Page 93 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 12
CHAPTER 12

CONCLUSION

We find ourselves in a bewildering world. We want to make sense of what we see around us and to ask: What
is the nature of the universe? What is our place in it and where did it and we come from? Why is it the way it is?

To try to answer these questions we adopt some “world picture.” Just as an infinite tower of tortoises supporting
the fiat earth is such a picture, so is the theory of superstrings. Both are theories of the universe, though the
latter is much more mathematical and precise than the former. Both theories lack observational evidence: no
one has ever seen a giant tortoise with the earth on its back, but then, no one has seen a superstring either.
However, the tortoise theory fails to be a good scientific theory because it predicts that people should be able to
fall off the edge of the world. This has not been found to agree with experience, unless that turns out to be the
explanation for the people who are supposed to have disappeared in the Bermuda Triangle!

The earliest theoretical attempts to describe and explain the universe involved the idea that events and natural
phenomena were controlled by spirits with human emotions who acted in a very humanlike and unpredictable
manner. These spirits inhabited natural objects, like rivers and mountains, including celestial bodies, like the
sun and moon. They had to be placated and their favor sought in order to ensure the fertility of the soil and the
rotation of the seasons. Gradually, however, it must have been noticed that there were certain regularities: the
sun always rose in the east and set in the west, whether or not a sacrifice had been made to the sun god.
Further, the sun, the moon, and the planets followed precise paths across the sky that could be predicted in
advance with considerable accuracy. The sun and the moon might still be gods, but they were gods who
obeyed strict laws, apparently without any exceptions, if one discounts stories like that of the sun stopping for
Joshua.

At first, these regularities and laws were obvious only in astronomy and a few other situations. However, as
civilization developed, and particularly in the last 300 years, more and more regularities and laws were
discovered. The success of these laws led Laplace at the beginning of the nineteenth century to postulate
scientific determinism; that is, he suggested that there would be a set of laws that would determine the
evolution of the universe precisely, given its configuration at one time.

Laplace’s determinism was incomplete in two ways. It did not say how the laws should be chosen and it did not
specify the initial configuration of the universe. These were left to God. God would choose how the universe
began and what laws it obeyed, but he would not intervene in the universe once it had started. In effect, God
was confined to the areas that nineteenth-century science did not understand.

We now know that Laplace’s hopes of determinism cannot be realized, at least in the terms he had in mind.
The uncertainty principle of quantum mechanics implies that certain pairs of quantities, such as the position and
velocity of a particle, cannot both be predicted with complete accuracy. Quantum mechanics deals with this
situation via a class of quantum theories in which particles don’t have well-defined positions and velocities but
are represented by a wave. These quantum theories are deterministic in the sense that they give laws for the
evolution of the wave with time. Thus if one knows the wave at one time, one can calculate it at any other time.
The unpredictable, random element comes in only when we try to interpret the wave in terms of the positions
and velocities of particles. But maybe that is our mistake: maybe there are no particle positions and velocities,
but only waves. It is just that we try to fit the waves to our preconceived ideas of positions and velocities. The
resulting mismatch is the cause of the apparent unpredictability.

In effect, we have redefined the task of science to be the discovery of laws that will enable us to predict events
up to the limits set by the uncertainty principle. The question remains, however: how or why were the laws and
the initial state of the universe chosen?

In this book I have given special prominence to the laws that govern gravity, because it is gravity that shapes
the large-scale structure of the universe, even though it is the weakest of the four categories of forces. The
laws of gravity were incompatible with the view held until quite recently that the universe is unchanging in time:

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