Does SR actually imply that FTL travel would allow for violations of causality? Here is some reasoning to suggest that it does not: (I would be surprised if there were no holes in the reasoning. My desire to find these holes is what motivated me to post this here.) Consider the following thought experiment (which demonstrates why FTL travel would violate causality): 1) Observers A and B are in a shared frame of reference at the same location. Let us call this location "the origin". 2) B teleports some distance away from the origin, but does so without accelerating (thus remaining in a shared frame of reference with A). 3) B then accelerates in a direction away from the origin (and A). By doing so, B effectively moves into A's past (to a time when B was still at the origin). 4) Finally, B teleports back to the origin. B has now returned to the origin before it left (and could stop itself from leaving if it so chose). On the surface, this violation seems to support the idea that FTL travel is forbidden by SR. However, it has a problem. Consider what happens when we add a third observer to the thought experiment: 1) Observer C joins A and B at the origin in their common reference frame. 2) B teleports away as before. 3) B accelerates away from the origin as before. At the same time, C accelerates in the same direction by the same amount, thus remaining in a shared reference frame with B. As before, B believes that it has traveled to A's past, but it remains in C's present. From C's perspective, however, both A (co-located) and B (same reference frame) share its same time. 4) B teleports back to the origin. Is B now in A's past, C's present, or both? When step 3) occurs, and B accelerates, a paradox arises. From B's perspective, A's time (At) is less than B's time (Bt), but C's time (Ct) is equal to B's time. Here are the perceived time relationships for each observer after step 3: A) At = Ct, At > Bt B) Bt = Ct, Bt > At C) At = Bt = Ct The solution to this paradox is that all of the above are true. We can go further, however, and say that from B's perspective, it simultaneously exists at every point in time at the origin, depending on an observer at the origin's frame of reference. This explanation works fine if you do not allow FTL travel. However, if B is able to teleport back to the origin (via entanglement or some undiscovered physics), SR does not actually predict what A's time will be when B arrives. While we could excuse the notion that B was simultaneously in A's past and C's present while B was far away from them, we can no longer do so when it is co-located. According to SR, all co-located observers must agree on the order of events (and time) at their shared location. I would suggest that this agreed upon time amongst all origin-located observers is, in fact, the "proper" time at the origin. If B were able to teleport back to the origin, this would be B's time as well (from B's step 3 perspective, this would be C's present and not A's past). If true, that would mean that SR does not forbid FTL travel. (Yes, the speed of light is still the limit to how fast matter can move, but there are other potential methods that get around this, such as entanglement and wormholes). If you made it this far, thanks for reading! Any thoughts?