I Is it possible to stretch a singularity?

[Darkmisc]

Hi everyone

When a smaller black hole gets sucked into a larger one, is it theoretically possible for the gravity of the larger black hole to stretch the smaller black hole so that it no longer has an infinitely dense centre?

I guess it won't matter once they are completely merged, but in theory, is it possible to "undo" a singularity in this way?Thanks

 

[PeterDonis]

is it possible to "undo" a singularity in this way?

No. The singularity doesn't have a size; it's not a place in space. It's a moment of time, which is to the future of all other moments inside the horizon.

In the case of two black holes merging, there actually is only one singularity, not two; there is also only one event horizon, not two. Remember that the event horizon is a surface in spacetime, not space. Heuristically, in the case of a black hole merger, the horizon is shaped like a pair of trousers instead of a cylinder.

 

[Darkmisc]

Cheers. Thanks

 

[nikkkom]

Hi everyone

When a smaller black hole gets sucked into a larger one, is it theoretically possible for the gravity of the larger black hole to stretch the smaller black hole so that it no longer has an infinitely dense centre?

I guess it won't matter once they are completely merged, but in theory, is it possible to "undo" a singularity in this way?

Theory (GR) is not actually able to say anything conclusive about singularity. Singularity is the point where math of GR gives infinite results for many predicted observables, and has other pathological properties (such as being a boundary of spacetime manifold: the time just "ends" there - when a particle arrives to the singularity, worldline does not continue into the future).

Most physicists take it as the indication that GR is not the final theory and will be superseded by a better theory (probably some sort of a quantum theory of gravity) which will give other, more sensible descriptions of gravity in high energy (equivalently, tiny distances) regime - hopefully there will be no singularity.

 

[PeterDonis]

Theory (GR) is not actually able to say anything conclusive about singularity.

Yes, it is. What I said in my earlier post is what GR predicts, and there is nothing imprecise or inconclusive about it.

Singularity is the point where math of GR gives infinite results for many predicted observables, and has other pathological properties

They are only "pathological" under certain assumptions about what "ought" to be the case physically in such a regime. Mathematically the theory is perfectly consistent and makes perfectly definite predictions.

Most physicists take it as the indication that GR is not the final theory and will be superseded by a better theory

Yes, but we don't have such a theory, so we don't know what it would predict.

 

[snorkack]

No. The singularity doesn't have a size; it's not a place in space.

It has infinite density, but does it have other measurements?
Like, can you define the length and radius of a ring singularity of Kerr black hole?
During the process of a black hole merger, can you have two ring singularities inside one event horizon?

 

[PeterDonis]

It has infinite density

No, it doesn't. The singularity itself is not part of spacetime at all, and doesn't have a density or any other "measurements".

can you define the length and radius of a ring singularity of Kerr black hole?

No.

During the process of a black hole merger, can you have two ring singularities inside one event horizon?

No.