  Although the Unruh and Hawking phenomena are commonly linked to field
quantization in "accelerated" coordinates or in curved spacetimes, we argue
that they are deeply rooted at the classical level. We maintain in particular
that these effects should be best understood by considering how the
special-relativistic notion of "particle" gets blurred when employed in
theories including accelerated observers or in general-relativistic theories,
and that this blurring is an instantiation of a more general behavior arising
when the principle of equivalence is used to generalize classical or quantum
special-relativistic theories to curved spacetimes or accelerated observers. A
classical analogue of the Unruh effect, stemming from the non-invariance of the
notion of "electromagnetic radiation" as seen by inertial and accelerated
observers, is illustrated by means of four gedanken-experimente. The issue of
energy balance in the various cases is also briefly discussed.
