# Circumstances of a superluminal causality violation [closed]

In the case of the Roman ring, a traveler is able to go through a wormhole mouth and emerge at some point in the past without violating causality, so long as the time traveled back is less than the time it would take for a speed of light signal to arrive at your previous reference frame. So, for a slower than light trip that traveled to some point, and then returned to its original reference frame at a faster than light speed, but respecting the above condition, would that qualify as a causality violation?

## closed as unclear what you're asking by Ben Crowell, Jon Custer, John Rennie, ZeroTheHero, Aaron StevensSep 17 at 14:38

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• Could you give us a reference for what you mean by a "Roman ring?" I assume this is the Roman of the Ford-Roman inequality? – Ben Crowell Sep 12 at 19:24
• Yes, the name was in honor of Tom Roman, but it was a system proposed in 1993 by Matt Vissier. Link: arxiv.org/abs/gr-qc/9702043 – CuriousDroid Sep 12 at 19:28
• When someone asks for clarification on a question, the thing to do is to edit the question, not provide the clarification in comments. – Ben Crowell Sep 12 at 21:41

A causality violation is a very specific condition. Causality is said to be violated if, given a point $$p$$ of your spacetime, there is a causal curve (ie a massive or massless particle) which can start from $$p$$ and end up at $$p$$ (a bit of a stronger condition is strong causality, where the curve starting at $$p$$ will come arbitrarily close to $$p$$ again). An observer can very much indeed go "back in time" without violating causality. In general relativity, as you may know, the notion of an event being in the past or the future is a bit shaky since it's always possible to rewrite coordinates in a way where they may be switched.