I’ve had the grandfather one. That may be why I prefer the terminology of stable vs. unstable time loops. Being stable implies to me that one should view events and their causes (which are also events) more like expressions in an equation than an incremental progression.
Because let’s face it; in a clockwork universe everything is predetermined, the future set in stone. And if you travel back in time, that’s still your future.