  The nature and properties of the vacuum as well as the meaning and
localization properties of one or many particle states have attracted a fair
amount of attention and stirred up sometimes heated debate in relativistic
quantum field theory over the years. I will review some of the literature on
the subject and will then show that these issues arise just as well in
non-relativistic theories of extended systems, such as free bose fields. I will
argue they should as such not have given rise either to surprise or to
controversy. They are in fact the result of the misinterpretation of the vacuum
as ``empty space'' and of a too stringent interpretation of field quanta as
point particles. I will in particular present a generalization of an apparently
little known theorem of Knight on the non-localizability of field quanta,
Licht's characterization of localized excitations of the vacuum, and explain
how the physical consequences of the Reeh-Schlieder theorem on the cyclicity
and separability of the vacuum for local observables are already perfectly
familiar from non-relativistic systems of coupled oscillators.
