0
$\begingroup$

Why are there different sized black holes? If they all contain a singularity with infinite mass, one cannot be more infinite than another. So there must be finite mass. Maybe there is a quantum particle that is only mass, with no spacial dimension, such that an infinite number of them can occupy a single space, and that a sufficient density of them will form a black hole?

$\endgroup$
  • $\begingroup$ There are probably different sized event horizons, but they are not the same thing as the singularities inside them.. en.wikipedia.org/wiki/Event_horizon $\endgroup$ – user108787 Aug 25 '16 at 14:40
  • 1
    $\begingroup$ It is the density, not the nass, that is infinite in a black hole singularity. $\endgroup$ – Lewis Miller Aug 25 '16 at 15:49
1
$\begingroup$

It's not that black holes have infinite mass. They have infinite mass density. In other words, if a star 6 times the mass of our sun dies and forms a black hole, and that black hole collapses down to a singularity, then all that mass is contained within a point (infinity mass density because the volume is zero). Same with a supper massive black hole which has 5,000,000 times the mass of our sun. All that mass is contained within a point of zero volume. So, infinite mass density. But the overall MASS is not the same as seen from afar. If you are orbiting the first black hole by a distance that is greater than the radius of the original star before it became a black hole, you would not notice any difference between that original star and the black hole. The gravity would be the same. Your orbit is well outside the event horizon. But if you orbit at that distance around a super massive black hole, you would probably be within the event horizon (didn't do the calculation here but just as an example), and you would be unable to escape. Does this help?

$\endgroup$
  • $\begingroup$ Worth noting that the mass-density isn't infinite; the spacetime curvature is. If there's even a singularity inside a black hole at all is debatable. The only real takeaway is that black holes break physics so completely that terms like "density" and "mass" no longer make sense. $\endgroup$ – UIDAlexD Aug 25 '16 at 20:19
  • $\begingroup$ @UIDAlexD: classically, you certainly do have a region of infinite mass density, at least if you start with something like Oppenheimer-Snyder collapse. $\endgroup$ – Jerry Schirmer Aug 25 '16 at 20:27
  • $\begingroup$ @JerrySchirmer Unfortunately classical mechanics don't work here. Singularities never form in our universe - massive objects are always swallowed by the event horizon before they can achieve infinite density. Since time effectively stops in a black hole, how can a true singularity ever form? $\endgroup$ – UIDAlexD Aug 25 '16 at 20:29
  • 1
    $\begingroup$ @UIDAlexD In an Oppenheimer-Snyder collapse, as Jerry says. $\endgroup$ – Rob Jeffries Aug 25 '16 at 20:41
  • $\begingroup$ @UIDAlexD: I'm not talking about experimentally observing a singularity, just the predictions of classical general relativity. Some theories of quantum gravity have singularities, some don't. Of course, there is no universally accepted theory of quantum gravity. $\endgroup$ – Jerry Schirmer Aug 25 '16 at 20:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.