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" In principle, a black hole can reach a point where it has as much charge or spin as it possibly can, given its mass. Such a black hole is called “extremal” —
by worldvoyageur 2y ago
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In principle, a black hole can reach a point where it has as much charge or spin as it possibly can, given its mass. Such a black hole is called “extremal” — the extreme of the extremes.
These black holes have some bizarre properties. In particular, the so-called surface gravity at the boundary, or event horizon, of such a black hole is zero.
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It had been thought impossible for such black holes to exist. However, new work now demonstrates that such black holes are indeed possible.
None have been found, however. Though this seems unsurprising. How would you detect one?
- floxy 2y ago>How would you detect one? I wonder if the gravitational wave signature of a merger between an extremal black hole with something else would give us any clues.
- observationist 2y agoYou might see something that appears to fall into nothing, with observable massive fluctuations in gravity - anything close by would get disintegrated, observing such an event would result in a treasure trove of data, as long as it's very, very far away. The secondary damage would be something like a particle accelerator bubble on the scale of multiple solar systems.
- dotancohen 2y agoBut it would not be falling into "nothing". There is an accretion disc.
- ben_w 2y agoThere can be an accretion disc, but it's not a necessary property for a black hole to have.
- mystified5016 2y agoAIUI the tidal forces and spacetime distortion are such that any matter falling in inevitably breaks apart into an accretion disk. It should be impossible for a whole body to enter a black hole intact. All this is for a spinning black hole, of course, which most (all?) are.
- floxy 2y agoTidal forces at the event horizon of supermassive black holes are negligible. https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf
- observationist 2y agoThis type of black hole is similar to dark matter, in that it warps local spacetime, but at the surface, the gravity is null, there's a weird spacetime topology to it, from what I can understand (I am not a physicist). It would be invisible, and any mass that went in would see an equivalent ejection of energy out, and the form of that energy would be fascinating. If you shot a planet into one of those at relativistic speeds it'd be a totally different, more catastrophically massive explosion than anything we've ever seen, and the volume of space around it would present opportunities to study extreme energy physics. One of the weird features is that because it cannot contain any more energy or mass, it has to immediately expel any additions, so the form of the energy coming out would be interesting to observe.
- bobbylarrybobby 2y agoNo surface gravity doesn't mean no gravity at all, right? It sounds like objects can orbit them at a distance, but would lose attraction as they got closer.
- marcosdumay 2y agoIt's a bit of a contradiction with the idea of a black hole, and brings the feature that you can observe its surface.
- dotancohen 2y agoFrom what I gather, you are correct. Furthermore, as there is no actual surface to stand on the theoretical forces such a surface would exert on a body (centrifugal due to lateral acceleration, and also normal) cannot exist anyway, so there is no physical predictive element to the idea anyway.
- mr_toad 2y agoBlack holes (all spinning masses) cause frame dragging, which effectively acts as a force on objects near it. The effect is normally tiny (but has been measured in Earth orbit). Close to a rotating black hole the force is so strong it can even prevent light from orbiting in the opposite direction to the spin of the black hole itself.
- chasil 2y ago"But black holes with a discernible charge have never been observed. It’s far more likely to see a black hole that’s quickly rotating." Three properties completely describe a black hole: mass, spin, and charge. It does not appear that charge will be useful in the question of an extremal black hole. In considering spin, as velocity of matter approaches c (the speed of light), more energy is required to achieve less gain as c is approached. Can a black hole spin at the speed of light? Can it spin faster?
- zvrba 2y ago> Three properties completely describe a black hole: mass, spin, and charge. So it's like an elementary particle?
- chasil 2y agoI think the precursor neutron star might be closer.
- The_Colonel 2y agoNeutron star is still composed of... neutrons. There is a hypothesised Quark star though.
- graycat 2y agoInside a black hole? Consider a neutron star with mass 1 gram less than needed for it to become a black hole. Since the neutron star is not (yet) a black hole, we can 'see' it. Send in the 1 gram and watch while the neutron star converts to a black hole where, as usually proposed, the mass that was the neutron star suddenly shrinks to the "singularity" at the center of the black hole. Now, it appears that there is a huge change -- neutron star to a black hole -- from a small input, the 1 gram, that is, in math terms, there is a jump discontinuity. There was something about the physics of the neutrons that kept the neutron star from shrinking to a singularity. Well, maybe that something also keeps that mass plus the 1 gram from shrinking to a singularity. That is, if there is no jump discontinuity, the inside of that black hole is essentially just like that neutron star.
- mr_toad 2y ago> In particular, the so-called surface gravity at the boundary, or event horizon, of such a black hole is zero. In space-like coordinates the event horizon is always an infinite distance away, so you can never reach the event horizon anyway. Like an infinitely deep hole in space.