  A local observer can measure only the values of fields at the point of his
own position. By exploring the coordinate transformation between two Fermi
frames, it is shown that two observers, having the same instantaneous position
and velocity, will observe the same values of covariant fields at their common
instantaneous position, even if they have different instantaneous
accelerations. In particular, this implies that in classical physics the notion
of radiation is observer independent, contrary to the conclusion of some
existing papers. A "freely" falling charge in curved spacetime does not move
along a geodesic and therefore radiates. The essential feature of the Unruh
effect is the fact that it is based on a noninstantaneous measurement, which
may also be viewed as a source of effective noncovariance of measured
quantities. The particle concept in Minkowski spacetime is clarified. It is
argued that the particle concept in general spacetime does not depend on the
observer and that there exists a preferred coordinate frame with respect to
which the particle number should be defined.
