5 ms·

Should be pointed out that this is a critique of common popsci journalism tropes and not a fancy new research result. Anyone who has taken a graduate level clas

Should be pointed out that this is a critique of common popsci journalism tropes and not a fancy new research result. Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same.

huflungdung 1mo ago
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kwoff 1mo ago
Or read Susskind's "The Theoretical Minimum: General Relativity". For a non-spinning blackhole at least, not only is the singularity not a point, it is a surface in time, not space (as the book explains, the space and time coordinates switch places as you cross the event horizon).
- dash2 1mo ago
  > the space and time coordinates switch places as you cross the event horizon I'm sorry but this is blowing my mind. What???
  - lstodd 1mo ago
    Yup. It's that weird. Also read Nick Gorkavyi: The Oscillating Universe: Einsteinian Cosmology of Black Holes and Gravitational Waves
  - MathMonkeyMan 1mo ago
    [This video][1] and the one before it on the playlist are a good no nonsense explanation of the topic. [1]: https://www.youtube.com/watch?v=O_2vnb_eVGE https://www.youtube.com/watch?v=O_2vnb_eVGE
  - metalliqaz 1mo ago
    it might help to think of the singularity as not a point in space but rather a future that cannot be avoided. All possible paths through space and time, no matter what happens, will go towards the singularity.
  - Kranar 1mo ago
    Because it's very misleading. Time and space do not switch places past the event horizon. What happens is that the direction/path between an object and the singularity becomes a timelike dimension, and the direction that plays the role of time outside of the event horizon becomes a spacelike dimension. That is not the same as them swapping or that time becomes space and space becomes time not to mention that space has 3 dimensions and time has only 1 dimension so how could they even swap places. Really what it means is that past the event horizon you can use the direction in space between you and the singularity as a way to measure time, specifically the amount of time left before you reach the singularity. It's not so mind blowing when you interpret it that way now is it? You can imagine many things in ordinary life that you use to measure time without claiming that time has literally swapped places with it. On a road trip, the number of kilometres to your exit tells you how long you have left, that's using space as a proxy for time... big deal. The notable difference between a road trip and a black hole is that on a road trip you could stop for a break, you could maybe take a detour, you could decide to go back home... and these would all break your use of space as a proxy for measuring time. Well with a blackhole you can't do any of those things, there is no going back, there is no detour, the relationship between the spatial direction towards the singularity and time is fixed and causal and there's nothing you can do about it. The phrasing used is used almost certainly to evoke some kind of voodoo mind-blowing mystery that completely disappears when you get down to the more strict formalism.
    - pdonis 1mo ago
      > Really what it means is that past the event horizon you can use the direction in space between you and the singularity as a way to measure time That's not correct. There is a relationship between the radial coordinate r you are at and the time it will take you, by your clock, to reach the singularity (at least assuming you are freely falling), but that relationship can't be described the way you are describing it. To put the issue with what you say as starkly as possible: at any event inside the black hole's horizon, there are spacelike curves in every direction from you that will hit the singularity! So "the direction in space between you and the singularity" is not even well-defined. To be clear, you are right that Susskind's statement is very misleading (I posted my own criticism of it upthread).
  - deleted 1mo ago
    [deleted]
  - _moof 1mo ago
    It's unfortunately misleading shorthand for what actually happens: space becomes timelike. It doesn't become time. All this means is that once you cross the event horizon, you can only ever move toward the center of the black hole, in the same way that outside of a black hole, you can only ever move toward the future. You might not take a direct route to the center, but no matter which way you move, you will be following a track that ends at the center. The reason this phenomenon has a spooky-sounding name is that it also affects whether two objects can be causally connected. If you can only ever move closer to the center of the black hole, then there are (conceivably) other objects inside the event horizon that you can never have a causal relationship with. But it doesn't mean that space and time literally switch places.
- pdonis 1mo ago
  > the space and time coordinates switch places as you cross the event horizon If Susskind's book does in fact say that, it's extremely disappointing to me, because, as a number of other GR textbooks will tell you (e.g., Misner, Thorne & Wheeler and Wald, the two great classic GR textbooks), the "switch places" is an artifact of a particular choice of coordinates (Schwarzschild coordinates), and does not represent anything physical. So it's not something that should be relied on. (Not to mention the confusion it causes when pop science sources repeat the statement and then draw all manner of wrong conclusions from it.) The part about being "a surface in time" might be all right, assuming that by that he means "a surface representing a moment in time, not a place in space"--in more technical language, a spacelike surface. That is correct, and it's an invariant that does not depend on any choice of coordinates. But that invariant fact can be described without having to talk about the "switch places" thing at all.
  - kwoff 1mo ago
    Susskind's book does also mention that the event-horizon shenanigans are due to coordinates and not a physical thing. Certainly I'd trust what he says rather than me, so sorry if I was misleading. (If anyone has the book, it is chapter 6 section "Interchange of Space and Time Dimensions at the Horizon" and the following section points out the singularity is a time (and you can't escape it (in a Schwartzschild model at least) just like you can't escape time). I'm sorry if my wording is still incorrect.).
    - pdonis 1mo ago
      > Susskind's book does also mention that the event-horizon shenanigans are due to coordinates and not a physical thing. That's good. However: > Interchange of Space and Time Dimensions at the Horizon This still seems misleading to me, because "Dimensions" makes it seem like it's not just an artifact of coordinates--but it is.
  - sigmoid10 1mo ago
    Kruskal-Szeres coordinates indeed get rid of the wonky coordinate stuff at the event horizon, but if you look at the corresponding diagrams, you'll just end up with the same confusion, because the singularity is still a point (or rather surface) in the future instead of a point in space. The issue is that these diagrams are for eternal, static black holes, which cause diagrams to have these weirdly stretched infinite regions that are quite useful for understanding details of the math, but are highly confusing to laypeople. In fact these diagrams make it look like you'll always fall into the black hole at t=infinity, no matter how far you are away, when in reality you could orbit a static black hole pretty close for eternity. If you really want to get a picture of what is happening, you can look at Eddington-Finkelstein coordinates. In particular at a light cone field diagram around a collapsing shell of matter that turns into a black hole. Then this whole stuff suddenly makes sense without even going into the math. You don't just see how an event horizon can form out of nothing, you also see how gravity starts to bend your causal forward light cone (i.e. all points in spacetime with events that you could interact with in the future) inward in such a way that you will necessarily always fall closer to the center of the mass once you pass a certain line (aka the event horizon). No need to deal with those weird infinities or points in time suddenly lying on a different axis. The great Roger Penrose (the same guy who also came up with some of the most confusing diagrams) published a beautiful, simple overview of exactly this stuff in Scientific American: https://www.wkbpic.com/wkbx/SA/1972/1972-05-01.pdf https://www.wkbpic.com/wkbx/SA/1972/1972-05-01.pdf (starting on page 38) Still one of the best things you can read if you don't just want the math.
- SoftTalker 1mo ago
  The way Brian Cox puts it, a singularity is a point in time: the end of time. I have trouble really conceptualizing black hole physics, I just think of it as a mass so great that nothing, including light, can escape its gravity. Works for me.
  - XorNot 1mo ago
    The more interesting component is that black hole physics is almost an anti-free will zone. Everywhere else in the universe with mass and energy you can do what you want (sort of). An event horizon throws a hard shroud over that and drastically reduces opportunities: your free will to use mass and energy is significantly curtailed (you must head towards the singularity).
    - teamonkey 1mo ago
      I’m not sure free will has anything to do with it. If you don’t have enough upward velocity to escape earths gravity, hitting the ground is also inevitable.
    - Sharlin 1mo ago
      Your free will outside a black hole is curtailed in exactly the same manner: you cannot do anything to avoid, or even delay, tomorrow.
      - XorNot 1mo ago
        Which is exactly my point. Under normal circumstances you inexorably move towards the future. But we generally believe we have freedom of action about the spatial coordinates. A black hole is interesting because you inexorably move towards the singularity - which is a defined location in spacetime, and also has a boundary - the event horizon. So now your freedom of action is reduced: you must move towards the singularity, but you also can't actually move outside of the event horizon either.
  - PaulHoule 1mo ago
    The singularity in a non-rotating, non-charged black hole is as you say. It’s like in a finite amount of time you “run out of time”, like there isn’t any more time on that trajectory. The singularity in a rotating black hole is entirely different but the interior of classical Kerr (rotating) black holes is one of the most controversial if inconsequential topics in theoretical physics because there are reasons to believe (without real proof mind you) the Kerr solution is unstable inside the inner event horizon so that whatever happens in there is not what that theory says. And of course black holes are quantum objects which might actually have an “interior” entirely different from the classical picture.
empath75 1mo ago
I think it's not even a valid critique of that and it's sort of playing games with what the definition of a singularity is to reach the claim that it's making. I think the topology of the singularity is not even a well defined question and certainly not well understood enough to bear the strong claims in the paper.
- ImHereToVote 1mo ago
  Singularities suggest incomplete theories.
  - pdonis 1mo ago
    This is the opinion of most physicists, yes, but it does not in any way justify the GP's claims or cast doubt on anything that is said in the paper. Note that the paper talks explicitly about the limitations of GR as the singularity is approached and how a quantum gravity theory, if we ever find and confirm one, might fix those issues.
- pdonis 1mo ago
  Unfortunately you are wrong. Everything the paper is saying about the singularity and its properties in GR, and more generally about the black hole solutions it describes, is well understood and has been for decades. The definition of "singularity" that the paper is using is perfectly fine, and its topology is perfectly well-defined. A good textbook treatment is that of Wald (1984). Some of the things the paper points out are not emphasized in other sources, which is probably why the authors chose to write it. But there is nothing in the paper that is in the least questionable or ill-defined; it's all standard General Relativity as applied to the Schwarzschild and Kerr black hole solutions.
rf15 1mo ago
As someone with basically only popsci knowledge of black holes: people claiming it would be a literal point never made much sense - fundamentally, common sense (as much as it can apply here) dictates that you cannot compress particles to an absolute point.
- fangspire 1mo ago
  [flagged]
  - jambalaya8 1mo ago
    This needs a joke about unbelievable denseness.
- goatlover 1mo ago
  Aren't fundamental particles like electrons and quarks treated as points?
  - kevindamm 1mo ago
    Kind of, but not really.. though there are simple models with electrons as a point charge, a more accurate model involves the electron field describing the probability of an electron existing at any region in space (not to be confused with the electromagnetic field, the medium in which photons propagate).
    - drdeca 1mo ago
      Sure the position of an electron is not definite, but neither is that of a buckyball, but a buckyball has a shape we can describe, an internal structure we regard as extended over space, in a way that is separate from the indeterminacy of its center of mass position. This is unlike an electron, for which, if we set aside the uncertainty as to its center of mass, my understanding is that the only internal degrees of freedom it has left (in the Standard Model) are its spin and whether it is left or right handed, with no other structure to it. It is in this sense that, AIUI, electrons are modeled as point particles. Of course, that doesn’t mean that if we zoom in enough, probing at higher and higher energy scales, that it can’t turn out to have some non-zero fundamental size outside of just uncertainty in its center of mass position. I think string theory would say that at the string scale it would be a string. But, AIUI, no experiment has shown it to have the kind of extent that would make it be called not a point particle (an extent in a sense beyond just uncertainty in COM position)
dhosek 1mo ago
I figure at least some of it comes from the idea that mathematically, a singularity is a point (e.g., in the graph of z=1/w, there is a singularity at the point w=0, and in the graph of z=(1-w)²/(1-w) there is a removable singularity at w=1 (that is, the function is undefined at w=1, but if you put a point at (1,0), the graph will be continuous and no longer have any holes in it). The fact that both have the same name and the similar behavior of a black hole singularity to a mathematical singularity¹ can lead people to make an incorrect assumption. ⸻ 1. I must admit to a lack of sufficient GR education to feel confident in this, but I think that one of the issues that made physicists unwilling to accept the idea of black holes when they were first postulated was that there ended up being a division by zero in the mathematics.
- sigmoid10 1mo ago
  >The fact that both have the same name They don't just have the same name, they are the same thing. A Schwarzschild black hole has both: a removable singularity at the event horizon that is just an artefact of a particular choice of coordinates and a true non-removable mathematical singularity at r=0 where curvature really does go to infinity. It also wouldn't be much of an issue in classical physics, because this singularity is always hidden from outside observers, so the mathematical weirdness there can't screw with your normal predictions in space outside the black hole. The problems start once you consider quantum mechanics, because any such singularity will break unitarity (a fancy way of saying that probabilities must add up to 1), which means your theory as a whole can no longer make predictions. This has opened a whole can of worms with a bunch of solution attempts, which are all sadly untestable for the foreseeable future.
  - NooneAtAll3 1mo ago
    OSM - slight generalization of Schwarzshild BH, where you take evolving spherically-symmetric mass distribution instead of point mass - shows that point singularity in the middle can be naked (aka observable), so it's not just QM that causes worms... https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model
deleted 1mo ago
[deleted]
bmitc 1mo ago
> Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same. You say that and yet this thread is full of people arguing about it, and there's an entire Wikipedia article on this: https://en.wikipedia.org/wiki/Gravitational_singularity https://en.wikipedia.org/wiki/Gravitational_singularity. In fact, that article says: > No complete and precise definition of singularities exist in the theory of general relativity, So which is it? It can't both be trivial to any grad student but also an open question. And things like naked singularities aren't proven to not exist either. Also, general relativity is a classical, geometric-only theory. It seems obvious that better understanding what a black hole's singularity is would require quantum mechanics because the singularity is effectively what's "left over" of the physical material once you go beyond a neutron star.
- GoblinSlayer 1mo ago
  > And things like naked singularities aren't proven to not exist either. What do you mean by not exist? If you postulate the right black hole with a naked singularity, it would have a naked singularity. > It seems obvious that better understanding what a black hole's singularity is would require quantum mechanics If you postulate a classical black hole, it won't require quantum mechanics to understand.
- sigmoid10 1mo ago
  That's what you get when you start reading wikipedia about such an advanced topic without knowing anything about it. Wikipedia actually starts to crack as a reliable source for laypeople and on top of that begins to mix science and metascience pseudobabble, because there is a lot of confusing and misinformed content out there about the topic. But if you're not an actual expert, there is no way to differentiate the quality of different sources, so they get stuck in wiki articles by normal editors. If you had checked the links from your quote, you'd have seen that this idea was not pulled from physics PhDs, but from philosophy PhDs. Take that as you want, but any actual physicist or mathematician would have told you the sentence right after the one you quoted is what actually matters. The basic math is not that hard. Neither are the implications of the math (as long as you want to remain in rigorous land and not step into quack territory). In contrast, the general implications on our reality as a whole is what gets certain people (i.e. philosophers) riled up, but they can't really contribute to the topic since philosophy is basically science without rigor. That's why these discussions are limited to wikipedia articles and philosophy departments. Everyone else is busy actually calculating things and passing this cumbersome concept of peer-review.
WhitneyLand 1mo ago
That’s only true regarding the one sentence about the singularity not being a point. People like to reduce papers to a simple hot take, but the paper is more than that, offers viewpoints that are non-standard and speculation about new possibilities.
BjoernKW 1mo ago
Anyone who has watched Interstellar should know this.
- sigmoid10 1mo ago
  The singularity is just a bookshelf and love can act as space-time GPS amirite? Slap it on a Möbius strip on and make it a flashy wearable, then you basically have the last Avengers movie too.

huflungdung 1mo ago

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kwoff 1mo ago

Or read Susskind's "The Theoretical Minimum: General Relativity". For a non-spinning blackhole at least, not only is the singularity not a point, it is a surface in time, not space (as the book explains, the space and time coordinates switch places as you cross the event horizon).

dash2 1mo ago
> the space and time coordinates switch places as you cross the event horizon I'm sorry but this is blowing my mind. What???
- lstodd 1mo ago
  Yup. It's that weird. Also read Nick Gorkavyi: The Oscillating Universe: Einsteinian Cosmology of Black Holes and Gravitational Waves
- MathMonkeyMan 1mo ago
  [This video][1] and the one before it on the playlist are a good no nonsense explanation of the topic. [1]: https://www.youtube.com/watch?v=O_2vnb_eVGE https://www.youtube.com/watch?v=O_2vnb_eVGE
- metalliqaz 1mo ago
  it might help to think of the singularity as not a point in space but rather a future that cannot be avoided. All possible paths through space and time, no matter what happens, will go towards the singularity.
- Kranar 1mo ago
  Because it's very misleading. Time and space do not switch places past the event horizon. What happens is that the direction/path between an object and the singularity becomes a timelike dimension, and the direction that plays the role of time outside of the event horizon becomes a spacelike dimension. That is not the same as them swapping or that time becomes space and space becomes time not to mention that space has 3 dimensions and time has only 1 dimension so how could they even swap places. Really what it means is that past the event horizon you can use the direction in space between you and the singularity as a way to measure time, specifically the amount of time left before you reach the singularity. It's not so mind blowing when you interpret it that way now is it? You can imagine many things in ordinary life that you use to measure time without claiming that time has literally swapped places with it. On a road trip, the number of kilometres to your exit tells you how long you have left, that's using space as a proxy for time... big deal. The notable difference between a road trip and a black hole is that on a road trip you could stop for a break, you could maybe take a detour, you could decide to go back home... and these would all break your use of space as a proxy for measuring time. Well with a blackhole you can't do any of those things, there is no going back, there is no detour, the relationship between the spatial direction towards the singularity and time is fixed and causal and there's nothing you can do about it. The phrasing used is used almost certainly to evoke some kind of voodoo mind-blowing mystery that completely disappears when you get down to the more strict formalism.
  - pdonis 1mo ago
    > Really what it means is that past the event horizon you can use the direction in space between you and the singularity as a way to measure time That's not correct. There is a relationship between the radial coordinate r you are at and the time it will take you, by your clock, to reach the singularity (at least assuming you are freely falling), but that relationship can't be described the way you are describing it. To put the issue with what you say as starkly as possible: at any event inside the black hole's horizon, there are spacelike curves in every direction from you that will hit the singularity! So "the direction in space between you and the singularity" is not even well-defined. To be clear, you are right that Susskind's statement is very misleading (I posted my own criticism of it upthread).
- deleted 1mo ago
  [deleted]
- _moof 1mo ago
  It's unfortunately misleading shorthand for what actually happens: space becomes timelike. It doesn't become time. All this means is that once you cross the event horizon, you can only ever move toward the center of the black hole, in the same way that outside of a black hole, you can only ever move toward the future. You might not take a direct route to the center, but no matter which way you move, you will be following a track that ends at the center. The reason this phenomenon has a spooky-sounding name is that it also affects whether two objects can be causally connected. If you can only ever move closer to the center of the black hole, then there are (conceivably) other objects inside the event horizon that you can never have a causal relationship with. But it doesn't mean that space and time literally switch places.
pdonis 1mo ago
> the space and time coordinates switch places as you cross the event horizon If Susskind's book does in fact say that, it's extremely disappointing to me, because, as a number of other GR textbooks will tell you (e.g., Misner, Thorne & Wheeler and Wald, the two great classic GR textbooks), the "switch places" is an artifact of a particular choice of coordinates (Schwarzschild coordinates), and does not represent anything physical. So it's not something that should be relied on. (Not to mention the confusion it causes when pop science sources repeat the statement and then draw all manner of wrong conclusions from it.) The part about being "a surface in time" might be all right, assuming that by that he means "a surface representing a moment in time, not a place in space"--in more technical language, a spacelike surface. That is correct, and it's an invariant that does not depend on any choice of coordinates. But that invariant fact can be described without having to talk about the "switch places" thing at all.
- kwoff 1mo ago
  Susskind's book does also mention that the event-horizon shenanigans are due to coordinates and not a physical thing. Certainly I'd trust what he says rather than me, so sorry if I was misleading. (If anyone has the book, it is chapter 6 section "Interchange of Space and Time Dimensions at the Horizon" and the following section points out the singularity is a time (and you can't escape it (in a Schwartzschild model at least) just like you can't escape time). I'm sorry if my wording is still incorrect.).
  - pdonis 1mo ago
    > Susskind's book does also mention that the event-horizon shenanigans are due to coordinates and not a physical thing. That's good. However: > Interchange of Space and Time Dimensions at the Horizon This still seems misleading to me, because "Dimensions" makes it seem like it's not just an artifact of coordinates--but it is.
- sigmoid10 1mo ago
  Kruskal-Szeres coordinates indeed get rid of the wonky coordinate stuff at the event horizon, but if you look at the corresponding diagrams, you'll just end up with the same confusion, because the singularity is still a point (or rather surface) in the future instead of a point in space. The issue is that these diagrams are for eternal, static black holes, which cause diagrams to have these weirdly stretched infinite regions that are quite useful for understanding details of the math, but are highly confusing to laypeople. In fact these diagrams make it look like you'll always fall into the black hole at t=infinity, no matter how far you are away, when in reality you could orbit a static black hole pretty close for eternity. If you really want to get a picture of what is happening, you can look at Eddington-Finkelstein coordinates. In particular at a light cone field diagram around a collapsing shell of matter that turns into a black hole. Then this whole stuff suddenly makes sense without even going into the math. You don't just see how an event horizon can form out of nothing, you also see how gravity starts to bend your causal forward light cone (i.e. all points in spacetime with events that you could interact with in the future) inward in such a way that you will necessarily always fall closer to the center of the mass once you pass a certain line (aka the event horizon). No need to deal with those weird infinities or points in time suddenly lying on a different axis. The great Roger Penrose (the same guy who also came up with some of the most confusing diagrams) published a beautiful, simple overview of exactly this stuff in Scientific American: https://www.wkbpic.com/wkbx/SA/1972/1972-05-01.pdf https://www.wkbpic.com/wkbx/SA/1972/1972-05-01.pdf (starting on page 38) Still one of the best things you can read if you don't just want the math.
SoftTalker 1mo ago
The way Brian Cox puts it, a singularity is a point in time: the end of time. I have trouble really conceptualizing black hole physics, I just think of it as a mass so great that nothing, including light, can escape its gravity. Works for me.
- XorNot 1mo ago
  The more interesting component is that black hole physics is almost an anti-free will zone. Everywhere else in the universe with mass and energy you can do what you want (sort of). An event horizon throws a hard shroud over that and drastically reduces opportunities: your free will to use mass and energy is significantly curtailed (you must head towards the singularity).
  - teamonkey 1mo ago
    I’m not sure free will has anything to do with it. If you don’t have enough upward velocity to escape earths gravity, hitting the ground is also inevitable.
  - Sharlin 1mo ago
    Your free will outside a black hole is curtailed in exactly the same manner: you cannot do anything to avoid, or even delay, tomorrow.
    - XorNot 1mo ago
      Which is exactly my point. Under normal circumstances you inexorably move towards the future. But we generally believe we have freedom of action about the spatial coordinates. A black hole is interesting because you inexorably move towards the singularity - which is a defined location in spacetime, and also has a boundary - the event horizon. So now your freedom of action is reduced: you must move towards the singularity, but you also can't actually move outside of the event horizon either.
- PaulHoule 1mo ago
  The singularity in a non-rotating, non-charged black hole is as you say. It’s like in a finite amount of time you “run out of time”, like there isn’t any more time on that trajectory. The singularity in a rotating black hole is entirely different but the interior of classical Kerr (rotating) black holes is one of the most controversial if inconsequential topics in theoretical physics because there are reasons to believe (without real proof mind you) the Kerr solution is unstable inside the inner event horizon so that whatever happens in there is not what that theory says. And of course black holes are quantum objects which might actually have an “interior” entirely different from the classical picture.

empath75 1mo ago

I think it's not even a valid critique of that and it's sort of playing games with what the definition of a singularity is to reach the claim that it's making. I think the topology of the singularity is not even a well defined question and certainly not well understood enough to bear the strong claims in the paper.

ImHereToVote 1mo ago
Singularities suggest incomplete theories.
- pdonis 1mo ago
  This is the opinion of most physicists, yes, but it does not in any way justify the GP's claims or cast doubt on anything that is said in the paper. Note that the paper talks explicitly about the limitations of GR as the singularity is approached and how a quantum gravity theory, if we ever find and confirm one, might fix those issues.
pdonis 1mo ago
Unfortunately you are wrong. Everything the paper is saying about the singularity and its properties in GR, and more generally about the black hole solutions it describes, is well understood and has been for decades. The definition of "singularity" that the paper is using is perfectly fine, and its topology is perfectly well-defined. A good textbook treatment is that of Wald (1984). Some of the things the paper points out are not emphasized in other sources, which is probably why the authors chose to write it. But there is nothing in the paper that is in the least questionable or ill-defined; it's all standard General Relativity as applied to the Schwarzschild and Kerr black hole solutions.

rf15 1mo ago

As someone with basically only popsci knowledge of black holes: people claiming it would be a literal point never made much sense - fundamentally, common sense (as much as it can apply here) dictates that you cannot compress particles to an absolute point.

fangspire 1mo ago
[flagged]
- jambalaya8 1mo ago
  This needs a joke about unbelievable denseness.
goatlover 1mo ago
Aren't fundamental particles like electrons and quarks treated as points?
- kevindamm 1mo ago
  Kind of, but not really.. though there are simple models with electrons as a point charge, a more accurate model involves the electron field describing the probability of an electron existing at any region in space (not to be confused with the electromagnetic field, the medium in which photons propagate).
  - drdeca 1mo ago
    Sure the position of an electron is not definite, but neither is that of a buckyball, but a buckyball has a shape we can describe, an internal structure we regard as extended over space, in a way that is separate from the indeterminacy of its center of mass position. This is unlike an electron, for which, if we set aside the uncertainty as to its center of mass, my understanding is that the only internal degrees of freedom it has left (in the Standard Model) are its spin and whether it is left or right handed, with no other structure to it. It is in this sense that, AIUI, electrons are modeled as point particles. Of course, that doesn’t mean that if we zoom in enough, probing at higher and higher energy scales, that it can’t turn out to have some non-zero fundamental size outside of just uncertainty in its center of mass position. I think string theory would say that at the string scale it would be a string. But, AIUI, no experiment has shown it to have the kind of extent that would make it be called not a point particle (an extent in a sense beyond just uncertainty in COM position)

dhosek 1mo ago

I figure at least some of it comes from the idea that mathematically, a singularity is a point (e.g., in the graph of z=1/w, there is a singularity at the point w=0, and in the graph of z=(1-w)²/(1-w) there is a removable singularity at w=1 (that is, the function is undefined at w=1, but if you put a point at (1,0), the graph will be continuous and no longer have any holes in it). The fact that both have the same name and the similar behavior of a black hole singularity to a mathematical singularity¹ can lead people to make an incorrect assumption. ⸻ 1. I must admit to a lack of sufficient GR education to feel confident in this, but I think that one of the issues that made physicists unwilling to accept the idea of black holes when they were first postulated was that there ended up being a division by zero in the mathematics.

sigmoid10 1mo ago
>The fact that both have the same name They don't just have the same name, they are the same thing. A Schwarzschild black hole has both: a removable singularity at the event horizon that is just an artefact of a particular choice of coordinates and a true non-removable mathematical singularity at r=0 where curvature really does go to infinity. It also wouldn't be much of an issue in classical physics, because this singularity is always hidden from outside observers, so the mathematical weirdness there can't screw with your normal predictions in space outside the black hole. The problems start once you consider quantum mechanics, because any such singularity will break unitarity (a fancy way of saying that probabilities must add up to 1), which means your theory as a whole can no longer make predictions. This has opened a whole can of worms with a bunch of solution attempts, which are all sadly untestable for the foreseeable future.
- NooneAtAll3 1mo ago
  OSM - slight generalization of Schwarzshild BH, where you take evolving spherically-symmetric mass distribution instead of point mass - shows that point singularity in the middle can be naked (aka observable), so it's not just QM that causes worms... https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model https://en.wikipedia.org/wiki/Oppenheimer–Snyder_model

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bmitc 1mo ago

> Anyone who has taken a graduate level class in General Relativity would have been able to tell you the same. You say that and yet this thread is full of people arguing about it, and there's an entire Wikipedia article on this: https://en.wikipedia.org/wiki/Gravitational_singularity https://en.wikipedia.org/wiki/Gravitational_singularity. In fact, that article says: > No complete and precise definition of singularities exist in the theory of general relativity, So which is it? It can't both be trivial to any grad student but also an open question. And things like naked singularities aren't proven to not exist either. Also, general relativity is a classical, geometric-only theory. It seems obvious that better understanding what a black hole's singularity is would require quantum mechanics because the singularity is effectively what's "left over" of the physical material once you go beyond a neutron star.

GoblinSlayer 1mo ago
> And things like naked singularities aren't proven to not exist either. What do you mean by not exist? If you postulate the right black hole with a naked singularity, it would have a naked singularity. > It seems obvious that better understanding what a black hole's singularity is would require quantum mechanics If you postulate a classical black hole, it won't require quantum mechanics to understand.
sigmoid10 1mo ago
That's what you get when you start reading wikipedia about such an advanced topic without knowing anything about it. Wikipedia actually starts to crack as a reliable source for laypeople and on top of that begins to mix science and metascience pseudobabble, because there is a lot of confusing and misinformed content out there about the topic. But if you're not an actual expert, there is no way to differentiate the quality of different sources, so they get stuck in wiki articles by normal editors. If you had checked the links from your quote, you'd have seen that this idea was not pulled from physics PhDs, but from philosophy PhDs. Take that as you want, but any actual physicist or mathematician would have told you the sentence right after the one you quoted is what actually matters. The basic math is not that hard. Neither are the implications of the math (as long as you want to remain in rigorous land and not step into quack territory). In contrast, the general implications on our reality as a whole is what gets certain people (i.e. philosophers) riled up, but they can't really contribute to the topic since philosophy is basically science without rigor. That's why these discussions are limited to wikipedia articles and philosophy departments. Everyone else is busy actually calculating things and passing this cumbersome concept of peer-review.

WhitneyLand 1mo ago

That’s only true regarding the one sentence about the singularity not being a point. People like to reduce papers to a simple hot take, but the paper is more than that, offers viewpoints that are non-standard and speculation about new possibilities.

BjoernKW 1mo ago

Anyone who has watched Interstellar should know this.

sigmoid10 1mo ago
The singularity is just a bookshelf and love can act as space-time GPS amirite? Slap it on a Möbius strip on and make it a flashy wearable, then you basically have the last Avengers movie too.