[SkittyDog]

So just to start with... Every legitimate physicist accepts that time travel and other major causality paradoxes are an inevitable result of ANY mechanism that provides a meaningful version of FTL travel or communication. This is provable from the math of special relativity. You need some algebra to show it, but the proof isn't super difficult if you already understand the math of special relativity.

• https://physics.stackexchange.com/questions/574395/why-would... • https://selfawarepatterns.com/2019/02/03/why-faster-than-lig...

There are a lot of YT videos that discusses it, too.

But none of this is likely to convince a skeptic if you don't already have a mathematical understanding of SR. So if you're serious, you should really start by learning that.

[zasdffaa]

> This is provable from the math of special relativity ... you already understand the math of special relativity

I'm not saying anything is possible or not possible, but maths is a model, no more. Asserting the conclusions of the maths is to demonstrate nothing until you also make a convincing case that the maths is an accurate representation of reality, which you haven't done. I suppose the shorter version is, the map isn't the territory. I guess.

[scatters]

But our universe doesn't obey special relativity, since it has gravity; you have to use general relativity.

FLRW has a preferred velocity at all points, right? Couldn't you use that to construct a global time coordinate and require FTL to experience that time coordinate as increasing? (This would be dependent on the metric; I realize that GR has solutions that have closed timelike curves inherently.)

[SkittyDog]

I don't know where you got the idea that our universe doesn't obey special relativity? That's an odd thing to have learned...

SR is a subset of general relatively. So SR can be used perfectly well in situations that obey the constraints under which it was designed to be used.

As for your second paragraph, I'm not qualified to answer your question. GR is an extremely rigorous subject that requires a very high level of math to understand.

But something about your tone makes me wonder if you might be engaged in some kind of motivated reasoning... That you desire the viability of FTL travel because it's exciting or romantic?

If you think that describes your feelings on the subject, then the answer is almost certainly going to be "No".

[scatters]

Yes, and the cosmological-scale universe does not satisfy the constraints of SR. SR is the limit of GR in the non-gravitating limit, but the stress-energy tensor is nonzero; we don't live in a Milne universe.

Motivated reasoning, really? Yes, a universe that censors CTCs but not FLT is more interesting than one where all FLT results in CTCs. I can't understand how that makes it less likely; the universe is in general extremely interesting, not dull.

[SkittyDog]

You should really make the effort to learn the math of SR, because I believe it would help keep your reasoning a little more grounded, here. At the very least, I think it would help you understand why SR still constrains FTL, even though we do live in a GR universe, at cosmological scale.

While the larger universe does depart from SR's constraints, it does not depart from the fundamental facts that disprove the possibility of FTL. The math is more complex in GR, but the conclusion is the same: FTL isn't compatible with the reality we observe... You have to invoke some kind of hidden, unobservable, untestable dicta in order to arrive at FTL.

As for excitement... If we decide to take that approach, what's wrong with me believing in Faeries and Santa Claus, because it makes the universe more interesting?

[xupybd]

One video with no maths required.

https://youtu.be/HUMGc8hEkpc

Without the math you can't get a deep understanding or reason about it too much but you can see the issue.