  Theoretical developments related to the gravitational interaction have
questioned the notion of particle in quantum field theory (QFT). For instance,
uniquely-defined particle states do not exist in general, in QFT on a curved
spacetime. More in general, particle states are difficult to define in a
background-independent quantum theory of gravity. These difficulties have lead
some to suggest that in general QFT should not be interpreted in terms of
particle states, but rather in terms of eigenstates of local operators. Still,
it is not obvious how to reconcile this view with the empirically-observed
ubiquitous particle-like behavior of quantum fields, apparent for instance in
experimental high-energy physics, or "particle"-physics. Here we offer an
element of clarification by observing that already in flat space there exist
--strictly speaking-- two distinct notions of particles: globally defined
$n$-particle Fock-states and *local particle states*. The last describe the
physical objects detected by finite-size particle detectors and are eigenstates
of local field operators. In the limit in which the particle detectors are
appropriately large, global and local particle states converge in a weak
topology (but not in norm). This observation has little relevance for
flat-space theories --it amounts to a reminder that there are boundary effects
in realistic detectors--; but is relevant for gravity. It reconciles the two
points of view mentioned above. More importantly, it provides a definition of
local particle state that remains well-defined even when the conventional
global particle states are not defined. This definition plays an important role
in quantum gravity.
