Einstein on the Impossibility of Superluminal Velocities
  In 1907 Einstein discussed with Wilhelm Wien the occurrences of superluminal velocities in dispersive and absorptive media. Einstein tried to present to Wien an expression for the group velocity in dispersive media that was based on his 1905 relativistic addition theorem for velocities, which he claimed to be valid for absorptive media, and to demonstrate the impossibility of superluminal velocities. However, he recognized that his expression needed an amendment and correction. Einstein did not convince Wien, and was finally confused, because he did not have the correct expression for the group velocity in dispersive media. However, he wrote Wien, "it is beyond doubt that our electromagnetic theories of dispersion can never yield superluminal velocity for the propagation of an optical signal". 
Galina Weinstein
physics.hist-ph