20 ms·

Black hole singularity is a surface not a point

• amelius 22d ago
  > Counterintuitively, in general relativity two points can be spatially close yet causally distant. Can the converse also be true in general relativity?
  • raattgift 22d 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 22d 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 21d 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.
• westurner 22d 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.
• sigmoid10 23d 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 23d ago
    [dead]
  • kwoff 23d 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 23d 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 23d ago
        Yup. It's that weird. Also read Nick Gorkavyi: The Oscillating Universe: Einsteinian Cosmology of Black Holes and Gravitational Waves
      • MathMonkeyMan 23d 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 23d 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.
• Certhas 23d 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 23d 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 23d ago
    Sorry man, that's not what this is about.
    • Certhas 23d 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 23d ago
        [flagged]
    • evanb 23d ago
      Yes it is?
  • greesil 23d ago
    I enjoyed this Veritasium video on the subject, which includes Penrose Diagrams. https://youtu.be/6akmv1bsz1M https://youtu.be/6akmv1bsz1M
• Eridanus2 23d 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 :}
  • deleted 23d ago
    [deleted]
  • Eridanus2 23d ago
    Downvoted back to the dark ages.
• ck2 23d 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 23d 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 23d 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 23d 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
        • 22d ago
• helf 23d ago
  [dead]
• jamesforestwest 23d 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?
• deleted 23d ago
  [deleted]
• 542458 23d ago
  [dead]
• dekdrop 23d ago
  How does one descr blackhole to a non-physicist without losing much accuracy? I just it of a very-dense-object.
  • icepush 23d ago
    A one-way door in space.
  • crooked-v 23d 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 23d 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 23d 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 23d 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 23d 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 23d 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 23d ago
  Isn't that how we're inside one.
  • neom 23d 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 23d 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 23d 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 23d 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 23d ago
      Doesn't that imply that, "everywhere" was, precisely, a single point?
      • Dylan16807 22d 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 22d 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 23d ago
  [flagged]
• sfink 23d 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 23d ago
    Kinda sounds like a "linguistic manifold."
  • PaulHoule 23d 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 22d 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 22d ago
    I believe that's called an echo chamber, and exemplified by a board meeting.
  • 22d ago
• yubblegum 23d ago
  Is there such a thing as a "point" in the universe?
  • throwawayffffas 23d ago
    That is the whole question. My gut says no, infinites and infinitesimals are the edges of our understanding not real.
  • goatlover 23d ago
    Depends on whether anything less than the Planck length has meaning.
    • yubblegum 23d ago
      An object sans semantics would be fine.
• dj_axl 23d ago
  scoffs Well yeah everyone and their aunt knows singularities have 5 dimensions.
  • throwawayffffas 23d ago
    You need at least 11 for the math to work out but it's okay most can be compacted.
• metalman 23d 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 23d 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 23d 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 23d 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 23d 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 23d 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 22d ago
  is hole
• DrJokepu 22d 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 22d 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 22d ago
      I was assuming the event horizon would also have a donut shape around the ring singularity.
  • fooker 22d 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 22d 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.
• jeisc 22d ago
  a point is a pancake of a sphere in space: everything has depth
• rfgplk 22d 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 22d 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 22d ago
  [dead]
• t_marsden 22d ago
  This kind of result is why GR still surprises after a century. Geometry beats intuition.
• wahid_seddiqi 22d ago
  [flagged]