A Brief History of Time - Stephen Hawking | Page 5

whether the universe had a beginning in time and whethe\
r 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, contr\
adictions) of pure reason
because he felt that there were equally compelling arguments for believi\
ng 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 a\
ny 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 t\
ime? 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 poin\
ted 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 univer\
se, the question of whether or not it
had a beginning was really one of metaphysics or theology. One could acc\
ount for what was observed equally
well on the theory that the universe had existed forever or on the theor\
y that it was set in motion at some finite
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time in such a manner as to look as though it had existed forever. But i\
n 1929, Edwin Hubble made the
landmark observation that wherever you look, distant galaxies are moving\
rapidly away from us. In other words,
the universe is expanding. This means that at earlier times objects woul\
d have been closer together. In fact, it
seemed that there was a time, about ten or twenty thousand million years\
ago, when they were all at exactly
the same place and when, therefore, the density of the universe was infi\
nite. This discovery finally brought the
question of the beginning of the universe into the realm of science.
Hubble’s observations suggested that there was a time, called the big\
bang, when the universe was
infinitesimally small and infinitely dense. Under such conditions all th\
e laws of science, and therefore all ability
to predict the future, would break down. If there were events earlier th\
an this time, then they could not affect
what happens at the present time. Their existence can be ignored because\
it would have no observational
consequences. One may say that time had a beginning at the big bang, in \
the sense that earlier times simply
would not be defined. It should be emphasized that this beginning in tim\
e is very different from those that had
been considered previously. In an unchanging universe a beginning in tim\
e is something that has to be
imposed by some being outside the universe; there is no physical necessi\
ty for a beginning. One can imagine
that God created the universe at literally any time in the past. On the \
other hand, if the universe is expanding,
there may be physical reasons why there had to be a beginning. One could\
still imagine that God created the
universe at the instant of the big bang, or even afterwards in just such\
a way as to make it look as though there
had been a big bang, but it would be meaningless to suppose that it was \
created before the big bang. An
expanding universe does not preclude a creator, but it does place limits\
on when he might have carried out his
job!
In order to talk about the nature of the universe and to discuss questio\
ns such as whether it has a beginning or
an end, you have to be clear about what a scientific theory is. I shall \
take the simpleminded view that a theory
is just a model of the universe, or a restricted part of it, and a set o\
f rules that relate quantities in the model to
observations that we make. It exists only in our minds and does not have\
any other reality (whatever that might
mean). A theory is a good theory if it satisfies two requirements. It m\
ust accurately describe a large class