As far as we know, time only travels forward. The grandfather paradox is a thought exercise that some argue suggests time travel is impossible, while others argue it suggests multiverses exist. The paradox asks what would happen if you traveled back in time and killed your grandfather when he was a child. If your grandfather died young enough, he wouldn't have been able to father your parent, thus you would not be alive. But if you weren't alive, you wouldn't be able to travel back in time to kill him, so he would be alive. And so on... The simplest solution to this paradox is that when you go back in time, you're not going back in time in your own history, but rather a copy of your history. Then, your actions don't influence future events in the that timeline you started with.
Share the knowledge!
Key Facts In This Video
What if you traveled back in time and killed your grandfather when he was a child? This is the grandfather paradox. 00:07
Quantum superposition is what makes the strange result of the double slit experiment possible. 01:11
Some schools of thought regarding the grandfather paradox maintain that the paradox suggests time loops and time travel into the past are impossible. 01:54