In the kinetic theory of gases in physics, the molecular chaos hypothesis (also called Stosszahlansatz in the writings of Paul and Tatiana Ehrenfest[1][2]) is the assumption that the velocities of colliding particles are uncorrelated, and independent of position.
James Clerk Maxwell introduced this approximation in 1867[3] although its origins can be traced back to his first work on the kinetic theory in 1860.
[4][5] The assumption of molecular chaos is the key ingredient that allows proceeding from the BBGKY hierarchy to Boltzmann's equation, by reducing the 2-particle distribution function showing up in the collision term to a product of 1-particle distributions.
This in turn leads to Boltzmann's H-theorem of 1872,[6] which attempted to use kinetic theory to show that the entropy of a gas prepared in a state of less than complete disorder must inevitably increase, as the gas molecules are allowed to collide.
This interpretation allows using the principle of maximum entropy in order to generalize the ansatz to higher-order distribution functions.