[mr_mitm]

If you "instantly" teleported to alpha centauri, then there would be a frame of reference in which you arrive before you left. To another observer, you would travel back in time. It's not something you can disagree on. Instead, convince yourself by studying the spacetime diagram: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...

Note that "instantly" is ill-defined in SR. Simultaneity of events is observer-dependent.

Edit: this is also known as the tachyonic antitelephone. It's described here: https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone

Instead of warning about shrimp you could easily tell your past self to buy a certain stock.

[jiggawatts]

> there would be a frame of reference in which you arrive before you left

There would be a frame of reference in which you appear to arrive before you left.

The analogy is with a sonic boom, where an observer using only sound for sensing the world would hear the plane arriving before it left.

This does not allow closed timelike loops, the type that would allow you to predict the stockmarket.

The observers can't create paradoxes through via a third observer with a high celerity in the same way that QM "action at a distance" doesn't allow causality violations either, or how moving a laser pointer across the surface of the moon doesn't allow FTL motion either.

It just looks like it does, that doesn't mean that it actually does.

PS: I'm fully aware that in the SR/GR model, appearance is reality, and hence the paradox within the framework of relativity. Clearly, if FTL were allowed, then relativity would be falsified by that, and a new framework would be required. I'm saying that that framework could be consistent, allowing FTL and relativity without allowing travel backwards in time.

[gtech1]

Does this assume a single universe ?

In other words, can causality be broken if we assume we live in a multi-verse and achieving FTL from Time-PointA in Universe 1 to Time-PointB in Universe 2 ?

[mr_mitm]

Yes it assumes a single universe. I'm not aware of any multiverse solution to this. There is, however, the Novikov self-consistency problem which could solve this. It's entirely speculative, of course.

Edit: IIRC Novikov only covers wormhole-type time travel, i.e. configurations of curved spacetime, while we were only talking about flat spacetime until now.

[superposeur]

And allowing non-flat space time refines but doesn’t alter the picture.

For instance, it’s a common misconception that wormholes circumvent the relativistic prohibition against ftl travel. Actually, if a wormhole could facilitate travel between two events faster than a light ray moving between the same two events on the outside of the wormhole, then all the same causality troubles would ensue. This just highlights that that the particular mode of transport between events is not important to the argument. (Whether it be warp drive or secret tunnel.)

[gnad_sucks]

Studying a spacetime diagram is hard work. It's a lot easier to just say "I never understood why it's so certain" and leave it at that.