Is it possible to stretch a singularity?

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Discussion Overview

The discussion centers around the theoretical implications of black hole mergers and the nature of singularities within general relativity (GR). Participants explore whether the gravitational influence of a larger black hole can "stretch" a smaller black hole's singularity, and what this means for the concept of singularities in the context of GR and potential future theories.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that during a merger, the singularity of the smaller black hole cannot be "undone" or altered by the larger black hole's gravity, as singularities are not spatial entities but rather points where GR's mathematics yield infinite results.
  • Others argue that while GR predicts singularities, it does not provide conclusive insights about their nature, suggesting that singularities indicate the limitations of GR and the need for a quantum theory of gravity.
  • There is a contention regarding the properties of singularities, with some participants asserting that singularities do not possess measurable characteristics like density, while others question whether specific measurements can be defined for ring singularities in Kerr black holes.
  • Participants discuss the implications of merging black holes, noting that there is only one singularity and one event horizon post-merger, challenging the notion of multiple singularities existing simultaneously.

Areas of Agreement / Disagreement

Participants express disagreement on the nature and properties of singularities, with some asserting that they lack measurable characteristics while others challenge this view. The discussion remains unresolved regarding the implications of GR and the potential for a more comprehensive theory.

Contextual Notes

Limitations include the dependence on the definitions of singularities and the unresolved nature of mathematical predictions in extreme conditions. The discussion reflects ongoing debates in theoretical physics about the validity and completeness of GR in describing singularities.

Darkmisc
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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
 
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Darkmisc said:
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.
 
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Cheers. Thanks
 
Darkmisc said:
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.
 
nikkkom said:
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.

nikkkom said:
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.

nikkkom said:
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.
 
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PeterDonis said:
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?
 
snorkack said:
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".

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

No.

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

No.
 
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