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Entropy

Most people are aware of entropy through learning about the Second Law of Thermodynamics at school. It is commonly viewed as representing the amount of disorder in a system, and that law states that in a closed system it can only ever increase with time and never decrease. On the face of it, it sounds like entropy has a preferred direction in time, and this is used as an explanation for the irreversibility of nature.

 

Unfortunately, entropy is commonly misunderstood. It is essentially a probabilistic phenomenon of systems involving many separate entities (e.g. atoms): if those entities can vary randomly then they will tend to an even arrangement of maximum disorder. But this is time-symmetric, contrary to some writings. If we stir a spoonful of coffee into hot water, its dispersal is a probabilistic consequence of the ordered state (low entropy) evolving to the disordered state (high entropy). If you reversed a film of this then it would look unreal as it un-mixed, but it is not violating any physical laws; it is simply that it is so unlikely as to be effectively impossible.[1] If you shuffle a deck of cards then you expect it to randomise them. The prospect of finishing in an ordered arrangement is just incredibly remote.

 

Essentially, the problem is that those examples are beginning with an ordered arrangement, and so the evolution would be to an unordered arrangement. Reversing time then appears to show something that’s virtually impossible. However, going from one disordered state to another (such as shuffling an already random deck of cards) would not

 

Although the probabilistic phenomenon is time-symmetric, the universe itself started in a state of extremely low entropy, and that’s where there is an asymmetric “arrow of time” one that accounts for memory only working from past-to-future and not the other way around.



[1] Having said that, if gravity appeared as a repulsive force then the reversed film would still look unreal, but for a different reason. As far as we know, gravity is always attractive.