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>Penrose ultimately showed that singularities are inevitable, with the implication that black holes are common in the universe. I'm not familiar with the proof

>Penrose ultimately showed that singularities are inevitable, with the implication that black holes are common in the universe. I'm not familiar with the proof. Did he show that in the THEORY of general relativity, singularity has to exist given our observation of the universe? Or there are something more to it. Would it be possible or plausible that singularity actually does not exist, but just that the theory of general relativity is not a correct description of space/time/matter in small scale? I am thinking in classical theory, when things were treated as point mass/charges, infinity exist in the solution of point sources.

Koshkin 6y ago
Well, a good theory should a) not contradict observations and b) allow to make predictions that are confirmed by subsequent observations. So far GR has been delivering on both points, including singularities, so currently there is no sensible reason to suspect that it is not correct.
- eggy 6y ago
  Here is an interesting paper that says a collapsing star sheds enough mass to not ever become a black hole or form a singularity. It's from 2014, and still controversial, but interesting, and her math seems to have been reviewed. I will follow this with interest to see how it fares peer review. Evidence of black holes is indirect at the moment, which is OK for now, but it will be interesting to see how it all pans out in the coming years. https://phys.org/news/2014-09-black-holes.html https://phys.org/news/2014-09-black-holes.html
  - ChrisLomont 6y ago
    There's plenty of papers by others past this one, showing a more accurate model yields black holes, for example [1]. Here's [2] about 25 others. [1] https://arxiv.org/pdf/1609.05775.pdf https://arxiv.org/pdf/1609.05775.pdf [2] https://scholar.google.com/scholar?cites=12026935191450315869&as_sdt=800005&sciodt=0,15&hl=en https://scholar.google.com/scholar?cites=1202693519145031586...
    - meowface 6y ago
      I understand that the existence of black holes is pretty widely believed, now, but how controversial is the idea of true singularities being inside? I know, for example, there's Carlo Rovelli's "Planck star" hypothesis, which posits that the black hole is effectively (from an outside observer's perspective) an extremely slow violent explosion and hits an energy density limit before ever reaching the singularity stage: https://en.m.wikipedia.org/wiki/Planck_star https://en.m.wikipedia.org/wiki/Planck_star
      - ChrisLomont 6y ago
        >but how controversial is the idea of true singularities being inside I don't know any physicist that would believe this. Singularities happen mathematically due to GR being continuous space and time and material, but we know material is not continuous, and it's possible space and time are not also. So pretty much every one I have ever read or talked to about this believes that once GR is corrected for the quantum level, the singularities will go away, since they by definition would require quantum scale behavior, and it's known GR is not a quantized theory. Note that the "Plank star" is a LQG theory, and LQG has so far failed to reproduce much of anything, so it's not clear at all how that will play out. It hasn't even been shown to reproduce GR at the appropriate scales, and it was recently a big deal when LQG was made consistent on a circle. It's no where ready to deal with 4D spacetime yet. My guess is LQG, like so many other theories of gravity, will die out as more and more pieces of it end up disagreeing with experiments.
        meowface 6y ago
        Thanks for the clarification on LQG. As a totally uninformed layman, your description of the singularity was also my understanding. I only brought up the question because this is the second paragraph of the original comment in this sub-thread: >Would it be possible or plausible that singularity actually does not exist, but just that the theory of general relativity is not a correct description of space/time/matter in small scale? I am thinking in classical theory, when things were treated as point mass/charges, infinity exist in the solution of point sources. Basically, I wasn't sure why they were getting so much criticism for this, because it sounded pretty reasonable to me.
zachf 6y ago
Its a short and elegant proof set in pure theoretical general relativity (GR). The idea is that if there’s a sphere of space where if you try to emit light rays and the light rays don’t initially start separating, then they can’t start separating due to gravity, because in classical GR, gravity is always attractive. You can then show that this implies that inside that sphere, spacetime must end, basically because you can’t outrun light. The proof is important because it was previously believed that black holes are not interesting because they require very special perfect conditions to create, like balancing a pencil on its tip is physically possible but requires perfect aim. But these aforementioned spheres are very common and easy to find so it turns out black holes are common too. If you think (as almost every physicist does) that GR is approximately correct to describe reality, but needs fixes at very tiny lengths because of poorly understood quantum effects, the proof does not directly carry over. One immediate problem is that the proof assumes that energy densities are positive, implying that that gravity is universally attractive, which for quantum matter can never be always true for every quantum state (this is a consequence of Reeh-Schlieder, that every QFT contains states with negative energy density). None of this invalidates Penrose’s work. Physicists have always used different physics to describe different scales. Newtonian physics is great to describe most physics on a human scale, but it’s “wrong” in the sense that GR supersedes it. Similarly GR is “wrong” but still approximately right for a ton of questions of cosmology. But if you fall into a black hole, once you wait long enough, we don’t know what will happen. In string theory, there are objects that are black hole-like. It is generally believed that the singularity is “resolved” (not truly present) in string theory but the details are very tricky to work out. It still is true that geometry breaks down near the singularity and whats left is some stringy stuff, something very new and confusing. Of course it might turn out that string theory does not describe our reality either...
- andrewon 6y ago
  Thanks for your reply. Sounds like the proof is an interesting piece to study.
- ssivark 6y ago
  Could you elaborate why Reeh-Schlieder implies negative energy states? Typically in QM we only care about the spectrum being lower bounded (existence of vacua), and ignoring additive energy constants.
deeeeplearning 6y ago
Why do random non-experts always think they have poked holes in the most fleshed out scientific theories?
- meowface 6y ago
  Speculation and asking questions is fun. Also, as I understand, it's not an uncommon belief among physicists that singularities may just be an artifact of GR being incomplete, kind of like a divide by zero error in the math which doesn't necessarily exist in reality. So I don't think that poster was questioning established science; I believe there's still a lot of open debate and uncertainty about if black hole centers contain infinitely dense singularities or just something that's incredibly but finitely dense. No one knows. There is indeed a common form of arrogant layman skepticism that confidently and ignorantly assumes scientists haven't thought about [some thing], but I don't think this is an example of it.
  - deeeeplearning 6y ago
    The problem is that the VAST majority of layman "theories" especially in Theoretical Physics fall into the Not Even Wrong category and are totally without merit or worth discussing.
    - meowface 6y ago
      That's true. But we're also kind of staring at a sheer wall at the moment. No one seems to be making significant theoretical progress in physics, and when the physicists are stumped, it rouses everyone else's curiosity. Personally, I think the more crackpot ideas, the better; as long as they're not presented in a crackpot way (e.g. the proposer having high confidence or even certainty in their random speculation). And even the ones that are not even wrong can at least lead to useful, intellectual discussion so that the proposer can understand the landscape a bit more and at least closer to the "mere wrong" category. Though I also agree it's much better for people to understand the existing state of affairs before proposing things as if the leading experts have somehow never considered [X] when [X] has been talked about for decades.

Koshkin 6y ago

Well, a good theory should a) not contradict observations and b) allow to make predictions that are confirmed by subsequent observations. So far GR has been delivering on both points, including singularities, so currently there is no sensible reason to suspect that it is not correct.

eggy 6y ago
Here is an interesting paper that says a collapsing star sheds enough mass to not ever become a black hole or form a singularity. It's from 2014, and still controversial, but interesting, and her math seems to have been reviewed. I will follow this with interest to see how it fares peer review. Evidence of black holes is indirect at the moment, which is OK for now, but it will be interesting to see how it all pans out in the coming years. https://phys.org/news/2014-09-black-holes.html https://phys.org/news/2014-09-black-holes.html
- ChrisLomont 6y ago
  There's plenty of papers by others past this one, showing a more accurate model yields black holes, for example [1]. Here's [2] about 25 others. [1] https://arxiv.org/pdf/1609.05775.pdf https://arxiv.org/pdf/1609.05775.pdf [2] https://scholar.google.com/scholar?cites=12026935191450315869&as_sdt=800005&sciodt=0,15&hl=en https://scholar.google.com/scholar?cites=1202693519145031586...
  - meowface 6y ago
    I understand that the existence of black holes is pretty widely believed, now, but how controversial is the idea of true singularities being inside? I know, for example, there's Carlo Rovelli's "Planck star" hypothesis, which posits that the black hole is effectively (from an outside observer's perspective) an extremely slow violent explosion and hits an energy density limit before ever reaching the singularity stage: https://en.m.wikipedia.org/wiki/Planck_star https://en.m.wikipedia.org/wiki/Planck_star
    - ChrisLomont 6y ago
      >but how controversial is the idea of true singularities being inside I don't know any physicist that would believe this. Singularities happen mathematically due to GR being continuous space and time and material, but we know material is not continuous, and it's possible space and time are not also. So pretty much every one I have ever read or talked to about this believes that once GR is corrected for the quantum level, the singularities will go away, since they by definition would require quantum scale behavior, and it's known GR is not a quantized theory. Note that the "Plank star" is a LQG theory, and LQG has so far failed to reproduce much of anything, so it's not clear at all how that will play out. It hasn't even been shown to reproduce GR at the appropriate scales, and it was recently a big deal when LQG was made consistent on a circle. It's no where ready to deal with 4D spacetime yet. My guess is LQG, like so many other theories of gravity, will die out as more and more pieces of it end up disagreeing with experiments.
      - meowface 6y ago
        Thanks for the clarification on LQG. As a totally uninformed layman, your description of the singularity was also my understanding. I only brought up the question because this is the second paragraph of the original comment in this sub-thread: >Would it be possible or plausible that singularity actually does not exist, but just that the theory of general relativity is not a correct description of space/time/matter in small scale? I am thinking in classical theory, when things were treated as point mass/charges, infinity exist in the solution of point sources. Basically, I wasn't sure why they were getting so much criticism for this, because it sounded pretty reasonable to me.

zachf 6y ago

Its a short and elegant proof set in pure theoretical general relativity (GR). The idea is that if there’s a sphere of space where if you try to emit light rays and the light rays don’t initially start separating, then they can’t start separating due to gravity, because in classical GR, gravity is always attractive. You can then show that this implies that inside that sphere, spacetime must end, basically because you can’t outrun light. The proof is important because it was previously believed that black holes are not interesting because they require very special perfect conditions to create, like balancing a pencil on its tip is physically possible but requires perfect aim. But these aforementioned spheres are very common and easy to find so it turns out black holes are common too. If you think (as almost every physicist does) that GR is approximately correct to describe reality, but needs fixes at very tiny lengths because of poorly understood quantum effects, the proof does not directly carry over. One immediate problem is that the proof assumes that energy densities are positive, implying that that gravity is universally attractive, which for quantum matter can never be always true for every quantum state (this is a consequence of Reeh-Schlieder, that every QFT contains states with negative energy density). None of this invalidates Penrose’s work. Physicists have always used different physics to describe different scales. Newtonian physics is great to describe most physics on a human scale, but it’s “wrong” in the sense that GR supersedes it. Similarly GR is “wrong” but still approximately right for a ton of questions of cosmology. But if you fall into a black hole, once you wait long enough, we don’t know what will happen. In string theory, there are objects that are black hole-like. It is generally believed that the singularity is “resolved” (not truly present) in string theory but the details are very tricky to work out. It still is true that geometry breaks down near the singularity and whats left is some stringy stuff, something very new and confusing. Of course it might turn out that string theory does not describe our reality either...

andrewon 6y ago
Thanks for your reply. Sounds like the proof is an interesting piece to study.
ssivark 6y ago
Could you elaborate why Reeh-Schlieder implies negative energy states? Typically in QM we only care about the spectrum being lower bounded (existence of vacua), and ignoring additive energy constants.

deeeeplearning 6y ago

Why do random non-experts always think they have poked holes in the most fleshed out scientific theories?

meowface 6y ago
Speculation and asking questions is fun. Also, as I understand, it's not an uncommon belief among physicists that singularities may just be an artifact of GR being incomplete, kind of like a divide by zero error in the math which doesn't necessarily exist in reality. So I don't think that poster was questioning established science; I believe there's still a lot of open debate and uncertainty about if black hole centers contain infinitely dense singularities or just something that's incredibly but finitely dense. No one knows. There is indeed a common form of arrogant layman skepticism that confidently and ignorantly assumes scientists haven't thought about [some thing], but I don't think this is an example of it.
- deeeeplearning 6y ago
  The problem is that the VAST majority of layman "theories" especially in Theoretical Physics fall into the Not Even Wrong category and are totally without merit or worth discussing.
  - meowface 6y ago
    That's true. But we're also kind of staring at a sheer wall at the moment. No one seems to be making significant theoretical progress in physics, and when the physicists are stumped, it rouses everyone else's curiosity. Personally, I think the more crackpot ideas, the better; as long as they're not presented in a crackpot way (e.g. the proposer having high confidence or even certainty in their random speculation). And even the ones that are not even wrong can at least lead to useful, intellectual discussion so that the proposer can understand the landscape a bit more and at least closer to the "mere wrong" category. Though I also agree it's much better for people to understand the existing state of affairs before proposing things as if the leading experts have somehow never considered [X] when [X] has been talked about for decades.