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Initial EH in Collapsing Star

  1. Jan 29, 2016 #1


    Staff: Mentor

    I was reading Stephen Hawking offers new resolution of black hole paradox. Most is over my head but when he mentions particles falling in through the EH, it raises a question.

    When a BH is created in a collapsing star, does the EH first appear at zero radius and grow outward, or does the EH first appear with non-zero radius? In the latter case, there would be some stellar mass captured inside that never did fall in through the EH.
    Last edited by a moderator: Jan 30, 2016
  2. jcsd
  3. Jan 29, 2016 #2
    The latter.
  4. Jan 29, 2016 #3


    User Avatar
    Science Advisor

    The event horizon is defined by the mass of the star. Once the collapsing star radius is smaller than the event horizon radius, it is a black hole.
  5. Jan 30, 2016 #4
    It is not that easy (even without rotation) because the event horizon appears inside the collapsing star.
  6. Jan 30, 2016 #5
    You might find this link useful- http://www.astro.ucla.edu/~wright/bh-st.html
  7. Jan 30, 2016 #6


    Staff: Mentor

    Thanks for the replies. But now I don't understand how Hawking can say that a hologram at the event horizon resolves the information paradox.

    The Information Paradox for Black Holes
    S. W. Hawking

    Hawking's reasoning seems to apply only to particles that fell in through the EH. It would not address the information of mass trapped inside during stellar collapse which never did cross the EH.
  8. Jan 30, 2016 #7


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    Science Advisor
    Gold Member

    Since the minimum mass necessary to form a black hole is the Planck mass, it appears safe to deduce all black holes begin with a non zero radius. I would expect them to grow rapidly, and possibly frenetically, from that point forward depending on the infall rate. I have no clue how you might go about calculating a minimum, or maximum infall rates. Given the smallest known black hole is about 5 solar masses, that is a pretty sizable discrepancy between any theoretical minimum and the observed minimum. I doubt many theorists have enough knowledge of condensed matter physics and happen to be maschistic enough to try and reconstruct a black hole from the ground up.
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