Oh, yeah, a non-spinning black hole presents no theoretical difficulties at all, and if you’re of a tech level that can make black holes in the first place, it should be possible to pretty much select almost any angular momentum you’d like, for negligible additional difficulty.
Sure, if you’re already aspiring to be a Kardashev Type III civilization. But why stop there? If you have that kind of energy and fine control of gravity, why not weave your matter into a closed 1D discontinuity to create an Einstein-Rosen bridge to another universe in order to escape the eventual heat-death of this one? What could possibly go wrong?
Stranger
I didn’t know what necessitates the snark.
I’m just saying that a non rotating black hole is something that could form but needs an unlikely combination of celestial bodies so could be induced by an advanced eti.
If two black holes had spins opposite the spin of their revolution around each other, would that suffice to produce a merged black hole of zero net spin? Or how would that affect their merger to begin with?
After some googling it appears that black holes are at least somewhat affected by tidal forces.
While this means that black holes must be far more rigid than planets and neutron stars, the duo predict that their deformation should be enough to influence merger dynamics. For example, in the case of a stellar-mass black hole spiralling into a supermassive black hole potentially billions of times more massive, the resulting tidal bulge in the larger object would generate a torque, slowing down its rotation.
Hopefully one of the more physics-educated posters will chime in, but if I had to guess what would happen with such a pair of counter-rotating, mutually orbiting black holes is that their rotation would slow down as they approached.
It would depend on their relative momentum since helicity is conserved.
Not sure where you got the notion there was some “purpose” to this as if the black hole had a will of its own. Weird.
That said, when we say spinning what is spinning as regards a black hole? The singularity I would think. Does that make the event horizon spin? Since the event horizon is not a “thing” and more a demarcation point I can’t imagine how it would spin but really asking because I do not know.
But if the singularity has no dimensions how does it “spin?”
If it has some spatial size then wouldn’t the speed of light limit how fast the singularity can spin?
And if that is the case I would suspect ALL singularities rotate and so near the speed of light that one could not be distinguished from another regardless of mass.
Which then gets weird because as you add mass nothing changes but the mass.
In somewhat the same fashion as higher mathematics questions “what exactly do you mean by ‘addition’?”, wrt black holes one starts to get down to “what exactly do you mean by ‘spinning’?” Black holes are at least externally nothing but a region of distorted spacetime. More or less one has to simply say that its a region of spacetime that according to the equations of General Relativity has the same gravitational effects as a visible spinning mass. What makes black holes unique is that we have nothing but the spacetime region to observe.
I’m with you on all that.
So how do we distinguish a “spinning” black hole from a non-spinning one and why is it important?
Same way all of physics works: you hit it with something watch how it interacts. See the above discussion about frame dragging and event horizons. That’s all measurable.
Things near a spinning black hole move differently from things near a non-spinning black hole. So, we can tell the two cases apart.
Various examples of how motion differs are given throughout the thread, but perhaps the least esoteric one is that an object orbiting the black hole at an angle or with some eccentricity will find its orbit precessing (i.e., its orientation rotating) in a way that is related to the angular momentum of the black hole.
All answers to “How can we tell?” will be in the form of “Stuff near the hole behaves differently.” If you’re thinking about sticking a probe up the rear end of the black hole to read it’s properties “internally”, that fails. The region inside the event horizon doesn’t connect in that way to the region outside. But that’s okay, yeah?
In what context? For understanding the behavior of objects near black holes, it is important.
The singularity has horizontal extent in a spinning black hole.
Sort of. There are limits, and the speed of light shows up, but the speed of light isn’t the cause of the limit. (Aside: the speed of light is never the cause of limits in relativity; it’s the consequence. I mean, one could choose it to be a postulate, but it would be a weird choice.)
Velocity does not equal angular momentum. Two frisbees with the same mass and the same edge velocity v have different angular momenta if they have different radii.
The angular momentum of the black hole is related to the radius of the singularity. Angular momentum determines the physics (along with mass).
Both the angular momentum and the mass of the black hole influence how spacetime is warped. It’s two separate dials, and both matter. If you throw in a non-spinning frisbee vs. a spinning frisbee, the resulting spacetime looks different in the two cases.
Helicity isn’t related to the context here.
Again it needs to be stressed that a singularity isn’t afawk an object, not even a very esoteric object. It’s simply what we’re left with when the equations of General Relativity predict that gravitational collapse inside an event horizon has no limit. It’s where the math breaks down and gives us the answer “infinity” to the density of the infalling matter. We’re pretty certain that if G.R. predicts that, then to that extent G.R. is wrong. We simply don’t have a good answer currently.
On a similar note: It’s worth remembering that a lot of language around “where”, “when”, “how fast”, “how large”, etc., brings in pitfalls when thinking about relativity, much more in GR than SR, and much more when singularities are involved.
Depending on the choice of coordinate system, the singularity of a rotating black hole may lie at r=0 but doesn’t occur on the “top” or “bottom” of r=0; it only happens around the equator of r=0. How does that make any sense? Exactly the point. You have zeros and infinities battling it out*, so one can’t rely on informal language to make hay. And if you calculate the equatorial distance around the singularity, you will find a finite answer, indicating that there is something ring-like about the (extremal, singular) geometry there. Other coordinate systems can make the ring-ness stand out in the metric more obviously (making all the behavior that is jammed into r=0 no longer jammed at a point, spreading the singularity out into an explicit ring in coordinate space), but the choice of coordinates is always arbitrary and does not define the behavior. Which makes intuiting about “where” the angular momentum “is” not so straightforward.
*This is not unlike dealing with indeterminate forms in calculus, where you have “zero divided by zero” type expressions that, in fact, have well-defined behavior once proper treatment is brought to bear.
I wouldn’t state it quite so strongly as to say that “we’re pretty certain” of that. But I don’t think anyone would be surprised to learn that a more complete theory doesn’t actually contain singularities, and we do certainly know that general relativity isn’t fully complete.