3 ms·

> 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, prob

by 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.
floxy 2y ago
>He is attacking a straw man. But the Nobel prize was awarded for the Singularity Theorem(?). Maybe Kerr is attacking something in between a straw-man and a steel-man? Clay-man?
- pdonis 2y ago
  > the Nobel prize was awarded for the Singularity Theorem Sure, because those theorems played a very important role in clarifying exactly what classical GR predicts in these regimes, and therefore in focusing the efforts of physicists in fruitful directions. That remains the case even if it turns out that in our actual universe, at least some of the assumptions of those theorems are violated so that there aren't any actual singularities. One of the key contributions of the singularity theorems was in making those assumptions clear, so that physicists who did not believe singularities were physically realistic knew exactly where to look for other possibilities.
ClumsyPilot 2y ago
Haven’t we basically proven that even horizon is real? At which point, isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc? I.e. if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
- pdonis 2y ago
  > Haven’t we basically proven that even horizon is real? No. All we have shown is that there are compact regions in our universe that look like event horizons--but could also be just apparent horizons. To know that they are actual, true event horizons, we would have to know the entire future of the universe, since an event horizon is a globally defined surface in spacetime and its presence and location cannot be known unless you know the entire spacetime, i.e., the entire future. If you don't know that--and of course we don't--you can't tell an actual event horizon from an apparent horizon. > isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc? None of these are actual features of actual event horizons (or apparent horizons, for that matter). The first is a missatement--the gravity of the object is not "sealed off" and can still be detected outside. The second is an illusion caused by an unfortunate choice of coordinates. The third is pure speculation, and speculation which does not appear to have gained much traction. > if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe? Most physicists do not believe that either of these (if I take "super heavy pokemon" to mean "some kind of ordinary compact object like a neutron star") is what actually happens.
- mr_toad 2y ago
  > Haven’t we basically proven that even horizon is real? We haven’t observed event horizons except as fuzzy highly processed images. They look a lot like what you’d expect from a black hole, but we have hardly ruled out all possibilities. And there are theoretical effects near an event horizon for charged and spinning black holes, that as far as I know we’ve never been able to observe. It doesn’t seem out of the question that a black hole without a singularity might exhibit behaviour near the event horizon that is measurably different from the theories we have now.

by pdonis 2y ago

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.

floxy 2y ago

>He is attacking a straw man. But the Nobel prize was awarded for the Singularity Theorem(?). Maybe Kerr is attacking something in between a straw-man and a steel-man? Clay-man?

pdonis 2y ago
> the Nobel prize was awarded for the Singularity Theorem Sure, because those theorems played a very important role in clarifying exactly what classical GR predicts in these regimes, and therefore in focusing the efforts of physicists in fruitful directions. That remains the case even if it turns out that in our actual universe, at least some of the assumptions of those theorems are violated so that there aren't any actual singularities. One of the key contributions of the singularity theorems was in making those assumptions clear, so that physicists who did not believe singularities were physically realistic knew exactly where to look for other possibilities.

ClumsyPilot 2y ago

Haven’t we basically proven that even horizon is real? At which point, isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc? I.e. if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?

pdonis 2y ago
> Haven’t we basically proven that even horizon is real? No. All we have shown is that there are compact regions in our universe that look like event horizons--but could also be just apparent horizons. To know that they are actual, true event horizons, we would have to know the entire future of the universe, since an event horizon is a globally defined surface in spacetime and its presence and location cannot be known unless you know the entire spacetime, i.e., the entire future. If you don't know that--and of course we don't--you can't tell an actual event horizon from an apparent horizon. > isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc? None of these are actual features of actual event horizons (or apparent horizons, for that matter). The first is a missatement--the gravity of the object is not "sealed off" and can still be detected outside. The second is an illusion caused by an unfortunate choice of coordinates. The third is pure speculation, and speculation which does not appear to have gained much traction. > if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe? Most physicists do not believe that either of these (if I take "super heavy pokemon" to mean "some kind of ordinary compact object like a neutron star") is what actually happens.
mr_toad 2y ago
> Haven’t we basically proven that even horizon is real? We haven’t observed event horizons except as fuzzy highly processed images. They look a lot like what you’d expect from a black hole, but we have hardly ruled out all possibilities. And there are theoretical effects near an event horizon for charged and spinning black holes, that as far as I know we’ve never been able to observe. It doesn’t seem out of the question that a black hole without a singularity might exhibit behaviour near the event horizon that is measurably different from the theories we have now.