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Hawking radiation and its significance

  1. Nov 15, 2013 #1
    Hello All,

    I have been trying to understand the exact contribution of our legendary physicist, Dr.Hawking and found that Hawking radiation is amongst one.

    Is it that Hawking radiation is the first attempt to unify, general relativity, quantum mechanics and thermodynamics which led to the search of quantum gravity (graviton)?

    Is is also that earlier to that the general notion was that black holes only absorb matter and does not radiate anything?

    Apart from this what are Dr.Hawking's other significant contributions?

  2. jcsd
  3. Nov 15, 2013 #2


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  4. Nov 15, 2013 #3


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    Staff: Mentor

    I don't think hawking radiation unifies anything. It's merely a prediction of Quantum Mechanics (or some other quantum theory) for black holes.

    Yes. Before hawking radiation it was believed that black holes would last forever since they only absorbed matter and radiation.
  5. Nov 15, 2013 #4
    Right now, no black holes are losing mass via Hawking radiation because they are colder than the surrounding universe. They are absorbing more energy than they are emitting.

    Hawking radiation is a thermal radiation; At the end of the universe when things are REALLY cold balck holes are expected to radiate and lose mass, but very, very slowly...and to last on the order of maybe ten times the age of the universe....depending on size....so they may very well be the last objects around.
  6. Nov 15, 2013 #5
    Thank you all for the answers.

    Well, Naty1, what I was reading and understood was that as black holes radiate energy through Hawking radiation (Hr), it looses energy. Well, what is energy? E=mc^2. Now there is something (the concept is still not clear to me), which is called negative energy. So we get -E=-m(c^2) as because c=constant. If we find this equation correct then -m will create a black hole to loose its' mass and vanish.

    Correct me if I am wrong.

    -- Thanks
  7. Nov 15, 2013 #6


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    I'm not quite sure what you're getting at with negative energy, but as a black hole loses energy from radiation it loses mass as well. (Due to the equation you mentioned above) Given a long enough time, a black hole will lose all its mass and disappear.
  8. Nov 16, 2013 #7
    Energy is the equivalent of mass, so when BH lose energy,yes, their observed mass also decreases.

    But there is a net loss only when the BH are warmer than their surroundings.....so far they are a roughly few tenths of a degree above absolute zero and the background radiation [CMBR] about 2.7 degrees.
  9. Nov 16, 2013 #8
    Thank you Naty1.

    In continuation of this I would like to mention that, with evaporation as the black holes starts loosing its' mass, we also arrive at the Information paradox. Whatever passes in the black hole, is finally lost, which again violates the basic tenet of physics. Consider quantum mechanics, the entire information is stored in the wave function, till the wave function collapses. In case of a black hole, we cannot see the collapse also. As far as I know that there was a scientific war between Hawking and Leonard Susskind, the latter strongly objected. It later got solved, where Hawking admitted that he was wrong.

    Can you please tell me how the information paradox was solved?

  10. Nov 16, 2013 #9
    Susskind describes the information loss controversy, with a lot of theoretical background and great analogies, and his unique perspective, in his book THE BLACK HOLE WAR. I loved the book..it's cheap used.

    A Hawking concession note, to another phycisist, Don Page, is copied in the book, handwritten by Hawking...it's a complex statement with a thumprint from Hawking at the bottom for authenticity!. There were several bets going about the resolution of the issue.

    The note means Hawking finally convinced himself information is NOT lost and Susskind writes:

    This essentially means the information is stored in encoded form within the black body Hawking radiation. Susskind's version is 'Black Hole Complementarity' and a 'stretched horizon', a Planck length or so outside the usual horizon:

    At a speech before phyicists Susskind described his analogy:

    and so while an outside observer IS permanently cut off from anything inside the horizon,
    the information is stored outside.....

    Wikipedia has a nice synopsis here:


    The string theory view is that the horizon is encoded with strings containing information....and bits break off due to quantum jitters...this is Hawking radiation with hidden information.

    As discussed in these forums, the additional conclusions of Roger Penrose and his Conformal Cyclic Cosmology discussed in the article are not widely accepted....which doesn't mean they are wrong, just unproven.
  11. Nov 20, 2013 #10
    Thank you Naty1 for this wonderful reply.

    It has helped me clearing doubts, also how the information paradox came into from evaporating black holes.

  12. Nov 20, 2013 #11
    As already noted, this is a bit of an overstatement...but BH do provide a unique common ground for interdisiplinary study. There is common ground regarding BH that brings together aspects of general relativity, quantum mechanics, thermodynamics, and string theory.....those are four that come to mind. Oh, and quantum gravity, not yet a completed theory.....

    If you want all the gory detailed concepts of different approaches, no math, try Kip Thorne's BLACK HOLES AND TIME WARPS...it's the most rigorous non mathematical discussion of black holes I've come across...a lot of GREAT descriptions quite different from Leonard Susskinds so the books complement each other conceptually and provide historial accountings as well.

    As Thorne describes it, Hawking started investigating BH radiation via an 'absolute' horizon rather than the 'apparent' horizon of the late 1960's. A bold step at the time. These were insights from gravity and general relativity. He and James Hartle were able to explain aspects of this new horizon and developed his 'area increase theorem'.....Not Beckenstein, but a fellow student of his studying under John Wheeler at Princeton, first noticed even before Hawking's work that GR formulas for BH looked like those of thermodynamics. When Hawking's work came out, according the Kip Thorne, it was enough to convince Beckenstein a thermodynamic approach could explain aspects of BH ....and so he did via the entropy of thermodynamics. Hawking did not believe it but utilized other approaches to convince himself Beckenstein was right..and in the process acheived the breakthru insight that BH do radiate.
    Had Beckenstein followed his own intuition, we would probably call it 'Beckenstein radiation'.
  13. Nov 21, 2013 #12
    Hello Naty1,

    It is very interesting to note what you have mentioned: "BH do provide a unique common ground for interdisiplinary study. "

    Yes I have the book of Thorne "Black Holes and Time Warps" and yet to go through it. Well, one thing I have in mind, but never dared to ask. It was said that when Hawking was going to sleep one day, he realized that the radiation takes place. I really wonder how such brilliant ideas come............
  14. Nov 21, 2013 #13
    I bet Hawking wonders, too.

    Do read the Kip Thorne book, you will not be disappointed!!
  15. Nov 21, 2013 #14
    I would do.

    Thanks for everything.
  16. Nov 26, 2013 #15
    Hello Naty1,

    Just to share with you that I am reading Susskind's "Black hole war", to get a clear idea of Hawking radiation and evaporation of black hole.
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