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Does the mass of a blackhole remain the same if it is not feeding ?

  1. May 20, 2013 #1
    Does the mass of a blackhole remain the same if it is not "feeding"?

    If matter in a blackhole keeps crushing down in size to infinity, does the mass stay the same, but the volume decreases? Isn't a blackhole constantly shrinking in size? Thanks in advance for the help!
     
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  3. May 20, 2013 #2

    phinds

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    We don't actually know WHAT is going on inside a black hole but the math says that the mass shrinks down to a point (but, yes, stays the same amount)
     
  4. May 20, 2013 #3
    You're thinking of Hawking radiation, which theoretically yes, over a great amount of time, a black hole will radiate off a small portion of it's mass. (Google heat death).

    But in the case of our sun, if it were big enough to form into a blackhole at collapse, would still have about the same gravitational pull as the sun pre-collapse. You might want to stay away from it in your space ship though, the event horizon might just swallow you before you find it.
     
  5. May 20, 2013 #4

    :eek: I actually got one right! :smile:


    If it shrinks down to a point, what stops it shrinking from there?
     
  6. May 20, 2013 #5
    Thats one aspect we don't know. QM feels the minimal size or state is the planck length. Or could be infitismally tiny. Opinions vary on that
     
  7. May 21, 2013 #6

    Chronos

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    Once the Pauli exclusion limit is exceeded, theoretically there is nothing to prevent the physical volume of mass in a black hole from shrinking to an infinitesimal point called a singularity. This is widely viewed as the mathematical consequence of an incomplete theory. A proper theory of quantum gravity should resolve this paradox.
     
  8. Feb 27, 2014 #7
    If thats true, why can light escape from stars with enough mass to form a black hole before they collapse? (If the gravitational pull remains the same, and light cant escape post collapse)
    As always, thanks in advance for any help!
     
  9. Feb 27, 2014 #8

    Drakkith

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    Imagine if the Earth were the same mass, but 3,000 km in radius instead of the 6,000 km that it is now. The matter on the far side of the Earth from you is now half the distance that it used to be, which means that the attractive force of gravity from that matter is four times as great due to the inverse square law. If we re-calculate the gravitational force on the Earth we would find that instead of accelerating at 9.8 m/s2 we are now accelerated at about 40 m/s2. If we keep compacting the Earth into a smaller and smaller volume, the force of gravity continues to increase as the average distance between the matter decreases. Eventually we would reach the point where the Earth is so compact that an event horizon forms.

    Note, however, that at distance much greater than the radius of the Earth the gravity barely changes. For example, the Moon orbits the Earth at a distance of about 380,000 km. Since that distance is much greater than the radius of the Earth, the gravitational force remains almost exactly the same before and after we compress the planet. This makes sense when you consider that even though parts of the Earth moved towards the Moon during compression, other parts moved away from it.
     
    Last edited: Feb 27, 2014
  10. Feb 27, 2014 #9

    phinds

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    Drakkith, I always thought the gravity at the moon didn't change at all if the Earth shrinks. Why do you say it is "almost" exactly the same? Is it because our simplification of taking the earth as a point object becomes more valid as it shrinks in size and that makes a small difference at the moon?
     
  11. Feb 27, 2014 #10

    Drakkith

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    Uh, yes. Let's go with that. :tongue:
     
  12. Feb 27, 2014 #11

    Great explanation! Thank you!
     
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