Page 64 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 8
one model, to represent our universe.

One such possibility is what are called chaotic boundary conditions. These implicitly assume either that the universe
is spatially infinite or that there are infinitely many universes. Under chaotic boundary conditions, the probability of
finding any particular region of space in any given configuration just after the big bang is the same, in some sense,
as the probability of finding it in any other configuration: the initial state of the universe is chosen purely randomly.
This would mean that the early universe would have probably been very chaotic and irregular because there are
many more chaotic and disordered configurations for the universe than there are smooth and ordered ones. (If each
configuration is equally probable, it is likely that the universe started out in a chaotic and disordered state, simply
because there are so many more of them.) It is difficult to see how such chaotic initial conditions could have given
rise to a universe that is so smooth and regular on a large scale as ours is today. One would also have expected the
density fluctuations in such a model to have led to the formation of many more primordial black holes than the upper
limit that has been set by observations of the gamma ray background.

If the universe is indeed spatially infinite, or if there are infinitely many universes, there would probably be some large
regions somewhere that started out in a smooth and uniform manner. It is a bit like the well-known horde of monkeys
hammering away on typewriters – most of what they write will be garbage, but very occasionally by pure chance they
will type out one of Shakespeare’s sonnets. Similarly, in the case of the universe, could it be that we are living in a
region that just happens by chance to be smooth and uniform? At first sight this might seem very improbable,
because such smooth regions would be heavily outnumbered by chaotic and irregular regions. However, suppose
that only in the smooth regions were galaxies and stars formed and were conditions right for the development of
complicated self-replicating organisms like ourselves who were capable of asking the question: why is the universe
so smooth.? This is an example of the application of what is known as the anthropic principle, which can be
paraphrased as “We see the universe the way it is because we exist.”
There are two versions of the anthropic principle, the weak and the strong. The weak anthropic principle states that
in a universe that is large or infinite in space and/or time, the conditions necessary for the development of intelligent
life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions
should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are
necessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty.

One example of the use of the weak anthropic principle is to “explain” why the big bang occurred about ten thousand
million years ago – it takes about that long for intelligent beings to evolve. As explained above, an early generation of
stars first had to form. These stars converted some of the original hydrogen and helium into elements like carbon and
oxygen, out of which we are made. The stars then exploded as supernovas, and their debris went to form other stars
and planets, among them those of our Solar System, which is about five thousand million years old. The first one or
two thousand million years of the earth’s existence were too hot for the development of anything complicated. The
remaining three thousand million years or so have been taken up by the slow process of biological evolution, which
has led from the simplest organisms to beings who are capable of measuring time back to the big bang.

Few people would quarrel with the validity or utility of the weak anthropic principle. Some, however, go much further
and propose a strong version of the principle. According to this theory, there are either many different universes or
many different regions of a single universe, each with its own initial configuration and, perhaps, with its own set of
laws of science. In most of these universes the conditions would not be right for the development of complicated
organisms; only in the few universes that are like ours would intelligent beings develop and ask the question, “Why is
the universe the way we see it?” The answer is then simple: if it had been different, we would not be here!

The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric
charge of the electron and the ratio of the masses of the proton and the electron. We cannot, at the moment at least,
predict the values of these numbers from theory – we have to find them by observation. It may be that one day we
shall discover a complete unified theory that predicts them all, but it is also possible that some or all of them vary
from universe to universe or within a single universe. The remarkable fact is that the values of these numbers seem
to have been very finely adjusted to make possible the development of life. For example, if the electric charge of the
electron had been only slightly different, stars either would have been unable to burn hydrogen and helium, or else
they would not have exploded. Of course, there might be other forms of intelligent life, not dreamed of even by
writers of science fiction, that did not require the light of a star like the sun or the heavier chemical elements that are
made in stars and are flung back into space when the stars explode. Nevertheless, it seems clear that there are
relatively few ranges of values for the numbers that would allow the development of any form of intelligent life. Most
sets of values would give rise to universes that, although they might be very beautiful, would contain no one able to
wonder at that beauty. One can take this either as evidence of a divine purpose in Creation and the choice of the

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