Entropy always increases with time, according to the second law of thermodynamics. But what does it have to do with the arrow of time? 

What is an Arrow of time?

An arrow of time is the one-directional flow of time. That means time only moves forward. However, it’s quite intuitive for a human mind to comprehend that time never flows backward. As a consequence, this forward movement of time differentiates the past from the future and vice versa. You are maybe aware of it, if you have read me this article TIME TRAVEL AND THE FREE WILL DEBATE?

How many types of arrows of time are there?

There are three types of arrows of time. They are:

                                     (i) Thermodynamic arrow of time

                                    (ii) Psychological arrow of time

                                    (iii) Cosmological arrow of time

What is Thermodynamic arrow of time?

Arrow of Time by NASA/GSFC

Second law of thermodynamics plays a crucial role in the universe. The thermodynamic arrow of time is the direction of time in which entropy increases as time increases. The entropy of a system never decreases. It may remain constant or increase. For instance, let’s consider a situation from our day-to-day lives. Suppose a glass of coffee falls on the floor and gets broken. The question here is why don’t we see the pieces of the cup getting together again? Does it have anything to do with entropy? Let’s delve a bit deeper into the “entropy” thing to clarify this thing.

What do we mean by disorder?

So, when the cup of coffee was first on the table, it was in the highest state of order. All the pieces of the cup were intact. They had a regular and ordered arrangement. But when it falls off the table onto the floor, all the intact pieces get away from each other. As a result, there occurs a disorder. As disorder increases, so does entropy. However, we can simply explain entropy as the number of arrangements or combinations one can make of small scale particles with large scale particles.

So, when the cup broke into pieces, the arrangement or combination was of the most disordered state. Hence, the entropy of the system increased as time increased. However, we never see the reverse occurring anywhere. Because that would mean that the coffee cup became more ordered or arranged than its previous version of being totally broken. That would mean the entropy had decreased. In fact, this violated the 2nd law of thermodynamics. Therefore, we don’t see that thing happening anywhere in the world.

The entropy of the universe is always increasing:

At the beginning of the universe, everything was condensed together in the state of a point of singularity. We can say it had the highest order. After that, the big bang occurred and everything turned into chaos or more disorder. Entropy only increased as time passed by. So, there’s nothing wrong in saying that the entropy of the universe is always increasing.

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