292 The Poetry of Physics and The Physics of Poetry
contraction. A third possible form of a collapsed state of a star is a black
hole. This state arises only if the star is massive enough, i.e. has a mass
greater than 1.4 solar masses. When a black hole forms the gravitational
collapse creates a density of matter so great that nothing can prevent the
total collapse of the star. The space in which the star is embedded is
warped to such a degree that the space closes in upon itself. Once this
happens nothing can escape the black hole because the space around it is
closed. In other words the gravitational field is so great everything is
pulled back to the black hole. This includes light, which is unable to
leave the star. This is why a black hole is black. Once an object falls into
a black hole it is lost. Nothing can retrieve it. The observation of a black
hole presents a problem because it does not send out any information, not
even light. The only way of detecting it is to observe the effects of its
gravitational field. If it is part of a binary system of stars one can observe
matter from the companion star being sucked out of it and falling into the
black hole with the consequence of emitting x-rays. One does not
observe the black hole directly merely the x-rays from the matter being
sucked into the black hole.
Supermassive black holes which contain any where from a thousand
to a billion of solar masses form due to the accretion of stellar black
holes, normal stars and clouds of gas and dust. They sit at the core of a
galaxy, as is the case with our galaxy, the Milky Way. The largest black
hole with a mass 18 billion times that of our Sun sits at the core of the
active galaxy, OJ 287.
The size of a black hole is determined by its event horizon, which is
defined as the radius of the black hole past which any thing that enters
never leaves including light. The radius or event horizon of a black hole
is determined by its mass. Its radius, known as the Schwarzchild radius,
in kilometers, is given approximately by 2.95 times its mass measured
in solar masses. The smallest possible black hole would have a mass of
1.4 solar masses and therefore a radius of 2.95 × 1.4 = 4.1 km. The
largest black hole with a mass of 18 billion solar masses has a radius
of 2.95 × 18 × 10^9 = 53 billion kilometers or 350 times the distance
between the Earth and the Sun.
Since the radius of a black hole is determined by its mass, it can be
described by only three numbers, its mass, its charge and its angular
momentum. Any two black holes with the same value of these numbers
cannot be distinguished from each other. This is consistent with the
notion that that time stops within a black hole since once its mass, charge
and angular momentum are fixed nothing can happen.