ABSTRACT
In any system, no matter how dreadfully off equilibrium it may be, this is the chemical potential. It is quite important that f, the mean value of the wave field, is a complex order parameter. The foundations of the subject of quantum coherence were laid long before there were any acceptable microscopic theories of superconducting electron gas in metals phenomena. The proposal O. Penrose and L. Onsager made was that one could essentially define superfluidity of a Bose system such as liquid He as a state in which the density matrix of the system factorized in a certain special way. If the energy were independent of the relative phase of the quantum field operator on the two sides the author would not have such a mean value because on the average the two phases would fluctuate completely independently. It is a different coherence, not a superposition of the original ones but a different coherent state.
