SUPERLUMINAL VELOCITIES 57 Sommerfeld[4] and Emil Wiechert[5] it appeared that for rigid electrons-the model championed by Max Abraham-stationary superluminal motion was only possible for volume-charged electrons under constant application of an external force. For particles with a surface charge such motion was excluded. The rival model of a de- formable electron, defended by H. A. Lorentz, did not allow superluminal speeds at all: it would need an infinite amount of energy to accelerate such an electron to the speed of light. Wilhelm Wien's participation in the debate started in 1904 in an exchange with Abraham in the Annalen der Physik.[6] His position was firmly on the side of the de- formable electron and thus on the impossibility of superluminal speeds. He stressed the successes the model of the deformable electron had achieved, in particular through the work of Lorentz. In a lecture at a meeting of the Gesellschaft Deutscher Naturforscher und Ärzte the next year he reaffirmed his belief and called the conse- quences drawn from the rigid electron model "physically of little probability."[7] The appearance of Einstein's 1905 relativity paper, Einstein 1905r (Vol. 2, Doc. 23), added a new aspect to the debate, as the theory did not allow superluminal speeds at all.[8] By the time of the Wien correspondence, Einstein had developed an argu- ment against superluminal velocities that is quite distinct from those featured in the earlier paper. First appearing in Einstein 1907h (Vol. 2, Doc. 45), this argument uses the relativistic velocity addition formula to show that a signal propagating superlu- minally from cause to effect in the rest frame of those two events will propagate from effect to cause in another frame moving relatively to the first.[9] Einstein concludes: Even though, in my opinion, this result contains no contradiction from a purely logical point of view, it conflicts so absolutely with the character of all our experience, that the impossibility of the assumption W V is suffi- ciently proved by this result.[10] The fundamental impossibility of superluminal speeds became all the more con- troversial, because it had become clear that in dispersive and absorptive media the phase velocity of a plane wave and even the group velocity of a superposition of waves could exceed the speed of light.[11] Thus, not only some electron theories, but, even more fundamentally, pure Maxwell theory seemed to contradict relativity. It is on this latter point that the debate with Wien centers. It is not unlikely that Wien's interest in these matters was stimulated by his reading of Einstein 1907h (Vol. 2, Doc. 45), which had appeared in June. II The exchange between Einstein and Wien starts with a letter by Einstein which responds to an inquiry made by Wien, presumably on the discrepancy between the occurrence of superluminal velocities in electromagnetic theory and Einstein's con-