The second law of thermodynamics states that the total entropy of an isolated system can only increase over time. It can remain constant in ideal cases where the system is in a steady state (equilibrium) or undergoing a reversible process. The increase in entropy accounts for the irreversibility of natural processes, and the asymmetry between future and past - wikipedia 
Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory. Statistical thermodynamics, classical or quantum, explains the microscopic origin of the law.
The second law has been expressed in many ways. Its first formulation is credited to the French scientist Sadi Carnot in 1824, who showed that there is an upper limit to the efficiency of conversion of heat to work in a heat engine.
# See also * Introduction * Various statements of the law * Corollaries * History * Statistical mechanics * Derivation from statistical mechanics * Living organisms * Gravitational systems * Non-equilibrium states * Arrow of time * Irreversibility * Quotations * See also * References * Further reading * External links