Have you ever seen Hamiltonians containing pairs of creation and
annihilation operators such as c^\dagger_i c^\dagger_j ? Vacuum is
nothing but a state annihilated by all the annihilation operators under
consideration. It is never really empty. For example, if a problem only
concerns electrons, all other particles, mu, tau, nu's are hidden in the
'vacuum'. If you do not consider electron-phonon scattering in a material,
phonons are hidden in the vacuum of the electron Hamiltonian. This is the
physical picture. For calculations, follow the strict mathematical
procedures in a good text book.
Have you ever seen Hamiltonians
containing pairs of creation and annihilation operators such as c^\dagger_i
c^\dagger_j ? Vacuum is nothing but a state annihilated by all the
annihilation operators under consideration. It is never really empty. For
example, if a problem only concerns electrons, all other particles, mu, tau,
nu's are hidden in the 'vacuum'. If you do not consider electron-phonon
scattering in a material, phonons are hidden in the vacuum of the electron
Hamiltonian. This is the physical picture. For calculations, follow the
strict mathematical procedures in a good text book.
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