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Black holes and light!

  1. May 2, 2013 #1
    Hello, I was wondering a thing or two about black holes, now I'm not any kind of physicist or hobby astronomer for that. But can a black hole consume infinite amounts of, lets say gamma rays without increasing in mass or any other effect? However gamma rays haven no mass since they are pure energy or light?

    Thank you for taking the time to answer! (PS! I googled it but couldn't find an answer)
    Last edited: May 2, 2013
  2. jcsd
  3. May 2, 2013 #2


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    A black hole can only absorb what is near enough to be drawn in. A popular misconception is a black hole can absorb the universe [or the earth], which is wildly inaccurate. Only things that wander in too close to a black hole misbehave. Everything else thinks a black hole is merely another gravitating object - like a star.
  4. May 2, 2013 #3
    massless particles are not necessarily massless. They have no rest mass.
    So photons can still contribute to the mass of the BH.
    There is no pure energy as per se. Energy and mass are essentially the same.
  5. May 2, 2013 #4
    Bear in mind that photons do have momentum, defined as P = hf

    One why to describe the effect would be:
    Turn 1kg into photons via nuclear explosion(mixing 0.5 kg matter with 0.5kg anti-matter)
    focus the radiation via a laser beam
    Fire the pulse at the black hole

    The laser apparatus and the black hole have now repelled each other through this exchange in momentum. The relatavistic mass change resulting from change in speed will be apparent as an increase in mass distributed between the gains in the black hole or the laser apparatus.
    If the laser apparatus were equal in mass to the black hole, I expect both would be accelerated in opposite directions to a speed at which they both gain 0.5kg each- so the original 1kg is conserved.

    I think this is correct, but would be interested if I'm wrong on this.
    Last edited: May 2, 2013
  6. May 2, 2013 #5
  7. May 3, 2013 #6
    "Black holes have no hair." is a famous quote about black holes [BH]. It means they are very 'simple' and display only mass, charge and spin. By "mass' is meant any energy, pressure or mass that either caused the BH to form in the first place or which were close enough to be absorbed after one formed. gamma rays are electromagnetic energy so if any were absorbed by a BH it would increase a bit to reflect the additional energy.

    Here are two descriptions to get you started:

    Simple answer.....yes, BUT!!!!....it depends on just what you mean by "surrounded' and also your frame of observer reference. One viewpoint is that everything that has ever been enclosed with the black hole event horizon is represented as information residing just outside the event horizon....maybe at Planck length outside the event horizon (per Leonard Susskind)….on a stretched horizon.

    But there is a lot more to it:

    from Roger Penrose:

    Watch clips of black holes colliding [simulations]

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