Classical roots of the Unruh and Hawking effects

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.