Entropy is a measure of disorder or randomness in a system. The second law of thermodynamics says that the total entropy of a closed system never decreases. It either stays the same or increases. That is why heat flows from hot to cold, why a broken glass does not reassemble itself, and why time seems to have a direction. Entropy is not a force. It is a statistical tendency. There are simply more ways for a system to be disordered than ordered.

Entropy is measured in joules per kelvin. Ludwig Boltzmann gave it a statistical definition: S = k ln W, where k is Boltzmann's constant and W is the number of microstates that correspond to a given macrostate. A macrostate with more microstates has higher entropy. That is why a gas expanding to fill a room has higher entropy than a gas compressed into a corner. Entropy is also used in information theory, where it measures the uncertainty or information content of a message. The two concepts are related, though they are not identical.

Entropy facts

• Definition. Measure of disorder or randomness.
• Second law. Total entropy of a closed system never decreases.
• Boltzmann. S = k ln W.
• Direction. Explains the arrow of time.
• Information. Measures uncertainty in a message.

Entropy is not the same as energy. Energy is conserved. Entropy tends to increase.