# Why is a spinning Black Hole distinct from one that doesn't? How does a Black Hole even spin? Why is it important? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [September 21, 2025, 6:09pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270 "2025-09-21T18:09:10Z")\ **Posts on this page:** 20\ **Page:** 3
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 22, 2025, 9:24pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/41 "2025-09-22T21:24:13Z")
> [@Pleonast](#): > > Slight hijack: does there exist a black hole with zero angular momentum? > > My gut says no. A topological transition from a toroidal singularity to a point-like one just feels wrong to me. There is no theoretical reason that a non-rotating black hole cannot exist, and in fact the Schwarzschild metric (the simplest exact model of a black hole) is exactly that. In reality, all matter in the universe is in some degree of large scale rotational motion in addition to any arbitrarily defined volumes of mass having net differential linear off-axis motion (creating a net gravitational torque on the masses), so the the likelihood of material converging to form a direct collapse black hole would exactly cancel out net angular momentum is practically nil. Stellar mass black holes form from the remnants of stars, and because of the way star systems form they have significant rotational motion, so any black hole that forms from a collapsar will have substantial angular momentum by default. Stranger ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 22, 2025, 9:34pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/42 "2025-09-22T21:34:25Z")
> [@Pleonast](#): > > Slight hijack: does there exist a black hole with zero angular momentum? The math certainly exists to describe one. And you don’t even have the “pick a random real number” problem, since angular momentum is quantized. Sure, going from a ring singularity to a point one would be weird, but then, going from not-a-singularity to any sort of singularity is also weird. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 22, 2025, 9:57pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/43 "2025-09-22T21:57:21Z")
> [@Pleonast](#): > > A topological transition from a toroidal singularity to a point-like one just feels wrong to me. I don’t see why. A point singularity is just the limiting case of a toroidal singularity with a radius of zero, just like a Schwarzschild black hole is the limiting case of a Kerr black hole, one with a spin rate of zero. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [September 22, 2025, 10:12pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/44 "2025-09-22T22:12:24Z")
> [@Chronos](#): > > The singularity of a spinning black hole is a ring, but the event horizon can’t ever be a torus. Even for a maximally-spinning black hole, the horizon is still an ellipsoid. No, but the ‘outer ergosphere’ of a spinning black hole is a weird shape something like a red blood cell, with dimples and everything. This might be one possible source of confusion. [![](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Kerr_surfaces.svg/640px-Kerr_surfaces.svg.png) ](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Kerr_surfaces.svg/640px-Kerr_surfaces.svg.png) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 22, 2025, 10:15pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/45 "2025-09-22T22:15:29Z")
The ergosphere is not in any sense a horizon: You can dip into the ergosphere and come back out again. But you can’t stay still in the ergosphere: If you try, you’ll end up falling. And the ergosphere is also where you can dump your garbage from profitably. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [September 22, 2025, 11:43pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/46 "2025-09-22T23:43:10Z")
> [@Chronos](#): > > going from not-a-singularity to any sort of singularity is also weird. Yeah, it’s why I doubt there’s any true singularities, only things that look like them at the precision we can measure. > [@Lumpy](#): > > > [@Pleonast](#): > > > > A topological transition from a toroidal singularity to a point-like one just feels wrong to me. > > I don’t see why. A point singularity is just the limiting case of a toroidal singularity with a radius of zero, The difference is in the topology–going from something with one “hole” to zero. There’s no continuous transition. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [September 23, 2025, 12:06am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/47 "2025-09-23T00:06:08Z")
> [@Pleonast](#): > > The difference is in the topology–going from something with one “hole” to zero. There’s no continuous transition. That may just mean that at some level of resolution that spacetime is not continuous. General relativity, as good as it has been in predicting the behavior of gravity in many extreme or subtle conditions, is almost certainly not a complete theory of gravity. Stranger ---
**Author:** ![Topologist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/topologist/32/3208_2.png) [@Topologist](https://boards.straightdope.com/u/Topologist)\ **Post date:** [September 23, 2025, 8:06pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/48 "2025-09-23T20:06:59Z")
> [@Stranger\_On\_A\_Train](#): > > > [@Pleonast](#): > > > > The difference is in the topology–going from something with one “hole” to zero. There’s no continuous transition. > > That may just mean that at some level of resolution that spacetime is not continuous. There’s also no real topological obstruction to this happening. Think, for example, of a horizontal plane slicing through a hollow sphere to produce a circle. As the plane rises, the circle shrinks to become a point just when the plane reaches the north pole. Nothing discontinuous going on in this picture. But a circle, with a hole, transitions to a point. ---
**Author:** ![EdwardFound](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@EdwardFound](https://boards.straightdope.com/u/EdwardFound)\ **Post date:** [September 25, 2025, 8:45pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/49 "2025-09-25T20:45:26Z")
What is the theoretical basis for a black hole retaining electric charge? Is is derived from GR or from theories of particle physics? In the model where charged particles interact by exchanging virtual photons it seems that charge within an event horizon would not be able to interact with external particles (unless virtual photons do not move through spacetime as real photons do). ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [September 27, 2025, 7:16pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/50 "2025-09-27T19:16:22Z")
> [@EdwardFound](#): > > What is the theoretical basis for a black hole retaining electric charge? Electric charge is a conserved quantity due to a symmetry of nature present in the fundamental laws as we understand them today. Matter going from “not a black hole” to “black hole” doesn’t undermine that charge conservation, so whatever charge was there must still quantitatively be in effect, even if qualitatively one has to be careful in the treatment due to spacetime being all jacked up. > [@EdwardFound](#): > > In the model where charged particles interact by exchanging virtual photons This approach to electrodynamics isn’t applicable to a charged black hole, or really any macroscopic object. The concept of virtual photons stems from a calculation approach whereby a discrete interaction can be connected quantitatively to an [infinite series](https://en.wikipedia.org/wiki/Series_(mathematics)); where the terms in that series can be connected to the famous Feynman diagrams involving both real and virtual particles (the latter of which are a echo of the calculation taking place); and where, in many important cases, the calculation is dominated by the first term or so in the series. For electromagnetism, the first term’s diagram will usually involve a virtual photon. However, this approach and the corresponding heuristics assume many things that do not hold even a little bit in the black hole case. The black hole is not a point-like object, nor is it small compared to the incoming particle’s wavelength such that its physical extent can be handled via a suitable “form factor” or other integration. The technique also assumes that the incoming and outgoing particles are fully free of the localized interaction, whereas the black hole would involve continuous scattering (in curved spacetime no less) of the incoming and outgoing particle states. Fortunately, classical treatments are generally applicable, even if cumbersome given the context. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 27, 2025, 9:41pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/51 "2025-09-27T21:41:20Z")
> [@EdwardFound](#): > > What is the theoretical basis for a black hole retaining electric charge? A combination of GR and Maxwell’s equations. Above my education level, but charged (non-spinning) black holes were addressed early on: > **[Reissner–Nordström metric](https://en.wikipedia.org/wiki/Reissner%E2%80%93Nordstr%C3%B6m_metric)** > > In physics and astronomy, the Reissner–Nordström metric is a static solution to the Einstein–Maxwell field equations, which corresponds to the gravitational field of a charged, non-rotating, spherically symmetric body of mass M. The analogous solution for a charged, rotating body is given by the Kerr–Newman metric. > The metric was discovered between 1916 and 1921 by Hans Reissner, Hermann Weyl, Gunnar Nordström and George Barker Jeffery independently. > In spherical coordinates ⁠ > > > > ... If I’m reading this correctly, essentially the space-time that a black hole is made of embodies the electric charge. Interestingly, charge is something that can defeat an event horizon: a maximally-charged black hole would have a naked singularity. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 28, 2025, 12:00am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/52 "2025-09-28T00:00:48Z")
A super-maximally charged black hole. One with charge equal to mass (in the appropriate units) is allowed (at least, allowed to exist, but there’s no known method for producing one). One with charge greater than mass would be naked. What’s perhaps more relevant than electric charge is magnetic charge, which black holes can also have. This is relevant because we don’t know of any other context in which magnetic charge can exist. And in principle, we even know how to make them: Just produce a strong enough magnetic field that it polarizes the vacuum into oppositely-charged black hole pairs. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [September 28, 2025, 12:04am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/53 "2025-09-28T00:04:08Z")
“Just” is doing some rather heavy lifting there… ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [September 28, 2025, 12:11am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/54 "2025-09-28T00:11:16Z")
> [@Lumpy](#): > > “Just” is doing some rather heavy lifting there…   Engineering problem… ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 28, 2025, 12:51am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/55 "2025-09-28T00:51:39Z")
Heavy lifting? Not at all! Why, it’s only a dozen times the Planck mass, or so! ---
**Author:** ![EdwardFound](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@EdwardFound](https://boards.straightdope.com/u/EdwardFound)\ **Post date:** [September 28, 2025, 2:57am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/56 "2025-09-28T02:57:23Z")
deleted ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [September 28, 2025, 3:01am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/57 "2025-09-28T03:01:23Z")
> [@EdwardFound](#): > > So charge cannot disappear inside a black hole - it must technically still exist within our universe. But is it still detectable by us on the outside? Would a negatively-charged probe be pulled towards a positively-charged black hole (besides gravitationally)? I believe so. There’s been theoretical proposals in the past to manipulate small black holes (assuming they exist) by giving them a charge and moving or spinning them using it. ---
**Author:** ![EdwardFound](https://avatars.discourse-cdn.com/v4/letter/e/4af34b/32.png) [@EdwardFound](https://boards.straightdope.com/u/EdwardFound)\ **Post date:** [September 28, 2025, 3:21am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/58 "2025-09-28T03:21:15Z")
Thanks, all. I had not known that GR equations incorporated electro-magnetism. So even though the original charged particles themselves are not observable, the charge itself would remain as an observable feature of the spacetime/EM-field along with mass and spin. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 28, 2025, 11:02pm UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/59 "2025-09-28T23:02:58Z")
It’s not precisely that the GR equations incorporate electromagnetism. Rather, we know how electromagnetism works in the context of GR. Roughly speaking, the ordinary equations of electromagnetism have a bunch of commas in them (which has a very specific meaning, in that system of notation), and GR says that you can take any such equation and replace all of the commas with semicolons (that also have a very specific, and very closely related, meaning). In other words, if you already know in detail how to describe a given electromagnetic situation in flat space, it’s really easy to adapt that to describing it in curved space. ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [September 29, 2025, 1:00am UTC](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270/60 "2025-09-29T01:00:46Z")
> [@Stranger\_On\_A\_Train](#): > > Stellar mass black holes form from the remnants of stars, and because of the way star systems form they have significant rotational motion, so any black hole that forms from a collapsar will have substantial angular momentum by default. And I guess a black hole formed from merging objects needs their spin and relative velocity to cancel out to a very unlikely precision. (I mention velocity because normally objects spiral in when forming a black hole, and I guess some of the angular momentum of the black hole comes from that?) It should be possible to artificially make a black hole of arbitrarily low angular momentum though. [Previous page](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270.md?page=2) [Next page](https://boards.straightdope.com/t/why-is-a-spinning-black-hole-distinct-from-one-that-doesnt-how-does-a-black-hole-even-spin-why-is-it-important/1023270.md?page=4)