THE NEW KINEMATICS 159Next we must return to Kaufmann. Stimulated by the new theoretical devel-
opments, he refined his experiments and in 1906 announced new results: The
measurements are incompatible with the Lorentz-Einstein postulate. The Abra-
ham equation and the Bucherer equation* represent the observations equally well
...'[K5].
These conclusions caused a stir among the theoretical experts. Planck discussed
his own re-analysis of Kaufmann's data at a physics meeting in 1906 [PI6]. He
could find no flaw, but took a wait-and-see attitude. So did Poincare in 1908
[PI7]. Lorentz vacillated: The experiments 'are decidedly unfavorable to the idea
of a contraction, such as I attempted to work out. Yet though it seems very likely
that we shall have to relinquish it altogether, it is, I think, worthwhile looking
into it more closely ...' [L12]. Einstein was unmoved: 'Herr Kaufmann has
determined the relation between [electric and magnetic deflection] of /3-rays with
admirable care. ... Using an independent method, Herr Planck obtained results
which fully agree with [the computations of] Kaufmann. ... It is further to be
noted that the theories of Abraham and Bucherer yield curves which fit the
observed curve considerably better than the curve obtained from relativity theory.
However, in my opinion, these theories should be ascribed a rather small proba-
bility because their basic postulates concerning the mass of the moving electron
are not made plausible by theoretical systems which encompass wider complexes
of phenomena' [E3]. Soon after this was written, experimental confirmation for
E = myc^2 was obtained by Bucherer [B7]. Minkowski was delighted. To intro-
duce a rigid electron into the Maxwell theory, he said, is like going to a concert
with cotton in one's ears [M8]. The issue remained controversial, however. Wien,
in his letter to the Nobel committee, commented early in 1912, 'Concerning the
new experiments on cathode and 0-rays, I would not consider them to have deci-
sive power of proof. The experiments are very subtle, and one cannot be sure
whether all sources of error have been excluded.' The final experimental verdict
in favor of relativity came in the years 1914-16.**
Special relativity killed the classical dream of using the ener-
gy-momentum-velocity relations of a particle as a means of probing the dynamic
origins of its mass. The relations are purely kinematic. The classical picture of a
particle as a finite little sphere is also gone for good. Quantum field theory has
taught us that particles nevertheless have structure, arising from quantum fluc-
tuations. Recently, unified field theories have taught us that the mass of the elec-
tron is certainly not purely electromagnetic in nature.
But we still do not know what causes the electron to weigh.*Alfred Bucherer [B5] and Langevin [LI 1] had independently invented an extended electron model
with FitzGerald-Lorentz contraction but with constant volume. This model was analyzed further
by Poincare [PI5] and by Ehrenfest [E37]. In 1908 Bucherer informed Einstein that his, Bucherer's,
experiments had led him to abandon his own model in favor of the relativity prediction [B6].
"See [PI] or [P2], Section 29, for detailed references to the experimental literature up to 1918.