5 ms·

This constraint is needed because having a shortcut would allow time travel, like all FTL does.

codethief 5y ago
But that's not a problem per se. The speed of light is only a limit locally but globally there is no such thing and in General Relativity there is nothing preventing time travel (closed timelike curves) at a global level, even though the existence of such curves is rather unlikely. The paper notes: > Interestingly, they are allowed in the quantum theory, but with one catch, the time it takes to go through the wormhole should be longer than the time it takes to travel between the two mouths on the outside. Does anyone know why exactly "quantum theory" would impose such requirements? A priori to me it sounds like quite a stretch to take a local theory like quantum mechanics to make claims about the global topology of the universe, given that QM and GR haven't been unified yet. Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all.
- ben_w 5y ago
  IIRC (I’m not a physicist), if you combine a closed timelike curve with quantum mechanics, then a random fluctuation in the photon field (which happens all the time) appearing in the middle of it will go all the way around, meet itself, and now there are two of them, then four, then eight, … But as this is in a closed loop of time, from the outside it goes to infinity instantly.
  - zozbot234 5y ago
    A closed timelike curve simply has to be consistent. It might reach a state where the fluctuations simply interfere with one another and reach some kind of non-infinity fixpoint, which always exists if there are no discontinuities.
    - ben_w 5y ago
      Given the way light works, I think the expectation is that all of the possible wavelengths will start doing this all the time — all the ones that interfere destructively will do so, all the ones that interfere constructively will also do so, but the former has a minimum of zero and the latter is unbounded, so one of these will just run off to infinity. (It does feel like trying and failing to make one, getting asymptotically close should release infinite free energy as virtual photons almost get into a CTC, but IANAP).
      - Beldin 5y ago
        Huh. So maybe gamma ray bursts are wormholes opening/closing? Yeah, that probably only makes sense in armchair physics. Which is all I'm licensed for anyway, so fine with me!
        ben_w 5y ago
        Possibly even the entire universe, assuming CTCs and quantum fields actually did the things I’ve suggested, but you shouldn’t take this as more than merely speculative — I’m already making far too many assumptions for this to be taken more seriously than a sci-fi plot device.
- joe_the_user 5y ago
  One thought I've had is that all time travel contradictions are based on conscious agents. Without a conscious agent who aims to change things, what happened is happened.
  - karpierz 5y ago
    That's not true, here's an example where it's billiard balls: https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
  - PeterisP 5y ago
    Consciousness is about self-awareness and it does not strictly require nondeterminism or true free will; it is possible to have a universe with conscious agents who aim to change things and experience these changes, however their aims, choices, attempts at change and any success is fully deterministic, what happened is happened.
- disentanglement 5y ago
  > Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all. The wormhole solutions from the paper are semi-classical: they are obtained by taking the expectations value of the energy configuration of a quantum theory and feeding that into the classical Einstein equations. Therefore, no string theory or AdS/CFT is needed for the construction.
  - codethief 5y ago
    Since it's part of the introductory chapter, my interpretation had been that the portion I quoted was supposed to be a general, well-known "law" implied by quantum theory, i.e. not a result specific to the situation at hand. Hence my question where that supposed law is coming from. But it seems they were indeed merely announcing a result specific to the wormhole they constructed since on p. 6 the authors say: > In deriving (2.12) we assumed that the distance d between the two black holes is smaller than l, d << l. Even when we do not make that assumption we find that the time through the wormhole is always longer than through the outside, πl > d.
arcastroe 5y ago
> FTL would imply time travel But not "time travel" in the sense that you could go back and kill your grandfather. Only in the sense that different observers could not agree on the order of events. And this doesn't seem so problematic to me. There always seems to exist some "true" order of events that results in the observations experienced by all observers, even if they don't agree based on their own individual knowledge.
- dghf 5y ago
  ETA: my comment was more than a little confused. Original stands below. What I should have said is that different observers can already, in the absence of FTL, disagree about the order of events with space-like separation, but it doesn't matter because by definition anything that happens at one such event can't affect anything happening at another. FTL removes that restriction -- if something can travel faster than light, it can be present at two space-like separated events -- and that's what threatens causality, assuming something like the Novikov consistency conjecture doesn't hold. ======== Original comment: > Only in the sense that different observers could not agree on the order of events. But that's already true of events with space-like separation anyway. You don't need FTL travel for that. With FTL, you run the risk of observers disagreeing about the order of events with time-like separation, so you are getting into grandfather-paradox territory.
  - Andrew_nenakhov 5y ago
    I still don't see why this would be a problem. Substitute light with sound and sight with hearing. We can exceed the speed of sound and produce noises that some observer would perceive in the incorrect order (in fact, happens all the time with supersonic jets). And the world doesn't seem to break because of that, no?
    - foerbert 5y ago
      It's not really about light. It's not so much the case that the speed of light is itself special, so much as it is that light (in a vacuum) travels at a special speed. That speed is effectively that of causality, in the 'speed of the ability to cause' sense. Trying to make conclusions about that by using simple analogies to light isn't likely to get you anywhere. You're not dealing with the actual thing you're interested in in the first place.
deleted 5y ago
[deleted]

But that's not a problem per se. The speed of light is only a limit locally but globally there is no such thing and in General Relativity there is nothing preventing time travel (closed timelike curves) at a global level, even though the existence of such curves is rather unlikely. The paper notes: > Interestingly, they are allowed in the quantum theory, but with one catch, the time it takes to go through the wormhole should be longer than the time it takes to travel between the two mouths on the outside. Does anyone know why exactly "quantum theory" would impose such requirements? A priori to me it sounds like quite a stretch to take a local theory like quantum mechanics to make claims about the global topology of the universe, given that QM and GR haven't been unified yet. Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all.

ben_w 5y ago
IIRC (I’m not a physicist), if you combine a closed timelike curve with quantum mechanics, then a random fluctuation in the photon field (which happens all the time) appearing in the middle of it will go all the way around, meet itself, and now there are two of them, then four, then eight, … But as this is in a closed loop of time, from the outside it goes to infinity instantly.
- zozbot234 5y ago
  A closed timelike curve simply has to be consistent. It might reach a state where the fluctuations simply interfere with one another and reach some kind of non-infinity fixpoint, which always exists if there are no discontinuities.
  - ben_w 5y ago
    Given the way light works, I think the expectation is that all of the possible wavelengths will start doing this all the time — all the ones that interfere destructively will do so, all the ones that interfere constructively will also do so, but the former has a minimum of zero and the latter is unbounded, so one of these will just run off to infinity. (It does feel like trying and failing to make one, getting asymptotically close should release infinite free energy as virtual photons almost get into a CTC, but IANAP).
    - Beldin 5y ago
      Huh. So maybe gamma ray bursts are wormholes opening/closing? Yeah, that probably only makes sense in armchair physics. Which is all I'm licensed for anyway, so fine with me!
      - ben_w 5y ago
        Possibly even the entire universe, assuming CTCs and quantum fields actually did the things I’ve suggested, but you shouldn’t take this as more than merely speculative — I’m already making far too many assumptions for this to be taken more seriously than a sci-fi plot device.
joe_the_user 5y ago
One thought I've had is that all time travel contradictions are based on conscious agents. Without a conscious agent who aims to change things, what happened is happened.
- karpierz 5y ago
  That's not true, here's an example where it's billiard balls: https://en.wikipedia.org/wiki/Novikov_self-consistency_principle https://en.wikipedia.org/wiki/Novikov_self-consistency_princ...
- PeterisP 5y ago
  Consciousness is about self-awareness and it does not strictly require nondeterminism or true free will; it is possible to have a universe with conscious agents who aim to change things and experience these changes, however their aims, choices, attempts at change and any success is fully deterministic, what happened is happened.
disentanglement 5y ago
> Unless of course by "quantum theory" Maldacena actually means "string theory" or "AdS/CFT" – which wouldn't surprise me at all. The wormhole solutions from the paper are semi-classical: they are obtained by taking the expectations value of the energy configuration of a quantum theory and feeding that into the classical Einstein equations. Therefore, no string theory or AdS/CFT is needed for the construction.
- codethief 5y ago
  Since it's part of the introductory chapter, my interpretation had been that the portion I quoted was supposed to be a general, well-known "law" implied by quantum theory, i.e. not a result specific to the situation at hand. Hence my question where that supposed law is coming from. But it seems they were indeed merely announcing a result specific to the wormhole they constructed since on p. 6 the authors say: > In deriving (2.12) we assumed that the distance d between the two black holes is smaller than l, d << l. Even when we do not make that assumption we find that the time through the wormhole is always longer than through the outside, πl > d.

arcastroe 5y ago

> FTL would imply time travel But not "time travel" in the sense that you could go back and kill your grandfather. Only in the sense that different observers could not agree on the order of events. And this doesn't seem so problematic to me. There always seems to exist some "true" order of events that results in the observations experienced by all observers, even if they don't agree based on their own individual knowledge.

dghf 5y ago
ETA: my comment was more than a little confused. Original stands below. What I should have said is that different observers can already, in the absence of FTL, disagree about the order of events with space-like separation, but it doesn't matter because by definition anything that happens at one such event can't affect anything happening at another. FTL removes that restriction -- if something can travel faster than light, it can be present at two space-like separated events -- and that's what threatens causality, assuming something like the Novikov consistency conjecture doesn't hold. ======== Original comment: > Only in the sense that different observers could not agree on the order of events. But that's already true of events with space-like separation anyway. You don't need FTL travel for that. With FTL, you run the risk of observers disagreeing about the order of events with time-like separation, so you are getting into grandfather-paradox territory.
- Andrew_nenakhov 5y ago
  I still don't see why this would be a problem. Substitute light with sound and sight with hearing. We can exceed the speed of sound and produce noises that some observer would perceive in the incorrect order (in fact, happens all the time with supersonic jets). And the world doesn't seem to break because of that, no?
  - foerbert 5y ago
    It's not really about light. It's not so much the case that the speed of light is itself special, so much as it is that light (in a vacuum) travels at a special speed. That speed is effectively that of causality, in the 'speed of the ability to cause' sense. Trying to make conclusions about that by using simple analogies to light isn't likely to get you anywhere. You're not dealing with the actual thing you're interested in in the first place.

deleted 5y ago

[deleted]