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Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. The recent Kerr paper [0] only adds fuel to this resistance wit

Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents. [0]: https://arxiv.org/abs/2312.00841 https://arxiv.org/abs/2312.00841

lawlessone 2y ago
Infinite density is pretty crazy though. Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
- newpavlov 2y ago
  A very simplified idea is that the denser you get, the slower time flows (for an "outside" observer). So while for an "inner" observer density rises "fast" to the point of singularity, for an "outside" observer time just freezes inside the forming black hole. Finally, if there is a way for black holes to "evaporate" (e.g. Hawking radiation), the "inner" observer will be able to observe shrinking black hole.
  - pdonis 2y ago
    No, this doesn't work. The "slower time flows" you is an optical illusion caused by the curvature of spacetime. It is not a way to avoid the formation of a black hole.
    - newpavlov 2y ago
      Are you talking about the Hawking-Penrose theorems? Have you read the Kerr paper?
      - pdonis 2y ago
        > Are you talking about the Hawking-Penrose theorems? No. Your wrong claim about the slowing of time flow has nothing to do with those. > Have you read the Kerr paper? Yes. It doesn't say anything like what you were saying. I have commented on it elsewhere in this discussion.
    - effie 2y ago
      Can you please give some reference for this claim? Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
      - pdonis 2y ago
        > Can you please give some reference for this claim? Any GR textbook. I referenced two classic ones elsewhere in this discussion. Or, for that matter, you could read the classic paper by Oppenheimer and Snyder which was the first published model of actual gravitational collapse to form a black hole. And was published in...1939. What I'm saying is not news. > Time stopping at the event horizon is quite the standard view in "science communication" of black holes. That's because "science communication" sucks at accurately portraying the actual science.
        effie 2y ago
        > "science communication" sucks at accurately portraying the actual science. Completely agree.
- pdonis 2y ago
  > Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center? There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon. The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
  - binary132 2y ago
    Pardon my ignorance, but why not? This is what I’ve long assumed is the true nature of black holes — not some kind of divide-by-zero error, but merely a very very heavy object.
    - pdonis 2y ago
      > why not? Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible. Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
      - binary132 2y ago
        Maybe I don’t know how compact these objects are, but I suppose I am assuming that the event horizon is always large enough to conceal a sufficiently massive object of extreme or arbitrary density, greater than than that of a neutron star, such as a “quark star” or similar. Is that erroneous? If so, then a followup: why can’t we imagine an object of arbitrary density sufficient that the massive object is also small enough? Why does it need to be a mathematical discontinuity or otherwise goofy universal exception-to-the-rule? We obviously can’t observe it, so isn’t it simply that the mathematics expresses the fact that we cannot observe it?
- contravariant 2y ago
  Kinda, you can't have the event horizon without a singularity. The one thing I'm not 100% sure on is whether an event horizon can form. I have never been able to shake the feeling that with Hawking radiation giving each black hole a large but finite lifetime and with matter falling in beyond the infinite future (to outside observers) you'd have the incoming matter encounter the death of the black hole before it can fall in. This would make a black hole a kind of explosion powered by Hawking radiation that looks frozen in time to outside observers. Lacking a complete theory that can adequately explain both Hawking radiation and general relativity it's quite hard to know if this makes any sense. Though I can't rule out an answer already exists but is simply not known to me.
pdonis 2y ago
> Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. That's because most physicists expect that new physics, probably some form of quantum gravity, will come into play before the singularity is reached, so that there won't actually be one. Which is a perfectly reasonable expectation. > The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents. Kerr misrepresents the actual opinion of most physicists in that paper. The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Practically nobody believes that there are actual curvature singularities; they believe that GR is not correct in that regime, and some kind of new physics is involved, as I noted above. So in the sense Kerr uses the term, there are practically no "singularity proponents" at all. He is attacking a straw man.
- newpavlov 2y ago
  >The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Maybe it was my misfortune, but the university professor who taught me GR thought that singularities are "real", so this anecdote (sadly) disproves your absolutist opinion. And there are heaps of real papers from real scientists which seriously discuss potential consequences of having "real" singularities. And if we'll take a look at the pop-sci coverage, it's orders of magnitude worse... Just see the Veritasium video linked in the other comment.
  - pdonis 2y ago
    > the university professor who taught me GR thought that singularities are "real" Did he teach it from Wald's classic textbook? Wald, Chapter 9, explicitly disclaims any belief that singularities are real and explicitly says the classical GR prediction of singularities indicates a breakdown in the theory in that regime. Other classic textbooks like Misner, Thorne, & Wheeler say similar things. So do many peer-reviewed papers published in the field. Sadly, I am not surprised, though I am disappointed, that many university professors do not know the subject they are teaching well enough to be aware of what I have just said. But the fact remains that the statements I have described are the ones Kerr (who certainly cannot claim ignorance on the part of whoever taught him GR as an excuse) should have been looking at to gauge the prevailing opinion of actual researchers in the field. But he didn't. > if we'll take a look at the pop-sci coverage Nobody should be relying on pop science to learn actual science. Nor should anyone claim that pop science coverage is an accurate gauge of the opinions of actual researchers in the field.

lawlessone 2y ago

Infinite density is pretty crazy though. Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?

newpavlov 2y ago
A very simplified idea is that the denser you get, the slower time flows (for an "outside" observer). So while for an "inner" observer density rises "fast" to the point of singularity, for an "outside" observer time just freezes inside the forming black hole. Finally, if there is a way for black holes to "evaporate" (e.g. Hawking radiation), the "inner" observer will be able to observe shrinking black hole.
- pdonis 2y ago
  No, this doesn't work. The "slower time flows" you is an optical illusion caused by the curvature of spacetime. It is not a way to avoid the formation of a black hole.
  - newpavlov 2y ago
    Are you talking about the Hawking-Penrose theorems? Have you read the Kerr paper?
    - pdonis 2y ago
      > Are you talking about the Hawking-Penrose theorems? No. Your wrong claim about the slowing of time flow has nothing to do with those. > Have you read the Kerr paper? Yes. It doesn't say anything like what you were saying. I have commented on it elsewhere in this discussion.
  - effie 2y ago
    Can you please give some reference for this claim? Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
    - pdonis 2y ago
      > Can you please give some reference for this claim? Any GR textbook. I referenced two classic ones elsewhere in this discussion. Or, for that matter, you could read the classic paper by Oppenheimer and Snyder which was the first published model of actual gravitational collapse to form a black hole. And was published in...1939. What I'm saying is not news. > Time stopping at the event horizon is quite the standard view in "science communication" of black holes. That's because "science communication" sucks at accurately portraying the actual science.
      - effie 2y ago
        > "science communication" sucks at accurately portraying the actual science. Completely agree.
pdonis 2y ago
> Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center? There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon. The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
- binary132 2y ago
  Pardon my ignorance, but why not? This is what I’ve long assumed is the true nature of black holes — not some kind of divide-by-zero error, but merely a very very heavy object.
  - pdonis 2y ago
    > why not? Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible. Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
    - binary132 2y ago
      Maybe I don’t know how compact these objects are, but I suppose I am assuming that the event horizon is always large enough to conceal a sufficiently massive object of extreme or arbitrary density, greater than than that of a neutron star, such as a “quark star” or similar. Is that erroneous? If so, then a followup: why can’t we imagine an object of arbitrary density sufficient that the massive object is also small enough? Why does it need to be a mathematical discontinuity or otherwise goofy universal exception-to-the-rule? We obviously can’t observe it, so isn’t it simply that the mathematics expresses the fact that we cannot observe it?
contravariant 2y ago
Kinda, you can't have the event horizon without a singularity. The one thing I'm not 100% sure on is whether an event horizon can form. I have never been able to shake the feeling that with Hawking radiation giving each black hole a large but finite lifetime and with matter falling in beyond the infinite future (to outside observers) you'd have the incoming matter encounter the death of the black hole before it can fall in. This would make a black hole a kind of explosion powered by Hawking radiation that looks frozen in time to outside observers. Lacking a complete theory that can adequately explain both Hawking radiation and general relativity it's quite hard to know if this makes any sense. Though I can't rule out an answer already exists but is simply not known to me.

pdonis 2y ago

> Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. That's because most physicists expect that new physics, probably some form of quantum gravity, will come into play before the singularity is reached, so that there won't actually be one. Which is a perfectly reasonable expectation. > The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents. Kerr misrepresents the actual opinion of most physicists in that paper. The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Practically nobody believes that there are actual curvature singularities; they believe that GR is not correct in that regime, and some kind of new physics is involved, as I noted above. So in the sense Kerr uses the term, there are practically no "singularity proponents" at all. He is attacking a straw man.

newpavlov 2y ago
>The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Maybe it was my misfortune, but the university professor who taught me GR thought that singularities are "real", so this anecdote (sadly) disproves your absolutist opinion. And there are heaps of real papers from real scientists which seriously discuss potential consequences of having "real" singularities. And if we'll take a look at the pop-sci coverage, it's orders of magnitude worse... Just see the Veritasium video linked in the other comment.
- pdonis 2y ago
  > the university professor who taught me GR thought that singularities are "real" Did he teach it from Wald's classic textbook? Wald, Chapter 9, explicitly disclaims any belief that singularities are real and explicitly says the classical GR prediction of singularities indicates a breakdown in the theory in that regime. Other classic textbooks like Misner, Thorne, & Wheeler say similar things. So do many peer-reviewed papers published in the field. Sadly, I am not surprised, though I am disappointed, that many university professors do not know the subject they are teaching well enough to be aware of what I have just said. But the fact remains that the statements I have described are the ones Kerr (who certainly cannot claim ignorance on the part of whoever taught him GR as an excuse) should have been looking at to gauge the prevailing opinion of actual researchers in the field. But he didn't. > if we'll take a look at the pop-sci coverage Nobody should be relying on pop science to learn actual science. Nor should anyone claim that pop science coverage is an accurate gauge of the opinions of actual researchers in the field.