Are Black Holes Necessarily Spherical?

sol invictus said:
It's not. There is a limit to how much a black hole with a given mass can spin, called the extremal limit. Near that limit the shape and spacetime structure of the hole is very different from that of a non-rotating one, and it is believed that nearly all astrophysical black holes come close to saturating the limit.
Click to expand...

Sol- given this sort of mass distorts the spacetime it sits in, can simplistic geometrical terms like "spherical" actually be applied meaningfully at all?
 
Singularitarian said:
Only when they get close together, are the tidal forces taken into account, otherwise, your structure can be uneffected, but the gravitational pull is pulling all the particles in your body at the same rate.
Click to expand...
What do you mean by "rate" in this context?
 
If you took the Earth and spun it up, you would expect it to go oblate (bulge outwards) along the equator. Similarly, the gravitational fiedl would also become oblate, because the mass in the bulge attracts to itself. The Earth, at the rate it is spinning actually is oblate, and the gravitational field follows along. It is the asymmetry in the field that cases the Earth to slowly exchange rotational momentum with the moon, slowing the Earth's spin and moving the moon's orbit further out.

There is no difference between this model of Earth and a black hole mass. The mass is mysterious only because we cannot see it; there is no reason to suppose it doesn't act in the same way, though the speeds may begin making relativistic effects apparent, just as Mercury orbiting the Sun is not perfectly classical.

This would also apply to binary systems. Picture two sub-black hole stars rotating around each other and how they are distorted by each other's gravitational fields. Black holes would react the same way, barring the relatavistic effects.
 
Dorfl said:
Do they sort of oscillate after absorbing something, or what happens?
Click to expand...

Yeah - they oscillate, damp, and radiate gravity waves while they ring down.

The brightest source for gravity wave detectors is supposed to be black hole mergers.
 
Dorfl said:
What happens if you try to make it spin faster?
Click to expand...

I think what will happen is either the object you try to spin it with will just scatter off the hole without imparting any additional angular momentum, or it will be absorbed and increase the mass enough to "make room" for the additional ang. mom.

Soapy Sam said:
Sol- given this sort of mass distorts the spacetime it sits in, can simplistic geometrical terms like "spherical" actually be applied meaningfully at all?
Click to expand...

Yes. The horizon of a non-rotating hole is a funny sort of sphere in that its surface area isn't related to its radius in the usual way. But it possesses the defining property of spherical symmetry; namely, it looks the same from all directions.
 
shadron said:
If you took the Earth and spun it up, you would expect it to go oblate (bulge outwards) along the equator. Similarly, the gravitational fiedl would also become oblate, because the mass in the bulge attracts to itself. The Earth, at the rate it is spinning actually is oblate, and the gravitational field follows along. It is the asymmetry in the field that cases the Earth to slowly exchange rotational momentum with the moon, slowing the Earth's spin and moving the moon's orbit further out.

There is no difference between this model of Earth and a black hole mass. The mass is mysterious only because we cannot see it; there is no reason to suppose it doesn't act in the same way, though the speeds may begin making relativistic effects apparent, just as Mercury orbiting the Sun is not perfectly classical.

This would also apply to binary systems. Picture two sub-black hole stars rotating around each other and how they are distorted by each other's gravitational fields. Black holes would react the same way, barring the relatavistic effects.
Click to expand...
It being that simple is both cool and disappointing. ;) Thanks! :)
 
sol invictus said:
I think what will happen is either the object you try to spin it with will just scatter off the hole without imparting any additional angular momentum, or it will be absorbed and increase the mass enough to "make room" for the additional ang. mom.
Click to expand...
Is this the "Penrose Effect"?
 
Dorfl said:
So they do not always have to be spherical?
Click to expand...

From a spin point of view, i guess not, but hey, you learn something new everyday.

From the point of view for two black hole coming relativlely close, you might expect them to pull on each other in such a way that one, or both stretch each other, hence the name, tidal effect.
 
Singularitarian said:
From a spin point of view, i guess not, but hey, you learn something new everyday.

From the point of view for two black hole coming relativlely close, you might expect them to pull on each other in such a way that one, or both stretch each other, hence the name, tidal effect.
Click to expand...
Fair enough.
 
sol invictus said:
Related, yes - although I think you're probably thinking of the reverse process, where you extract some energy (and some angular momentum) from a rotating hole.
Click to expand...
I thought that was what you meant—a particle absorbs some energy from the Black Hole and therefore gets scattered away from it.
 
Singularitarian said:
From a spin point of view, i guess not, but hey, you learn something new everyday.

From the point of view for two black hole coming relativlely close, you might expect them to pull on each other in such a way that one, or both stretch each other, hence the name, tidal effect.
Click to expand...
How could you not know this with your advanced physics education?
 
KingMerv00 said:
What's funny is most, if not all, black holes probably spin so most black holes are not spherical.
Click to expand...
True. I am not sure if there are any non-rotating black holes. If there were, it would sure make the math alot simpler.
 
Hmm what does charge do a back hole's shape?
 
~enigma~ said:
Charge can only be very insignificant. Think about it, if the charge were large would it remain so for long?
Click to expand...

A neutron star is the less massive cousin of a black hole and is composed almost entirely of neutrons. In a neutron star, protons and electrons are squeezed so tightly, they form neutrons and thus lose their charge. I imagine the same thing must happen in a black hole so how could a significant charge exist?
 
Last edited:
Am I correct in thinking that non-rotating black holes are just a neat maths solution (makes it easier not handling angular momentum) whereas any existing black hole would statistically have at least some angular momentum as the original star would have some as it formed?
From some reading it looks like a black hole could lose spin by the penrose process but the universe would hit heat death first?
I suspect much of what I recall is hideously outdated.
 
Wudang said:
Am I correct in thinking that non-rotating black holes are just a neat maths solution (makes it easier not handling angular momentum) whereas any existing black hole would statistically have at least some angular momentum as the original star would have some as it formed?
Click to expand...

That's what Phil's latest book said.
 
You must log in or register to reply here.

Back
Top Bottom