A Brief History of Time 9, by Stephen Hawking

 Chapter 9

The arrow of time

Our views of the nature of time have changed over the years.

Up to the beginning of the 20^th century people believed in an absolute time. 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 so we had to abandon the idea of absolute time.

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 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, there is a big difference between the forward and the backward directions, in “real” time, as we know.

 Where does this difference between the past and the future come from?

 Why do we remember the past but not the future?

 

The second law of thermodynamics says that in any closed system disorder or entropy, always increases with time: “things always tend to go wrong”.

 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.

The second law of thermodynamics results from the fact that there are always more disordered states than there are ordered ones.

The laws of science do not distinguish between the forward and backward directions of time- between the past and the future, in the imaginary time.

However, there are at least three arrows of time that do distinguish the past from the future:

·         The thermodynamic arrow of time, the direction of time in which disorder increases

·         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.

 

The psychological arrow is essentially the same as the thermodynamic arrow, so that the two would always point in the same direction.

The “no boundary” proposal for the universe predicts the existence of a well-defined thermodynamic arrow of time because the universe must start in a smooth and ordered state. And the reason we observe this thermodynamic arrow to agree with the cosmological arrow is that intelligent beings can exist only in the expanding phase.

The contracting phase will be unsuitable because it has no strong thermodynamic arrow of time.

 The progress of human race in understanding the universe has established a small corner of order in an increasingly disordered universe.