The Arrow Of Time~ STEPHEN HAWKING

 Up to the beginning of this century, people believed in an absolute time. That is, each event could be described by a number called 'time' in a unique way, and all the good clocks would agree on the time interval between two events. However, the discovery that the s[peed 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 see his own measure of time as recorded by a clock that he carried. 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?

Imagine a cup of water falling off a table and breaking into pieces on the floor. If you make 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. Moreover, it is forbidden by the second law of thermodynamics. This says that in any closed system disorder, or entropy, always increases with time. An intact cup on the table is in a disordered state. The increase of disorder or entropy with time is one example of what is called an 'arrow of 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, according to the present shreds of evidence we have.

According to SIR STEPHEN HAWKING, all three arrows of time point in the same direction. The question may arise, why does disorder increase in the same direction of time as that in which the universe expands?  Let's meet up in my next blog to find the answer of this question...till then stay tuned😊