Page 6 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 1
gravitational force repulsive at very large distances. This did not significantly affect their predictions of the
motions of the planets, but it allowed an infinite distribution of stars to remain in equilibrium – with the attractive
forces between nearby stars balanced by the repulsive forces from those that were farther away. However, we
now believe such an equilibrium would be unstable: if the stars in some region got only slightly nearer each
other, the attractive forces between them would become stronger and dominate over the repulsive forces so
that the stars would continue to fall toward each other. On the other hand, if the stars got a bit farther away
from each other, the repulsive forces would dominate and drive them farther apart.

Another objection to an infinite static universe is normally ascribed to the German philosopher Heinrich Olbers,
who wrote about this theory in 1823. In fact, various contemporaries of Newton had raised the problem, and the
Olbers article was not even the first to contain plausible arguments against it. It was, however, the first to be
widely noted. The difficulty is that in an infinite static universe nearly every line of sight would end on the
surface of a star. Thus one would expect that the whole sky would be as bright as the sun, even at night.
Olbers’ counter-argument was that the light from distant stars would be dimmed by absorption by intervening
matter. However, if that happened the intervening matter would eventually heat up until it glowed as brightly as
the stars. The only way of avoiding the conclusion that the whole of the night sky should be as bright as the
surface of the sun would be to assume that the stars had not been shining forever but had turned on at some
finite time in the past. In that case the absorbing matter might not have heated up yet or the light from distant
stars might not yet have reached us. And that brings us to the question of what could have caused the stars to
have turned on in the first place.

The beginning of the universe had, of course, been discussed long before this. According to a number of early
cosmologies and the Jewish/Christian/Muslim tradition, the universe started at a finite, and not very distant,
time in the past. One argument for such a beginning was the feeling that it was necessary to have “First Cause”
to explain the existence of the universe. (Within the universe, you always explained one event as being caused
by some earlier event, but the existence of the universe itself could be explained in this way only if it had some
beginning.) Another argument was put forward by St. Augustine in his book The City of God. He pointed out
that civilization is progressing and we remember who performed this deed or developed that technique. Thus
man, and so also perhaps the universe, could not have been around all that long. St. Augustine accepted a
date of about 5000 BC for the Creation of the universe according to the book of Genesis. (It is interesting that
this is not so far from the end of the last Ice Age, about 10,000 BC, which is when archaeologists tell us that
civilization really began.)

Aristotle, and most of the other Greek philosophers, on the other hand, did not like the idea of a creation
because it smacked too much of divine intervention. They believed, therefore, that the human race and the
world around it had existed, and would exist, forever. The ancients had already considered the argument about
progress described above, and answered it by saying that there had been periodic floods or other disasters that
repeatedly set the human race right back to the beginning of civilization.

The questions of whether the universe had a beginning in time and whether it is limited in space were later
extensively examined by the philosopher Immanuel Kant in his monumental (and very obscure) work Critique of
Pure Reason, published in 1781. He called these questions antinomies (that is, contradictions) of pure reason
because he felt that there were equally compelling arguments for believing the thesis, that the universe had a
beginning, and the antithesis, that it had existed forever. His argument for the thesis was that if the universe did
not have a beginning, there would be an infinite period of time before any event, which he considered absurd.
The argument for the antithesis was that if the universe had a beginning, there would be an infinite period of
time before it, so why should the universe begin at any one particular time? In fact, his cases for both the thesis
and the antithesis are really the same argument. They are both based on his unspoken assumption that time
continues back forever, whether or not the universe had existed forever. As we shall see, the concept of time
has no meaning before the beginning of the universe. This was first pointed out by St. Augustine. When asked:
“What did God do before he created the universe?” Augustine didn’t reply: “He was preparing Hell for people
who asked such questions.” Instead, he said that time was a property of the universe that God created, and
that time did not exist before the beginning of the universe.

When most people believed in an essentially static and unchanging universe, the question of whether or not it
had a beginning was really one of metaphysics or theology. One could account for what was observed equally
well on the theory that the universe had existed forever or on the theory that it was set in motion at some finite

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