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Black Holes?

  1. Jun 21, 2007 #1
    IF nothing can escape from a black hole's event horizon,
    then how do x-rays and Gamma rays escape? and how does it emits light?
    it says in "Brief History of Time" that the light it emits is just at the boundary of the event horizon. So then how does it escape the gravity and reach earth? (x-rays) since Gamma rays can't get through the atmosphere. :uhh:
  2. jcsd
  3. Jun 22, 2007 #2


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    In most cases it will be a superheated accretion disk that is generating the X-ray emmisions. As you can imagine when somehing falls toward a black hole it loses an enormous amount of potential energy and this will heat an accretion disk to the point where it emits x-rays. Black holes do not emit light, but stuff that has not crossed the event horizon can.

    You may find the following link interesting as it has a lot of information about X-ray binaries.

  4. Jun 22, 2007 #3
    Even a black hole without accretion disk will emmit extremely faint radiation - the so called Hawking radiation. It is a speculative quantum calculation first done by Hawking showing that the observer at infinity detects flux of particles comming from the black hole. The flux was formed during the formation of the black hole just outside its horizon.

    An eternal blackhole (not formed by collapse of matter) doesn't emmit Hawking radiation.

    The calculation is speculative because it was done in a 'semiclassical approximation' to Quantum gravity, a theory yet to be created.
  5. Jun 22, 2007 #4


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    Space observatories? Sensitive, Earth-based observatories? This seems an unrelated question to your first one about BHs.
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