MACROSCOPIC MOTION IN SYSTEMS AT THERMAL-EQUILIBRIUM

A theorem regarding the macroscopic motion of systems states that unde r thermal equilibrium the only possible motions are those of a rigid b ody. We show that this is true even for relativistic movements of the system. However for systems like superfluids relative motions between different components (superfluid and normal components) can take place even while maintaining thermal equilibrium. The existing theorem cont radicts this. We generalize the theorem so as to eliminate this incons istency. (C) 1995 American Association of Physics Teachers.