A "Loschmidt demon" is exhibited which effectively reverses the spin-spin relaxation of a system of interacting magnetic dipoles in a strong external field, thereby demonstrating that this system does not approach internal thermodynamic equilibrium in a time $T_2$ as was implicitly recognized by Philippot.

• Received 5 May 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.218