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Dumb Black Hole Question

  1. Nov 30, 2004 #1
    Okay, I'm proberly missing something very silly, but if gravity is thought to go the speed of light, how come it can escape a black hole, and light can not?
     
  2. jcsd
  3. Nov 30, 2004 #2

    selfAdjoint

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    This is a subtlety. The fields are carried by VIRTUAL particles. For example the electromagnetic field is carried by virtual photons, while real ones carry light, or other EM radiation. Similarly gravitation, if it is carrried by gravitons at all, is carried by virtual ones.

    Virtual particles cannot be observed as individuals, therefore they are exempt from the ordinary laws of physics, which only apply to real, observable, particles. In particular virtual gravitons can move faster than light. Since it's just the speed limitation that traps real photons, the FTL virtual particles aren't trapped. This goes for virtual photons too; black holes can be electrically charged, and nearby bodies can feel and respond to the charge.
     
  4. Nov 30, 2004 #3

    mathman

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    The story of black holes can be very complicated. The gravitation force felt outside a black hole is a residue of what was there before the black hole formed. To an outside observer, the black hole never forms - it takes forever for it to become, although in the black hole frame, it is done. Thus the gravitons come from what is, to the outside observer.
     
  5. Nov 30, 2004 #4

    marcus

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    Amen.
    Fields extend out from charges and from masses.
    But if they are unchanging then nothing travels.
    It is the trembling of the field that cannot travel faster than light.
    As ripples cannot spread on a pond faster than some speed.

    But the electric force field that extends out from a static charge is not traveling----it is simply there---

    and the gravity field that extends from a stationary mass into surrounding space is not traveling either.

    Something has to be changing: only the undulations travel, and only they are governed by the speed limit.

    Personally I cannot say for certain if I think VIRTUAL photons and gravitons really exist or whether they are merely computational conveniences invented by humans, which work approximately in certain circumstances. Maybe all a virtual photon is, at bottom, is a wavy line in a Feynmann diagram that helps somebody doing calculation keep track of some aspect of the field.
     
    Last edited: Nov 30, 2004
  6. Dec 1, 2004 #5
    The posts on this thread are an example of how little we understand nature. I see above at least three different answers to why gravity escapes black holes :
    1. It is carried by virtual particles that travel faster than light
    2. It is a residue of an objects gravity before it collapsed into a black hole
    3. Static or nearly static fields just are- they need not travel

    I would suggest that gravity is not at all like light. General Relativity says that the gravitational fields of massive objects determine the configuration of space, which in turn determines how light travels in it. Light is passive. And gravity is fundamental, even more fundamental than space. Mass creates gravity, and gravity creates the black hole which traps light, but not gravity. The gravitational field must exist both in and outside the black hole, just as space must.
     
  7. Dec 1, 2004 #6

    marcus

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    I am in close agreement with all or most of what yanniru says here.
    I dont think those three descriptions are at odds---they look to me like 3 alternative ways of talking about the same thing.

    the gravitational field is the geometry of spacetime---the world-shape

    the mass that collapsed to form the hole leaves a lasting imprint in the field (so 2 is right)

    "virtual gravitons" provide one possible way of describing the field (so 1 is right) although virtual gravitons are not always a mathematically good way of talking in every situation. they're probably more useful, as mathematical devices, in flat or nearly flat space than they are for analyzing the highly curved space around a hole.

    =======
    I also agree with yanniru that we dont know very much for sure about nature. But I see points 1.2.3 not as contradictory but as converging on the same reality.
    =======

    Now to comment on yanniru's own point. (Yanniru I think you know all this, but just to spell it out) If the matter that collapses to form a hole has some net static electric charge then the hole will have a static electric field (like that of a conventional point-charge placed at the center of the hole) which is analogous to the gravitational field.

    It is true that we use different mathematics to describe the electromagnetic field (static and vibrating) and the gravitational field (static and vibrating).
    You are quite right to point out the big difference----the grav. field is described thru spacetime geometry, that is, thru shape---the electric (etc.) field is written down on that space whose shape the gravitational field defines.

    But in a very primitive naive sense the gravitational mass and the static electric charge of a black hole are analogous. One can feel the effect of them both, standing outside the hole's horizon. Just as with any ordinary gravitational mass and ordinary concentration of static electric charge.

    Light is an undulation in the electric (etc.) field
    and it should be compared not to gravity itself but to gravity waves.
    The point I've been trying to make is that the effects of mass AND charge conventionally located at the center of the hole CAN be felt.

    But undulations of mass and charge that has fallen in cannot be felt
    (one way of looking at this, point 2, says that the matter bearing the mass and charge is frozen in infinitely dilated time right at the horizon caught in the act of falling in---so it can't wiggle) and there are several equivalent ways to say this: signals, whether gravity wave or electromagnetic (light) wave, cannot get out.

    Anyone see any mistakes? yanniru is this an ok clarification?
     
    Last edited: Dec 1, 2004
  8. Dec 1, 2004 #7

    Garth

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    Where did the idea that virtual particles can travel faster than light come from?

    The question of whether a virtual particle has mass or not determines its range, because its short lifetime is governed by the uncertainty principle. A massive particle such as the gluon has a short lifetime and therefore the range of the strong nuclear force is restricted so that it acts only within the nucleus, likewise the massless photon has an infinite range because it travels on a null-geodesic.

    Garth
     
  9. Dec 1, 2004 #8

    Chronos

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    I have to agree with Garth. Virtual particles still obey the universal speed limit of 'c'. Propagation of any wave form is constrained by physics as is currently known.
     
    Last edited: Dec 1, 2004
  10. Dec 1, 2004 #9
    ahhhh... but the gluon is massless, my friend
    I disagree. I remember to have read in some Baez's page that virtual gravitons can travel faster than c. If Baez said it, it must be true
     
    Last edited: Dec 1, 2004
  11. Dec 1, 2004 #10

    jcsd

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    Viryual particles can travel faster than c, but they are not real particles and this does not mean that information is being propagated faster than c.
     
  12. Dec 1, 2004 #11

    pervect

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    I looked up the reference on Baez's homepage about virtual particles travelling faster than 'c'.

    here is the link, and this is what it says

    Overall, I have to agree with the position that virtual particles can exceed 'c', but not by "very much".

    The FAQ on "how does gravity get out of a black hole" is also worth reading. It's located
    here


    I'm personally not really very fond of the argument that gravity gets out of a black hole because virtual particles can travel faster than 'c', but it does seem to have a certain amount of support in the sci.physics.faq. But it is not presented as the sole explanation, either. Note that the first argument in the FAQ is that strictly from a GR point of view, gravity doesn't have to get out of a black hole, as it's a curvature of space-time. Only when people try and graft quantum mechanical ideas onto gravity does the problem arise.

    One very good point the FAQ does make, though, is to point out that there is a closely related question that doesn't involve quantum gravity at all, so it should be able to be answered unambiguously. This is the question of how the electric field gets out of a charged black hole. Here, there is no argument that the electric field can be considered to be carried by virtual particles (if desired).

    I would really like to see the math on this, however. I find the verbal explanations just a bit too vague to be really illuminating. The FAQ entry on how one expalins how unlike charges attract via virtual particles was somewhat illuminating, but not enough for me to be really comfortable with the virtual particle explanations.
     
  13. Dec 2, 2004 #12

    Garth

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    Thank you, it is normally accepted that gluons are massless; I was using the Higgs-Kibble mechanism to give mass to gauge particles, such as the gluon, in order to explain confinement, otherwise they would have infinite range.
    As we have never weighed a free gluon, they don't exist, the matter is one of conjecture.
    Alright, I suppose if they are virtual and gravitons they can be made to do whatever you want, Baez did not seem convinced. Virtual gravitons are beyond observational physics.

    Observationally the loss of orbital energy from the double pulsar is consistent with the existence of gravitational waves predicted by GR and therefore with the idea that gravitation and the supposed graviton do travel at c.

    Garth
     
  14. Dec 7, 2004 #13
    For Marcus: Sorry for the delay

    "But undulations of mass and charge that has fallen in cannot be felt
    (one way of looking at this, point 2, says that the matter bearing the mass and charge is frozen in infinitely dilated time right at the horizon caught in the act of falling in---so it can't wiggle) and there are several equivalent ways to say this: signals, whether gravity wave or electromagnetic (light) wave, cannot get out.

    Anyone see any mistakes? yanniru is this an ok clarification?[/QUOTE]"

    I like your clarification, especially the idea that both the static gravitational and electric field can be felt outside the black hole.

    But I tend to now accept point 2 as most realistic; and I think that point 2 is really at odds with the other points. Using point 2 nothing escapes from the hole, not gravity, not the electric field, not even virtual particles. This is because as you say: "matter bearing the mass and charge is frozen in infinitely dilated time right at the horizon ". The source of the gravitational and electris fields is therefore right at the event horizon as far as the outside world is concerned. So there is EFFECTIVE mass and charge accumulation on the surface of the event horizon as far as a distant observer is concerned. Dark Matter would be continiously accumulated.

    So then I wonder if GR solutions inside the event horizon are trustworthy. I guess they have to be except at the singularity. But I have difficulty understanding how time become a spatial dimension and how this relates to inflow and outflow of matter as suggested by Hamilton.

    Richard
     
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