A Brief History of Time - Stephen Hawking | Page 7

planets in order to calculate their orbi\
ts.
Today scientists describe the universe in terms of two basic partial the\
ories – the general theory of relativity
and quantum mechanics. They are the great intellectual achievements of t\
he first half of this century. The
general theory of relativity describes the force of gravity and the larg\
e-scale structure of the universe, that is,
the structure on scales from only a few miles to as large as a million m\
illion million million (1 with twenty-four
zeros after it) miles, the size of the observable universe. Quantum mec\
hanics, 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 – the\
y cannot both be correct. One of the
major endeavors in physics today, and the major theme of this book, is t\
he search for a new theory that will
incorporate them both – a quantum theory of gravity. We do not yet ha\
ve 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 predicat\
ions 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 de\
scribe 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 ob\
serve 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 i\
t! And why should it determine that we
come to the right conclusions from the evidence? Might it not equally we\
ll 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 vari\
ations 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 an\
d to act accordingly. These individuals
will be more likely to survive and reproduce and so their pattern of beh\
avior and thought will come to dominate.
It has certainly been true in the past that what we call intelligence an\
d 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 d\
ifference to our chances of survival.
However, provided the universe has evolved in a regular way, we might ex\
pect that the reasoning abilities that
natural selection has given us would be valid also in our search for a c\
omplete unified theory, and so would not
lead us to the wrong conclusions.
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Because the partial theories that we already have are sufficient to make\
accurate predictions in all but the most
extreme situations, the search for the ultimate theory of the universe s\
eems difficult to justify on practical
grounds. (It is worth noting, though, that similar arguments could have\
been used against both relativity and
quantum mechanics, and these theories have given us both nuclear energy \
and the microelectronics
revolution!) The discovery of a complete unified theory, therefore, may\
not aid the survival of our species. It
may not even affect our lifestyle. But ever since the dawn of civilizati\
on, people have not been content to see
events as unconnected and inexplicable. They have craved an understandin\
g of the underlying order in the
world. Today we still yearn to know why we are here and where we came fr\
om. Humanity’s deepest desire for
knowledge is justification enough for our continuing quest. And our goal\
is nothing less than a complete
description of the universe we live in.
 
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CHAPTER 2
SPACE AND TIME
 
Our present ideas about the