I am wondering about the size of a black hole singularity. We know that a classical black hole is infinitely dense. I am not asking about size of event horizon. I am asking about actual size of the black hole singularity.
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4$\begingroup$ This is very poorly worded. Please clarify what you mean by the "actual size of black hole". $\endgroup$– Vincent ThackerCommented Jul 14 at 12:33
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$\begingroup$ I took the liberty of polishing the format of the question slightly. I think it is a worthwhile question (that many new learners or readers of lay accounts might have), and that the OP or others like him perhaps cannot clarify what they mean by the "actual size of a black hole" because that is what they want to know. Perhaps this is a duplicate, is there some question/answer about the difference between the idealized model of a black hole in GR (singularity) versus speculative quantum gravity models (where there is not necessarily a singularity nor pointlike concentration of mass)? $\endgroup$– jwimberleyCommented Jul 14 at 12:48
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$\begingroup$ It sounds like he is asking about the size of the singularity at the center. $\endgroup$– mmesser314Commented Jul 14 at 13:31
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1$\begingroup$ As others have said, nobody really knows. But physicist Carlo Rovelli has written an interesting paper on planck black holes that it's worth reading phys.org/news/… $\endgroup$– foolishmuseCommented Jul 14 at 13:42
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2$\begingroup$ The first problem in your headline is that there are no such things as "fundamental particles" and the second problem is that the size of a singularity is not measurable. It will swallow your yard sticks. It's a non-physical question. $\endgroup$– FlatterMannCommented Jul 14 at 18:33
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4 Answers
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The very short answer to this is: We have no idea.
General relativity predicts that the singularity of a Schwarzschild black hole (which I assume is what you mean by "actual black hole") is a single point, which by definition has no "size" to speak of. However, physicists expect that before you actually get to the point-sized singularity, quantum gravitational effects would become significant. The catch, of course, is that we don't have a theory that describes quantum gravitational effects at the energy scales usually expected in black holes. So until such a theory is developed and experimentally confirmed, your question cannot be answered.
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2$\begingroup$ Even if we had a theory, why would it be credible? Physics is founded on experimental results. What experiment could elucidate this? $\endgroup$ Commented Jul 14 at 16:31
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1$\begingroup$ Can you tell me how you would measure the size of that point? How would you get a physical yard stick inside the event horizon and then back out? $\endgroup$ Commented Jul 14 at 18:34
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2$\begingroup$ @FlatterMann that is obviously impossible. A credible theory of quantum gravity must be testable, so there must be another experiment, otherwise the theory isn't worth a lot. $\endgroup$– paulinaCommented Jul 15 at 9:49
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$\begingroup$ @paulina You have a testable theory of quantum gravity that can measure the size of the black hole singularity? Can you give us citations? $\endgroup$ Commented Jul 15 at 13:10
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$\begingroup$ @FlatterMann I don't know if you are intentionally misunderstanding me. What I mean to say is that a useful (meaning experimentally testable) theory of quantum gravity would allow us to predict the "size of the singularity". This prediction would obviously not be testable, just like the predictions of GR concerning the black hole area inside the event horizon. Therefore the usefulness of such predictions is up for debate, but it is by no means a new debate. Until we have such a theory however, we cannot make such predictions other than "probably there's more going on". $\endgroup$– paulinaCommented Jul 15 at 17:13
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While @paulina's answer: we don't know is correct, because quantum gravity is not understood, I'll answer for a classical Schwarzschild blackhole as described by Kip Thorne.
The size is zero, however it is not infinitely dense. There is a point singularity where in-falling matter goes and disappears. The only remnant of what falls in is contained in the infinite curvature at the singularity. Note that the singularity is not part of the space-time manifold anymore, it becomes a point defect surrounded by spacetime curvature. The mass of the blackhole is contained in the self energy of the spacetime curvature. The blackhole is made entirely from spacetime, no matter, no density, no nothing.
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$\begingroup$ An issue is that the descriptor: infinite, and quantities such as density, size, etc. cannot reasonably coexist. Another is that without a something that keeps curving the spacetime, it has no reason to stay curved. If the point-defect is not connected as the source of the curvature, the curvature ought to dissipate. $\endgroup$– WookieCommented Dec 1 at 13:06
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We know that black hole is infinitely densed.
More exactly: The theory of general relativity predicts that the center of a black hole is infinitely dense. This theory predicts very well everything about gravity (including black holes). But it does not account for elementary particles in any way. Hence we cannot trust this theory on the particle level.
On the other hand, we have the standard model of particle physics. This theory explains very well everything about elementary particles and their interactions (electromagnetic, strong nuclear, and weak nuclear). But it does not account for gravity in any way. Hence we cannot trust this theory in strong gravitational fields.
Until now there is no confirmed theory of quantum gravity which would explain both gravity and elementary particles. There are several candidate theories, but all of them are still speculative, and none of them is confirmed by experimental facts. Hence we don't know what is at the center of a black hole on the particle level. We don't even know if our concept of elementary particles still makes sense there. May be particles are only valid as an approximation in weak gravitational fields.
answered Jul 14 at 13:13
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The singularity is a point. Literally, according to current scientific consensus it is the 0 dimensional object my fourth grade teacher tried to explain by making a dot on the overhead projector… Mass is conserved as is energy but everything else supposedly goes out the window. No length, No width, nor height, the point being literally A POINT in terms of normal concepts of what “things” or “objects” are, is simply a concept. A set of coordinates; not a “thing”. On the other hand, this non-thing, purely meta, set of coordinates concept, this dot on the grid, is the equivalent of several times the mass of our sun as a minimum in most cases. Personally, I believe that it may be The fundamental particle, naysayers be damned. Frankly the physics that are cited to illustrate why the singularity is NOT a particle consistently completely fail to offer an alternative, primarily, the lack of any dimensions is pointed to as some sort of proof. Ok, cool… What are the dimensions of a photon? A quark? An electron even? Can any of these so called particles be pinned down to be fitted for a tuxedo? What is the literal size of any of them? I do not believe the reality is as neat as their instincts tell them. Consider for a moment a neutron star. When this object formed that needed only but a relatively little bit of additional mass to become a singularity instead. Instead, we look at this object that cannot be compressed any further due to neutron degeneracy pressure. Driven the right amount of mass would overcome this exclusion principles that it cannot, the quantum state that being the electron within the neutrons within the soup that this substance is not true. Mass is conserved energy is conserved everything else compressed to a single quantum steak much like the state occupied by a single electron. What quantum field
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$\begingroup$ Does this mean that gigantic stars can literally become too small to see? $\endgroup$– WookieCommented Dec 1 at 13:13