Talk:Stationary state
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[edit] Vacuum
It is not true that the ground state of a quantum field theory is always the vacuum. Indeed, in (for example) non-interacting many-body quantum field theory, the ground state is the state where all the levels are occupied up to the Fermi level. The Fock-space state corresponding to the ground state is hence something like | 1....1 > up to the Fermi level. I propose to clarify that point, by stating that it holds only for QED, QCD or other subnuclear physics quantum field theory.