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The Question

(Submitted December 27, 1996)

Understanding that a singularity is the theoretical remnant of a supernova, and that the singularity has mass, the question I have is in regards to the accretion disk that is formed when the singularity is near a star (Cygnus X-1 as an example). If matter is being drawn into the singularity from the exterior source, what happens to the accreted matter once it has collapsed into the singularity? Linear inference dictates that the singularity must be accruing mass from this exterior source. Is it possible that the singularity would eventually acquire enough "donor" matter to rebuild a physical structure, eventually reversing the object's status from a singularity to a supermassive body (analogous to a neutron star)? The question is academic, but this is a phenomenon which has puzzled me for a very long time. Thanks!

The Answer

Black holes can be produced by supernovae, but other production mechanisms are possible. Many galaxies for instance, including our own, may have super-massive black holes at their centers, which have grown by accretion where the galactic densities were highest. Wherever sufficient mass is crammed into a sufficiently small space a black hole will result. If matter is added to a neutron star for example, at some point (somewhere between 1.4 and 3 solar masses) the internal pressure within the star cannot resist gravity and a black hole is formed. Isolated black holes will be almost impossible to detect. There are a number of binary stars however, where one of the pair is a compact object (white dwarf or neutron star or black hole) accreting material from its companion (and generating X-rays and gamma-rays in the process) and studies of the binary system motion (using the Doppler shifts of spectral lines) suggest that the compact object is too massive to be a neutron star. Cygnus X-1 is just such a binary, where the likely mass of the compact object appears to be considerably more than 3 solar masses.

Adding mass to a black hole just makes it more massive. It doesn't fill it up. Quasars may represent instances where black holes have swallowed significant fractions of entire galaxies - billions of solar masses! Once matter has entered a black hole, it is not accessible to observation. All we can know about that black hole is its mass, charge and angular momentum. Everything else is open to untestable speculation.

In 1974 Stephen Hawking made the surprising discovery that quantum mechanics permits black holes to emit particles, an effect entirely forbidden under classical mechanics. (There are many situations in nuclear physics where quantum particles can similarly 'tunnel' through what would otherwise be impenetrable barriers.) For massive black holes the rate of particle escape is very low. A singularity with the mass of the Sun, for example, would lose an utterly insignificant fraction of its mass over many billions of solar lifetimes. It's still an interesting effect though!

Hawking has some fine discussions of black holes in his two popular books 'A Brief History of Time' and 'Black Holes and Baby Universes'. Imagine the Universe! includes other good references in its Black Holes section. The X-ray Binaries section is also relevant. I hope this answer has been helpful.

Paul Butterworth

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