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

B Why do black holes have infinite density, mass, and gravity?

  1. Jan 16, 2017 #1
    If stars have finite mass, gravity, and density, why does a black hole have infinite density, mass, and gravity and why doesn't it attract everything around it with such infinite gravity? Also, with infinite density, why are black holes all different sizes?
     
  2. jcsd
  3. Jan 16, 2017 #2

    russ_watters

    User Avatar

    Staff: Mentor

    As far as we can tell, black holes don't have any of those characteristics.
     
  4. Jan 16, 2017 #3
    In a science documentary, Michio Kaku mentioned that the singularity of black holes have infinite density and infinite gravity. How can laymen expect to learn anything or trust scientists if what some scientists say is not what other scientists believe and scientists never agree with each other about fundamental ideas about the Universe? If what he said is true, it seems then that scientists cannot agree with each other, either that or scientists just make stuff up even if it doesn't make any sense. Same with string theory.
     
  5. Jan 16, 2017 #4

    russ_watters

    User Avatar

    Staff: Mentor

    There is some debate on PF as to what "the singularity" means, however Kaku was almost certainly referring to a finite mass in zero volume. In either case, since we can't ever go and check, that doesn't impact what a black hole looks like to us (a region of space with a finite volume, finite mass and finite gravitational field). There is no debate about that. So does that clear it up...?
    Sorry, but the misunderstanding there is yours; You extrapolated something beyond what he said.
    [also, a link would help...]
     
  6. Jan 16, 2017 #5

    berkeman

    User Avatar

    Staff: Mentor

    Why does an electron have infinite electric field at zero distance?
     
  7. Jan 16, 2017 #6
    That makes no sense. If Kaku meant that, he should have said so instead of making it confusing and misleading for the laymen. Saying a black hole has infinite density is not the same as saying finite mass in zero volume.

    << Mentor Note -- Insult removed from post >>
     
  8. Jan 16, 2017 #7

    Bandersnatch

    User Avatar
    Science Advisor

    Pay attention to what was said: singularities have infinite density and infinite gravity. This is not the same as black holes having those characteristics. Nor was anything said about infinite mass.

    Black hole singularities have 0 volume, so any finite mass they might have will net infinite density, and infinite gravity (since you can get infinitely close to the central mass).

    Black holes themselves are regions surrounding the singularity, within which escape velocity is greater than c. Since those regions have finite volume, their density and gravity are finite too.

    The extent of that region is dependent on mass encompassed within, so it will vary with varying mass.
     
  9. Jan 16, 2017 #8

    russ_watters

    User Avatar

    Staff: Mentor

    Yes, actually it is:
    What happens to "d" in the following function as "v" approaches zero?
    d=m/v

    And please slow your roll; I'm trying to help.
     
  10. Jan 16, 2017 #9

    berkeman

    User Avatar

    Staff: Mentor

    You beat me to it...
     
  11. Jan 17, 2017 #10

    DrClaude

    User Avatar

    Staff: Mentor

    You now understand why most physicists roll their eyes at the mention of Michio Kaku...
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do black holes have infinite density, mass, and gravity?
Loading...