Is the event horizon of a black hole a perfect sphere?

[TobiasTheViking]

I don't have the ability to grasp the math, so i can't really figure this out on my own...


A few thought experiments

A) Dead black hole.. hasn't had any matter for millenia, that means, all the matter should(or shouldn't, depend on how you look at the theory and frames of reference) be confined to the singularity at the middle. Since there would be no granuality in the singularity, wouldn't the event horizon be a perfect sphere around it?

B) An extremely fast spinning singularity(i believe you can get a black hole that spins so fast that the singularity is actually a ring.. can't remember the name of said ring, or know if it even has one). Would such a monster(considering it would be a naked singularity) have the event horizon as a perfect sphere?

CD) the same as AB, but with the black holes having consumed some mass within the last thousand or so years

EF) the same as AB with the black hole consuming mass right now.


Just an idle thought.. Does anyone know? can anyone know?

I'm thinking, C and E may have bumps in the event horizon because of infaling matter(miniscule bumps, but there none the less).

And perhaps B would warp space around it in a way that gravitational attraction is different because of it(wait, isn't this the entire idea of how a naked singularity can be formed).

actually, this brings up another question.. if naked singularites are real, that would mean that framedragging has an effect on gravity, wouldn't that mean that a sufficiently large black hole would not have any gravity escape? or is the naked singularity just "space moving so you get speed enough to escape from the gravitational attraction of the black hole"

Wow, this post looks like crap, sorry bout that.

I promise, i'm not going ATM with this.. this is basically all my questions, no agenda or anything else in mind(Though one more black hole question may surface later).

Sincerely
Me.

[grant hutchison]

The quiescent, non-rotating black hole has a spherical event horizon.
A rotating black hole has a horizon shape which depends on your choice of coordinates: one choice makes it spherical, another choice gives it an equatorial bulge. You'll find both depictions used on the internet and in books. A naked singularity, by definition, has no event horizon. (Black holes resist being spun up to the point at which this happens.)

Black holes absorbing mass develop transient distortions of the event horizon, most dramatically when two black holes merge. A dumbell-shaped event horizon very rapidly rings down to an equilibrium shape, radiating gravitational waves. The ring-down happens very quickly, so you're not going to see effects persisting for years, AFAIK.

Grant Hutchison

[astromark]

I think you have your answers...
The slow rotating black hole would have a spherical event horizon.
The faster the rotation the flatter the disk and its event horizon.
Any other anomaly would not last. The all conquering gravity would deal to it.

[TobiasTheViking]

So, it won't be a perfect sphere because of frame dragging.. oki.

And i guess any infalling matter would have a neglible influence of the eventhorizon as well..

Thanks

[tdvance]

Quantum fluctuations--"should" cause microscopic wrinkles on the event horizon.

[TobiasTheViking]

Oki, thanks for the information...

as i promied, no atm

[Cougar]

You ought to read Kip Thorne's Black Holes and Time Warps, Einstein's Outrageous Legacy if you're really into it. He pins this stuff down.

[spratleyj]

You ought to read Kip Thorne's Black Holes and Time Warps, Einstein's Outrageous Legacy if you're really into it. He pins this stuff down.

I couldn't agree more - it's a great book especially for someone like me (and apparently you) whom doesn't yet (hope to soon ) have higher mathematical knowledge, but still wants to understand the concepts of black holes.

[TobiasTheViking]

I'll add it to my list.. thanks

[Digix]

I think you have your answers...
The slow rotating black hole would have a spherical event horizon.
The faster the rotation the flatter the disk and its event horizon.
Any other anomaly would not last. The all conquering gravity would deal to it.

that is interesting possiblity to suck energy away from blackhole.

if we make some very flat disc shaped horizon, then there is very little distance left to singularity and this would increase amount of tunneling or hawking radiation in many orders of magnitude

also some gravimagnetic field may appear (same as with circular electric current)
so some gravitational Lorentz forces should appear too

[mugaliens]

Grant partially touched on one aspect of the OP, but not fully: "A) Dead black hole.. hasn't had any matter for millenia, that means, all the matter should(or shouldn't, depend on how you look at the theory and frames of reference) be confined to the singularity at the middle."

Mathematically speaking, a singularity is "a point at which a given mathematical object is undefined."

Put simply, y/x becomes undefined as x --> infinity. Similarly, mathematics becomes undefined as one's location approaches the event horizon.

Therefore, it's not that all matter, once passing the event horizon, compresses down to an infinately small point, wrongly termed "the singularity."

Rather, it's that the singularity occurs at the event horizon. One might say that the event horizon is the black hole's only known singularity, as we just don't quite know what happens beyond the event horizon.

that is interesting possiblity to suck energy away from blackhole.

if we make some very flat disc shaped horizon, then there is very little distance left to singularity and this would increase amount of tunneling or hawking radiation in many orders of magnitude

How much energy are you willing to put into that black hole to get it spinning so fast that it becomes the "very flat disc" from which you're trying to extract energy?

[grant hutchison]

Therefore, it's not that all matter, once passing the event horizon, compresses down to an infinately small point, wrongly termed "the singularity."

Rather, it's that the singularity occurs at the event horizon. One might say that the event horizon is the black hole's only known singularity, as we just don't quite know what happens beyond the event horizon.

A singularity at the event horizon certainly shows up in the Schwarzschild metric, in which it takes an infalling object infinite time to arrive at the event horizon. But this is a coordinate singularity, which can be disposed of by adopting a different set of coordinates. So it's not an intrinsic property of spacetime under GR, and it doesn't indicate that there's anything peculiar about spacetime at that location. This makes it a rather different beast from the singularity that shows up at the centre of mass of the black hole.

As an analogy, we can find coordinate singularities at the north and south poles of the Earth, where longitude becomes undefined at 90 degrees north and south latitude. This doesn't indicate that there's anything singular about the surface of the Earth at such points, and we can simply make the singularities disappear by adopting a new set of coordinates: Cartesian, for example.

Grant Hutchison