Microstate

A microstate is a full, exact description of the instantaneous physical configuration of a system: for a classical gas, the position and velocity of every single molecule, all six numbers per particle. For a mole of gas that is roughly 6 × 10²³ molecules, so a microstate is a single point in a phase space of about 3.6 × 10²⁴ dimensions. It is the most detailed answer physics can give to the question 'what is the system doing right now?'

Microstates are inaccessible in practice. No instrument can read off 10²⁴ positions and velocities, and the configuration changes completely on the timescale of a molecular collision, about 10⁻¹³ seconds. Their importance is not as measurable quantities but as objects to be counted: statistical mechanics works by asking how many microstates are compatible with a given coarse, measurable description (a macrostate), and assuming each accessible microstate is equally likely.

The distinction between the unobservable microstate and the observable macrostate is the conceptual hinge of statistical mechanics. Entropy, temperature, and the second law all emerge from counting microstates, not from tracking any one of them.

The microstate–macrostate language crystallised in the work of Boltzmann and Gibbs in the late nineteenth century, as the statistical interpretation of thermodynamics took shape. Maxwell's 1860 treatment of molecular velocities was an early step toward treating a gas as a statistical ensemble of microscopic configurations.