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Comment by midasuni | Hacker News Reader

Imagine a wormhole which dumped you from New York to LA instantly. How would that provide time travel? Ok you beat your message by a few milliseconds, HFTs love it, but that’s now different to the ping routing via Sydney. how do you get the message back to the location it left before it left?

If an FTl spaceship went to a different universe then presumably a ship in orbit of Earth aiming at mars would simply vanish and never appear in mars orbit, telescopes on earth wouldn’t see it 8, 16, 30 minutes or even years later

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It comes down to special relativity not allowing an absolute frame of reference. Without any absolute frame of reference there is no instantaneous.

You may think of instantaneous as being 90 degrees away on a spacetime diagram. But someone else's frame of reference will observe your instantaneous at a different angle than 90 degrees on their own spacetime diagram, perhaps they'll even see it as going backwards in time. Their frame of reference is valid too. So if they sent a message back at a 90 degree angle on their own spacetime diagram using the same mechanism they'd be sending a message to us back in time in our frame of reference. It gets really funky when you do this with 3 different observers all observing each other communicating backwards in time to the point where they can cycle a message between the three of them that goes further and further back in time.

You'd have to first throw away frames of reference to make such travel possible but that's the entire basis of relativity and good luck finding a better explanation as to why light always appears to go at the same speed for any observer in the universe.

The geometric description of the universe is a high-level description of the universe, but its just a description, its not what fundamentally is. The large-scale universe emerges from the low-level behaviours of its smallest parts, the particles and the vacuum.

Causality is not a product of the universe, but the other way around.

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.

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.)

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".

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.

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?

> 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.

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.

What if the other end of the wormhole is travelling close to the speed of light? What if one end of the wormhole is accelerating?

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