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Black Hole entropy

  1. Feb 13, 2015 #1
    How exactly did Hawking compute that black hole entropy is 1/4 that of a planck area and concluded about the holographic principle where information of a volume is located on the area of black hole? And if there was no holographic principle, how big should entropy of the black hole be with reference to planck area (would it be say 1/8 or twice that of planck area for example)? How is this computed?

    What observational evidence is there for these? Do all observations still obey the holographic principle?
  2. jcsd
  3. Feb 13, 2015 #2


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    Staff: Mentor

    Black Holes and Entropy
    Bekenstein, Phys. Rev. D 7, 2333, abstract

    Black hole explosions?
    Hawking, Nature 248, 30 - 31 (01 March 1974), abstract

    There is no observational evidence for hawking radiation from black holes at all - but that is not unexpected as the radiation is so weak. There are some experiments that see similar effects in other types of effective horizons (water waves, I think currents, and whatever).
  4. Feb 13, 2015 #3


    Staff: Mentor

    Hawking didn't come up with this principle, and it is a separate concept from the computation of entropy in terms of the horizon area. See here:


    The computation of the black hole entropy, as above, does not depend on the holographic principle; it is the same whether that principle is true or false.
  5. Feb 13, 2015 #4


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    Yes, the primary evidence for Hawking Radiation from black holes is that when we create a horizon in some other wave medium (such as sound waves in a fluid), we get Hawking radiation emitted from the horizon.
  6. Feb 13, 2015 #5
    Ok I realized now, but I wondered this (and caused of my initial confusion). Since Hawking was the first one to discover about the computation of entropy in the terms of the horizon area, how come he didn't conclude that all information of physical systems can be located in the area and not volume. How come it needs others (Susskind) to realize this?
  7. Feb 13, 2015 #6


    Staff: Mentor

    I couldn't say. Anyway, that's not a question of physics, it's a question of history and human psychology.
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