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A Brief History of Time - Stephen Hawking... Chapter 9
CHAPTER 9
THE ARROW OF TIME
In previous chapters we have seen how our views of the nature of time have changed over the years. Up to the
beginning of this century people believed in an absolute time. That is, each event could be labeled by a number
called “time” in a unique way, and all good clocks would agree on the time interval between two events.
However, the discovery that the speed of light appeared the same to every observer, no matter how he was
moving, led to the theory of relativity – and in that one had to abandon the idea that there was a unique
absolute time. Instead, each observer would have his own measure of time as recorded by a clock that he
carried: clocks carried by different observers would not necessarily agree. Thus time became a more personal
concept, relative to the observer who measured it.
When one tried to unify gravity with quantum mechanics, one had to introduce the idea of “imaginary” time.
Imaginary time is indistinguishable from directions in space. If one can go north, one can turn around and head
south; equally, if one can go forward in imaginary time, one ought to be able to turn round and go backward.
This means that there can be no important difference between the forward and backward directions of
imaginary time. On the other hand, when one looks at “real” time, there’s a very big difference between the
forward and backward directions, as we all know. Where does this difference between the past and the future
come from? Why do we remember the past but not the future?
The laws of science do not distinguish between the past and the future. More precisely, as explained earlier,
the laws of science are unchanged under the combination of operations (or symmetries) known as C, P, and T.
(C means changing particles for antiparticles. P means taking the mirror image, so left and right are
interchanged. And T means reversing the direction of motion of all particles: in effect, running the motion
backward.) The laws of science that govern the behavior of matter under all normal situations are unchanged
under the combination of the two operations C and P on their own. In other words, life would be just the same
for the inhabitants of another planet who were both mirror images of us and who were made of antimatter,
rather than matter.
If the laws of science are unchanged by the combination of operations C and P, and also by the combination C,
P, and T, they must also be unchanged under the operation T alone. Yet there is a big difference between the
forward and backward directions of real time in ordinary life. Imagine a cup of water falling off a table and
breaking into pieces on the floor. If you take a film of this, you can easily tell whether it is being run forward or
backward. If you run it backward you will see the pieces suddenly gather themselves together off the floor and
jump back to form a whole cup on the table. You can tell that the film is being run backward because this kind
of behavior is never observed in ordinary life. If it were, crockery manufacturers would go out of business.
The explanation that is usually given as to why we don’t see broken cups gathering themselves together off the
floor and jumping back onto the table is that it is forbidden by the second law of thermodynamics. This says that
in any closed system disorder, or entropy, always increases with time. In other words, it is a form of Murphy’s
law: things always tend to go wrong! An intact cup on the table is a state of high order, but a broken cup on the
floor is a disordered state. One can go readily from the cup on the table in the past to the broken cup on the
floor in the future, but not the other way round.
The increase of disorder or entropy with time is one example of what is called an arrow of time, something that
distinguishes the past from the future, giving a direction to time. There are at least three different arrows of
time. First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy
increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, the
direction in which we remember the past but not the future. Finally, there is the cosmological arrow of time. This
is the direction of time in which the universe is expanding rather than contracting.
In this chapter I shall argue that the no boundary condition for the universe, together with the weak anthropic
principle, can explain why all three arrows point in the same direction – and moreover, why a well-defined arrow
of time should exist at all. I shall argue that the psychological arrow is determined by the thermodynamic arrow,
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