  The history of the particle concept is briefly reviewed, with particular
emphasis on the `foliation dependence' of many particle creation models, and
the possible connection between our notion of particle and our notion of
simultaneity. It is argued that the concept of `radar time' (originally
introduced by Sir Hermann Bondi in his work on k-calculus) provides a
satisfactory concept of `simultaneity' for observers in curved spacetimes. This
is used to propose an observer-dependent particle interpretation, applicable to
an arbitrary observer, depending solely on that observers motion and not on a
choice of coordinates or gauge. This definition is illustrated with application
to non-inertial observers and simple cosmologies, demonstrating its generality
and its consistency with known cases.
