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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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