140 RELATIVITY, THE SPECIAL THEORYand the relativity principle of classical mechanics. This principle, already long
known by then, states that all mechanical laws should be the same in any two
coordinate systems (x,y,z,t) and (x',y',z',t') related by*
x' = x - vt y' = y z' = z t' = t (7.1)
Since 1909 these transformations have been called Galilean transformations.**
(Recall that in 1905 there existed as yet no evidence against the general validity
of Galilean invariance in pure mechanics.) The conflict arises if one attempts to
elevate Galilean invariance to a universal principle. An aether at absolute rest
hardly fits this scheme of things. Some physicists believed therefore that the very
foundations of electrodynamics should be revised.f Einstein opted for the alter-
native: 'The phenomena of electrodynamics and mechanics possess no properties
corresponding to the idea of absolute rest' [El]. In the June paper, he gave two
concrete reasons for this view: first, the absence of experimental evidence for an
aether drift and second, the existence of 'asymmetries which do not appear to be
inherent in the phenomena.' As an example of such an asymmetry, he considered
a system consisting of a magnet and a conductor. If the magnet moves in the pres-
ence of a resting conductor, then an electric field is generated which induces a
current in the conductor. If, on the other hand, the conductor moves in the pres-
ence of the resting magnet, then an electromotive force (proportional to ~u X H)
is generated, which again causes a current. Transcribed rather freely, one might
say that Einstein cared for neither the logical disconnectedness of electricity and
magnetism nor the asymmetry between the two coordinate systems just described.
I argued in Chapter 6 that Einstein rejected the nineteenth century explanations
of the first-order aether drift effects as unconvincing and artificial and that the
second-order Michelson-Morley paradox was to some extent secondary to him.
Add to this his remark that 'Maxwell's electrodynamics—as usually understood
at the present time—when applied to moving bodies, leads to asymmetries which
are not inherent in the phenomena' and one has the motivation for the June paper:
Einstein was driven to the special theory of relativity mostly by aesthetic argu-
ments, that is, arguments of simplicity. This same magnificent obsession would
stay with him for the rest of his life. It was to lead him to his greatest achievement,
general relativity, and to his noble failure, unified field theory.
- The Two Postulates. The new theory is based in its entirety on two pos-
tulatesf [El]:
*As in the previous chapter, I shall, for simplicity, consider relative motions in the x direction only.
**This term was introduced by Philipp Frank [Fl].
fFor details, see Section 3 of Pauli's encyclopedia article, in German [PI], or in its English trans-
lation [P2].
^1 do not copy Einstein verbatim. The term inertial frame gained currency only some time later, as
did the terms Galilean invariance and Lorentz invariance, which I freely use from now on.