Oops, forgot to finish the title, but don't see how I can edit it. Sorry. How does one reconcile the idea that, according to relativity, one cannot talk of simultaneity, yet one can talk of a slice of spacetime to be space where all the points are at the same time? That is my main question. While I am at it, can one say that a particle could stop existing at one point and instantaneously (?) start appearing at another (not constrained by causality) ? Not only does the problem of "instantaneous" appear again, but since the only thing that is going to distinguish one particle (OK, excitation of the field) from another with otherwise identical characteristics (Energy, charge, spin, etc.) is going to be its positions in spacetime, but if now that changes, how can one say that this is the same particle that reappeared?