Time traveling is just full of crap, innit?

[Alex K]

It's called the theory of relativity. The faster you travel through space, the slower time travels for you compared to people traveling at slower speeds.

There is another layer of subtlety to this that is barely ever discussed in popular treatments: travel speed itself is not an absolute observer-independent quantity in relativity, so there's no unique objective way to say who is faster and who is slower. This is why in two space ships passing each other, each could claim to be at rest while the other is fast, hence both could equally claim that their time runs faster. This seems paradoxical. The way the paradox is avoided in nature is very interesting: there is no way to compare the "time speed" of two observers unless 1. the two observers meet twice, or 2. if one observer prepares several synchronized clocks set up a distance apart, which are passed in succession by the other observer.

In the first case, the paradox is avoided bc one observer has to turn around and go back, breaking the symmetry of the situation. In the second case likewise, there is no paradox because the synchronicity of spatially separated clocks is not observer-independent either and the observer in the passing space ship will simply see the two clocks as not synchronous in the first place, explaining why they seemingly show more time passing even though they are the ones who are moving from his vantage point .