Does the entropy of a closed system always increase, or could it possibly decrease?

Asked by: Johannes Pollanen


The standard answer to your question from the laws of thermodynamics is that entropy (disorder) will increase, but there are at least two ways I believe entropy can decrease in a closed system.

First, you used the word 'possibly'. The laws of probability allow a closed system's entropy to decrease, but with such a low likelihood that the odds would make it very unlikely. Making the system small enough, however, by decreasing the number of its possible states can help improve the odds.

Take, for example, a movie of a billiards game 'break' shot. The ordered arrangement of balls becomes disordered, but running the film in reverse would show each individual collision obeying the usual physical laws. The time reversal would be apparent, however, when all the balls ended up in an ordered collection. Although that result could conceivably occur by chance, it is very unlikely. Reducing the example to just two balls would make the odds of an orderly arrangement occurring more likely.

For a second example of decreasing entropy, start with a closed system large enough to allow significant gravitational forces among its components. Gravity provides a 'negative energy' that can take a completely disordered system and organize it into a radically symmetric arrangement around a common center of gravity.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor