# Why does time have a direction? Why did the universe have such low entropy in the past, and time correlates with the universal (but not local) increase in entropy, from the past and to the future, according to the second law of thermodynamics?[4]  Why are CP violations observed in certain weak force decays, but not elsewhere? Are CP violations somehow a product of the second law of thermodynamics, or are they a separate arrow of time? Are there exceptions to the principle of causality? Is there a single possible past? Is the present moment physically distinct from the past and future, or is it merely an emergent property of consciousness? What links the quantum arrow of time to the thermodynamic arrow?

Arun
25750 Points
5 years ago
Dear student

1).

Time does have a direction in macroscopic scale. People get old, a new car becomes shabby, an egg splatters but never the opposite, everywhere things get disordered rather than ordered with time. This gives time a direction. An arrow of time.

All these things are a direct result of the second law of thermodynamics. According to this law there is a quantity called entropy which is always associated with an isolated system and which can only increase or stay equal but never decrease in any physical process. Our universe is also an isolated system and its entropy can only increase in any process until it reaches a maximum value. Therefore everything that happens inevitably increase its entropy.

But what is entropy? You can think of it as a measure of disorder in a system. Since the number of disordered states are much larger than the ordered states, a system left to itself, will have much larger probability to get disordered than ordered.

Consider a deck of cards arranged in a neat order. Now if you shuffle it for some time the cards will be arranged in a haphazard manner. The reason is the cards can be arranged in haphazard manner in so many ways but there are only a few arrangements which make the cards well ordered. You can say any process like shuffling increases the disorderliness of the cards.

Similarly, every process that takes place in our world make it more disordered. You can locally create some order but that creation process will make the world far more disordered. So, as a whole disorder always increases.

This gives time an arrow and it is usually called a thermodynamic arrow of time. It exists in our world because our universe started in a very special condition with extremely low entropy and ever since it is increasing its entropy to reach its maximum. A state of thermal equilibrium.

Apart from the thermodynamic arrow there are other two arrows of time. The arrow defined by our ability to remember the past but not the future called the psychological arrow of time and the other called cosmological arrow of time the direction of time in which the universe is expanding rather than contracting.

The thermodynamic and psychological arrow of time can be very well related in the sense that our memory may well depend on the thermodynamic arrow to function and remember the past rather than the future. So basically it is the thermodynamical arrow which gives time a direction and the reason it exist is the universe started in a very very special condition. Why or how it started in that very special condition is still a mystery.