Wikipedia has this interesting article about a physics "paradox" that Einstein came up with: https://en.wikipedia.org/wiki/Tachyonic_antitelephone It shows that if you were able to send a signal faster than the speed of light, then you could send a message back in time leading to a causality paradox. Einstein said this wasn't a problem since you can't send FTL signals to begin with. I don't understand why the result comes out the way it does though. The one way communication example doesn't really have an explanation, other than if you pick the right variables to plug into this equation then you can get a negative result. But I can't figure out how to translate that to a hypothetical experiment that someone could carry out. The provided illustration didn't really help me either. Then there's the two way communication example. I found that one to be even more confusing, because you have to keep track of two different "timelines", and the example keeps switching back and forth between them. But what really confused me about the two way communication example, is that it seems to me that you can play out all the events from just one person's perspective, and if you do you end up with a consistent (paradox free) accounting of the events. But obviously that must not be possible for some reason, and I don't know why.