Earlier quoted context omitted.
> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical sys…
> GR, even without FTL, allows for closed time-like loops, which violate causality. The theory may "allow" them, but this is not to say they actually exist in reality. http://en.wikipedia.org/wiki/Causality_(physics) Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause , even if the spacetime…
And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic.
On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances.
It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.