Page 8 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 1
approach most scientists actually follow is to separate the problem into two parts. First, there are the laws that
tell us how the universe changes with time. (If we know what the universe is like at any one time, these physical
laws tell us how it will look at any later time.) Second, there is the question of the initial state of the universe.
Some people feel that science should be concerned with only the first part; they regard the question of the
initial situation as a matter for metaphysics or religion. They would say that God, being omnipotent, could have
started the universe off any way he wanted. That may be so, but in that case he also could have made it
develop in a completely arbitrary way. Yet it appears that he chose to make it evolve in a very regular way
according to certain laws. It therefore seems equally reasonable to suppose that there are also laws governing
the initial state.

It turns out to be very difficult to devise a theory to describe the universe all in one go. Instead, we break the
problem up into bits and invent a number of partial theories. Each of these partial theories describes and
predicts a certain limited class of observations, neglecting the effects of other quantities, or representing them
by simple sets of numbers. It may be that this approach is completely wrong. If everything in the universe
depends on everything else in a fundamental way, it might be impossible to get close to a full solution by
investigating parts of the problem in isolation. Nevertheless, it is certainly the way that we have made progress
in the past. The classic example again is the Newtonian theory of gravity, which tells us that the gravitational
force between two bodies depends only on one number associated with each body, its mass, but is otherwise
independent of what the bodies are made of. Thus one does not need to have a theory of the structure and
constitution of the sun and the planets in order to calculate their orbits.

Today scientists describe the universe in terms of two basic partial theories – the general theory of relativity
and quantum mechanics. They are the great intellectual achievements of the first half of this century. The
general theory of relativity describes the force of gravity and the large-scale structure of the universe, that is,
the structure on scales from only a few miles to as large as a million million million million (1 with twenty-four
zeros after it) miles, the size of the observable universe. Quantum mechanics, on the other hand, deals with
phenomena on extremely small scales, such as a millionth of a millionth of an inch. Unfortunately, however,
these two theories are known to be inconsistent with each other – they cannot both be correct. One of the
major endeavors in physics today, and the major theme of this book, is the search for a new theory that will
incorporate them both – a quantum theory of gravity. We do not yet have such a theory, and we may still be a
long way from having one, but we do already know many of the properties that it must have. And we shall see,
in later chapters, that we already know a fair amount about the predications a quantum theory of gravity must
make.

Now, if you believe that the universe is not arbitrary, but is governed by definite laws, you ultimately have to
combine the partial theories into a complete unified theory that will describe everything in the universe. But
there is a fundamental paradox in the search for such a complete unified theory. The ideas about scientific
theories outlined above assume we are rational beings who are free to observe the universe as we want and to
draw logical deductions from what we see.

In such a scheme it is reasonable to suppose that we might progress ever closer toward the laws that govern
our universe. Yet if there really is a complete unified theory, it would also presumably determine our actions.
And so the theory itself would determine the outcome of our search for it! And why should it determine that we
come to the right conclusions from the evidence? Might it not equally well determine that we draw the wrong
conclusion.? Or no conclusion at all?

The only answer that I can give to this problem is based on Darwin’s principle of natural selection. The idea is
that in any population of self-reproducing organisms, there will be variations in the genetic material and
upbringing that different individuals have. These differences will mean that some individuals are better able
than others to draw the right conclusions about the world around them and to act accordingly. These individuals
will be more likely to survive and reproduce and so their pattern of behavior and thought will come to dominate.
It has certainly been true in the past that what we call intelligence and scientific discovery have conveyed a
survival advantage. It is not so clear that this is still the case: our scientific discoveries may well destroy us all,
and even if they don’t, a complete unified theory may not make much difference to our chances of survival.
However, provided the universe has evolved in a regular way, we might expect that the reasoning abilities that
natural selection has given us would be valid also in our search for a complete unified theory, and so would not
lead us to the wrong conclusions.

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