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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

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?

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.

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.

> pretty much all of current cosmology rules out FTL

Except current cosmology is full of strange things that feel to me like they are only there as placeholders for our gaps in the understanding of the universe. The expansion of space only exists because we need something to explain redshift. Dark matter only exists because we need something to make the galaxy rotation speed match our formulas. I do have a feeling that some of these ideas may turn out wildly wrong in the future and we'll be looking at "the universe is expanding" the same way we look at "combustion releases phlogiston out of whatever you're burning" today.

I completely agree and often get told I'm an idiot for it. But some of it just doesn't ring true for me.

If you're not convinced, you should look in to the evidence and the deductions that lead to what is now presented as fact. Very few popular presentations cover the reasons behind the wacky stuff scientists say, which is to blame for you evaluating that stuff on the basis of whether it rings true - because you've been given no other way to evaluate it.

Expansion is the Occam's razor explanation of redshift, and dark matter is, unfortunately, the Occam's razor explanation of the half dozen lines of evidence commonly cited, including but not limited to galaxy rotation curves.

Honestly skepticism of dark matter is understandable, but what the heck else are you expecting to pop up to explain redshift, a phenomenon that happens always and only when an object is moving away from you? What's the mystery there? You might be confusing universe expansion with dark energy (which IMO might well disappear, being vulnerable to improvements in the cosmic distance ladder), or cosmic inflation (apparently well substantiated but quite ad hoc, but if so you should really learn your terminology before you try to make predictions about future discoveries.

> but what the heck else are you expecting to pop up to explain redshift

For example, some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform and we're in an area that has properties slightly different from the surrounding space.

What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?

> some kind of phenomenon that happens to photons but only when they travel great distances. Or that maybe the space(-time) is not uniform

I think people have explored the photons thing with regard to “solving” dark matter, and found the evidence lacking. As for spacetime… trivially yes, but I don’t think this is what you meant? I mean, spacetime being non-uniformly curved just means there’s stuff in it, and that is literally one of the things we can directly observe, and we see that in e.g. the Bullet Cluster the dark matter kept moving when the visible matter collided and slowed down.

> What happens if JWST sees galaxies that, according to their redshift, were formed before the supposed moment of the big bang?

If, after careful work to exclude all other possibilities, then a party and a Nobel prize.

> some kind of phenomenon that happens to photons but only when they travel great distances

Tried, found wanting. Repeatedly. https://en.wikipedia.org/wiki/Tired_light

Anisotropy of space is (a) again adding complications and (b) unlikely to produce such a spherically symmetric redshift. The CMB is infamous for how smooth it is. The different region of space would have to be perfectly spherical with us in the dead center. May as well revive geocentrism.

Look, if we did find evidence like that from JWST or whatever, we'd re-evaluate what best meets Occam's razor for the totality of the evidence. But that doesn't change the fact that with our current evidence, that explanation is expansion of space. Other stuff is sci-fi at best. Fun to think about, but no reason to actually believe it.

The story for dark matter is more complicated, but similar in broad outline. The more the alternative theories try to fit all the evidence, the closer they get to reinventing dark matter. You have something that walks like matter and quacks like matter, so the smart money says it's matter.

Redshift also occurs to light emitted from within a gravity well. Obviously that isn't what's happening at a cosmic scale (our light isn't blueshifted to other observers) but it's at least conceivable that some third phenomenon could be the cause.

It’s not just redshift, it’s also measurements of the CMB, and other effects. The evidence for metric expansion is considered overwhelming today: https://en.m.wikipedia.org/wiki/Expansion_of_the_universe#Ob...

Similarly, for dark matter it’s not just galaxy rotation, it’s also observations of gravitational lensing and again the CMB, formation and evolution of galaxies, mass location during galaxy collisions, and movement of galaxies within galaxy clusters. Again, the observational evidence today is overwhelming: https://en.m.wikipedia.org/wiki/Dark_matter#Observational_ev...

I always struggled with Fermi paradox. We've been floating through insanely vast universe for laughably short time and to me its just bizarre to assert that if we couldn't find evidence of other civilizations so far, that for sure means we're the only intelligent species there is.

As much as I appreciate Fermi's work, this is something I can't at all agree with.

1 - we think that a highly advanced alien civilisation would build massive startships that move at increadible speeds, Radio beacons that signal to alien civilisations, dyson spheres, etc. These things would be massive, easy to detect, and stick out like a sore thumb.

2 - with our instruments, we can observe vast swathes of space, many galaxies, quite precisely. So it appears that no highly advanced alien civilisation is in our galaxy or the nearby galaxies

3 - Even at our technology level, If we wanted to signal out into the void on purpose, we could signal quite far - thousands of lightyears, and if we wanted to target a particular star we could signal precisely with a large laser and that signal would be detectable across most of the galaxy.

So it stands to reason that there is no civilisation similar to ours in the near couple hundred lightyears and that there is no advanced civilisation, the way we envision them, in our or nearby galaxies. It could be that everyone is hiding, or our idea of advanced civilisation is wrong. Or anytthing else really. That;s hwy it's a paradox/

One obvious answer is that any civilization that is advanced enough to be at the dyson sphere level probably has no problem hiding their existence from humanity if they want to.

That’s quite the coordination problem and it doesn’t seem like you can solve that by being an advanced civilization.

(Inventing cars didn’t stop us from littering.)

I assume that any advanced civilization has already created AI that is several orders of magnitude smarter than humans (and able to think ceaselessly, without being tired etc.)

At that point, it seems fairly trivial to do direct things like messing with the very small number of telescopes and other methods of detection that we have rather than actually trying to hide by coordinating the entire civ to act a certain way.

There's no reason to believe that "being really smart" lets you do things better, much less that it gives you magic powers over other people's telescopes. Evil superintelligent AI deities are inventions of people whose egos are attached to the idea that they're the "most intelligent" and don't want to lose the title they gave themselves.

Ftl means there's one global reference frame most likely the cmbr one.

The reason relativity predicts weird effects like time travel is because it doesn't allow ftl as an assumption, so the only way to hack ftl into it is negative time. It's baffling how many people seriously argue about consequences of something that completely disproves the theory itself while still using it to generate predictions.

"Every warp drops you in a universe where you are the only sentient life." Well that explains the Fermi Paradox.

That last paragraph would make an interesting setting for a book.

Or a scifi short on Dust