  The vacuum is the lowest energy state of a field in a certain region of
space. This definition implies that no particles can be present in the vacuum
state. In classical physics, the only features of vacuum are those of its
geometry. For example, in the general theory of relativity the geometry is a
dynamical structure that guides the motion of matter, and, in turn, it is bent
and curved by the presence of matter. Other than this, the classical vacuum is
a structure void of any physical properties, since classically properties are
strictly associated with physical objects such as particles and
finite-amplitude fields. The situation is very different in quantum physics. As
I will show in this paper, the difference stems from the fact that in quantum
physics the properties are not strictly tied to objects. We know for example
that physical properties come into existence - as values of observables - only
when the object is measured. Thus, quantum physics allows us to detach
properties from objects. This has consequences: one does not need pre-existing
real objects to create actual properties, and indeed under certain
perturbations the quantum vacuum produces observable effects such as energy
shifts and creation of particles. An open question is if by necessity the
vacuum comes with an embedded geometry, and if it is possible to construct
viable physical theories in which geometry is detached from the vacuum.
