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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?
No 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.
From 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.
Black 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.
You 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.
This 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.
AIUI 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.
Inside 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.
> 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.
This "conversion" doesn't imply matter transitioning from one state to another. The main thing happening during transition to a black hole is that the light can't escape anymore - you see the star in one moment, and can't see it in another moment. Not necessarily because of some matter transition, but because it stops radiating light.
Singularity is a mathematical artifact, we don't know what's happening in the blackhole with matter and don't really care since it has no effect on the outside world.
We don’t know that an object with 1 gram less than needed to become a black hole will be a neutron star. There may be other denser states or matter in between, like quark stars or strange stars that are still not dense enough to become black holes.
Under GR it doesn’t matter since as the mass increases beyond a critical point an event horizon will form and all that matter will be compressed into a singularity regardless.
I am trying to imagine what happens to a single contiguous mass that has both a part that is travelling at relativistic speeds, and a part that is not?
Maybe nothing much? From each parts point of view it is still simply in contact with it's neighbor and neither is moving relative to the other?
> 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.
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. "
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?