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Thread: Stupid questions about black holes

  1. #1 Stupid questions about black holes 
    Forum Freshman MrStupid's Avatar
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    Doesn't SR require that gravity not move faster than light?

    So if photons cannot escape a black hole, how can gravitons escape?

    If gravitons can't escape a black hole, how can we know it's there?

    If we can't know it's there, how can it exist?


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by MrStupid View Post
    So if photons cannot escape a black hole, how can gravitons escape??
    Firstly, gravitons are purely hypothetical. There is, as yet, no quantum theory of gravity.

    However, if it turns out that gravity can be explained as a force mediated by force carriers (gravitons) then they will be "virtual gravitons" which are not constrained in the same way. By way of analogy, a black hole can also (in principle, at least) have charge. The electromagnetic force is carried by photons ... by virtual photons ... and so the electric charge could exert a force on charged bodies nearby.

    If gravitons can't escape a black hole, how can we know it's there?

    If we can't know it's there, how can it exist
    Currently, the gravitational effects of a black hole (or any mass) are described in terms of the curvature of space-time. We detect the gravitational effects, e.g. on stars orbiting nearby.


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  4. #3  
    Forum Freshman MrStupid's Avatar
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by MrStupid View Post
    So if photons cannot escape a black hole, how can gravitons escape??
    Firstly, gravitons are purely hypothetical. There is, as yet, no quantum theory of gravity.

    However, if it turns out that gravity can be explained as a force mediated by force carriers (gravitons) then they will be "virtual gravitons" which are not constrained in the same way. By way of analogy, a black hole can also (in principle, at least) have charge. The electromagnetic force is carried by photons ... by virtual photons ... and so the electric charge could exert a force on charged bodies nearby.

    If gravitons can't escape a black hole, how can we know it's there?

    If we can't know it's there, how can it exist
    Currently, the gravitational effects of a black hole (or any mass) are described in terms of the curvature of space-time. We detect the gravitational effects, e.g. on stars orbiting nearby.
    What does it mean then when people say the speed of gravity is the same as the speed of light (in a vacuum)?

    Are they wrong?

    Isn't GR a classical theory in which matter is modeled as a fluid without charge?

    Does a photon somehow propagate differently through the gravitational field then a graviton (or whatever you call a little bit of gravity) does?
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  5. #4  
    Forum Radioactive Isotope MagiMaster's Avatar
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    The speed of gravity is the speed at which changes in gravity propagate. That is, if the sun vanished, how long would it take for us to notice the change in gravity. This is what's equal to the speed of light.

    And GR handles charge as part of the total mass-energy of an object.

    Disclaimer: I am not a physicist. If someone who knows better comes along and corrects me, take their word over mine.
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  6. #5  
    Malignant Pimple shlunka's Avatar
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    A stupid question would be "what color are they?". Your questions are perfectly reasonable.
    KALSTER, Strange and Dinesh balu like this.
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  7. #6  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by MrStupid View Post
    What does it mean then when people say the speed of gravity is the same as the speed of light (in a vacuum)?
    Actually, the speed of the wave depends somewhat on the form of the wavefront itself - it would be more accurate to say that it can at most propagate at the speed of light, but it can be less than that too.

    Are they wrong?
    No. Changes in the field propagate at most at the speed of light, static fields act instantaneously.

    Isn't GR a classical theory in which matter is modeled as a fluid without charge?
    It can handle fluids carrying electric charge, too

    Does a photon somehow propagate differently through the gravitational field then a graviton (or whatever you call a little bit of gravity) does?
    Graviton packets would be equivalent to gravitational waves, so they are just disturbances of the gravitational field.
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  8. #7  
    KJW
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    Quote Originally Posted by Markus Hanke View Post
    Changes in the field propagate at most at the speed of light, static fields act instantaneously.
    It's worth noting that static fields aren't really acting instantaneously, but that the field is already present at the location by the time the object has arrived. Being static implies quite simply that the field has the same value now as it was when it was emitted (if it can be regarded as being emitted - I'm not making that claim).
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  9. #8  
    KJW
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Markus Hanke View Post
    Changes in the field propagate at most at the speed of light, static fields act instantaneously.
    It's worth noting that static fields aren't really acting instantaneously, but that the field is already present at the location by the time the object has arrived. Being static implies quite simply that the field has the same value now as it was when it was emitted (if it can be regarded as being emitted - I'm not making that claim).
    Basically it is changes that propagate because it is only changes that can be observed to propagate.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  10. #9  
    Forum Freshman MrStupid's Avatar
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by KJW View Post
    Quote Originally Posted by Markus Hanke View Post
    Changes in the field propagate at most at the speed of light, static fields act instantaneously.
    It's worth noting that static fields aren't really acting instantaneously, but that the field is already present at the location by the time the object has arrived. Being static implies quite simply that the field has the same value now as it was when it was emitted (if it can be regarded as being emitted - I'm not making that claim).
    Basically it is changes that propagate because it is only changes that can be observed to propagate.
    I don't think of the static field as being truly static, but as being constantly refreshed by unceasing flows of gravitons moving at the speed of light from the source mass.

    Gravitons refer to particles or simply infinitesimal bits of wavefront.
    Then when a mass moves the signal propagates at the speed of light because the not-truly-static nature of the field is being revealed.
    There really aren't any static fields, because everything is accelerating in one way or another.
    So, I think of static fields as a purely theoretical case while the reality is a constant flow of gravitons at c.

    What I don't understand is whether "a" graviton from a test particle (extremely tiny mass) would move on the same trajectory as a photon emitted at the same time.
    I also wonder if that would cause a problem for relativity because if they don't follow the same trajectory, it seems you could arrange a path from A to B, say, where the time would be different for the graviton and the photon.

    This motivates my question about black holes. I had been assuming, maybe incorrectly, this model of a constant flow of gravitons. But if light can't escape from a black hole, then gravitons couldn't escape either, I was thinking, and therefore the field could not be refreshed, effectively causing the black hole to disappear.

    It would disappear first in its remote effects, from the point of view of a distant observer, because gravitons there are moving faster in the Schwarzschild sense, but would work its way inward over time, in the limit disappearing completely.
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by MrStupid View Post
    This motivates my question about black holes. I had been assuming, maybe incorrectly, this model of a constant flow of gravitons. But if light can't escape from a black hole, then gravitons couldn't escape either, I was thinking, and therefore the field could not be refreshed, effectively causing the black hole to disappear.
    Gravitons only mediate changes in the gravitational field, so there is no steady flow of them eminating from a static object. And how do you tell whether the mass of a black hole has changed ? By observing the area of the event horizon; when it changes, the gravitational influence changes. Hence, gravitons actually originate from the event horizon, at least for an external observer.
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  12. #11  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by MrStupid View Post
    I don't think of the static field as being truly static, but as being constantly refreshed by unceasing flows of gravitons moving at the speed of light from the source mass.
    Then you think wrong. A static magnet (or charge) does not continuously emit photons.

    What I don't understand is whether "a" graviton from a test particle (extremely tiny mass) would move on the same trajectory as a photon emitted at the same time.
    If such a thing were possible, then yes they would follow the same geodesic as they are both massless particles.
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  13. #12  
    Forum Freshman MrStupid's Avatar
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by MrStupid View Post
    I don't think of the static field as being truly static, but as being constantly refreshed by unceasing flows of gravitons moving at the speed of light from the source mass.
    Then you think wrong. A static magnet (or charge) does not continuously emit photons.

    What I don't understand is whether "a" graviton from a test particle (extremely tiny mass) would move on the same trajectory as a photon emitted at the same time.
    If such a thing were possible, then yes they would follow the same geodesic as they are both massless particles.
    That's an interesting point about magnets, but I'm not sure gravity works the same way as magnets. (I am particularly stupid about magnets.)
    However, magnets and all other massive bodies are moving all the time, and so the field is changing all the time, and so something must be moving all the time from the moving magnet or other mass to refresh the field, no?
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  14. #13  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by MrStupid View Post
    That's an interesting point about magnets, but I'm not sure gravity works the same way as magnets.
    If gravitons exist, then in principle the way they operate as force carriers would be the same.

    However, magnets and all other massive bodies are moving all the time, and so the field is changing all the time, and so something must be moving all the time from the moving magnet or other mass to refresh the field, no?
    Magnets are not necessarily moving all the time. Remember, all movement is relative. The magnet on the desk in front of me is not moving (relative to me) so the magnetic field is not changing and it is not emitting photons. I don't think linear motion is enough for it to emit photons either. If I shook it backwards and forwards really fast, I suppose, in principle, it would emit radio wave photons.
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