'SUBTLE is THE LORD' 121medium might perhaps be dispensed with. Thus Paul Drude wrote in 1900: 'The
conception of an ether absolutely at rest is the most simple and the most natural—
at least if the ether is conceived to be not a substance but merely space endowed
with certain physical properties' [D3]; and Emil Cohn in 1901, 'Such a medium
fills every element of our space; it may be a definite ponderable system or also the
vacuum' [Cl].
Of the many papers on the subject treated in this section, the following in par-
ticular have been of great help to me: Tetu Hirosige on the aether problem [H5],
McCormmach on Hertz [Ml4], Bork [B3] and Brush [B4] on FitzGerald, and
Miller [M15] on Poincare.
As to Einstein himself, in his first relativity paper he mentions only three phys-
icists by name: Maxwell, Hertz, and Lorentz. As he repeatedly pointed out else-
where, in 1905 he knew Lorentz's work only up to 1895. It follows—as we shall
see—that in 1905 Einstein did not know of Lorentz transformations. He invented
them himself. Nor did he know at that time those papers by Poincare which deal
in technical detail with relativity issues.- Voigt. It was noted in 1887 [V2] by Woldemar Voigt that equations of the
type
retain their form if one goes over to the new space-time variables
These are the Lorentz transformations (Eq. 6.3) up to a scale factor. Voigt
announced this result in a theoretical paper devoted to the Doppler principle. As
an application of Eq. 6.7, he gave a derivation of the Doppler shift, but only for
the long-familiar longitudinal effect of order v/c. His new method has remained
standard procedure to this day: he made use of the invariance of the phase of a
propagating plane light wave under Eq. 6.7 [P3]. Since the Doppler shift is a
purely kinematic effect (in the relativistic sense), it is irrelevant that Voigt's argu-
ment is set in the dynamic framework of the long-forgotten elastic theory of light
propagation, according to which light is propagated as a result of oscillations in
an elastic incompressible medium.
Lorentz was familiar with some of Voigt's work. In 1887 or 1888, the two men
corresponded—about the Michelson-Morley experiment [V3]. However, for a
long time Lorentz seems not to have been aware of the Voigt transformation (Eq.
6.7). Indeed, Lorentz's Columbia University lectures, given in 1906 and published
in book form in 1909, contain the following comment: 'In a paper... published
in 1887. .. and which to my regret has escaped my notice all these years, Voigt
has applied to equations of the form [of Eq. 6.5] a transformation equivalent to
[Eq. 6.3]. The idea of the transformations [Eq. 6.3] ... might therefore have been