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I wasn’t aware of the constraint that travel within wormhole must take longer than travel between the mouths. Given that, is there any real practical use for

I wasn’t aware of the constraint that travel within wormhole must take longer than travel between the mouths. Given that, is there any real practical use for them? In scifi they are usually presented as shortcuts, but this constraint makes that seem less likely to ever be true.

beecafe 5y ago
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]
xwdv 5y ago
Well I think you might still experience time as a short trip but everything else would age as if you had completed the physical distance?
- remcob 5y ago
  This holds true for regular travel nearing light speed too: For an external observer the spaceship never goes faster than light, but the passengers clocks slow down and they can experience arbitrarily high faster-than-light speeds. For some reason I never see this discussed when people talk about FTL travel, maybe I'm wrong?
  - hiptobecubic 5y ago
    I think because we use words like "faster" to mean "more stuff in less time," not just "feels like less time to you, but isn't."
  - pcl 5y ago
    Orson Scott Card’s sequels to Ender’s Game take this into account.
  - bno1 5y ago
    It's because distances and time durations contract the faster you travel. The passangers don't experience faster than light speeds, just shorter distances. This is how the twin paradox gets solved. The twin that leaves earth sees the trip as if it was shorter in both directions, so from their perspective it makes sense that they aged less than the twin that remained on earth. Also, if you had infinite energy and you could travel at the speed of light you wouldn't feel any movement or time passing during your trip, it would feel like instantly teleporting from one place to another. Photons wouldn't feel their existence if they could. From their perspective they are produced in one place and instantly absorbed in another place.
GordonS 5y ago
I was just about to say that same thing - any time I've seen wormholes in sci-fi (I'm reading Peter F Hamilton right now!), you go through in an instant (for all observers). It's like sci-fi uses wormholes as a practical alternative to travelling at sub-C, relativistic speeds.
- tokai 5y ago
  In the game Free Space there is travel time through wormholes (subspace jump nodes). In the finale the player intercepts and destroy the enemy flag ship as it is travelling through a wormhole to Earth. Only scifi example I can think of wormholes with travel time.
  - Aerroon 5y ago
    Stargate has had time travel through wormholes in a few cases.
  - JaimeThompson 5y ago
    The wormhole in Star Trek Deep Space 9 had a travel time.
  - ben_w 5y ago
    Farscape has an episode where wormhole time travel happens. How does time travel get presented in that game? Searching got me unrelated results.
- Rhinobird 5y ago
  Yeah. Think teleporter instead of warp drive
  - GordonS 5y ago
    Yes, exactly what I meant, but said more succinctly!
- dylan604 5y ago
  In Contact, <spoilerAlert> the pod drops instantly through from everyone's perspective outside the wormhole, but inside the wormhole 99 hours elapsed (potentially).
teeray 5y ago
> is there any real practical use for them? An infinite source of power. Place one wormhole at the top of a hill, another at the bottom. Send rolling generators through them, stop them once in awhile to swap dead batteries for charged ones.
- ben_w 5y ago
  I suspect that (if wormholes can actually be made) gravitational potential would be smooth throughout, and that you’d be pulled up as much while falling out of the top as you get pulled down while approaching the bottom.
  - ars 5y ago
    Keep in mind you can not create or destroy gravitational potential - you can only move it. Otherwise you have a discontinuity. So you would have to start your wormhole with both mouths at the same place, and then move one mouth to the new location, with gravity "flowing" between them all the while.
    - ben_w 5y ago
      The word “flowing” is ambiguous, I don’t know if you’re agreeing with me or disagreeing with me.
      - ars 5y ago
        A change in gravitational potential (because you moved a mass) flows through normal space. It would do the same through the wormhole. So I'm agreeing with you, and adding how this gravitational slope is created: It's created at the start of the wormhole and changes as you move the mouths of the wormhole, rather than popping into existence at a later time with the two wormhole mouths separated.
- teekert 5y ago
  Something tells me the wormhole also costs energy.
imglorp 5y ago
The most practical constraint is getting to one. The paper indicates they will "resemble intermediate mass charged black holes". The nearest black hole at the moment, maybe a worm hole candidate, is V723 Monocerotis at 1500 LY away. This would be tens of thousands of years of travel. So perhaps we can learn from afar but not visit.
deleted 5y ago
[deleted]

beecafe 5y ago

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]

xwdv 5y ago

Well I think you might still experience time as a short trip but everything else would age as if you had completed the physical distance?

remcob 5y ago
This holds true for regular travel nearing light speed too: For an external observer the spaceship never goes faster than light, but the passengers clocks slow down and they can experience arbitrarily high faster-than-light speeds. For some reason I never see this discussed when people talk about FTL travel, maybe I'm wrong?
- hiptobecubic 5y ago
  I think because we use words like "faster" to mean "more stuff in less time," not just "feels like less time to you, but isn't."
- pcl 5y ago
  Orson Scott Card’s sequels to Ender’s Game take this into account.
- bno1 5y ago
  It's because distances and time durations contract the faster you travel. The passangers don't experience faster than light speeds, just shorter distances. This is how the twin paradox gets solved. The twin that leaves earth sees the trip as if it was shorter in both directions, so from their perspective it makes sense that they aged less than the twin that remained on earth. Also, if you had infinite energy and you could travel at the speed of light you wouldn't feel any movement or time passing during your trip, it would feel like instantly teleporting from one place to another. Photons wouldn't feel their existence if they could. From their perspective they are produced in one place and instantly absorbed in another place.

GordonS 5y ago

I was just about to say that same thing - any time I've seen wormholes in sci-fi (I'm reading Peter F Hamilton right now!), you go through in an instant (for all observers). It's like sci-fi uses wormholes as a practical alternative to travelling at sub-C, relativistic speeds.

tokai 5y ago
In the game Free Space there is travel time through wormholes (subspace jump nodes). In the finale the player intercepts and destroy the enemy flag ship as it is travelling through a wormhole to Earth. Only scifi example I can think of wormholes with travel time.
- Aerroon 5y ago
  Stargate has had time travel through wormholes in a few cases.
- JaimeThompson 5y ago
  The wormhole in Star Trek Deep Space 9 had a travel time.
- ben_w 5y ago
  Farscape has an episode where wormhole time travel happens. How does time travel get presented in that game? Searching got me unrelated results.
Rhinobird 5y ago
Yeah. Think teleporter instead of warp drive
- GordonS 5y ago
  Yes, exactly what I meant, but said more succinctly!
dylan604 5y ago
In Contact, <spoilerAlert> the pod drops instantly through from everyone's perspective outside the wormhole, but inside the wormhole 99 hours elapsed (potentially).

teeray 5y ago

> is there any real practical use for them? An infinite source of power. Place one wormhole at the top of a hill, another at the bottom. Send rolling generators through them, stop them once in awhile to swap dead batteries for charged ones.

ben_w 5y ago
I suspect that (if wormholes can actually be made) gravitational potential would be smooth throughout, and that you’d be pulled up as much while falling out of the top as you get pulled down while approaching the bottom.
- ars 5y ago
  Keep in mind you can not create or destroy gravitational potential - you can only move it. Otherwise you have a discontinuity. So you would have to start your wormhole with both mouths at the same place, and then move one mouth to the new location, with gravity "flowing" between them all the while.
  - ben_w 5y ago
    The word “flowing” is ambiguous, I don’t know if you’re agreeing with me or disagreeing with me.
    - ars 5y ago
      A change in gravitational potential (because you moved a mass) flows through normal space. It would do the same through the wormhole. So I'm agreeing with you, and adding how this gravitational slope is created: It's created at the start of the wormhole and changes as you move the mouths of the wormhole, rather than popping into existence at a later time with the two wormhole mouths separated.
teekert 5y ago
Something tells me the wormhole also costs energy.

imglorp 5y ago

The most practical constraint is getting to one. The paper indicates they will "resemble intermediate mass charged black holes". The nearest black hole at the moment, maybe a worm hole candidate, is V723 Monocerotis at 1500 LY away. This would be tens of thousands of years of travel. So perhaps we can learn from afar but not visit.

deleted 5y ago

[deleted]