Photon under repeated transverse Lorentz boosts: An apparent paradox

Abstract

We investigate the effects of the repeated application of Lorentz-boosts to the four momentum of a photon in the transverse direction and observe that this can take us to a reference frame in which the direction of the photon's momentum is apparently reversed. We further extend this to an infinite succession of infinitesimal transverse Lorentz-boosts and show it amounts to a rotation of the photon's momentum, while the transformation is not a simple rotation but a Lorentz transformation. These possibly surprising results can be understood in light of the Wigner rotation: the combination of Lorentz-boosts along different directions amounts to a Lorentz-boost combined with a rotation. The presented exercises are some more illustrations of the counterintuitive behavior of Lorentz-boost combinations.