Good news for Doctor Who fans, bad news for those who want to kill Hitler.
Don't go back with the sole intention of changing the past. No matter how much you would like to kill Hitler, it won't work.
It's a common storyline in science fiction: A time traveller goes back in time, commits some action like sneezing on a dinosaur or killing Hitler, then returns to see that due to their actions, reality in their own time is entirely different – like birds don’t exist because the dinosaurs they were meant to evolve from died out in a flu epidemic, or everyone speaks German.
But – bad news, guys – your dream of assassinating Hitler is not logically possible in a single timeline. The late, and great, Princeton philosopher David Lewis argued that for time travel to exist in a single-history world (that is, not across several different possible/parallel universes), we must adhere to a block theory of time. So, if time travel will be physically possible in the future, then we may have already been visited by a time traveller (theoretically). In a block theory of time, the future and the past are equally true. Basically, anything you, the time traveller, will go on to do when you travel back in time would have already happened before you actually stepped into the time machine. Therefore, in a single timeline, you cannot change the past. Which sucks, because lots of bad things happened in the past.
Universal Pictures
Also, don't go back with the intention of killing your grandfather, because you will fail.
Your friends may laugh at your plans to travel back in time. They may say "Why don't you go to Thailand during your gap year with us, instead of 1933?"
Then being the good, albeit sceptical, friends that they are, they will try to dissuade you from travelling to 1933 by reminding you of the grandfather paradox, which is basically this:
You decide to travel back in time and assassinate Hitler. Unbeknown to you, prior to meeting your grandmother, your grandfather was living in Berlin in 1933 and had a fondness for unusual moustache shapes. Mistaking him for Hitler, you shoot your grandfather, in a time before he has any children. So that means your father or mother would never have been born, and in turn you would never have been born. But that would mean you would never go back in time and kill your grandfather in the first place. So your grandfather would meet your grandmother, your parent would be born, you’d be born, and you’d be able to kill your grandfather. So you cannot logically kill your grandfather (just like how it is logically impossible to change history).
But you're a trained time-travelling assassin with top marks in marksmanship, and your grandfather is mortal, so surely, you should be able to kill him, right? Yet, how can you kill your grandfather and consequently prevent your own existence coming about?
Well, my friend, David Lewis's theory of Time Travel means that it simply cannot logically follow that you shot your grandfather in the past.
It is possible for you, a skilled assassin, to kill a mortal fleshy man, and your grandfather is a mortal fleshy man. However, it is not logically possible for you to prevent your own existence. Therefore, these two possibilities are incompatible. Your coming into existence in the future means that you had to fail in 1933 at your Hitler/grandfather assassination. No matter how unlikely (the gun backfired, an unfortunate dog got in the way, your grandfather happened to be wearing a bulletproof vest that day), your grandfather/Hitler assassination is doomed to failure in a single-history block theory of time in which your grandfather lived to father your parent. So, there is no paradox at all.
So yeah, screw Thailand and your sceptical friends: Time travel is going to look so much better on your UCAS application.
20 Century
But, just because you cannot change the future, doesn't mean you cannot affect it.
Just because whatever you do in the past has to be consistent with the history you came from, doesn’t mean you can't have any effect on the course of events. Imagine going back in time to a historic battle. You land in one general's camp where he is discussing the strategy for the battle. Impressed with the technology you possess and convinced that you will be the secret weapon he needs to win the war, he tells you his strategy and asks for your help. Of course, he also threatens to kill you if you refuse. Naturally, you decide this man has no chill, and you flee. You quickly get back into your time machine and transport yourself to the rival general’s camp. This dude also threatens you, so you offer the first general's plans in return for your life. This convinces the rival general and he lets you live, and he goes on to win the war.
You then find out that the general whom you just aided was the Duke of Wellington in his fight against Napoleon in the Battle of Waterloo. So you ensured Napoleon's defeat. Good job.
At this point, you may be asking: "If my actions in the past are limited by requirements of logical consistency, then doesn't that mean I don't have free will, and am forced to act in a deterministic, fatalistic universe?" Well, not quite, my dear inquisitive time traveller. You are free to act however you choose. You could have taken Napoleon's side, for example, but you didn't. The reason why it appears that you do not have free will is that you already know what you did or did not do in the past; the outcome was already true. But there was nothing actually forcing you to act in the way that you did. The confusion arises because of the way we sometimes conflate the indeterminacy of the future with our freedom to act as per our will.
Anyway, the bottom line is that you can make history, but you cannot change it. Which is still pretty neat.
Amicus Productions / Via rhetthammersmithhorror.tumblr.com
But hey, no worries if you accidentally bone a grandparent and become your parent's parent.
So, while you're packing for your gap year trip to 1933, your concerned friend asks you, “OK, but what if you travelled back in time and instead of assassinating your grandfather, you became him?"
That is (assuming you have all the right and functioning body parts to perform the task), you travel back in time, take your grandmother out to a nice dinner, then finish the evening off with some sweet, sweet lovin’. Your grandmother consequently falls pregnant, and gives birth to your mother, who gives birth to you. You consequently become your own grandfather, creating a causal loop.
Though not an entirely satisfying philosophical defence of causal loops, it is no more unlikely than other causal justifications. As senior lecturer at the University of Edinburgh and philosophy of time travel expert Alasdair Richmond puts it, “We have no complete explanation for any causal sequence (closed or linear) and we may just have to accept spontaneous creation of information in other cases.” In other words, though it seems weirder, causal loops are no more problematic than explaining any event with an infinite string of "it happened because this earlier thing happened", or claiming that the causal root of all events is one spontaneous event, like the Big Bang, which in itself has no causal explanation.
TL;DR: Time is weird, y’all. ¯\_(ツ)_/¯
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