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A Blackhole Paradox

  1. Jul 13, 2008 #1
    If we can agree that a blackhole can exist, and that its gravitational field is intensified with its mass, the we can also agree that its gravity can create one hell of a gravitational lens.

    If that is the case, and EM radiation, whatever the frequency, is directed towards the blackhole, how can we say that EM radiation cannot escape if it approaches the center? It would make sense that because of the gravitational lens effect, that EM radiation would less likely ever reach the event horizon, as it is guided around the intense gravity field.

    I guess I am saying that there is a paradox in which gravity from a blackhole cannot indeed repel EM radiation if the gravity itself will turn around and make the EM radiation not escape its event horizon.

    Perhaps the center of the black hole is black, not because no EM radiation can escape it, but perhaps because no EM radiation can ever get to it, as it is pushed around the intense gravity field of the black hole itself.
  2. jcsd
  3. Jul 13, 2008 #2


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    You misunderstand what gravitational lensing is and does.

    It does not "guide EM rays around the BH", it bends rays that otherwise would have passed the BH by a distance into rays that now converge.

    See attached diagram.

    Attached Files:

    Last edited: Jul 13, 2008
  4. Jul 14, 2008 #3
    Wow, a picture is worth a thousand words. I understand what you are saying now.

    I guess this is what I was visualizing:


    I got it from this link:

    http://imagine.gsfc.nasa.gov/docs/features/news/grav_lens.html [Broken]

    I believe I was visualizing a distortion in space time in which gravity would bend light.

    The picture you posted is the first representation of light vectors pointing in that direction that I have seen.

    Would that picture apply only to black holes if the event horizon was horizonal to the observer, of would it apply to planets and stars as well?
    Last edited by a moderator: May 3, 2017
  5. Jul 14, 2008 #4


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    Yes, that is the most common representation, and is also correct. I'd started off describing the effect just like that but then I realized where it was steering you wrong. If interpreted wrong, it could look like the rays are "pushed" apart by the BH.

    I've attached another image with the missing rays added in. Note the star is radiating diverging rays in all directions. Some of those rays that would have diverged are bent back toward Earth.

    The event horizon around a black hole is spherical; it will appear the same from all directions.

    (Now, an accretion disc OTOH is in a plane, but that has nothing to do with lensing.)

    Gravitational lensing will occur, at least in principle, around any mass, though it is proportional to the mass of the object, so only very massive objects have a noticeable effect.

    Attached Files:

    Last edited: Jul 14, 2008
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