Albert Einstein (1879-1955).

Autograph letter signed ('A. Einstein') to Wilhelm Wien, Berne, 29 July 1907.

In German. Two pages, 215 x 134mm. Provenance: by descent from Wilhelm Wien.

'The equation "signal velocity ≤ velocity of light L" cannot contain anything new for us'. Einstein had written two letters in the previous week responding to a question raised by Wien in relation to group velocity and signal velocity. However, he now ruefully retracts what he had suggested: 'Unfortunately, upon closer examination almost everything that I over-hastily reported to you in my previous letters has proved to be false . In fact, the only thing that is to be retained as correct is that Maxwell's theory rules out the possibility of the propagation of a signal with superluminal velocity'. The situation for absorptive bodies where the absorption coefficient depends on the frequency is particularly difficult, and Einstein's calculations had implicitly oversimplified it. In reality, 'extremely complicated investigations would be needed in order to define and calculate clearly the propagation velocities of optical signals in metals. As moreover according to Wiechert's results there can be no doubt that our electromagnetic theories of dispersion can never produce a superluminal velocity for the propagation of an optical signal, the equation "signal velocity ≤ velocity of light L" cannot therefore contain anything new for us'. Einstein concludes that it was an error in calculation that had led him astray.

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