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  • Oshin Mittal

Ch 7: The Arrow of Time | A Brief History of Time | Summary

The theory and concept of time have changed from time to time. From absolute time to imaginary time, that the journey that time has travelled. But the difference between real-time and imaginary time is the direction i.e., past and future. The laws of science don’t distinguish between the past and the present. So, the basic explanation of why we don’t travel behind in time can be well explained by the second law of thermodynamics.


The increase of entropy or disorder with time is one example of what is called an arrow of time. Arrow of time is referred to as something that distinguishes the past from the future, giving a direction to time. There are at least three different arrows of time. Firstly, the thermodynamic arrow, the direction of time in which entropy increases. Secondly, the psychological arrow of time, the direction in which remember the past and not the future. Thirdly, we have the cosmological arrow of time, the direction in which the universe is expanding rather than contracting. It is pre-requisitely assumed that the universe is no-boundary and finite.


The thermodynamic arrow of time is governed by the second law of thermodynamics which states the direction of a reaction i.e., towards randomness. Extending the same argument to physical sciences, the universe always tends to be in the state of disorder. There is more randomness in this universe than the orderings. We can discuss the psychological arrow with respect to the computer’s memory. A computer memory contains elements that can exist in either of two states. Before being recorded in a computer’s memory, the memory is in a disordered state, with equal probabilities for the two possible states. After the memory interaction with the system, it will definitely be in one state or the other. So, the memory has passed from a disordered state to an ordered one. Our subjective sense of the direction of the time, the psychological arrow of time, is therefore determined by the thermodynamic arrow of time. the reason for the cosmological arrow to be in the accordance with the thermodynamic arrow of time can be explained is that living beings that we see today can exist only in the expanding phase of the universe. The contracting universe would have been unsuitable because it has no strong evidence with the thermodynamic arrow of time.



[ A chapter’s summary from the great book A Brief History of Time -By Stephen Hawkins].


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