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Q & A: quantum gravity and singularities

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Most recent answer: 12/28/2013
Q:
We know the size (diameter) of the singularity inside a black hole is zero or almost infinitesimal, that's why it has infinite density. For small objects, on the other hand, the uncertainty principle dominates, so for singularity, quantum fluctuation in singularity mass and location will cause substantial changes in its gravity and The Schwarzschild radius. If this is the case, has anyone discovered this phenomenon in physics? thanks in advance, Behrooz Hariri
- Behrppz (age 40)
Mashhad, Khorasan Razavi, Iran
A:

Great question.

The problem you're describing is at the heart of the "quantum gravity" issue. So far, there's no complete theory of quantum gravity. Many people believe that string theory/M-theory is the most promising route to one. In these theories there are no point objects, and the singularity is removed while the structure of quantum mechanics is maintained. General Relativity would then only emerge as the long-wavelength approximation to the full theory.

Mike W,


(published on 12/28/2013)

Follow-Up #1: quantum gravity

Q:
Thanks Mike W., for the answer. But my question still remains: Let's say singularity is removed and replaced by string/M theory in future. We have still a very tiny object at the heart of the black hole, and quantum mechanics is still there. The size, mass and location of this quantum-sized object would not be certain and fluctuates dramatically. A black hole would be then unstable and change its radius accordingly. Is my reasoning correct, and has this phenomenon observed physically to this time?
- Behrooz Hariri (age 40)
Mashhad, Khorasan Razavi, Iran
A:

We still don't know what happens on that small scale, called the Planck scale. The Planck distance scale is about 10-33 cm and the time scale is about 10-44 s. Those are far, far smaller than any direct probe available in the the foreseeable future.

Perhaps we observe quantum gravity all the time, in that the solutions for how it behaves may be responsible for what we call the elementary particles. Without some better theory, however, we don't even know if we're observing the symptoms of the Planck-scale physics.

So yes, your question remains- for all of us, not just you.

I should straighten out one point concerning the descriptive language often used to describe quantum states. People often say that something is "fluctuating", implying that it's changing in time. The position of a little mass on a spring (simple harmonic oscillator) in its ground state doesn't fluctuate. It's spread out and it stays spread out in the same pattern. The same goes for other quantum variables, including ones that are somewhat more abstract.

Mike W.


(published on 12/28/2013)

Follow-up on this answer.