The second law of thermodynamics states that the entropy of an isolated system cannot decrease; during spontaneous change, it increases until equilibrium is reached. Entropy measures how energy and matter are distributed among the possible microscopic states of a system.
The law gives thermodynamics its direction of time. Heat naturally flows from a hotter object to a cooler one, gases spread through available space, and friction converts organized mechanical energy into dispersed thermal energy. These processes are not forbidden by the first law, which accounts for energy conservation, but the second law determines their natural direction.
The law was developed through nineteenth-century work on heat engines. Sadi Carnot studied the limits of engine efficiency, while Rudolf Clausius introduced entropy and formalized the idea that heat does not spontaneously flow from cold to hot. Ludwig Boltzmann later connected entropy with the number of microscopic arrangements available to a system.
A common oversimplification says entropy always increases everywhere. The precise statement concerns an isolated system. A local region can become more ordered, such as a refrigerator making its interior colder, provided the surroundings gain enough entropy to produce a net increase overall.