4 ms·

sigh Here, can we reset a bit? Obviously we didn't start this conversation on the best note, so I guess it shouldn't be surprising that it's gone sour, but th

sigh Here, can we reset a bit? Obviously we didn't start this conversation on the best note, so I guess it shouldn't be surprising that it's gone sour, but that doesn't mean it has to stay that way, right? I'm sorry for my own jerkiness. Let's start over: Howdy! I'm YellowApple, and I have some questions about the implications of superluminal communication... > There are no tachyons, it's just a way to talk about literally any forms of superluminal communication. So I think this the core of my question. If we can agree that these signals are not themselves tachyonic, then why are we subjecting them to mathematics that do assume them to be tachyonic? That is: the math on that Wikipedia page seems to specifically describe signals that are moving faster than c within their own reference frames, whereas we're discussing spacetime topologies where that shouldn't be necessary - i.e. where from the signal's perspective it's moving slower than the speed of light. So if from the signal's point of view it ain't moving faster than c, how would it still result in a negative t? > Are you familiar by "proof by contradiction" I am, yes.

SideburnsOfDoom 6y ago
Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate. Unless you're thinking of "loopholes", e.g. "the Rocinante travels through the stargate to the Illus system, and since it travels through the gate at 100 km/h with respect to the gate, it's clearly not superluminal". The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that. Same applies to "the Enterprise travels in warp bubble where it's at rest". If it's 8 light years to Wolf 359 and you get there before lunchtime, it's superluminal regardless of warp bubbles. Debating how your signals or spaceship carrying a message travel is pointless - the only question is, did it get there faster than light would or not? You can't have your cake and eat it too here. BTW: I already said off of that: > Literally any method of getting from A to B faster than light would, has a causality paradox. it does not matter if you cover the miles between FTL, or pop out of a hypothetical wormhole gate: If you end up outside of your own light cone, no matter how, there's a causality paradox.
- yellowapple 6y ago
  > Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate. Okay, but that's the thing: from one perspective (the signal's) the signal is indeed subluminal, but from another (an observer's, e.g. the sender or recipient) it's not. So which math applies? > The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that. Except there is a way around that, no? Specifically that from the signal's point of view it was not actually superluminal, and therefore not subject to the infinite energy requirements of locally-superluminal travel? If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert. That is, going back to the start of this conversation: > If you end up outside of your own light cone, no matter how, there's a causality paradox. From which frame of reference is that light cone authoritative? If I'm B, and A sends me two copies of a message - one by some wormhole or warp pigeon or whatever, and one by radio - and the W message takes 1 second while the R message takes 10, then where does that leave room for the W message to reach me before it was sent? The math behind a tachyonic antitelephone - as far as I can tell - only answers this for cases where the signal is itself traveling faster than c within its own frame of reference, but that wouldn't be the case in this situation.
  - SideburnsOfDoom 6y ago
    > Okay, but that's the thing: from one perspective (the signal's) the signal is indeed subluminal, but from another (an observer's, e.g. the sender or recipient) it's not. So which math applies? All observers are valid. The maths answers differ due to different inputs but they apply the same formula. All should get self-consistent results, though not necessarily the same results: the idea that two events are "simultaneous" is observer-dependant. Details here https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity If _any_ observer, including one not on the fast spaceship, can communicate FTL, that will lead to paradoxes. > Except there is a way around that, no? No. > Specifically that from the signal's point of view it was not actually superluminal, Irrelevant. Another observer - for whom the signal is superluminal - can leverage that. > and therefore not subject to the infinite energy requirements of locally-superluminal travel? I didn't mention infinite energy, and it's a problem for FTL, but not the problem that we were talking about. > If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert. If a signal can travel at lightspeed or lower from A to B, causality of A -> B cannot invert. if it _cannot travel_ , then all bets are off, which is pretty much the point. Basically by stretching "Relativity of simultaneity" to breaking point. > From which frame of reference is that light cone authoritative? All frames of reference are equally valid. > where does that leave room for the W message to reach me before it was sent? An observer moving with respect to A and B can "hack" this to produce effects before cause in their frame of reference. It's no co-incidence that Alice and Bob in the Tachyonic antitelephone are moving with respect to each other, therefor with different frames of reference. > the signal is itself traveling faster than c within its own frame of reference, That's not a thing. The tachyonic antitelephone is a dialog between two observers with different frames of reference. The "Tachyons" could be miniature starship enterprises in "stationary" warp bubbles, flying back and forth; and it would work the same. So in line with "this has been said before" it's what is here: https://news.ycombinator.com/item?id=26617487 https://news.ycombinator.com/item?id=26617487 Note "I can find a frame of reference" - all frames of reference are valid, and should be free of paradox.

SideburnsOfDoom 6y ago

Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate. Unless you're thinking of "loopholes", e.g. "the Rocinante travels through the stargate to the Illus system, and since it travels through the gate at 100 km/h with respect to the gate, it's clearly not superluminal". The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that. Same applies to "the Enterprise travels in warp bubble where it's at rest". If it's 8 light years to Wolf 359 and you get there before lunchtime, it's superluminal regardless of warp bubbles. Debating how your signals or spaceship carrying a message travel is pointless - the only question is, did it get there faster than light would or not? You can't have your cake and eat it too here. BTW: I already said off of that: > Literally any method of getting from A to B faster than light would, has a causality paradox. it does not matter if you cover the miles between FTL, or pop out of a hypothetical wormhole gate: If you end up outside of your own light cone, no matter how, there's a causality paradox.

yellowapple 6y ago
> Signals slower than the speed of light (or even at the speed of light, e.g. radio signals in space) are ordinary and have no paradox, so there's nothing to debate. Okay, but that's the thing: from one perspective (the signal's) the signal is indeed subluminal, but from another (an observer's, e.g. the sender or recipient) it's not. So which math applies? > The answer is that light takes 104 years to travel from our solar system to the Illus system, so yes it very much is superluminal, there's no way around that. Except there is a way around that, no? Specifically that from the signal's point of view it was not actually superluminal, and therefore not subject to the infinite energy requirements of locally-superluminal travel? If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert. That is, going back to the start of this conversation: > If you end up outside of your own light cone, no matter how, there's a causality paradox. From which frame of reference is that light cone authoritative? If I'm B, and A sends me two copies of a message - one by some wormhole or warp pigeon or whatever, and one by radio - and the W message takes 1 second while the R message takes 10, then where does that leave room for the W message to reach me before it was sent? The math behind a tachyonic antitelephone - as far as I can tell - only answers this for cases where the signal is itself traveling faster than c within its own frame of reference, but that wouldn't be the case in this situation.
- SideburnsOfDoom 6y ago
  > Okay, but that's the thing: from one perspective (the signal's) the signal is indeed subluminal, but from another (an observer's, e.g. the sender or recipient) it's not. So which math applies? All observers are valid. The maths answers differ due to different inputs but they apply the same formula. All should get self-consistent results, though not necessarily the same results: the idea that two events are "simultaneous" is observer-dependant. Details here https://en.wikipedia.org/wiki/Relativity_of_simultaneity https://en.wikipedia.org/wiki/Relativity_of_simultaneity If _any_ observer, including one not on the fast spaceship, can communicate FTL, that will lead to paradoxes. > Except there is a way around that, no? No. > Specifically that from the signal's point of view it was not actually superluminal, Irrelevant. Another observer - for whom the signal is superluminal - can leverage that. > and therefore not subject to the infinite energy requirements of locally-superluminal travel? I didn't mention infinite energy, and it's a problem for FTL, but not the problem that we were talking about. > If A sends a message at tA=1, and it takes one second for the message to reach B (so, tB=2), then how does that result in tA > tB? No matter which frame of reference we're using (A's, B's, the signal's), I'm failing to see where causality would invert. If a signal can travel at lightspeed or lower from A to B, causality of A -> B cannot invert. if it _cannot travel_ , then all bets are off, which is pretty much the point. Basically by stretching "Relativity of simultaneity" to breaking point. > From which frame of reference is that light cone authoritative? All frames of reference are equally valid. > where does that leave room for the W message to reach me before it was sent? An observer moving with respect to A and B can "hack" this to produce effects before cause in their frame of reference. It's no co-incidence that Alice and Bob in the Tachyonic antitelephone are moving with respect to each other, therefor with different frames of reference. > the signal is itself traveling faster than c within its own frame of reference, That's not a thing. The tachyonic antitelephone is a dialog between two observers with different frames of reference. The "Tachyons" could be miniature starship enterprises in "stationary" warp bubbles, flying back and forth; and it would work the same. So in line with "this has been said before" it's what is here: https://news.ycombinator.com/item?id=26617487 https://news.ycombinator.com/item?id=26617487 Note "I can find a frame of reference" - all frames of reference are valid, and should be free of paradox.