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Reason for a black hole's gravity?

  1. Dec 28, 2013 #1
    Hey, we all know that when a massive star dies it forms a black hole - an object with a great amount of gravitational force. So where does this gravity come from ? Because according to newton's equation of gravity , gravity depends directly on mass but the mass of the black hole is still the same as of the original star. It's just the density which increases. So, shouldn't the strength of gravitational force be same as before?
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  3. Dec 28, 2013 #2
    Yes. From far away it is the same. Only close up does it get crazy. How close is "close up"? Well, its usually closer than the size of the non-blackhole star's radius. For example, if the earth were a black hole its schwarzschild radius would be well inside the surface of the current earth!
  4. Dec 28, 2013 #3


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    Not quite. A supernova blows off some of the mass of the star so what's left is less.
  5. Dec 28, 2013 #4


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    Exactly. As ModusPwnd confirmed, the gravitational interaction remains the same in the 'big picture'.

    The significant difference is that this collapse of density crosses a threshold that gives a black hole unique characteristics. One of these is the 'event horizon'. That's the line in the sand that even light cannot escape from.

    That's probably why black holes get their reputation for being so strong, gravitationally. But as you wrote, and as ModusPwnd confirmed, if you are far enough away to observe without becoming a part of the experiment yourself, everything is the same a before. :wink:
  6. Dec 28, 2013 #5


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    No, it's NOT exactly. Reread post #3
  7. Dec 28, 2013 #6


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    I read your post the first time through, before I replied. I didn't think it addressed what the OP was looking to understand. I thought the OP wanted to better understand the gravitational differences between a star, and a star that collapsed to form a black hole. ModusPwnd appears to have thought similarly.

    From my own experience learning here at PF, extraneous information can be confusing if the OP thinks you're providing useful information to what they're asking for help to understand.

    No worries. By now I'm sure the OP is aware that supernovas blow off some of their mass, and me, too.
  8. Dec 29, 2013 #7
    OK...that means black holes are not as dangerous as they seem...
  9. Dec 29, 2013 #8


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  10. Dec 29, 2013 #9


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    Good question!

    Two things:

    1) A famous result from classical physics is that the gravity outside a sphere is the same as it would be if all the mass were concentrated at the centre. So, if the Earth, or any object, were to shrink to a small size, the gravity beyond where the surface used to be would remain the same. (*)

    2) The gravitational force is proportional to the inverse square of the distance to the centre of the massive object. So, if something shrinks to, say, 1000th of its diameter, then the gravity at its new surface would be 1,000,000 greater than at its previous surface.

    So, as already mentioned, it's only "close" to a black hole that its gravity is very large.

    (*) When you model planetary motion, you model the sun and planets as points where all the mass is concentrated. Like they were massively dense black holes!
  11. Dec 29, 2013 #10


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    No, they are exactly as dangerous as they seem, it's just that your perception was incorrect in thinking that they are somehow more dangerous at a distance than other large masses. This is very common, that people think black holes have some kind of magical power or something, whereas, as mentioned above, unless you are very close a black hole is no different than any other large mass.
  12. Dec 29, 2013 #11
    Actually a massive star can collapse down to a neutron star and past that, a quark star. The gravitational pressure must break the force that holds atoms together. Once this happens the proton and electrons collide forming a neutron. Then, when the gravitational pressure is even greater, the neutron breaks down to quarks. Forming a quark star. My intuition on black holes is that the gravitational pressure surpasses the force that holds a quark together. Maybe a black hole is made of the same thing quarks are made of.

    This explains why black holes are so dense. Since atoms are pretty hollow, the potential density of the space one atom holds is enormous.

    Maybe this explains things for you, maybe is doesn't. Just making sure you understand this.
  13. Dec 29, 2013 #12
    The volume a typical hydrogen atom has a potential weight of 10,000 times the weight of the atom. Maybe more. But you can fit 10,000 protons in the space between the nucleus and electron cloud. (Yes the electron could appear at the nucleus but this is not probable)
  14. Dec 29, 2013 #13


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    Quark stars are speculative; we don't know for sure that there is a stable state of matter that is denser than neutron star matter. (More precisely, we don't know that there is a "quark star" state that is discontinuous with neutron star matter: the core of a neutron star may well be in a state that is better described as "quark matter" because the neutrons aren't really separate entities any more.)

    No, it has to break the force that keeps atoms *apart*--more precisely, that keeps electrons and protons separate. In white dwarf matter, the degeneracy pressure comes from electrons; the atoms are collapsed, but the electrons and protons are still separate.

    No, a black hole is made of spacetime curvature; the inside of a black hole is vacuum (except for the singularity at the center).

    Not really. Black holes aren't "objects" the way you're used to thinking of ordinary objects, and they don't have a "volume" the way ordinary objects do, so the concept of "density" doesn't really apply to them. Also, their insides are vacuum, as above, not any kind of matter--it's impossible for anything to exist in static equilibrium inside a black hole anyway. So it's not correct to think of them as made of a "substance" that's even denser than neutrons or quarks.

    This explains why white dwarfs and neutron stars can be so much more dense than ordinary matter, yes; but it doesn't really apply to black holes. See above.
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