  The assumption that the vacuum is the minimum energy state, invariant under
unitary transformations, is fundamental to quantum field theory. However, the
assertion that the conservation of charge implies that the equal time
commutator of the charge density and its time derivative vanish for two
spatially separated points is inconsistent with the requirement that the vacuum
be the lowest energy state. Yet, for quantum field theory to be gauge
invariant, this commutator must vanish. This essay explores how this conundrum
is resolved in quantum electrodynamics.
