On the Existence of Undistorted Progressive Waves (UPWs) of Arbitrary   Speeds $0 \leq v< \infty$ in Nature
  We present the theory, the experimental evidence, and fundamental physical consequences concerning the existence of families of undistorted progressive waves (UPWs) of arbitrary speeds $0\leq v < \infty$, which are solutions of the homogeneous wave equation, Maxwell equations, and Dirac and Weyl equations. 
W.A. Rodrigues Jr. and J.-Y. Lu