Electrically charged particles emit Cherenkov radiation when they move faster than the speed of light in a medium. Theory predicts electric and magnetic dipoles to radiate as well, with a puzzling behaviour for magnetic dipoles pointing in transversal direction [I. M. Frank, Izv. Akad. Nauk SSSR, Ser. Fiz. 6, 3 (1942)]. A discontinuous Cherenkov spectrum should appear at threshold, where the particle velocity matches the phase velocity of light. Ilya Frank had been struggling with this problem for over 40 years, and there is no experimental verifications for this discontinuity.


In my talk I will explain why point-like light bullets [Y. Silberberg, Opt. Lett. 15, 1282 (1990)] partly behave like magnetic dipoles, and should emit an analogous radiation with exactly the same spectral discontinuity. For extended sources, this discontinuity turns into a spectral peak at threshold. I will argue that this Cherenkov radiation has been experimentally observed in the first attempt to measure Hawking radiation in optics [F. Belgiorno et al., Phys. Rev. Lett. 105, 203901 (2010)] thus giving experimental evidence for a puzzle in Cherenkov radiation instead.