120 RELATIVITY, THE SPECIAL THEORYpressibility, and the extent to which the earth dragged the aether along. This
explains largely (though not fully) why there was such a variety of post-Max-
wellian Maxwell theories, the theories of Hertz, Lorentz, Larmor, Wiechert,
Cohn, and probably others.
Hertz was, of course, aware of these options [Ml4]. After all, he had to choose
his own aether (the one he selected is dragged along by the earth). Indeed, his
dictum referred to earlier reads more fully: 'Maxwell's theory is Maxwell's system
of equations. Every theory which leads to the same system of equations, and there-
fore comprises the same possible phenomena, I would consider as being a form or
special case of Maxwell's theory.'
The most important question for all these authors of aethers and makers of
Maxwell theories was to find a dynamic understanding of the aberration of light,
of Fresnel drag, and, later, of the Michelson-Morley experiment. In a broad
sense, all these men were precursors of Einstein, who showed that theirs was a
task both impossible and unnecessary. Einstein's theory is, of course, not just a
Maxwell theory in the sense of Hertz. Rather s Einstein's resolution of the diffi-
culties besetting the electrodynamics of moving bodies is cast in an all-embracing
framework of a new kinematics. Going beyond Lorentz and Poincare, he based
his theory on the first of the two major re-analyses of the problem of measurement
which mark the break between the nineteenth and the twentieth centuries (the
other one being quantum mechanics).
It is not the purpose of this section on precursors to give a detailed discussion
of the intelligent struggles by all those men named above. Instead I shall mainly
concentrate on Lorentz and Poincare, the precursors of the new kinematics. A
final comparison of the contributions of Einstein, Lorentz, and Poincare will be
deferred until Chapter 8. Nor shall I discuss Lorentz's finest contribution, his
atomistic interpretation of the Maxwell equations in terms of charges and currents
carried by fundamental particles (which he called charged particles in 1892, ions
in 1895, and, finally, electrons in 1899), even though this work represents such a
major advance in the development of electrodynamics. Rather, I shall confine
myself largely to the evolution and the interpretation of the Lorentz
transformation:
which relates one set of space-time coordinate systems (x',y',2?,t') to another,
(x,y,z,t), moving with constant velocity v relative to the first. (For the purpose of
this section, it suffices to consider only relative motion in the x direction.)
The main characters who will make their appearance in what follows are:
Voigt, the first to write down Lorentz transformations; FitzGerald, the first to
propose the contraction hypothesis; Lorentz himself; Larmor, the first to relate the
contraction hypothesis to Lorentz transformations; and Poincare. It should also be
mentioned that before 1900 others had begun to sense that the aether as a material