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

Asked by: Johannes Pollanen

Answer

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