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Charged Black Holes
It is difficult to conceive of how a star-size or larger object could carry a
net electrical charge because if any such object formed with positive and negative charges separated spatially
from each other there would be strong forces that would act to
recombine the charges. Thus, it is usually assumed that
the black holes of interest in astronomy have zero charge, leaving
only mass and angular momentum as distinguishing characteristics.
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Properties of Black Holes
One might expect that anything as exotic as a black hole would be a very complicated thing. Surprisingly,
black holes are in a certain sense some of the simplest things that we can think of, because they have a very
limited set of distinguishing features. The black hole solutions to general relativity that are known in fact
display only three characteristics to external observers: mass, electrical charge, and angular
momentum.
This is remarkable. No matter what mass was originally collapsed to form a black hole--stars, planets,
dust, or green
cheese--the resulting black hole differs from another only
in these three quantities: mass, charge, and angular
momentum.
As noted in the adjacent box, we usually assume that
only mass and angular momentum are distinguishing characteristics for black holes that are
likely to exist.