Page 44 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 6
CHAPTER 6

BLACK HOLES

The term black hole is of very recent origin. It was coined in 1969 by the American scientist John Wheeler as a graphic
description of an idea that goes back at least two hundred years, to a time when there were two theories about light:
one, which Newton favored, was that it was composed of particles; the other was that it was made of waves. We now
know that really both theories are correct. By the wave/particle duality of quantum mechanics, light can be regarded as
both a wave and a particle. Under the theory that light is made up of waves, it was not clear how it would respond to
gravity. But if light is composed of particles, one might expect them to be affected by gravity in the same way that
cannonballs, rockets, and planets are. At first people thought that particles of light traveled infinitely fast, so gravity
would not have been able to slow them down, but the discovery by Roemer that light travels at a finite speed meant that
gravity might have an important effect.

On this assumption, a Cambridge don, John Michell, wrote a paper in 1783 in the Philosophical Transactions of the
Royal Society of London in which he pointed out that a star that was sufficiently massive and compact would have such
a strong gravitational field that light could not escape: any light emitted from the surface of the star would be dragged
back by the star’s gravitational attraction before it could get very far. Michell suggested that there might be a large
number of stars like this. Although we would not be able to see them because the light from them would not reach us,
we would still feel their gravitational attraction. Such objects are what we now call black holes, because that is what
they are: black voids in space. A similar suggestion was made a few years later by the French scientist the Marquis de
Laplace, apparently independently of Michell. Interestingly enough, Laplace included it in only the first and second
editions of his book The System of the World, and left it out of later editions; perhaps he decided that it was a crazy
idea. (Also, the particle theory of light went out of favor during the nineteenth century; it seemed that everything could
be explained by the wave theory, and according to the wave theory, it was not clear that light would be affected by
gravity at all.)
In fact, it is not really consistent to treat light like cannonballs in Newton’s theory of gravity because the speed of light is
fixed. (A cannonball fired upward from the earth will be slowed down by gravity and will eventually stop and fall back; a
photon, however, must continue upward at a constant speed. How then can Newtonian grav-ity affect light?) A
consistent theory of how gravity affects light did not come along until Einstein proposed general relativity in 1915. And
even then it was a long time before the implications of the theory for massive stars were understood.

To understand how a black hole might be formed, we first need an understanding of the life cycle of a star. A star is
formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As
it contracts, the atoms of the gas collide with each other more and more frequently and at greater and greater speeds –
the gas heats up. Eventually, the gas will be so hot that when the hydrogen atoms collide they no longer bounce off
each other, but instead coalesce to form helium. The heat released in this reaction, which is like a controlled hydrogen
bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it is
sufficient to balance the gravitational attraction, and the gas stops contracting. It is a bit like a balloon – there is a
balance between the pressure of the air inside, which is trying to make the balloon expand, and the tension in the
rubber, which is trying to make the balloon smaller. Stars will remain stable like this for a long time, with heat from the
nuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen and
other nuclear fuels. Paradoxically, the more fuel a star starts off with, the sooner it runs out. This is because the more
massive the star is, the hotter it needs to be to balance its gravitational attraction. And the hotter it is, the faster it will
use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive
stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When a star
runs out of fuel, it starts to cool off and so to contract. What might happen to it then was first understood only at the end
of the 1920s.

In 1928 an Indian graduate student, Subrahmanyan Chandrasekhar, set sail for England to study at Cambridge with the
British astronomer Sir Arthur Eddington, an expert on general relativity. (According to some accounts, a journalist told
Eddington in the early 1920s that he had heard there were only three people in the world who understood general
relativity. Eddington paused, then replied, “I am trying to think who the third person is.”) During his voyage from India,
Chandrasekhar worked out how big a star could be and still support itself against its own gravity after it had used up all
its fuel. The idea was this: when the star becomes small, the matter particles get very near each other, and so
according to the Pauli exclusion principle, they must have very different velocities. This makes them move away from
each other and so tends to make the star expand. A star can therefore maintain itself at a constant radius by a balance
between the attraction of gravity and the repulsion that arises from the exclusion principle, just as earlier in its life

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