Entropy and the Direction of Natural Change

Binny J Cheryil is an Associate Professor at Department of Physical Chemistry, Indian Institute of Science, Bnagalore. He works on the statistical mechanics of polymers.

For the king of Corinth who was condemned forever to roll a large rock repeatedly up a hill only to have it roll down again when he had reached the summit, it would have been natural to conclude that the more things change the more they stay the same. But outside the pages of Greek myth, the world doesn't quite work this way. More often than not, the more things change the more they become different. A balloon filled with a coloured gas, for instance, will release its contents to the air in a room as soon as its mouth is opened or its skin punctured, but an empty balloon in the same room with just the same amount of coloured gas will never fill up with the gas on its own. Irreversibility is generally the norm, not the exception, to the way ordinary matter behaves, and there are certain directions of change that we know instinctively are allowed and certain others that we know are not. Why should that be? After all, transformations between different kinds of matter ultimately involve nothing more than the rearrangement of atoms and molecules, and that requires just the same expenditure of work in one direction as it does in another. We know this because the total supply of energy in the Universe is fixed and unchanging, so if some of it is used up at point A, exactly the same amount will be returned at point B. Neverthless, emptying a balloon is easy, filling it up is hard. Mere respect for the constancy of energy is apparently not enough to prevent nature's preference for certain directions of change over others. What then is?

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Address for Correspondence
Binny J Cherayil 
Department of Inorganic and Physical Chemistry 
Indian Institute of Science Bangalore 560012, India.