Systems in thermal equilibrium with their environment have uncertainty in energy, and are instead described by the canonical ensemble or the grand canonical ensemble, the latter if the system is also in equilibrium with its environment in respect to particle exchange.
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:: : : : StuRat, and everyone really : Feynman diagrams are ridiculously oversold in popular books, to the point that a lot of people seem to think that quantum field theory is about virtual particle exchange and you can't have a quantum field theory without virtual particles.
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:Someone else here who better understands the subject should arrive to address this question shortly, but in the meantime I will direct you to our Static forces and virtual-particle exchange article for some background on the subject .-- ToE 00 : 36, 21 June 2015 ( UTC)
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For example, if we consider a simple gas with " N " particles, at sufficiently low density that it is practically certain that each sublevel contains either one particle or none ( i . e . a Maxwell Boltzmann gas ), this means that a simple container of gas will be in one of " N ! " detectably different " exchange macrostates ", one for each possible particle exchange.
