  It is often remarked in the literature that particles in QFT on curved
spacetime are akin to coordinates in general relativity and hence are
physically meaningless. This moral is given an explicit demonstration by giving
the correspondence between the coordinates on phase space for a field theory
and the particle number. Usually the ambiguity in particle definitions is only
as varied as the possible sets of observers on the spacetime. However, there is
a greater choice in coordinates on the phase space, especially for a field
system with infinite degrees of freedom. Hence, for one set of coordinates on
the spacetime (one class of comoving observers) there are many different
coordinates to choose on the phase space. This demonstrates the true
vacuousness of the concept of particles when defined as energy levels of the
harmonic oscillator. In order to give a definition of particles we must specify
the apparatus that detects them. The Unruh-Dewitt detector is one such
apparatus, so we are not surprised to find that it gives a physically
meaningful definition of particles. We give an explicit example on de Sitter
spacetime and explain how the definition of particles as energy levels of the
harmonic oscillator is meaningless even in simple cases. This is done first by
comparing the response of an Unruh-Dewitt detector to the expectation of the
number operator. Second, we demonstrate that by a choice of coordinates on
phase space one can turn the Hamiltonian of a free Klein-Gordon field on FRW
with flat spatial sections into a set of harmonic oscillators with
time-independent mass and frequency. Also, we demonstrate a new method for
determining the wave functional of known states such as the conformal vacuum.
