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Are black holes cold?

  1. Dec 14, 2007 #1

    new here, my first post. Like many people here, I'm no astrophysist, but I have an active interest in our solar system, cosmology in general and I guess astrophysics. Here's my first question.

    "Are black holes cold?".

    Given my amateur understanding, I would say yes, they are near absolute zero (they leak very slowly). But if light can't escape it, then I dont see how other wave forms can escape it. Or am I missing something?

    Thanks for any opinions.

  2. jcsd
  3. Dec 14, 2007 #2


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    The temperature of a black hole is inversely related to its mass. But generally speaking, yes, black holes are cold, as it takes a less than stellar mass BH to have a temp greater than even the Cosmic Background radiation.
  4. Dec 14, 2007 #3
    This is an application of the first law of thermodynamics, energy in is energy out. The black hole doesn't 'suck' in heat the same as it 'sucks' in matter.
  5. Dec 14, 2007 #4

    jim mcnamara

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    Assuming Hawking's quantum black holes exist, what about temperature in that realm?
    I do not see how anything near Planck Length in size can have measurable properties like that.

    It just seems odd to me that someone would postulate being able to measure temperature inside a region of highly curved spacetime, where the other physical laws on which we base the concept of temperature (here in flat spacetime) are inapplicable. Since this is clearly not my field I would appreciate some correction.
    Last edited: Dec 14, 2007
  6. Dec 14, 2007 #5
    The Hawking radiation is a speculative theoretical derivation. Hawking considered what happens to light radiation before, during and after creation of a black hole from gravitationally collapsing body. The light rays that escape after the BH formation almost glazed the forming and expanding event horizon but never got inside so there is no contradiction with the idea that nothing can escape from inside an even horizon. The result of the calculation was that after the BH formation, the rays that manage to glaze the horizon and escape back to infinity will have a thermal spectrum characterized by a temperature that is inversely proportional to the BH mass. I think there is still controversy going on because close to the horizon, the photons were very high in energy (blue shift), even above Planck's scale, and we can't be sure as you said that the laws we know at lower energies still apply.

    Also I've seen statements in the literature that the classical eternal black hole of GR that exists forever, never created or destroyed, is symmetric in time therefore it doesn't radiate like a BH formed from gravitational collapse. They say that the eternal black hole has another natural choice of vacuum for the radiation which doesn't produce Hawking radiation.
    Last edited: Dec 14, 2007
  7. Dec 16, 2007 #6
    where is kenetic energy of accreted matter?

    the black hore is formed by accreted matter from second star. the speed of accreted matter can reach the light speed at the last orbit [tex] R_g\sffamily\approx \sqrt{\frac{G M}{c^2}}[/tex]. After this limit neither light no matter can escape from the system , my question where is passed the kenitic energy of the matter accreted at this high speed?
  8. Dec 17, 2007 #7
    the matter is accreted with its energy into black hole, and no things can escape from there after this moment. logicaly the black is not so cold. matter cross the horizon limit with a high speed and consequently its kenitic energy is very high and will be converted to thermal energy.

  9. Dec 17, 2007 #8

    jim mcnamara

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    I guess I don't understand - I thought anything "operating" beyond Planck's scale was considered either impossible or dismissed as an artifact. Therefore, a photon with wavlength shorter than Planck's length either could not be observed or doesn't exist? Or maybe we cannot describe it....
  10. Dec 17, 2007 #9
    yes i agree.............black holes could be a lot warmer than previously thought
  11. Dec 27, 2007 #10
    a blackholes event horizon bares an inverse ratio with its temperature. the bigger the event horizon the less the temp it has.this says that smaller blackholes are red hot while large blackholes are cooler as compared to the small ones.
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