A Brief History of Time - Stephen Hawking | Page 4

universe in which gravity is always attractive.
It is an interesting reflection on the general climate of thought before\
the twentieth century that no one had
suggested that the universe was expanding or contracting. It was general\
ly accepted that either the universe
had existed forever in an unchanging state, or that it had been created \
at a finite time in the past more or less
as we observe it today. In part this may have been due to people’s te\
ndency to believe in eternal truths, as well
as the comfort they found in the thought that even though they may grow \
old and die, the universe is eternal
and unchanging.
Even those who realized that Newton’s theory of gravity showed that t\
he universe could not be static did not
think to suggest that it might be expanding. Instead, they attempted to \
modify the theory by making the
A Brief History of Time - Stephen Hawking... Chapter 1
file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of ti\
me/n.html (3 of 7) [2/20/2001 3:14:06 AM]

gravitational force repulsive at very large distances. This did not sign\
ificantly 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 domi\
nate over the repulsive forces so
that the stars would continue to fall toward each other. On the other ha\
nd, if the stars got a bit farther away
from each other, the repulsive forces would dominate and drive them fart\
her 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 aga\
inst it. It was, however, the first to be
widely noted. The difficulty is that in an infinite static universe near\
ly 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 eventuall\
y 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 shinin\
g forever but had turned on at some
finite time in the past. In that case the absorbing matter might not hav\
e 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 star\
ted 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 alw\
ays 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 bo\
ok 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 th\
at 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, wh\
ich 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, there\
fore, 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 civilizatio\
n.
The questions of