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Question about Black Holes.

  1. Sep 22, 2009 #1
    I have been thinking about this for a few weeks and can not come up with an answer. As I understand it a black hole is essentially a gravitational field so powerful not even light can escape it. The last part is what I don't understand. I thought light(photons) was massless. If light has no mass how can it possibly be trapped by a gravitational field? Sorry if this is a trivial question.
  2. jcsd
  3. Sep 22, 2009 #2


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    Light responds to gravity - general relativity explains it. It doesn't requite a black hole.
  4. Sep 22, 2009 #3
    Gravitational fields are created by, and affect, anything with energy and momentum as those are the parameters gravity couples to. In the newtonian model it only couples to mass and so light is unperturbed by gravity whereas in GR it does as photons definitely have energy and momentum. This is why the sun bends starlight from behind it and why gravitational lensing occurs.
  5. Sep 22, 2009 #4


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    Another way of saying the same thing:

    Gravity is not a force. Mass bends space and that curvature is gravity. Light travels in straight lines through space. If the space is curved, so is the light's path.

    Inside the event horizon of a black hole, the space is so curved that all paths curve back into the BH, and thus so do the light rays.
  6. Sep 23, 2009 #5
    thanks for the answers.
  7. Oct 5, 2009 #6
    A black hole is NOT a gravitational field. It is a spherical body, like a planet or star, whose mass is so great that the gravity produced is enough to pull in light.

    We know that gravity affects light, and some believe that light does indeed carry a small amount of mass, but reguardless, we know it is affected. Thus, a black hole is a gravity sphere with enough pull to keep any light emitted from its surface from getting free of its gravity field. Think of it like throwing a baseball straight up into the air...it gets away from the surface, but then gravity pulls it back in. Same thing with light.

    But a black hole is definitely a mass...not just a gravity field.
  8. Oct 5, 2009 #7


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    With the exception of the last sentence, this is all incorrect.

    A BH is not a spherical body and not like a planet or star. It is a singularity, theoretically zero-size. There is a radius defined - the event horizon, defined as the radius below which nothing can escape. This radius has no physical manifestation; it is entirely a definition.

    A BH does not treat light like a baseball. Light paths within the EH will bend back towards the BH, true, but a light beam that managed to find itself pointing directly out of the BH would not be pulled back down, it would be inifinitely red-shifted.
    Last edited: Oct 5, 2009
  9. Oct 5, 2009 #8
    Well, given that we have ZERO information from inside the event horizon all of this is speculation, but we do know that gravity pulls matter into spheres, so my assertion of the shape shouldn't be an issue.

    As for matter, it exists, therefore it has some volume no matter how densely packed it becomes. So, the discussion can be how small of a sphere, but there has got to be one at the center.
  10. Oct 5, 2009 #9


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    We have mathematical models of what matter does under the extreme forces in a BH. This is the prevailing model, which is what Physics Forums supports, and what you agreed to when you registered.

    If you have some other ideas, you are welcome to publish them in a journal.
  11. Oct 5, 2009 #10
    Mathematical models are just guesses. Without at least some raw data it's just speculation.

    We know that gravity results from mass...mass has volume...therefore a blackhole must have volume. It's simple logic.
  12. Oct 5, 2009 #11


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    This is not true. You may be on the wrong board.

    It is currently understood that, at the centre of the black hole, the singularity, mass is compressed to zero volume and infinite density. Your second statement is false.
  13. Oct 5, 2009 #12
    I think you should be using the word 'matter' here. You can't compress mass as mass is a property of matter, think I'm right in saying that.

    I would have thought that in the extreme environment of a black hole matter could not structurally exist. Possible a mathematical model could be contrived to allow the individual atomic components to get ever closer but it would seem unlikely in reality.
  14. Oct 5, 2009 #13


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    Good point.
    Yes. Our understadning of atomic structure says that the atoms will lose their identity and be compressed to a point.
  15. Oct 6, 2009 #14


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    Isn't there a theorem proven in the context of classical GR that states that an object which collapses below its schwarzschild radius must end up as a singularity? I want to say it was proven by Hawking and someone else, but I can't be sure of that.

    At any rate, in the frame of classical (non-quantum) gravity, we must then accept singularities will form. As far as I'm aware of, no quantum theory of gravity has anything meaningful to add to this discussion at this time, so in the spirit of sticking to physics, we should accept the singularity.
  16. Oct 6, 2009 #15


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    It is conceivable mass can only be compressed to the planck density in a black hole. It would occupy a finite, albeit tiny volume of space in such a state.
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