Black holes and causality

Discussion in 'Science' started by Pacifist, Jul 18, 2008.

  1. Pacifist

    Pacifist Member

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    I was thinking, how can black holes possibly exist?
    Information has to travel faster than the speed of light to escape a black hole right. And it's impossible for information to travel faster than the speed of light (causality).
    So how is a black hole about to transmit information about its mass in the form of gravity to the outside world?
     
    Last edited: Jul 22, 2008
  2. von.Gunst

    von.Gunst Member

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    I didn't know that information could escape black holes, however heat apparently can - when in doubt, blame quantum theory. I tried reading a bit more to help you out, but my brain started melting - stupid physics-less science degree.
     
  3. hlokk

    hlokk Member

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    Radiation can be emitted by Hawking radiation from a black hole, so they eventually disappear.

    As for the information of the mass, the black hole, like any massive object (used in the actual sense of the word) distorts spacetime, which then gives the illusion of gravity. There is not some invisible hand that reaches out and pulls something down, the objects are just in warped spacetime.

    Visualise it like a bowling ball on a trampoline, and you have a remote control car. The bowling ball is like the sun and distorts the fabric of the trampoline. The car going in an apparant straight line will deviate from the expected line, as light is deviated as it passes by the sun (ever so slightly). Now, a black hole is like a incredibly dense, small object, so for the trampoline, just imagine someone has taken a point and attached it to the floor using a bolt or something. Say we reverse the car down into the hole, at some point the car wont have enough power/speed to stop itself slipping. The transition point is the event horizon. Now, just because the car cant escape from below that line, doesnt mean that the car is not affected by the warped spacetime.

    Changing the mass might be a little different, gravity waves and all that.
     
  4. OP
    OP
    Pacifist

    Pacifist Member

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    Yeah but regardless of how the gravitational information is transmitted it is still going faster than the speed of light to escape the black hole (i'm guessing it travels instantly considering the information is the warping of space-time itself).
    If i was to move a mass around and someone could detect the gravitational effects of that mass moving around then that would be a faster than light form of communication.
    But my whole life i've been taught faster than light communication = time travel (link) so i'm really having trouble getting my head around this.
     
  5. Zylatis

    Zylatis Member

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    Quantum Theory to the rescue! Basically what happens is the gravitational field loses energy via virtual pair produciton (and gravity waves but only if its a kerr black hole ie rotating or changing somehow).

    In QM there is the well known Heisenberg uncertainty principle which manifests itself most prominently in the position momentum uncertainty equation which says that

    (change in x)(change in p) >= hhbar/2 However there is also the energy-time uncertainty relation which is of a similar form, although the RHS is different.

    Basically this allows for virtual particles. Now dont be fooled, they do actually exist and can be detected in scattering experiments so virtual is a pretty poorly picked name if you ask me. ALl it means is that as per the ETU relation (rar acronyms ftw) they can only exist for a certain amount of time based on their energy. I cant remember the specifics as i did particle physics a while ago but basically if you get them having negative energy they are virutal, i think. Anyway people often say that virtual particles just pop into existence from nowhere and then annihilate each other. This isnt strictly true as 'from nothing' would violate thermodynamics. In a vacuum you have vacuum energy to provide for it. Around a black hole you have its gravitational field and all the energy contained therein (its mass basically).

    So, what happens is the gravitational field loses a bit of energy and in doing so creates virtual particles (always produced in matter/antimatter pairs afaik). There is a chance that one of the pair is created on one side of the event horizon and the other on the other side. So one falls into the black hole and the other can escape which is what we see as Hawking radiation. This process seems to undo the 'virtualness' of the particles, as to why im not entirely sure, i think it has to do with measurement postulates or something.

    Anyway, we detect this hawking radiation and say ooh ah but more importantly the gravitational field has lost some energy. This means the BH has lost some mass, thus the BH will slowly 'evaporate', which i think is pretty friggin cool to be honest =)
     
  6. hlokk

    hlokk Member

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    Ok, back the trampoline example. If you hit a trampoline, the resulting waves will travel at some speed. (Lets assume the trampoline is practically infinitely big, so the distortion doesnt apply more tension to the edges). Now, you cannot transmit information faster than these waves (analogous to the speed of light). If we drop a bowling ball on there, waves will propagate out.
    If we leave the ball on the trampoline, the trampoline stays distorted around the ball. So, the 'gravity does not need to travel to the object everytime to influence it'. Say, if you had some pulsating mass, then there would be a time delay, as gravitational waves travel at the speed of light, but you dont get gravitational waves with a non-changing mass (rotating or mass change).

    Due to the speed of the waves, moving a mass around would not result in FTL information transfer (providing you could detect it).

    The gravity properties of a black hole is such that it behaves as a normal mass, with diameter outside the event horizon with whatever mass it has (in other words, the gravitational field is identical, outside of the event horizon to an non-black hole object of the same mass)
     
    Last edited: Jul 18, 2008
  7. OP
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    Pacifist

    Pacifist Member

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    But then could the virtual particles be used for faster than light information transfer? It seems they could if they can be created on each side of the infinite space-time distance that is the event horizon.

    Edit: Also it seems scientists say virtual particles do not count as information transfer? The mass of the black hole is information. The virtual particles tell me about the mass from the other side of the event horizon. That's information isn't it?
     
    Last edited: Jul 18, 2008
  8. OP
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    Pacifist

    Pacifist Member

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    Put it this way. Why doesn't light escape using the same method?
    ie. light is always propagating but as far as it's concerned there is an infinite distance from one side of the event horizon to the other. The same should hold for gravity waves unless they don't move at the same speed as light.
     
  9. Bastard Child

    Bastard Child RIP

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  10. Zylatis

    Zylatis Member

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    Thats a bit different, that was about information being destroyed. Im not sure that virtual particles cant be used as information transfer but like i said i think in this process they become 'real'
     
  11. Kow

    Kow Member

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    Information /particles can travel faster than the speed of light, at least in the quantum world. check out these biatches right here :p

    http://en.wikipedia.org/wiki/Tachyon * may or maynot actually exsist yet :p
     
  12. Whisper

    Whisper Member

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    The guys who really know their stuff are still arguing about this.

    So don't expect an answer here, one doesn't really exist, I'm not even sure they have something that amounts to a theory yet. Whatever is below a theory is what they are arguing about.
     
  13. Zylatis

    Zylatis Member

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    Well afaik Hawking Radiation and the way its produced is fairly concrete, as far as these theories go.
     
  14. DaveHimself

    DaveHimself Member

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    a: no 100% proof blackholes do or don't exist.
    b: no 100% proof we cannot go faster than light....yet.
    c: 100% proof that this shit is so damn cool to think about.:D
     
  15. Sniper

    Sniper Member

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    There such a thing as dark matter as well.
     
  16. irR4tiOn4L

    irR4tiOn4L Member

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    Correct me if im wrong (which i inevitably am), but light cannot escape a black hole because a photon's mass is not zero, and therefore is affected by gravity. The way i understand it, a photon is a separate entity to the space-time continuum itself, and due to its mass, subject to it.

    What your really asking, the way i understand it, is whether a space-time distortion can be affected by a space time distortion, which seems nonsensical to me (with my pleb understanding). Its a continuum, which means any change in one part directly affects all the other parts. Its not separate 'waves' that travel along the continuum they create (like light would be), its the continuum itself!

    If your version held some water, then I would expect that black holes would have a maximum 'distortion' they could create, before any future distortions could not escape. That would imply a universal mass for all black holes, which hasnt been borne out in the literature ive seen on the subject.
     
  17. von.Gunst

    von.Gunst Member

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    Photons have zero mass, but are still affected by gravity. See the thread "shining a light perpendicular to the earth" for discussion on that, if you're interested.
     
  18. irR4tiOn4L

    irR4tiOn4L Member

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    Theyre half-half is all i know. Half wave, half particle. The thread you pointed out suggested 'four-momentum' is the relevant attribute, which i take to mean inertial-mass - the resistance of a particle to further changes in velocity. However, my understanding is that something must be matter - a particle - (or at least half-so, like light) for it to have inertial-mass at all, and be affected by space-time distortions. Since the propagation of space-time distortions, though they move at light-speed, are not matter with an inertial-mass, but rather part of the space-time continuum itself, as i previously pointed out, i believe these should not be subject to themselves - the distortion of space-time should not hinder the propagation of further distortions.

    Do you agree with the rest of what i said, or did i go wrong there too?
     
    Last edited: Jul 20, 2008
  19. Zylatis

    Zylatis Member

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    Check out the big argument i had with someone else a while ago =)
     
  20. oldnewby

    oldnewby Member

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    A black hole "causes" a warp in space/time, as does any massive object.
    Why do you assume it transmits any information at all?
     

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