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B Singularity vs. Black Hole

  1. May 31, 2016 #1
    What has a higher density? Black Holes or Singularities?

    I looked online and all I saw was that they both have infinite densities.
     
  2. jcsd
  3. Jun 1, 2016 #2
    The density of a black hole would be computed as its mass over its volume - the volume within the event horizon. So that would be far from infinite.

    A black hole with zero angular momentum should have an embedded singularity where almost all of the black hole's mass will be accumulated. I'm not sure if makes sense to talk about its volume at all, but if it has one perhaps it is zero. Of course, if it is zero, that would give it "infinite density".
     
  4. Jun 1, 2016 #3
    I heard that a black hole can have a extremely low density. Also I heard that the bigger the black hole the lower the density. This does not make much sense to me, is it true? And if so can someone explain why?
     
  5. Jun 2, 2016 #4
    As I said in the previous post the "size of a black hole" is generally taken to be the diameter of its event horizon. The actual geometry of the black hole is pretty involved - but as a simplification, imagine that all of the mass of the black hole is concentrated in a singularity, a point, at the center. The size of the event horizon is the sphere around that point where light cannot escape. If you double the mass at the center, the distance to the event horizon increases so the total volume of the black hole increases. That total volume will more than double. So, heavier black holes are less dense because they can enclose more empty space.

    Interestingly, it is the surface area of the black hole (the event horizon) that increases proportionately to the mass.
     
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