Cherenkov radiation has nothing to do with X-shaped Localized Waves (Comments on "Cherenkov-Vavilov Formulation of X-Waves")

Abstract: The Localized Waves are nondiffracting ("soliton-like") and self-reconstructing solutions to the wave equations, and are known to exist with subluminal, luminal and superluminal peak-velocities. The most studied ones are the "X-shaped" superluminal waves; which are associated with a cone, so that some authors [e.g., Walker and Kuperman, PRL 99 (Dec.2007) 244802] have been tempted to link them with Cherenkov radiation. However, the "X-waves" belong to a different realm, and exist even in the vacuum, independently of any media, as verified in a number of papers [listed e.g. in "Localized Waves" (J.Wiley; Jan.2008)]. We want to clarify the whole question on the basis of rigorous formalism and clear physical considerations. In particular, by explicit calculations based on Maxwell equations only, we show that: (i) the X-waves exist also inside both the front and the rear part of their double cone (that has nothing to do with Cherenkov's); (ii) they are to be found not via ad hoc assumptions, but by use of strict mathematical (or experimental) procedures; (iii) the ideal X-waves possess infinite energy, but finite-energy X-waves can be easily constructed (even without space-time truncations): And we do construct exact finite-energy solutions (totally free from backward-traveling waves); (iv) an actual attempt at comparing Cherenkov radiation with X-waves would lead one to consider the very different situation of the (X-shaped, too) field generated by a superluminal point-charge, a question exploited in previous papers [Recami et al., PRE 69 (2004) 027602]: We show explicitly, here, that the point-charge would not lose energy in the vacuum, and its field would not need to be continuously feeded by incoming side-waves (as it is the case, indeed, for an ideal, ordinary X-wave).