I lost the edit window so I'll tack on that I've been thinking about this with respect to conventional faster than light travel. I think that sort of relativistic speed travel basically requires traveling back in time, because time is supposed to slow as one gets close to the speed of light. If hitting the speed of light was actually possible for a massive particle, then presumably time would stop (I think)? So for that case I guess time would have to reverse if speed could increase, though I don't know that causality would have to reverse. Maybe under these conditions there could be paradoxes of various kinds.

I'm not a relativity physicist, and don't even know if these questions and suppositions are even the right things to be asking. If one could travel faster than the speed of light by teleportation, I don't think time travel or an unsyncing of the fabric of reality seems like it would need to happen. But what do I know. Thought experiments don't always reflect reality.

Since fast moving objects appear to contract (to an outside observer) the faster they go, if you have an object traveling at near light speed, it will appear vanishingly short, regardless of its true length.

So let there be a one mile long object, let it travel at nearly light speed for five seconds. As God, freeze time after 5 seconds and place two measuring stations: one touching the head of Object from Observer's point of view, and one touching the tail of Object from Object's point of view (we can teleport around instantly so we can verify that we've got everything set up correctly with no penalty).

Observer Head thinks Object Head and Object Tail are at about 5 light-seconds + 1 mile from origin (because Object is vanishingly short, and Object Head started at 1 mile from origin). But within Object, the Tail is 1 mile behind.

So Object Tail will signal Observer Tail at about 1 mile/c before it reaches where it supposedly is, and Observer Tail instantly signals Observer Head, thus Observer Head receives causality from its signal before it sends its signal.

Not sure how the universe resolves that. Unless the universe can "overclock" and become indefinitely fast at propagating and processing information when need be.

I think my example isn't entirely sound because I forget the exact math (it's been a long time), so you might have to tweak the example to be fully mathematically sound, but the general concept of causal contradiction within one's own frame of reference should be sound, I think.

I think I've understood your example. I'm just not sure how important it is. We have things such as optical illusions already. Does this have a material effect on the structure of the universe? On objects in the universe? Or is it just causing mathematical paradoxes?

I've played around with the fantastic idea of instant portals between places (or simple teleportation), and assume this would have unusual effects on gravity when, for instance, portalling from one side of a planet to the other.

I think I lack the theoretical basis to really understand whether this is a problem for the universe as it is. And I don't think a lay explanation can be satisfactory. I appreciate your attempt.

An optical illusion is an appearance of something amuck without it actually being so.

In this case, something is actually (not illusory) happening before it happens, if we assume some actor that can act faster than causality can keep up.

I have also come to the conclusion I have forgotten too much of my math haha, was a fun exercise trying to recover some part of it.