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Special relativity must apply everywhere for general relativity to work. You can't arbitrarily separate them.

Sure we can. That's the whole point behind Shapiro delay and gravitational lensing, after all - spacetime is no longer flat/Euclidean and c no longer (always) a constant. Yes, with proper modifications the mathematical equations still work but most people don't factor these things in and end up producing nonsense.

It would be more accurate to say C remains constant but the distance is not. Which leads to a lot of confusing questions about consistency in this universe.

lol, the idea that we're in a "slow" part of the universe where the speed of light is constant, but somewhere out there, trillions of light years away, C is in flux, is morbidly hilarious.

That's bad luck on a cosmic scale.

In fact, the only way the universe can remain consistent if the speed of light is constant is for distances and times to change. This allows all observers that are causally connected to agree on the same sequence of events.

Gravitational lensing doesn't violate this. The light may take different paths and arrive at different times, but there are no discontinuities and it is always possible to trace the light rays back to their point of origin and agree on that. Such tracing would also agree with hypothetical observers riding along with the rays at very near the speed of light.

Something moving FTL through space would create discontinuities. If the Enterprise passes you at Warp 10, you would see it suddenly appear in front of you and then speed away in both directions. An observer at the destination would see it pop into existence and then race backwards. It doesn't matter that the "warp bubble" maintains causality within itself; it creates a discontinuity between the inside and the outside.

I still think that a wormhole should have the same problem. Even if it doesn't violate special relativity on a local level, an observer far away from either end could and would see events occurring in a different causal order. Even on a local scale, light paths traced through the wormhole would arrive quickly while light paths going through normal space would take much longer to make the transit.

It would be possible to go through the wormhole, move a short distance away, and watch yourself departing from the origin. This seems like it must be a causality violation, which illustrates my other point. It's not enough that you be able to construct a path for information that doesn't violate causality. You must be able to show that there is no possible path that violates causality.

Edited by Fighteer on Apr 28th 2021 at 6:28:11 AM

How would it be a causality violation to watch yourself depart?

Isnt adding distance more like turning the light into a recording, you have already left via the wormhole that's why you are in a position to observe.

The image caused by you leaving has already been made as such, it's just that it took the slow route to get to you.

Okay but counterpoint. There's a giant mirror 1 lightsecond away. How does seeing yourself in that mirror differ from seeing yourself before you enter a wormhole?

If I understood his earlier posts on the matter, the problem isn't the sight of your departure traveling at lightspeed, but that the fact of your departure is ALSO traveling at lightspeed due to relativity.

It isn't about visual information, it's about the universe at your current location being unaware of your departure, which means you arrived before you departed, which is impossible as far as cause-and-effect logic is concerned. It's not "happened but the sight hasn't arrived yet", it's literally "not happened yet" because the universe does not have a universal clock everything sticks to.

This is why people in the scientific community 100 years ago thought Einstein was nuts: what he proposed sounds like a completely bonkers violation of common sense to anyone without a degree in physics.

Edited by amitakartok on Apr 28th 2021 at 1:19:03 PM

But that's not how it works... just because you cant observe something doesnt mean it hasnt happened yet.

Facts dont have to propagate, information does.

Edited by Imca on Apr 28th 2021 at 4:16:34 AM

That's exactly what I've been saying. Information isn't what the semi-transparent balls of goo plugged into the brain of a sentient being report to said being's conscious mind, information is what is.

People just keep confusing it because some people like reusing existing words for new concepts vaguely related to something else without bothering with the context of which meaning they're referring to. Information as defined by IT and information as defined by quantum mechanics is not the same.

Yes! This conversation again! Because I never tire of repeating myself! Yay!

The mere fact of FTL travel, by itself, won't create any temporal paradoxes. Provided that no one at the destination can send back information to the point of origin before the departure took place, causality is undisturbed, regardless of what the spaceship is observed doing. It doesn't make any difference how fast the Enterprise is going: if it travels 100 light years in one year, and then turns around and travels back at the same speed, it will arrive at the destination two years after it left. There is no violation.

There are specific thought-experiment scenarios in which a violation of causation does occur, but that may simply mean that something prevents those specific scenarios from occurring. What that might be is highly speculative.

Case in point, supernovae. Many have already occurred but the light and image of it happening simply hasn’t yet reached us.

In terms of real time, you warping away FTL a short cosmic distance say 10 light minutes away and then “waiting” for the image of you warping away to catch up doesn’t violate causality because your light image of you warping away already happened 10 minutes earlier.

In that mirror scenario, you aren't seeing the original you; you're seeing the light that you emitted being reflected from the mirror. Everyone who could possibly look at it would agree on the sequence of events.

In this wormhole scenario, it doesn't matter that you aren't actually traveling back in time. It matters that there are perspectives from which you could travel back in time.

Nope. This is fundamentally incorrect. There is no such thing as "ten minutes earlier". Time, like distance, is relative. There is no universal clock on which things happen. It is not necessary that observers agree on when things occur; it is necessary that observers agree on the order in which they occur.

The speed of light is the speed of causality. No part of the universe can influence any other part of the universe faster than this. You are part of the universe; hence you cannot influence other parts of it faster than light.

Edited by Fighteer on Apr 28th 2021 at 8:05:49 AM

Which means that the problem isn't FTL itself, but certain ways in which multiple observers could interact with each other.

In Uncle Tom's scenario, causality isn't violated. After warping out some distance and then looking back, you see an image of yourself launching off of the planet you just came from, and then traveling toward you at the speed of light. This doesn't create a paradox, because there is no way to send information back to those previous versions of you.

Edited by DeMarquis on Apr 28th 2021 at 8:07:52 AM

Uhhh... that is precisely what relativity tells us. What matters is exactly and specifically how "observers" interact with each other — or rather, how each individual bit of the universe causally interacts with other bits of the universe.

Edited by Fighteer on Apr 28th 2021 at 8:07:20 AM

Yes, but FTL itself doesn't require that a problem like that occur. Relativity prevents certain types of FTL relationships from happening—it does not make those violations inevitable.

Quantum entanglement does...

Okay. How is time not somewhat universal?

I live on the East Coast.

I have friends that live in Singapore. It takes roughly 22 hours of flight time to get there from my location. If they’re going to work right now (it’s after 8 am there as of now) and I’m winding down, how is that not universal time? In a sense? Distance doesn’t matter.

Likewise why wouldn’t the same thing apply to Mars? Europa? Alpha Centauri.

Sure the way we *communicate* might or will have Light lag, but our actions should happen at roughly the same time.

Ugh. I'm about to be very busy and don't have time to keep up with this. I'll do my best.

The two parts of your last sentence are synonymous. Relativity isn't a traffic cop handing out tickets if you cheat causality. Those violations fundamentally break reality as defined by relativity, and therefore you cannot have FTL and relativity in the same universe. One has to give.

This is a misunderstanding. Quantum phenomena are non-local (we think), but the information that propagates from them must obey the speed of light. You cannot use the collapse of a waveform to transmit information faster than light. (Also, waveforms don't exactly "collapse", but that's an entirely different discussion. They decohere.)

You only think this because we humans operate on a very narrow range of velocity. Our common frame of reference is almost perfectly Newtonian and so we don't see relativity in our daily lives. However, it does exist. We can see it in satellites orbiting the Earth. We can observe relativistic effects in high-energy particles within accelerators and from cosmic rays striking the planet.

Edit: On the quantum thing... I've seen (well, skimmed) some very interesting papers saying that we can't be perfectly sure that the speed of light is equal in all directions. This sort of merges with quantum non-locality to suggest that in a very literal sense, a photon or other massless particle never actually transits the space that it passes through. Rather, its waveform may spontaneously resolve to its final location, with the information about that resolution traveling at the speed of light.

Edited by Fighteer on Apr 28th 2021 at 9:03:12 AM

And if its not equal in that way, the wormhole wouldn't violate it either, because it being unequal also allows you to arive at the destination before the observation of you does.

The order of events isn't changing either way, you just receive the information out of order, which is possible with.... well almost any thing.

If I send two messages acoss my table, and one just has to go across the table, but the other has to circle the world back to it, the order of them may reverse, but they still happened in the correct order.

Edited by Imca on Apr 28th 2021 at 6:13:17 AM

@Fighter: I don't think you quite understand what I'm saying. Relativity does not prevent FTL travel per se, the only problem is that one can propose a set of origin points and destinations that seemingly allow one to send a message back to the point of origin before it is sent. That would violate causality, creating a paradox. But it isn't relativity that is preventing this, it's the outcome of sending the message across space in a certain way. If you then propose that there is some additional barrier that prevents the message from being sent back that way (ie, you dont make the trip) causality isn't violated. If one does try to make that trip anyway, there are serious scientific papers which propose that field forces ill align in such a way as to prevent the paradox (the most famous example being Stephan Hawking's "chronology protection conjecture", but here's another. For yet another, more speculative proposal see this site and scroll down to "9.5.3 "Producing" Restricted Space-Time Areas").

These ideas do not violate General Relativity, although they do impose some provisions on the types of FTL travel that can occur in order that Special Relativity not be violated either.

Edited by DeMarquis on Apr 28th 2021 at 10:01:08 AM

I think you are misunderstanding the Chronology Protection Conjecture. It doesn't say that the universe will contrive ways to preserve causality if we use FTL; it says that anything we can conceive of that might violate causality simply won't work. Also, it's a conjecture, not a theory.

All of these solutions in GR that might allow FTL are interesting mathematical ideas, but just because they're ideas doesn't mean they can actually work. For example, it may simply not be possible to keep a wormhole stable long enough for something to transit it. It may not be possible to accelerate an Alcubierre warp bubble past the speed of light. The math governing it suggests no way to actually push it past that barrier. It has to already be FTL to begin with, a paradox.

Edit: There's another way out of this. If we finally figure out a unification of general relativity and quantum mechanics, it could either eliminate all possible ways of going FTL or tell us how to achieve it without breaking causality. My money is on the former, but it would be cool if I'm wrong.

Edited by Fighteer on Apr 28th 2021 at 10:27:19 AM

The Chronology Protection Conjecture proposes that closed timelike curves cannot form. "Closed timelike curve" is mathematically equivalent to "time travel" but not necessarily to "time paradox" because in a non-deterministic universe you may have more than one past for the same present.

I think it's important to distinguish between what I call a "Weak Chronology Protection" (i.e you can time travel to meet your granddad, but you cannot shoot him) and "Strong Chronology Protection" (i.e you cannot time travel, period). Yes, I know that Wikipedia does not consider the former an example of CPC but I think it should be considered one. From what I know "semi-classical" quantum gravity^note Assuming that GR and quantum mechanics both apply while ignoring singularities and UV catastrophes for a moment the "Weak CPC" probably applies to Real Life while the "Strong CPC" is more murky.

That has nothing to do with my issue, which is people discussing wormholes by uncritically applying the mathematical formulations of special relativity to them without considering the topological variation. In particular, gravitational lensing can be noticed from a distance. So it's not true that you can approximate the larger-scale structure as flat just because the distortion is small relative to the larger scale structure. So no we cannot in fact treat a trip through a wormhole as the equivalent to a "warp jump" - as Wikipedia notes, GR light cones cannot be rotated in the way you do to demonstrate that FTL breaks causality. Of course it doesn't give a source and I can't find any discussion on how light cones operate in the context of a wormhole, but for one thing you would need to draw a branch across the hole.

Regarding quantum gravity, from what I can tell only entropic gravity theories (which are among the less mainstream QG theories) have something like an effective FTL ban and not all of them. I don't think string or loop quantum theories specifically disallow FTL but they put restrictions on causality violations.

Well, your talking mostly above my head there, Septimus, which isn't surprising since you're basically a researcher yourself. Can you explain the implications of the difference between wormholes and warp travel in layman's terms, since I have been thinking of them as exactly identical. If you can't apply SR to a particular setting, does that imply FTL can exist, that it can't, or neither?

Actually, my speciality is biology, not physics. And in spotting arguments that rely on premises (flat spacetime with the parallel axiom) that don't apply in a certain context (topological anomalies such as wormholes) and yet kept being applied there. And reading enough scientific papers to know that nobody discusses wormholes in this manner. Usually, that's a good sign that it is not correct to discuss them in this manner.

The problem I see here though is that I've been looking through scientific publications and none of them shows or discusses a spacetime diagram of how FTL would look like with a wormhole. Plenty that discuss how non-wormhole FTL would look like and how it leads to observers with different reference frames disagreeing on the order of events. But none that discusses whether these apply to GR scenarios, the just ignore the issue or blithely assert it.

The fact that gravitational lensing is equivalent to observers disagreeing on the source of an arrow does imply that the diagrams are modified (arrows tilted) by a gravitational field. Thus the SRT diagrams need to be modified which is more explicitly stated here.

Edited by SeptimusHeap on Apr 29th 2021 at 3:23:01 PM