What if the Halo Ring from "Halo" turned into a blackhole?

[Justice Hunter]

Just a question originating from pure curiosity.

But what would happen if we had a large ring in outer-space, akin to the ring world in the game "Halo", so massive that it's minutes from collapsing into a black hole.

If it were to uniformly gain mass on all parts of its structure, how exactly would it collapse into a black hole, would it even be spherical in shape? or would the black hole take the form of a ring. I'm just morbidly curious as to what would occur if such an object reached critical densities.

 

[HallsofIvy]

I'm no physics expert but I don't see how your premises could be physically valid. That is, how could a "ring" that massive not collapse into its center?

 

[mfb]

The geometry of forming black holes can be complicated, and does not have to be spherical.
To avoid a "normal" collapse of the structure, you would have to add a lot of mass everywhere at the same time - there are fundamental limits on material strength, and they don't allow to get too close to black holes.

 

[PeterDonis]

There's a limit to how close a stable structure can be to a black hole. For a spherically symmetric structure, its radius can't be less than 9/8 of the horizon radius of a black hole with the same mass.

 

[Justice Hunter]

The geometry of forming black holes can be complicated, and does not have to be spherical.
To avoid a "normal" collapse of the structure, you would have to add a lot of mass everywhere at the same time - there are fundamental limits on material strength, and they don't allow to get too close to black holes.

So then if such a ring structure would collapse, given the right initial conditions it would be a ring shaped black hole? at least initially?

 

[PeterDonis]

if such a ring structure would collapse, given the right initial conditions it would be a ring shaped black hole?

No. The shape of the hole's horizon might not be spherical (or ellipsoidal if the hole was rotating) to start with (though it would fairly rapidly become so as gravitational waves were emitted to remove any asymmetries), but it would not have a "hole" in the center the way the ring did.

 

[Justice Hunter]

No. The shape of the hole's horizon might not be spherical (or ellipsoidal if the hole was rotating) to start with (though it would fairly rapidly become so as gravitational waves were emitted to remove any asymmetries), but it would not have a "hole" in the center the way the ring did.

Any reason why the gravitational field wouldn't take the shape of the ring?

 

[PeterDonis]

Any reason why the gravitational field wouldn't take the shape of the ring?

I didn't say the gravitational field; I said the event horizon. The event horizon can't take the shape of a ring; it can't have a "hole" in the middle of it. Or, to put it another way, if light can't escape to infinity from a ring-shaped region, it won't be able to escape to infinity from the "hole" in the center of that ring-shaped region either.