Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Black hole question

  1. Oct 25, 2012 #1
    So are all black holes infinitely small, which is what they call the singularity point? I'm talking about the actual distance the matter of a black hole takes up. Does a super massive black hole have the same physical size as any other black hole(which i guess would be infinitely small) and just a must larger radius of effective gravity?
  2. jcsd
  3. Oct 25, 2012 #2
    The event horizon, the only part of the black hole that you can "see", is proportional to the mass that generated the black hole. The best theory of gravity we have, General Relativity, predicts that there is a singularity at the centre. General Relativity is thought to break down in this regime and quantum effects take over.
  4. Oct 25, 2012 #3
    The singularity at the very center of a BH is a point of virtually zero size....a singularity.
    About all we know is that it appears space and time get mixed up there in quantum fluctuations...a quantum foam. No matter is believed to exist inside a BH....it has been compressed out of existence.

    'Big' and 'small' BH refer to the size of the external event horizon....It's usually quoted with a radius r dependent of the mass M which has been swallowed: r = 2M called the Schwarszchild radius. This turns out to be a solution of formulas from Einstein's general relativity....Once something goes inside the event horizon, nothing gets out.

    Check out the first section here for more:

  5. Oct 25, 2012 #4
    So the event horizon is proportional to the mass that generated the black hole(so maybe a collapsing star...). but is all of that mass that used to be the star condensed into an infinitely small point, or is it just packed more tightly together than it was in the star, making it have a measurable radius of actual matter that makes it up.(say a couple kilometers or something...)
  6. Oct 25, 2012 #5


    User Avatar
    Gold Member

    It's really called a "singularity", not in the sense of a point, but in the sense of "we really don't know WHAT is going on in there".

    As was pointed out, the math says it is a dimensionless point with infinite density, but that just means it's where the math model that we currently use breaks down and again, "we really don't know WHAT is going on in there".

    The hope is that some day a theory of quantum gravity will explain what's REALLY happening.
  7. Oct 25, 2012 #6
    I still havent really got my answer. (Im probably not being clear enough). Are ALL black holes an infitely small point and their area of gravitational influence is what makes them big or small? Or are there black holes the size of, say, a marble, or the earth, but much much denser? (But theres actual matter making it up and to get to the singularity of the BH you would have to "dig" through physical matter).
  8. Oct 25, 2012 #7


    User Avatar
    Gold Member

    According to the math, the only difference in "size" of a black hole is due to the amount of matter that goes in. The singularity is exactly what I described in my previous post, regardless of the amount of matter.

    EDIT: and the "size" is the diameter of the event horizon.
  9. Oct 25, 2012 #8
    If we don't know what is going on in there in the inside, how can we say with any conviction what's going on, on the outside.That could apply to the size of the event horizon regardless of how much mass has been used to make it.
    We therefore won't know what the speed of mass at the singularity might be moving at, and therefore the size of it's event horizon might not give an indication of the amount of mass used to make it.
    Last edited: Oct 26, 2012
  10. Oct 25, 2012 #9
    It's been stated several times that General Relativity works for the outside of the event horizon, but not the inside. There's no current theory that explains what goes on in the inside.
  11. Oct 26, 2012 #10
    That does not explaine what happens on the outside of the event horizon for the reasons given.
    That's a fudge.
  12. Oct 26, 2012 #11
    You are having a problem with 'infinite'. Some infinitely small black holes are infinitely smaller than others. ;)

    Black holes aren't just some sort of black 'star' hanging in the sky that swallow anyone who gets too close. They are places where time and space literally end. We can't even guess what that might mean for the existence of matter.

    Long before you get to the singularity itself, ordinary matter ceases to exist as first the orbiting electrons are forced into the nucleus then the nucleus itself is crushed into ??? and then snuffed out as space itself ceases to exist and time ends.
  13. Oct 26, 2012 #12
    If you want you could impose Planck units which are suppose to be the boundary that length, mass and time lose meaning (some theories predict that the mass within black holes collapse to within about 50% of Planck density) though due to rotation, I think the whole idea of an absolute singularity is mute (in some ways, an absolutely static black hole is on par with the likes of reaching absolute zero K or reaching the speed of light, i.e. it's not possible) every BH, no matter how slight, will have some degree of rotation, which means there may be an inner horizon where time-like worldlines are reinstated, the singularity may reside at the surface of this horizon or within where due to the time-like worldlines, mass wouldn't have to collapse to r=0.
    Last edited: Oct 26, 2012
  14. Oct 26, 2012 #13


    User Avatar
    Science Advisor

    Just to clarify...not all black holes have a point - singularity. Rotating (kerr) black holes have ring singularities.
  15. Oct 26, 2012 #14
    This is incorrect.

    That is NOT what has been stated.

    GR works inside the event horizon down to the singularity...at the singularity GR and QM break down...it is only at the singularity that we have no theory...this is pretty clearly described in the above posts....
  16. Nov 30, 2012 #15
    How can something have infinite density? That would mean the volume is zero, right? If the volume of something is zero, then how can it even exist? Zero volume would be literally nothing.
  17. Nov 30, 2012 #16


    User Avatar
    Gold Member

    So do you think maybe that's why we say that the singularity is the place where the math / models just break down and don't give meaningful answer?
  18. Nov 30, 2012 #17
    It's a leap of imagination to allow something that does not have a meanigfull answer to have one.
    If the math on on the inside is not right how can the math be right on the outside unless one allows for wishfull thinking.
  19. Nov 30, 2012 #18
    Only at the singularity!

    How can GR work if it does not explain the singularity.It's nonesense, you would be better off explaining how an egg boils.
    Sorry if this sounds abrupt but to state with conviction that we know how GR works inside the event horizon down to the singularity is just plain wrong.
    Last edited: Nov 30, 2012
  20. Nov 30, 2012 #19


    User Avatar
    Science Advisor

    Physics isn't meant to be a blueprint to reality. Being part of reality, we have no way of verifying anything to be absolutely correct. Physics is meant to describe what we can observe. General Relativity describes everything we can observe perfectly, as far as we can tell. Certainly, with higher precision than you could hope for with "wishful thinking".

    Whether GR describes anything bellow the event horizon properly is an open question. But it's also irrelevant. Absolutely nothing that's happening bellow event horizon could influence anything above. So frankly, as far as Physics goes, it's not relevant. GR works up to event horizon, and that's all it needs to do. Unless we find a flaw in GR's predictions we can test, to the best of our knowledge, it just works. And since absolutely no experiment can be performed on anything bellow event horizon, any failures of GR in that region are of no significance.
  21. Dec 1, 2012 #20
    Tosh! how can you arrive at any conclusion is beyond any known Physics.We don't know if it's of any influence or if it's relivant or not.Why bother looking, might as well say we know as much as we ever will about gravity inside an event horizon so it is of no importance.
    We have not actualy performed any experiments just outside of any event-horizons so it's jumping the gun to say the least that non can be performed on anything bellow.
    We just don't know at this moment.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook