A common justification for prohibiting many unusual phenomena such as faster than light travel is that if they were possible, causality would be violated.
Let's define causality as:
You cannot change the past.
Meaning that at any given moment $t_1$, it is impossible to influence any event which took place at $t_0<t_1$.
Obviously, no one has ever heard of this being violated. Is there a reason why? Is it just because nobody has managed to build a time machine yet, or does something in physics expressly forbid causality from being violated? In other words, is causality a law of physics, and how much of physics would have to be rolled back and re-written if I built a time machine (it's probably easier to say what physics there would be left)?
There are some (in)famous paradoxes that arise if you are able to influence the past. There is probably no need to restate these for this question, unless these paradoxes per se are the justification for expecting causality to not be violated.
Question motivated by John Rennie's comment.
Some examples of how causality can be violated:
- A device which can travel faster than light
- A device which can travel into the past
- Newcomb's paradox