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Black hole transparency

  1. Apr 11, 2013 #1
    Hey, before people start throwing rotten vegetables at me, I'm not any good in general relativity!
    So my question is: are black holes transparent to electromagnetic waves of wavelengths on the same order as the Schwarzchild radius of the BH?
  2. jcsd
  3. Apr 11, 2013 #2


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    They will scatter the electromagnetic waves and absorb some fraction. I would not call this "transparent" - there is no part of the wave going through the black hole itself.
  4. Apr 11, 2013 #3
    What is your observer? Different observers make different observations.

    A hovering exterior observer is causally disconnected from all events INSIDE the horizon....while a free falling observer, in sharp contrast, can 'see inside'....something just ahead,not the singularity.....if those two observers pass each other they observe very different things and, for example, time passes very differently for them. It's analogous, if not identical, to an Unruh [accelerated] observer making different observations than an inertial observer right alongside.

    I surprised to find mfb thinks some of the radiation is 'scattered'.....I'd like to know
    more about that. As I understand it, free falling observers pass right into the black holes
    observing nothing. Is electromagnetic energy different?? Or does this refer to the electromagnetic fields outside the BH horizon....

    Quoting from "Quantum Fields in Curved Space" by Birrell and Davies,
    for outgoing radiation:

    So saying the particles are emitted by the horizon, or asking what particles you see when you get near the horizon - both of these are meaningless.
    Last edited: Apr 11, 2013
  5. Apr 11, 2013 #4
    I found the description I could not recall:


    So mfb is correct.....but I still don't really understand why....
  6. Apr 11, 2013 #5
    This is an area I only know a little about, essentially a perfect blackbody never exists in nature, their is always some absorbtion, reflection etc.

    I found on that same page with the references listed an article which provides some coverage


    As I said I'm not familiar enough with it to make any detailed explanation
  7. Apr 11, 2013 #6
    Apparently you have been here long enough to know fools are not suffered lightly,
    but perhaps not yet long enough to knew that pleas for 'love and understanding and
    kindness' will go unheeded!!

    If that's what you are looking for, as they say in Washington, DC, Get a dog! [LOL]
  8. Apr 11, 2013 #7
    Thanks for all the answers. It was sort of easy for me to imaging macroscopic black holes partially absorbing km radio waves, but when I tried to think about microscopic black holes in particle accelerators and shorter wavelengths, my imagination just didn't want to have part in it at all.

    So should we call them grey holes instead?

    Well, the more I look at it, the less I get it!%) Apparently, I'm just not cut out for Washington, DC.
  9. Apr 11, 2013 #8


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    What is inside, cannot get out, but what is outside can go around the black hole, be scattered, get reflected, ...
  10. Apr 11, 2013 #9
    I'm not sure I'd go that far lol

    again its not an area I'm too familiar with although I've been studying Hawkings radiation I still have a considerable ways to go in my studies on various blackbody radiations. I have seen published papers on arxiv.com that describe blackholes are more grey than black but I wouldn't consider it as mainstay regardless of its source
  11. Apr 11, 2013 #10


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    I am assuming an incident wave were some fraction would "miss" the black hole if we would not have gravity. With gravity, that fraction gets deflected, and everything gets complicated.
  12. Apr 11, 2013 #11
    Yeah I've yet to see a good article other than the one I posted covering transparent body, grey body, blackbody and white body. I know what each term means but not clear on the underlying mathematics as of yet. Those terms are in the link
  13. Apr 11, 2013 #12

    A small piece of black cardboard hovering near the event horizon of a black hole has no problem emitting radiation that will have a very large wavelength when observed far away. (Redshift helps here)

    Therefore it is possible to send, from far away, radiation with a very large wavelenght onto the same piece of cardboard. (we know this because there is such thing as time-reversal symmetry)

    From that it follows that it is possible to send, from far away, radiation with very large wavelenth into the same black hole. (Because the black hole is larger than the small piece of cardboard)

    (I am less sure about small black objects away from large masses. The radiation they emit does not contain very large wavelengths??)
    Last edited: Apr 11, 2013
  14. Apr 11, 2013 #13
    That would be an extremely long wavelength. I don't know. I feel certain that there would be some absorption. As to whether the fraction that is not absorbed passed through the black hole, it might or might not be possible to measure experimentally.

    That inspired a question of my own. Can gravitational waves pass through a black hole?
  15. Apr 12, 2013 #14
    I don't think anyone knows the wavelength of a gravity wave lol

    I was thinking about this a bit more and read that link I posted in more detail, I can see how it could apply for microblackholes but I did not see how any of the examples of reflection and absorbtion apply with regards to a BH. No where did that article discuss the light paths generated from a BH twisting of spacetime .
    The surface bouncing of waves I would think would be quite different than the cardboard example above. The other important factor is can a wavelength exceed the EH of the average size BH, I'm not sure that it does.
    Anyways I would surmise that any reflection would occur at the outer portion of the ergosphere
    most of the refelction I can see happening via the accretion disk but thats not the same as whats described by the OP on wavelength size.
  16. Apr 12, 2013 #15
    If they do, they likely move to another universe.....but the general answer remains:

    'No mass-energy escapes a black hole'.....especially in our lifetimes.....
  17. Apr 12, 2013 #16


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    It is no problem to calculate the wavelengths of gravitational waves in free space. The formula is the same as for electromagnetic waves, all you need is the frequency.
  18. Apr 12, 2013 #17
    Yeah I know I have to calculate wavelengths for work regularly

    any takers on the frequency of gravity lol
  19. Apr 13, 2013 #18
    Suppose a gravitational wave is generated by two mutually orbiting neutron stars. Then the frequency of the wave would be the frequency of the orbit (or maybe half of that frequency.) So the wave length could be extremely long. That's probably one reason they are so hard to measure.

    As to what happens when such a wave encounters a black hole, I have no idea. I think it would be necessary to solve the equations to be sure.
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