The invariance of the speed of light is one of the foundational pillars of our current understanding of the Universe. It implies a series of consequences related to our perception of simultaneity and, ultimately, of time itself. Whilst these consequences are experimentally well studied in the case of subluminal motion, the kinematics of superluminal motion lack direct evidence or even a clear experimental approach. Here we investigate kinematical effects associated with the superluminal motion of a light source. By using high temporal resolution imaging techniques, we directly demonstrate that if the source approaches an observer at superluminal speeds, the temporal ordering of events is inverted and its image appears to propagate backwards. Moreover, for a source changing its speed and crossing the interface between sub- and super-luminal propagation regions we observe image pair annihilation and creation, depending from the crossing direction. These results are very general and show that, regardless of the emitter speed, it is not possible to unambiguously determine the kinematics of an event from imaging and time-resolved measurements alone. This has implications not only for light but also, for example, for sound and other wave phenomena.
Clerici, M., Spalding, G. C., Warburton, R. E., Lyons, A., Aniculaesei, C., Richards, J. M., Leach, J., Henderson, R., & Faccio, D. F. A. (2016). Observation of image pair creation and annihilation from superluminal scattering sources. Science Advances, 2(4), [e1501691]. https://doi.org/10.1126/sciadv.1501691