entropy

Entropy is a property of a physical system that quantifies the number of microscopic configurations that correspond to its macroscopic state. In classical thermodynamics, entropy is a state function with units of energy per temperature (joules per kelvin) and is denoted by S. The second law of thermodynamics states that for an isolated system, entropy tends to increase or remain constant over time, guiding the system toward thermodynamic equilibrium.

In statistical mechanics, entropy relates to the multiplicity of microstates. Boltzmann's relation S = k_B ln W expresses entropy as proportional to the logarithm of the number W of distinct microstates compatible with the macrostate. An alternative form uses probabilities p_i of microstate i: S = -k_B sum p_i ln p_i. At fixed energy, volume, and particle number, the entropy is maximized at the equilibrium distribution.

In information theory, entropy measures the uncertainty or average information content of a random variable. For outcomes i with probabilities p_i, H = - sum p_i log p_i. The base of the logarithm sets the unit: bits for base 2, nats for natural log. Although conceptually distinct, thermodynamic and information entropies share mathematical structure, and operations such as information erasure have physical consequences (Landauer's principle).

Entropy also features in other domains, such as black hole thermodynamics, where the Bekenstein–Hawking entropy is proportional to the area of the event horizon, linking gravity and thermodynamics. In practice, entropy provides a unifying language for irreversibility, equilibrium, and statistical likelihood across physics, chemistry, and information science.