Abstract:
Our main topic is the classical phenomenon of gravitational redshift, the decrease in the frequency of a photon moving away from a massive body (e.g. the Earth). Of the two current inter-pretations, one is that at higher altoitudes the frequency-measuring clocks (atoms or atomic nuclei) run faster, i.e. their characteristic frequencies are higher, while the photon frequency in a static gravitational field is independent of the altitude a so the photon reddens only relative to the clocks. The other approah is that the photon reddens because it loses energy when overcoming the attraction of the gravitational field. This view, which is especially widespreadin popular science literature, ascribes such notions as "gravitational mass" and "potential energy" to the photon. Unfortunately, even scientific papers and serious books on relativity often employ the second interpretation.We shall show that this approach is misleading and only serves to create confusion on the subject.

The talk is meant to be pedagogical. If time allows it, we shall also discuss the bending of light and the radar echo in elementary terms.