  In analog gravity the recent experiment of Drori {\it et al.} [Phys. Rev.
Lett. {\bf 122}, 010404 (2019)] is impressive, as it shows how the emission of
two Hawking quanta emitted in opposite directions lead to measurable
consequences in the medium's rest system in a straightforward way. This result
raises however the following problem: how can this experiment be explained in
terms of classical electrodynamics? There must necessarily exist such an
explanation (the experiment is after all classical); otherwise classical
electrodynamics would be an incomplete theory. This is the main topic of the
present paper. We propose that the measured effect is a demonstration of the
spacelike character of the Minkowski four-momentum. Moreover, we extend the
discussion by analyzing a Gedanken experiment (making use of the Kerr effect as
a formal agency), to illustrate the transition from subluminal to superluminal
phenomena in a straightforward way. Finally, we emphasize the close
relationship that exists between the spacelike Minkowski momentum and the
anomalous Doppler effect.
