  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".
