Acausality and Retrocausality in Four- and Higher-Dimensional General Relativity

Abstract

Causality is a very important ingredient of physical theories and is considered to be a fundamental principle. However strict definitions cannot be substituted by slogans. If causality is true in its strictest possible sense as used everywhere in physics outside of General Relativity (GR), then retrocausality is impossible, so either retrocausal observations are untrue or physics is. However we do not have to take this standpoint here, since GR does not seem to support it. For example, Novikov’s paper [1] discusses world lines which are nonmonotonic functions of time within orthodox GR. We argue that GR does not support free and unrestricted travel backward in time. GR claims that spacetime has a unique, objective structure, i.e., geometry, and that this is valid for any observer. Retro motions should be possible for anyone if they are at all possible on spacetime, otherwise for no one. (We are not speaking here about facts; anyway, exactly what are facts and what are not facts is a matter for debate.) This does not mean that retro travel would be equally easy for everybody.