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I'm guessing from these responses that 'singularity' has turned into kind of pop-culture term expressing the center of a black hole. A singularity is just wher

I'm guessing from these responses that 'singularity' has turned into kind of pop-culture term expressing the center of a black hole. A singularity is just where physical quantities go to infinity. There are many singularities at the event horizon. For example, as you fall into a black hole and approach the event horizon the time of an observer a distance away begins to speed up and approaches infinity - the universe comes to an end before you actually reach the event horizon. Many unphysical things like that happen. These are all singularities. Importantly also, the energy density of space-time approaches the plank energy and this is a region where general relativity and quantum field theory do not agree with each other. So we know the equations are non-sense in this region for sure.

pdonis 5y ago
> I'm guessing from these responses that 'singularity' has turned into kind of pop-culture term expressing the center of a black hole. It's not a pop culture term at all. It's a precisely defined technical term in GR. Everything I said about it is taken from that precise technical definition. > A singularity is just where physical quantities go to infinity. Correct. (Although this itself is something of a pop science version of the actual technical definition. But it will do for this discussion.) > There are many singularities at the event horizon. Wrong. All physical quantities are finite at the event horizon. Your counterexamples are based on misconceptions which have been well understood and corrected in the GR literature for decades. > as you fall into a black hole and approach the event horizon the time of an observer a distance away begins to speed up and approaches infinity Wrong. If you are free-falling into a black hole, light from the rest of the universe is redshifted, not blueshifted. If you are hovering at a constant altitude above a black hole's horizon, light from the rest of the universe is blueshifted; but the blueshift is finite for any altitude above the horizon. It is impossible to hover at the horizon. > the universe comes to an end before you actually reach the event horizon. Wrong. > Many unphysical things like that happen. Wrong. > the energy density of space-time approaches the plank energy Wrong. The black hole is a vacuum solution: the stress-energy tensor is zero everywhere. > this is a region where general relativity and quantum field theory do not agree with each other. So we know the equations are non-sense in this region for sure. Wrong. The area at which GR and QM create compatibility problems is within a Planck time of the singularity, not the horizon. As I said above, all of these misconceptions have been known of and corrected in the GR literature for decades.
- CRConrad 5y ago
  > It's not a pop culture term at all. Wrong. > It's a precisely defined technical term in GR. Sure. But it is also a pop culture term, with a slightly different meaning.
- mrjangles 5y ago
  I didn't believe you so I looked this all up and indeed everything you said is correct. My GR professor was famously one of the worst academics at the department and he never taught us any of that. His specialty was GR too. Anyway, your response was needlessly hyperbolic. The only two misconceptions you are correcting here is that tidal forces can be quite weak at the event horizon and the universe does not come to an before passing through it.
  - pdonis 5y ago
    > My GR professor was famously one of the worst academics at the department and he never taught us any of that. His specialty was GR too. This is disappointing, but unfortunately I don't find it surprising. I have learned GR by self study over many years, starting with borrowing my office mate's copy of Misner, Thorne & Wheeler when I was in graduate school; I never actually took a course in it. But the reports I had from people who did were that the professors did not do a good job of explaining things. Unfortunately being good at research and being good at teaching are two very different things and not many people have both. > The only two misconceptions you are correcting here is that tidal forces can be quite weak at the event horizon and the universe does not come to an before passing through it. I did correct those two misconceptions, but those weren't the only ones in your post.

pdonis 5y ago

> I'm guessing from these responses that 'singularity' has turned into kind of pop-culture term expressing the center of a black hole. It's not a pop culture term at all. It's a precisely defined technical term in GR. Everything I said about it is taken from that precise technical definition. > A singularity is just where physical quantities go to infinity. Correct. (Although this itself is something of a pop science version of the actual technical definition. But it will do for this discussion.) > There are many singularities at the event horizon. Wrong. All physical quantities are finite at the event horizon. Your counterexamples are based on misconceptions which have been well understood and corrected in the GR literature for decades. > as you fall into a black hole and approach the event horizon the time of an observer a distance away begins to speed up and approaches infinity Wrong. If you are free-falling into a black hole, light from the rest of the universe is redshifted, not blueshifted. If you are hovering at a constant altitude above a black hole's horizon, light from the rest of the universe is blueshifted; but the blueshift is finite for any altitude above the horizon. It is impossible to hover at the horizon. > the universe comes to an end before you actually reach the event horizon. Wrong. > Many unphysical things like that happen. Wrong. > the energy density of space-time approaches the plank energy Wrong. The black hole is a vacuum solution: the stress-energy tensor is zero everywhere. > this is a region where general relativity and quantum field theory do not agree with each other. So we know the equations are non-sense in this region for sure. Wrong. The area at which GR and QM create compatibility problems is within a Planck time of the singularity, not the horizon. As I said above, all of these misconceptions have been known of and corrected in the GR literature for decades.

CRConrad 5y ago
> It's not a pop culture term at all. Wrong. > It's a precisely defined technical term in GR. Sure. But it is also a pop culture term, with a slightly different meaning.
mrjangles 5y ago
I didn't believe you so I looked this all up and indeed everything you said is correct. My GR professor was famously one of the worst academics at the department and he never taught us any of that. His specialty was GR too. Anyway, your response was needlessly hyperbolic. The only two misconceptions you are correcting here is that tidal forces can be quite weak at the event horizon and the universe does not come to an before passing through it.
- pdonis 5y ago
  > My GR professor was famously one of the worst academics at the department and he never taught us any of that. His specialty was GR too. This is disappointing, but unfortunately I don't find it surprising. I have learned GR by self study over many years, starting with borrowing my office mate's copy of Misner, Thorne & Wheeler when I was in graduate school; I never actually took a course in it. But the reports I had from people who did were that the professors did not do a good job of explaining things. Unfortunately being good at research and being good at teaching are two very different things and not many people have both. > The only two misconceptions you are correcting here is that tidal forces can be quite weak at the event horizon and the universe does not come to an before passing through it. I did correct those two misconceptions, but those weren't the only ones in your post.