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Black hole singularity is a surface not a point

300 points by raattgift 1mo ago

sigmoid10 1mo ago
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 29d 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
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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.
Certhas 1mo ago
As is nicely visualised by it's Penrose Diagram, e.g. https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif
- shagie 1mo ago
  Some PBS Space Time episodes featuring the Penrose Diagram (in order - the first two are from 9 years ago, the last from 6) What Happens at the Event Horizon? - https://youtu.be/mht-1c4wc0Q https://youtu.be/mht-1c4wc0Q Escape The Kugelblitz Challenge - https://youtu.be/v3hd3AI2CAA https://youtu.be/v3hd3AI2CAA Mapping the Multiverse - https://youtu.be/4v9A9hQUcBQ https://youtu.be/4v9A9hQUcBQ
- moralestapia 1mo ago
  Sorry man, that's not what this is about.
  - Certhas 1mo ago
    It's not? Schwarzschild has a space-like singularity. That's the wiggly horizontal line at the top left of the diagram. If you are in the black hole you can't avoid hitting it. Seems to be exactly what the paper is remarking on.
    - DeluluDon 1mo ago
      [flagged]
  - evanb 1mo ago
    Yes it is?
- greesil 1mo ago
  I enjoyed this Veritasium video on the subject, which includes Penrose Diagrams. https://youtu.be/6akmv1bsz1M https://youtu.be/6akmv1bsz1M
Eridanus2 1mo ago
Contains 08 rendered frames of a free falling observer's view while crossing the event horizon. This is not reddit, but plz someone animate it :}
- Eridanus2 1mo ago
  Downvoted back to the dark ages.
- deleted 1mo ago
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ck2 1mo ago
what's really going to blow your mind is while you probably assumed or knew spinning black holes move space around them spinning black holes also move TIME around them * https://www.science.org/doi/10.1126/sciadv.ady9068 https://www.science.org/doi/10.1126/sciadv.ady9068 so in theory a spinning black hole that's been around for billions of years has a time drag around it in a path that is billions of years old (no we can't navigate it because yes that would be time travel to the past and violates causality) black holes are just so weird with every new detail even more weird oddly more interesting to me to try to grasp neutron stars (densest objects before black holes and are still visible, our entire solar system in a neutron star would be only 10km 6.2miles across)
- srean 1mo ago
  Consider the magnetar. https://en.wikipedia.org/wiki/Magnetar https://en.wikipedia.org/wiki/Magnetar "A magnetar's 10^10 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead."
  - 1970-01-01 1mo ago
    Yes, magnetars are considerably more rare than black holes and considerably more interesting to study in terms of raw horsepower. Imagine a type-2 civilization using them as engines or launchers for spacecraft to zip around the galaxy.
    - ck2 1mo ago
      the radiation from a magnetar exceeds any other star, overcoming that would seem implausible still trying to wrap my mind around kilonovas (colliding neutron stars) ie. they can pop out earth-sized chunks of gold, in theory, and since they aren't black holes that would be VISIBLE, albeit also "in theory" lol * https://www.nasa.gov/image-article/unfolding-story-of-kilonova-told-x-rays/ https://www.nasa.gov/image-article/unfolding-story-of-kilono... maybe Roman can spot one someday, that would be something
      - 1mo ago
helf 1mo ago
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jamesforestwest 1mo ago
Interesting work. The idea that the singularity is a surface rather than a point was unexpected to me even though it seems to follow logically from the theory of relativity. I wonder how this reconciles with quantum gravity. If the singularity is truly a two-dimensional surface, perhaps it's related to Hawking radiation and the thermodynamics of black holes?
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542458 1mo ago
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dekdrop 1mo ago
How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.
- icepush 1mo ago
  A one-way door in space.
- crooked-v 1mo ago
  The problem there is that not even the physicists completely agree on the details, because we know black holes definitely exist, but every explanation breaks one rule or another that should apply from different disciplines. It's part of why they get so much ongoing attention.
  - lazide 1mo ago
    Black holes are essentially where our knowledge of spacetime breaks, and we can’t even see into it. It’s hard to really concretely know much about it directly.
- pantulis 1mo ago
  It's a region of space from where not even light can scape. You can get a region like that by squashing a lot of mass in a small space, like happens when a star collapses under its own gravity. So here the intuition of "high density" makes sense. But at the center of galaxies you have the so called "supermassive black holes" which are more or less comparable in size to the solar system and yes, they have a lot of mass but they are not very dense, a pop-sci trope is comparing it's density to cotton candy or even the air we're breathing right now. So it's a matter of how you distribute mass/energy in a given diameter, not exactly of density.
- ben_w 1mo ago
  Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in. A black hole happens when there is enough gravity that space gets pulled inwards somewhere, at at least the speed of light. Gravity falls off with distance, and the distance where space is being pulled inwards at exactly the speed of light is called the "event horizon". It has this name because speed of light is the speed of causality: events that happen further in, are "over the horizon" for you, they cannot causally influence you.
Groxx 1mo ago
Well... yeah? That's describing the event horizon. It's a term roughly as widely used as "singularity". Talking about the inside of a black hole is indeed rather pop-misunderstood though, yes. But it's not like physicists are especially confident about the details either. Theoretical astrophysics changes a lot as time goes on and our instruments improve, and it's a rather hard field to do experiments on to get better data quicker.
- evanb 1mo ago
  No, sorry, the singular surface in a Schwarzschild spacetime is not the event horizon. Nothing particularly interesting (from the GR point of view) happens at the event horizon.
dboreham 1mo ago
Isn't that how we're inside one.
- neom 1mo ago
  Good PBS spacetime episode that looks at this: https://www.youtube.com/watch?v=jeRgFqbBM5E https://www.youtube.com/watch?v=jeRgFqbBM5E ( Could The Universe Be Inside A Black Hole?) Also, this spacetime episode is interesting the context of the paper and your statement: https://www.youtube.com/watch?v=x4TdColoIu8 https://www.youtube.com/watch?v=x4TdColoIu8 (We Thought Black Holes Created Event Horizons. It Might Be the Opposite)
imzadi 1mo ago
Would this apply also to the singularity at the beginning of the universe? I guess I thought that the singularity was where all matter is compressed so much that it occupies a zero dimension point. I'm not sure if that applies equally to black holes and the singularity at the beginning of the universe. I'm kind of dumb on this stuff even though it fascinates me.
- measurablefunc 1mo ago
  It could also be an infinite dimensional ball which technically also has 0 surface area & volume even though it has a non-zero radius.
- mrkeen 1mo ago
  The big bang didn't happen at a single point, it happened everywhere. You can look out from anywhere and see the cosmic background radiation having expanded from your location, wherever that location might be.
  - 725686 1mo ago
    Doesn't that imply that, "everywhere" was, precisely, a single point?
    - Dylan16807 1mo ago
      No, why would it imply that? One theory that might help you visualize an alternative is that the big bang was basically two 3D universes (floating in higher-dimensional space) slapping against each other really hard. That creates an explosion everywhere even if everywhere is quite big or even infinite.
  - sfink 1mo ago
    > The big bang didn't happen at a single point, it happened everywhere. A bit of an odd thing to say, since "everywhere" implies there are multiple places to be, and at the instant of the big bang, there was only place to be. So it was both everywhere and at a single point: it was at all of the single place there was to be. But your point (sorry) about expansion being from everywhere isn't specific to the big bang; space was expanding well after the big bang and it doesn't seem like the expansion ever had a "center" (at least, not since not-center places existed). It's expanding from everywhere. (But evenly everywhere? I have no idea. Hey, maybe black holes are like buttons in cloth, and it expanded everywhere except for where the buttons were holding things still at a rate relative to distance from the button. A brilliant hypothesis that explains exactly zero unexplained phenomena, at least none that I know of.)
Yiin 1mo ago
[flagged]
sfink 1mo ago
Off-topic, but it makes me think of "reasoning black holes": you get enough like-minded people together that they start reinforcing each other's logic and beliefs until not only those people get completely detached from reality, but anyone who interacts with them gets sucked in as well unless their own logic ("velocity") is adequate to skirt the edge and escape, forever altered by the experience. Similar questions arise: how would you know if you were inside one? The laws of logic ("physics") seemingly don't apply, but there's no way to test them in that environment.
- beeandapenguin 1mo ago
  Kinda sounds like a "linguistic manifold."
- PaulHoule 1mo ago
  I dunno. Those rationalists seem to be in some kind of intellectual black hole which never had any danger of sucking me in. Like I have seen many strands of posthumanism and transhumanism, speculations about an intelligence explosion circa 1970, and figure it would have been just as much fun to sit around the campfire, pass a joint around, and talk about crazy stuff with these guys https://en.wikipedia.org/wiki/Russian_cosmism https://en.wikipedia.org/wiki/Russian_cosmism as it would be to do with anyone contemporary. In their orbit I get periodically annoyed but changed forever, no.
  - sfink 1mo ago
    Well, the analogy is to something flying by a black hole and having its trajectory altered.[1] It sounds like it fits you, then -- you interacted, and are now forever irritated by them or by things that sound like them, even things that you would not have previously noticed. [1] This is just gravity, nothing specific to black holes, so the analogy isn't doing a lot of work here.
- inigyou 1mo ago
  I believe that's called an echo chamber, and exemplified by a board meeting.
- 1mo ago
yubblegum 1mo ago
Is there such a thing as a "point" in the universe?
- goatlover 1mo ago
  Depends on whether anything less than the Planck length has meaning.
  - yubblegum 1mo ago
    An object sans semantics would be fine.
- throwawayffffas 1mo ago
  That is the whole question. My gut says no, infinites and infinitesimals are the edges of our understanding not real.
dj_axl 1mo ago
scoffs Well yeah everyone and their aunt knows singularities have 5 dimensions.
- throwawayffffas 1mo ago
  You need at least 11 for the math to work out but it's okay most can be compacted.
metalman 1mo ago
treating a black hole as a point, or ;) pointicle,may be valid as matter may be condensed, to the, I mean ,in such a way that there is NO space left whatsoever, and therefor no room for physical dimensions to exist in, and the entire body functions as an undiferentiated body of stuff, or giant pointicle that destroys time and space. Hopefully very soon we will get news about our galaxys central black holes interaction with a star that is orbiting very close but at 8% light speed, which may reveal if our blackhole is spinning, and if that is the case the we would have proof of energy/information/gravity waves? escaping from a black hole, along with gravity which while apparently imune to its self, nothing else is so far.
kazinator 1mo ago
> Black hole singularity is a surface not a point Might they be trying to say this? 1. The boundary of the black hole which traps light, etc, is called the event horizon, and sits at the Schwarzschild radius. This is a geometric surface. 2. There is no singularity at this surface. 3. In models of black holes, there is a gravitational singularity at a point in the centre: https://en.wikipedia.org/wiki/Gravitational_singularity https://en.wikipedia.org/wiki/Gravitational_singularity which is a topic with nuances.
- Mithriil 1mo ago
  They do speak of the gravitational singularity (not the event horizon). It's within the horizon, there is a contradiction between the need for two infallers' position to reach the singularity (thus hitting a point) and the causal impossibility of them actually meeting (following General Relativity?). This suggests that the point should maybe be a surface. I don't follow most of the arguments however.
- throwawayffffas 1mo ago
  No, they are theorizing about the Kerr metric. The Kerr metric describes rotating black holes and instead of a point like singularity predicts a ring of zero thickness with infinite density additional work posits the ring is unstable and collapses to a surface all inside the event horizon, they then proceed to make predictions about the nature and behavior of that surface. By the way the Kerr metric predicts a ring because the centrifugal acceleration due to the rotation partially counteracts the gravity. As far as I understand, not a physicist.
lukeify 1mo ago
I know this isn't a new discovery but I fully expect the next major theoretical physics breakthrough to be discovered by a frontier LLM at this point, given their aptitude at solving a lot of the recent mathematical conjectures.
throwawayffffas 1mo ago
Or you know there is no such thing as a singularity, and stars collapse to fuzzballs[1] https://en.wikipedia.org/wiki/Fuzzball_(string_theory) https://en.wikipedia.org/wiki/Fuzzball_(string_theory)
chendawenplus 1mo ago
is hole
DrJokepu 1mo ago
I have a question for any physicists here. Due to my engineering background, I know just enough physics and mathematics to completely misunderstand general relativity and quantum mechanics. However, one pattern I have noticed is that one favorite past time of physicists is looking at the mathematical models, trying to find insane ass edge cases and then trying to interpret them. With that in mind, do these equations allow black holes whose singularities extend beyond their event horizons?
- automatic6131 1mo ago
  Yeah. I can't do the physics myself, so I take on faith (heh) that a sufficiently fast rotating blackhole has a ring shaped singularity that can extend outside its' event horizon. This was news in the 90s and it was a plot point in at least two scifi books, though I don't think I can recommend either
  - davidgrenier 1mo ago
    I was assuming the event horizon would also have a donut shape around the ring singularity.
- fooker 1mo ago
  The reason is that these edge cases often expose where the current models can break down. If the current accepted theory is predicting negative mass or infinite mass, it doesn't really mean that a physicist deeply believes we are going to be finding particles with negative mass. It's more likely we'll find a better theory. In some rare cases, these mathematical oddities do turn out to be real. We found equations producing negative energy as solutions long before we discovered antimatter.
- jiggawatts 1mo ago
  This is the "cosmic censorship hypothesis": singularities are always hidden by an event horizon and can never be observed without falling victim to them. Or more accurately, any observer of a singularity can never communicate that information to someone who is not a victim. This is hypothetical, and not at all "proven" in any meaningful sense.
amelius 1mo ago
> Counterintuitively, in general relativity two points can be spatially close yet causally distant. Can the converse also be true in general relativity?
- raattgift 1mo ago
  Yes, during cosmic inflation for example: two test objects initially close can end up many light-years apart. If we make these test objects null (i.e., lightlike) then we can always contrive an inflation that stretches them apart in such a way that they still meet again. Some exotic spacetimes involving pp-wave sandwiches can focus initially non-converging and spatially distant light pencils onto each other at a caustic shortly after the passing of the stack of plane-parallel gravitational waves. One can hide some such processes in the early cosmos.
  - Obscurity4340 1mo ago
    Where or encroaching upon what does the univerise expand into? If the universe is all the real estate of physical reality, what is it subsuming as it expands in order to accomodate that growth?
    - raattgift 1mo ago
      Not sure what level of answer you want here. I don't think there's a good slogan that you could memorize and repeat like "the night sky is black because Olber's 'paradox' is badly formulated in that the universe's star formation has a finite history and radiation from before the first stars is redshifted too low to activate visual opsins" or "matter tells spacetime how to curve", and anyway most such slogans are likely to induce misunderstanding (a major theme of the article linked at the very top). Ethan Siegal (a former theoretical cosmologist who has lots of practice in his second career doing science outreach) did it well enough at a pop-sci level that I'll just point to his https://bigthink.com/starts-with-a-bang/what-universe-expanding-into/ https://bigthink.com/starts-with-a-bang/what-universe-expand... (I don't think I could do better [*]). Here's a sketch for a crash syllabus that would take you closer to an answer I'd write, not being a practiced science communicator: My approach would be to teach you some differential geometry on a differentiable Euclidean plane (mostly relating the classic Euclidean distance to the integration of a line element), then what a Riemann manifold is, then how a 3+1-d pseudo-Riemannian one differs from a 4-d Riemannian manifold (and understanding the Ricci curvature in an Einstein manifold), and take you to understanding the simplest of metrics on the Lorentzian manifold, and the concept of geodesics and how they separate into spacelike, timelike, and null. I'd also teach you early about affine distance so that you don't stumble into problems understanding that a pulse of light from the ground to a mirror on the moon and back to the ground takes about two seconds, and how a pulse of matter -- including a pulse of light -- loses energy in an expanding spacetime. (That's another where does it go question, and a good one to think about.) Then I'd introduce Raychaudri-equation-style thinking, with a spray of timelike geodesics separating, as a way of understanding the metric expansion of space and the FLRW metric (where each Friedmann-equation dust represents an enormous number of timelike and lightlike geodesics). I'd also teach you about the Lagrangian and Eulerian specifications of the flow field, and how they relate to one another. We can have a idealized (freely-falling, feels-no-forces) Lagrangian observer follow one line in a spray of geodesics which are initially extremely close to each other, and which separate with the metric expansion of space. Some of the initially-close geodesics causally disconnect from our chosen Lagrangian observer, with close-but-less-close ones disconnecting quickly, and very-close ones staying practically parallel for a very very long time. This is basically the Raychaudri equation, as applied to cosmology. We'd want to explore radar distances between our Lagrangian observer and ideal reflective objects attached to other geodesics on the spray. We then can relate all that to a spacetime-slicing approach where we track what's on 3-d spacelike hypersurfaces, in a Eulerian style, going from our Rachaudhrian spray to a collection of space-filling dusts or fluids that dilute away differently over time. This is the usual picture cosmology students operate with. Understanding that, especially how expansion generates several cosmological horizons, is half of the key to answering your question. The other half is understanding that one can run the relevant equations under a time-reversal, with initially enormously distant objects freely falling towards each other and ending up practically on top of each other in the early history of expansion. Along the way we'd also be talking about the thermodynamics, as expansion is adiabatic. Our causal physics are all related to an extremely hot, extremely dense, extremely low-entropy volume in our billions-of-years-ago past, which we retrodict by studying fractions of later volumes (fractions as small as careful laboratory experiments and as big as large scale galaxy surveys). Anything close to that patch causally disconnected from us very early, and we'll never be able to hear from those parts of a big spray, and they'll never hear from us. Just outside our very early universe, things probably look very similar to things just outside it. The logic here is that as our galaxy crosses out of a cosmic horizon of somone far away, our galaxy doesn't do anything weird, and likewise there are many galaxies currently crossing out of our cosmic horizons, and they probably aren't doing anything weird either. Studies of the expansion history, still-viable cosmic inflation scenarios, and global spatial curvature have led to estimates (e.g. Guth's work) that some our early hot dense patch is at most 10^-23 of basically the same early hot dense stuff. That's fairly comparable to the number of atoms of water in the North Atlantic ocean, all of which are interchangeable, although they all have different histories of where they've been in Earth's oceans, the pressures and densities they've experienced on their travels, and so on. The pre-inflationary patch's tiny elements are pretty interchangeable although they'll have slightly different histories of expansion, galaxy formation, and so on, given tiny differences in their very early histories ("initial conditions"). Some may be overdense and quickly collapse. Some may be underdense and thus produce few if any stars. Now, is that primordial hot dense patch embedded into something bigger? Good question! Does it even matter, given that it causally decoupled from us so early? Good question! How do we even begin to investigate that? Good question! That's all live postgrad and postdoc research, with a lot of focus on trying to make the low entropy part of our hot dense early universe seem un-special. Siegel again: https://bigthink.com/starts-with-a-bang/cosmic-inflation-past-hypothesis/ https://bigthink.com/starts-with-a-bang/cosmic-inflation-pas... Once you have that under your belt you can join the manifold (pardon the pun) papers exploring the physical implications of various guesses about what's outside the everything-everywhere-everywhen fully determined ("block universe") picture painted by a notional exact solution of the Einstein Field Equations of General Relativity, which we can only successively approximate by sampling signals from our past. But at least you'd then understand what it means to say that mean energy-densities fall over cosmological time, and that the centres of mass of galaxy clusters are separating over cosmological time, and that our distant distant descendants won't see any galaxies not presently in our local group. For extra credit you could play around with embeddings of de Sitter space in higher-dimensional manifolds and run into the usual frustrations of it being quite hard to recover known physics -- one can even largely justify a statement like embedding a 3+1d spacetime into a higher dimensional spacetime is generally not possible. Of course, many people still attempt to make that work not so much to answer your question, but to find ways of more easily calculating the way our visible universe behaves. [*] I'd have maybe said "its own future" and otherwise present a wordier version of what Siegel wrote (explicitly raising time-orientability), but really I'd want to explain why I'm mostly a blockworlder in spite of how us small temporary knots of atomic nuclei feel about that <https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)#Objections https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...> and that maybe the real question is why our brains encode the concept of expansion at all. Anyway our puny brains can't hold all knowledge, we can't just pour in mathematical physicslike kung fu, helicopter piloting, or motorcycle-hotwiring skills in The Matrix movies, and the behaviour of the universe at scales of billions of lightyears didn't change once humans started printing cosmology textbooks. And it's OK if you haven't worked through any of those; just be careful of memorizing factoids from people who haven't worked through any of them either.
jeisc 1mo ago
a point is a pancake of a sphere in space: everything has depth
rfgplk 1mo ago
The primary issues with black holes and singularities (as well as the numerous endless debates about them) is due to admitting only a single temporal dimension as the sole representation of the universe. Once you allow multiple temporal dimensions (at least two non gauge constrained ones) the issues disappear entirely.
- DexesTTP 1mo ago
  The issue is that we only observe one dimension of time and three dimensions of space. So allowing more than that would require a lot of evidence. Postulating that there's multiple times dimensions is the same thing as postulating that there's more than three space dimensions in string theory. You make the maths "easier" by postulating that there's more dimensions, but you don't make any predictions that the 3+1 spacetime theory doesn't make and that can be observed experimentally.
kelvo_ran 1mo ago
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t_marsden 1mo ago
This kind of result is why GR still surprises after a century. Geometry beats intuition.
wahid_seddiqi 1mo ago
[flagged]
westurner 1mo ago
There is no black hole singularity in SQG (Fedi). Spacetime is a shear-thickening (dilatant) non-Newtonian fluid, and that's why the speed of light c is what it is.

300 points by raattgift 1mo ago

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 29d 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
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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.

Certhas 1mo ago

As is nicely visualised by it's Penrose Diagram, e.g. https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif https://jila.colorado.edu/~ajsh/insidebh/penrose_schw.gif

shagie 1mo ago
Some PBS Space Time episodes featuring the Penrose Diagram (in order - the first two are from 9 years ago, the last from 6) What Happens at the Event Horizon? - https://youtu.be/mht-1c4wc0Q https://youtu.be/mht-1c4wc0Q Escape The Kugelblitz Challenge - https://youtu.be/v3hd3AI2CAA https://youtu.be/v3hd3AI2CAA Mapping the Multiverse - https://youtu.be/4v9A9hQUcBQ https://youtu.be/4v9A9hQUcBQ
moralestapia 1mo ago
Sorry man, that's not what this is about.
- Certhas 1mo ago
  It's not? Schwarzschild has a space-like singularity. That's the wiggly horizontal line at the top left of the diagram. If you are in the black hole you can't avoid hitting it. Seems to be exactly what the paper is remarking on.
  - DeluluDon 1mo ago
    [flagged]
- evanb 1mo ago
  Yes it is?
greesil 1mo ago
I enjoyed this Veritasium video on the subject, which includes Penrose Diagrams. https://youtu.be/6akmv1bsz1M https://youtu.be/6akmv1bsz1M

Eridanus2 1mo ago

Contains 08 rendered frames of a free falling observer's view while crossing the event horizon. This is not reddit, but plz someone animate it :}

Eridanus2 1mo ago
Downvoted back to the dark ages.
deleted 1mo ago
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ck2 1mo ago

what's really going to blow your mind is while you probably assumed or knew spinning black holes move space around them spinning black holes also move TIME around them * https://www.science.org/doi/10.1126/sciadv.ady9068 https://www.science.org/doi/10.1126/sciadv.ady9068 so in theory a spinning black hole that's been around for billions of years has a time drag around it in a path that is billions of years old (no we can't navigate it because yes that would be time travel to the past and violates causality) black holes are just so weird with every new detail even more weird oddly more interesting to me to try to grasp neutron stars (densest objects before black holes and are still visible, our entire solar system in a neutron star would be only 10km 6.2miles across)

srean 1mo ago
Consider the magnetar. https://en.wikipedia.org/wiki/Magnetar https://en.wikipedia.org/wiki/Magnetar "A magnetar's 10^10 tesla field, by contrast, has an energy density of 4.0×1025 J/m3, with an E/c2 mass density more than 10,000 times that of lead."
- 1970-01-01 1mo ago
  Yes, magnetars are considerably more rare than black holes and considerably more interesting to study in terms of raw horsepower. Imagine a type-2 civilization using them as engines or launchers for spacecraft to zip around the galaxy.
  - ck2 1mo ago
    the radiation from a magnetar exceeds any other star, overcoming that would seem implausible still trying to wrap my mind around kilonovas (colliding neutron stars) ie. they can pop out earth-sized chunks of gold, in theory, and since they aren't black holes that would be VISIBLE, albeit also "in theory" lol * https://www.nasa.gov/image-article/unfolding-story-of-kilonova-told-x-rays/ https://www.nasa.gov/image-article/unfolding-story-of-kilono... maybe Roman can spot one someday, that would be something
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helf 1mo ago

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jamesforestwest 1mo ago

Interesting work. The idea that the singularity is a surface rather than a point was unexpected to me even though it seems to follow logically from the theory of relativity. I wonder how this reconciles with quantum gravity. If the singularity is truly a two-dimensional surface, perhaps it's related to Hawking radiation and the thermodynamics of black holes?

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dekdrop 1mo ago

How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.

icepush 1mo ago
A one-way door in space.
crooked-v 1mo ago
The problem there is that not even the physicists completely agree on the details, because we know black holes definitely exist, but every explanation breaks one rule or another that should apply from different disciplines. It's part of why they get so much ongoing attention.
- lazide 1mo ago
  Black holes are essentially where our knowledge of spacetime breaks, and we can’t even see into it. It’s hard to really concretely know much about it directly.
pantulis 1mo ago
It's a region of space from where not even light can scape. You can get a region like that by squashing a lot of mass in a small space, like happens when a star collapses under its own gravity. So here the intuition of "high density" makes sense. But at the center of galaxies you have the so called "supermassive black holes" which are more or less comparable in size to the solar system and yes, they have a lot of mass but they are not very dense, a pop-sci trope is comparing it's density to cotton candy or even the air we're breathing right now. So it's a matter of how you distribute mass/energy in a given diameter, not exactly of density.
ben_w 1mo ago
Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in. A black hole happens when there is enough gravity that space gets pulled inwards somewhere, at at least the speed of light. Gravity falls off with distance, and the distance where space is being pulled inwards at exactly the speed of light is called the "event horizon". It has this name because speed of light is the speed of causality: events that happen further in, are "over the horizon" for you, they cannot causally influence you.

Groxx 1mo ago

Well... yeah? That's describing the event horizon. It's a term roughly as widely used as "singularity". Talking about the inside of a black hole is indeed rather pop-misunderstood though, yes. But it's not like physicists are especially confident about the details either. Theoretical astrophysics changes a lot as time goes on and our instruments improve, and it's a rather hard field to do experiments on to get better data quicker.

evanb 1mo ago
No, sorry, the singular surface in a Schwarzschild spacetime is not the event horizon. Nothing particularly interesting (from the GR point of view) happens at the event horizon.

dboreham 1mo ago

Isn't that how we're inside one.

neom 1mo ago
Good PBS spacetime episode that looks at this: https://www.youtube.com/watch?v=jeRgFqbBM5E https://www.youtube.com/watch?v=jeRgFqbBM5E ( Could The Universe Be Inside A Black Hole?) Also, this spacetime episode is interesting the context of the paper and your statement: https://www.youtube.com/watch?v=x4TdColoIu8 https://www.youtube.com/watch?v=x4TdColoIu8 (We Thought Black Holes Created Event Horizons. It Might Be the Opposite)

imzadi 1mo ago

Would this apply also to the singularity at the beginning of the universe? I guess I thought that the singularity was where all matter is compressed so much that it occupies a zero dimension point. I'm not sure if that applies equally to black holes and the singularity at the beginning of the universe. I'm kind of dumb on this stuff even though it fascinates me.

measurablefunc 1mo ago
It could also be an infinite dimensional ball which technically also has 0 surface area & volume even though it has a non-zero radius.
mrkeen 1mo ago
The big bang didn't happen at a single point, it happened everywhere. You can look out from anywhere and see the cosmic background radiation having expanded from your location, wherever that location might be.
- 725686 1mo ago
  Doesn't that imply that, "everywhere" was, precisely, a single point?
  - Dylan16807 1mo ago
    No, why would it imply that? One theory that might help you visualize an alternative is that the big bang was basically two 3D universes (floating in higher-dimensional space) slapping against each other really hard. That creates an explosion everywhere even if everywhere is quite big or even infinite.
- sfink 1mo ago
  > The big bang didn't happen at a single point, it happened everywhere. A bit of an odd thing to say, since "everywhere" implies there are multiple places to be, and at the instant of the big bang, there was only place to be. So it was both everywhere and at a single point: it was at all of the single place there was to be. But your point (sorry) about expansion being from everywhere isn't specific to the big bang; space was expanding well after the big bang and it doesn't seem like the expansion ever had a "center" (at least, not since not-center places existed). It's expanding from everywhere. (But evenly everywhere? I have no idea. Hey, maybe black holes are like buttons in cloth, and it expanded everywhere except for where the buttons were holding things still at a rate relative to distance from the button. A brilliant hypothesis that explains exactly zero unexplained phenomena, at least none that I know of.)

Yiin 1mo ago

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sfink 1mo ago

Off-topic, but it makes me think of "reasoning black holes": you get enough like-minded people together that they start reinforcing each other's logic and beliefs until not only those people get completely detached from reality, but anyone who interacts with them gets sucked in as well unless their own logic ("velocity") is adequate to skirt the edge and escape, forever altered by the experience. Similar questions arise: how would you know if you were inside one? The laws of logic ("physics") seemingly don't apply, but there's no way to test them in that environment.

beeandapenguin 1mo ago
Kinda sounds like a "linguistic manifold."
PaulHoule 1mo ago
I dunno. Those rationalists seem to be in some kind of intellectual black hole which never had any danger of sucking me in. Like I have seen many strands of posthumanism and transhumanism, speculations about an intelligence explosion circa 1970, and figure it would have been just as much fun to sit around the campfire, pass a joint around, and talk about crazy stuff with these guys https://en.wikipedia.org/wiki/Russian_cosmism https://en.wikipedia.org/wiki/Russian_cosmism as it would be to do with anyone contemporary. In their orbit I get periodically annoyed but changed forever, no.
- sfink 1mo ago
  Well, the analogy is to something flying by a black hole and having its trajectory altered.[1] It sounds like it fits you, then -- you interacted, and are now forever irritated by them or by things that sound like them, even things that you would not have previously noticed. [1] This is just gravity, nothing specific to black holes, so the analogy isn't doing a lot of work here.
inigyou 1mo ago
I believe that's called an echo chamber, and exemplified by a board meeting.
1mo ago

yubblegum 1mo ago

Is there such a thing as a "point" in the universe?

goatlover 1mo ago
Depends on whether anything less than the Planck length has meaning.
- yubblegum 1mo ago
  An object sans semantics would be fine.
throwawayffffas 1mo ago
That is the whole question. My gut says no, infinites and infinitesimals are the edges of our understanding not real.

dj_axl 1mo ago

scoffs Well yeah everyone and their aunt knows singularities have 5 dimensions.

throwawayffffas 1mo ago
You need at least 11 for the math to work out but it's okay most can be compacted.

metalman 1mo ago

treating a black hole as a point, or ;) pointicle,may be valid as matter may be condensed, to the, I mean ,in such a way that there is NO space left whatsoever, and therefor no room for physical dimensions to exist in, and the entire body functions as an undiferentiated body of stuff, or giant pointicle that destroys time and space. Hopefully very soon we will get news about our galaxys central black holes interaction with a star that is orbiting very close but at 8% light speed, which may reveal if our blackhole is spinning, and if that is the case the we would have proof of energy/information/gravity waves? escaping from a black hole, along with gravity which while apparently imune to its self, nothing else is so far.

kazinator 1mo ago

> Black hole singularity is a surface not a point Might they be trying to say this? 1. The boundary of the black hole which traps light, etc, is called the event horizon, and sits at the Schwarzschild radius. This is a geometric surface. 2. There is no singularity at this surface. 3. In models of black holes, there is a gravitational singularity at a point in the centre: https://en.wikipedia.org/wiki/Gravitational_singularity https://en.wikipedia.org/wiki/Gravitational_singularity which is a topic with nuances.

Mithriil 1mo ago
They do speak of the gravitational singularity (not the event horizon). It's within the horizon, there is a contradiction between the need for two infallers' position to reach the singularity (thus hitting a point) and the causal impossibility of them actually meeting (following General Relativity?). This suggests that the point should maybe be a surface. I don't follow most of the arguments however.
throwawayffffas 1mo ago
No, they are theorizing about the Kerr metric. The Kerr metric describes rotating black holes and instead of a point like singularity predicts a ring of zero thickness with infinite density additional work posits the ring is unstable and collapses to a surface all inside the event horizon, they then proceed to make predictions about the nature and behavior of that surface. By the way the Kerr metric predicts a ring because the centrifugal acceleration due to the rotation partially counteracts the gravity. As far as I understand, not a physicist.

lukeify 1mo ago

I know this isn't a new discovery but I fully expect the next major theoretical physics breakthrough to be discovered by a frontier LLM at this point, given their aptitude at solving a lot of the recent mathematical conjectures.

throwawayffffas 1mo ago

Or you know there is no such thing as a singularity, and stars collapse to fuzzballs[1] https://en.wikipedia.org/wiki/Fuzzball_(string_theory) https://en.wikipedia.org/wiki/Fuzzball_(string_theory)

chendawenplus 1mo ago

is hole

DrJokepu 1mo ago

I have a question for any physicists here. Due to my engineering background, I know just enough physics and mathematics to completely misunderstand general relativity and quantum mechanics. However, one pattern I have noticed is that one favorite past time of physicists is looking at the mathematical models, trying to find insane ass edge cases and then trying to interpret them. With that in mind, do these equations allow black holes whose singularities extend beyond their event horizons?

automatic6131 1mo ago
Yeah. I can't do the physics myself, so I take on faith (heh) that a sufficiently fast rotating blackhole has a ring shaped singularity that can extend outside its' event horizon. This was news in the 90s and it was a plot point in at least two scifi books, though I don't think I can recommend either
- davidgrenier 1mo ago
  I was assuming the event horizon would also have a donut shape around the ring singularity.
fooker 1mo ago
The reason is that these edge cases often expose where the current models can break down. If the current accepted theory is predicting negative mass or infinite mass, it doesn't really mean that a physicist deeply believes we are going to be finding particles with negative mass. It's more likely we'll find a better theory. In some rare cases, these mathematical oddities do turn out to be real. We found equations producing negative energy as solutions long before we discovered antimatter.
jiggawatts 1mo ago
This is the "cosmic censorship hypothesis": singularities are always hidden by an event horizon and can never be observed without falling victim to them. Or more accurately, any observer of a singularity can never communicate that information to someone who is not a victim. This is hypothetical, and not at all "proven" in any meaningful sense.

amelius 1mo ago

> Counterintuitively, in general relativity two points can be spatially close yet causally distant. Can the converse also be true in general relativity?

raattgift 1mo ago
Yes, during cosmic inflation for example: two test objects initially close can end up many light-years apart. If we make these test objects null (i.e., lightlike) then we can always contrive an inflation that stretches them apart in such a way that they still meet again. Some exotic spacetimes involving pp-wave sandwiches can focus initially non-converging and spatially distant light pencils onto each other at a caustic shortly after the passing of the stack of plane-parallel gravitational waves. One can hide some such processes in the early cosmos.
- Obscurity4340 1mo ago
  Where or encroaching upon what does the univerise expand into? If the universe is all the real estate of physical reality, what is it subsuming as it expands in order to accomodate that growth?
  - raattgift 1mo ago
    Not sure what level of answer you want here. I don't think there's a good slogan that you could memorize and repeat like "the night sky is black because Olber's 'paradox' is badly formulated in that the universe's star formation has a finite history and radiation from before the first stars is redshifted too low to activate visual opsins" or "matter tells spacetime how to curve", and anyway most such slogans are likely to induce misunderstanding (a major theme of the article linked at the very top). Ethan Siegal (a former theoretical cosmologist who has lots of practice in his second career doing science outreach) did it well enough at a pop-sci level that I'll just point to his https://bigthink.com/starts-with-a-bang/what-universe-expanding-into/ https://bigthink.com/starts-with-a-bang/what-universe-expand... (I don't think I could do better [*]). Here's a sketch for a crash syllabus that would take you closer to an answer I'd write, not being a practiced science communicator: My approach would be to teach you some differential geometry on a differentiable Euclidean plane (mostly relating the classic Euclidean distance to the integration of a line element), then what a Riemann manifold is, then how a 3+1-d pseudo-Riemannian one differs from a 4-d Riemannian manifold (and understanding the Ricci curvature in an Einstein manifold), and take you to understanding the simplest of metrics on the Lorentzian manifold, and the concept of geodesics and how they separate into spacelike, timelike, and null. I'd also teach you early about affine distance so that you don't stumble into problems understanding that a pulse of light from the ground to a mirror on the moon and back to the ground takes about two seconds, and how a pulse of matter -- including a pulse of light -- loses energy in an expanding spacetime. (That's another where does it go question, and a good one to think about.) Then I'd introduce Raychaudri-equation-style thinking, with a spray of timelike geodesics separating, as a way of understanding the metric expansion of space and the FLRW metric (where each Friedmann-equation dust represents an enormous number of timelike and lightlike geodesics). I'd also teach you about the Lagrangian and Eulerian specifications of the flow field, and how they relate to one another. We can have a idealized (freely-falling, feels-no-forces) Lagrangian observer follow one line in a spray of geodesics which are initially extremely close to each other, and which separate with the metric expansion of space. Some of the initially-close geodesics causally disconnect from our chosen Lagrangian observer, with close-but-less-close ones disconnecting quickly, and very-close ones staying practically parallel for a very very long time. This is basically the Raychaudri equation, as applied to cosmology. We'd want to explore radar distances between our Lagrangian observer and ideal reflective objects attached to other geodesics on the spray. We then can relate all that to a spacetime-slicing approach where we track what's on 3-d spacelike hypersurfaces, in a Eulerian style, going from our Rachaudhrian spray to a collection of space-filling dusts or fluids that dilute away differently over time. This is the usual picture cosmology students operate with. Understanding that, especially how expansion generates several cosmological horizons, is half of the key to answering your question. The other half is understanding that one can run the relevant equations under a time-reversal, with initially enormously distant objects freely falling towards each other and ending up practically on top of each other in the early history of expansion. Along the way we'd also be talking about the thermodynamics, as expansion is adiabatic. Our causal physics are all related to an extremely hot, extremely dense, extremely low-entropy volume in our billions-of-years-ago past, which we retrodict by studying fractions of later volumes (fractions as small as careful laboratory experiments and as big as large scale galaxy surveys). Anything close to that patch causally disconnected from us very early, and we'll never be able to hear from those parts of a big spray, and they'll never hear from us. Just outside our very early universe, things probably look very similar to things just outside it. The logic here is that as our galaxy crosses out of a cosmic horizon of somone far away, our galaxy doesn't do anything weird, and likewise there are many galaxies currently crossing out of our cosmic horizons, and they probably aren't doing anything weird either. Studies of the expansion history, still-viable cosmic inflation scenarios, and global spatial curvature have led to estimates (e.g. Guth's work) that some our early hot dense patch is at most 10^-23 of basically the same early hot dense stuff. That's fairly comparable to the number of atoms of water in the North Atlantic ocean, all of which are interchangeable, although they all have different histories of where they've been in Earth's oceans, the pressures and densities they've experienced on their travels, and so on. The pre-inflationary patch's tiny elements are pretty interchangeable although they'll have slightly different histories of expansion, galaxy formation, and so on, given tiny differences in their very early histories ("initial conditions"). Some may be overdense and quickly collapse. Some may be underdense and thus produce few if any stars. Now, is that primordial hot dense patch embedded into something bigger? Good question! Does it even matter, given that it causally decoupled from us so early? Good question! How do we even begin to investigate that? Good question! That's all live postgrad and postdoc research, with a lot of focus on trying to make the low entropy part of our hot dense early universe seem un-special. Siegel again: https://bigthink.com/starts-with-a-bang/cosmic-inflation-past-hypothesis/ https://bigthink.com/starts-with-a-bang/cosmic-inflation-pas... Once you have that under your belt you can join the manifold (pardon the pun) papers exploring the physical implications of various guesses about what's outside the everything-everywhere-everywhen fully determined ("block universe") picture painted by a notional exact solution of the Einstein Field Equations of General Relativity, which we can only successively approximate by sampling signals from our past. But at least you'd then understand what it means to say that mean energy-densities fall over cosmological time, and that the centres of mass of galaxy clusters are separating over cosmological time, and that our distant distant descendants won't see any galaxies not presently in our local group. For extra credit you could play around with embeddings of de Sitter space in higher-dimensional manifolds and run into the usual frustrations of it being quite hard to recover known physics -- one can even largely justify a statement like embedding a 3+1d spacetime into a higher dimensional spacetime is generally not possible. Of course, many people still attempt to make that work not so much to answer your question, but to find ways of more easily calculating the way our visible universe behaves. [*] I'd have maybe said "its own future" and otherwise present a wordier version of what Siegel wrote (explicitly raising time-orientability), but really I'd want to explain why I'm mostly a blockworlder in spite of how us small temporary knots of atomic nuclei feel about that <https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)#Objections https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time...> and that maybe the real question is why our brains encode the concept of expansion at all. Anyway our puny brains can't hold all knowledge, we can't just pour in mathematical physicslike kung fu, helicopter piloting, or motorcycle-hotwiring skills in The Matrix movies, and the behaviour of the universe at scales of billions of lightyears didn't change once humans started printing cosmology textbooks. And it's OK if you haven't worked through any of those; just be careful of memorizing factoids from people who haven't worked through any of them either.

jeisc 1mo ago

a point is a pancake of a sphere in space: everything has depth

rfgplk 1mo ago

The primary issues with black holes and singularities (as well as the numerous endless debates about them) is due to admitting only a single temporal dimension as the sole representation of the universe. Once you allow multiple temporal dimensions (at least two non gauge constrained ones) the issues disappear entirely.

DexesTTP 1mo ago
The issue is that we only observe one dimension of time and three dimensions of space. So allowing more than that would require a lot of evidence. Postulating that there's multiple times dimensions is the same thing as postulating that there's more than three space dimensions in string theory. You make the maths "easier" by postulating that there's more dimensions, but you don't make any predictions that the 3+1 spacetime theory doesn't make and that can be observed experimentally.

kelvo_ran 1mo ago

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t_marsden 1mo ago

This kind of result is why GR still surprises after a century. Geometry beats intuition.

wahid_seddiqi 1mo ago

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westurner 1mo ago

There is no black hole singularity in SQG (Fedi). Spacetime is a shear-thickening (dilatant) non-Newtonian fluid, and that's why the speed of light c is what it is.