If time travel into the past were possible, which at the moment it isn't but let's say it was, there's a famous paradox which pretty much sums up the difficulty of going back in time. It deals with the idea of going back, messing with events and altering the outcome of things that have already taken place.
This time travelling quandary is called the Grandfather Paradox and versions of it date back to early sci-fi stories from the 1930s and 40s. It goes like this, what if you went back in time and killed your grandfather when he was a child. Regardless of the fact it would be a psycho thing to do, the problem it sets off is if your grandfather died as a child your mother or father wouldn't have been born so you wouldn't have been born.
So you wouldn't be able to go back in time to kill your grandfather in the first place. Ergo a paradox. How do you solve something like that?
Even though it seems impossible to figure out, there are some solutions that have been posited. The main one that YouTube channel MinutePhysics presents in this video is this: say you go back and kill your grandfather, that means you're not born, which then means you can't go back in time, so then your grandfather isn't killed, so then you are born. So then you do go back in time and kill him, and it sets off the chain of events again and the whole situation is forever stuck in that loop.
It means that two entangled histories are happening in parallel in their own loop—which if you're wondering if that's even possible, the video uses the example of subatomic particles which regularly do multiple things in parallel and it's called quantum superposition. Among other things it's responsible for fusion in the sun's core.
So the solution to the paradox is, that the universe would have to exist in a superposition of two states, one where your grandad is dead and one where he's alive, which logically is possible. Even if time travel isn't.
There you go, is your brain hurting yet? Watch the video above and it most definitely will.