THE LIGHT-QUANTUM 385when the evidence is incontrovertible. If they do not, physics tends to pass them
by.
I often argued with Einstein about reliance on experimental evidence for con-
firmation of fundamental new ideas. In Chapter 25, I shall have more to say on
that issue. Meanwhile, I shall discuss next the influence of experimental devel-
opments on the acceptance of the ideas of Planck, Bohr, and Einstein.
First, Planck. His proximity to the first-rate experiments on blackbody radia-
tion being performed at the Physikalisch Technische Reichsanstalt in Berlin was
beyond doubt a crucial factor in his discovery of 1900 (though it would be very
wrong to say that this was the only decisive factor). In the first instance, experi-
ment also set the pace for the acceptance of the Planck formula. One could (and
did and should) doubt his derivation, as, among others, Einstein did in 1905. At
the same time, however, neither Einstein nor any one else denied the fact that
Planck's highly nontrivial universal curve admirably fitted the data. Somehow he
had to be doing something right.
Bohr's paper [B2] of April 1913 about the hydrogen atom was revolutionary
and certainly not at once generally accepted. But there was no denying that his
expression 2ir^2 e^4 m/h}c for the Rydberg constant of hydrogen was remarkably
accurate (to within 6 per cent, in 1913). When, in October 1913, Bohr was able
to give for the ratio of the Rydberg constants for singly ionized helium and hydro-
gen an elementary derivation that was in agreement with experiment to five sig-
nificant figures [B3], it became even more clear that Bohr's ideas had a great deal
to do with the real world. When told of the helium/hydrogen ratio, Einstein is
reported to have said of Bohr's work, 'Then it is one of the greatest discoveries'
[H6].
Einstein himself had little to show by comparison.
To be sure, he had mentioned a number of experimental consequences of his
hypothesis in his 1905 paper. But he had no curves to fit, no precise numbers to
show. He had noted that in the photoelectric effect the electron energy E is con-
stant for fixed light frequency v. This explained Lenard's results. But Lenard's
measurements were not so precise as to prevent men like J. J. Thomson and Som-
merfeld from giving alternative theories of the photoeffect of a kind in which Len-
ard's law does not rigorously apply [S4]. Einstein's photoelectric equation, E =
hv — P, predicts a linear relation between E and v. At the time Einstein proposed
his heuristic principle, no one knew how E depended on v beyond the fact that
one increases with the other. Unlike Bohr and Planck, Einstein had to wait a
decade before he saw one of his predictions, the linear E-v relation, vindicated,
as was discussed in the previous section. One immediate and salutary effect of
these experimental discoveries was that alternative theories of the photoeffect van-
ished from the scene.
Yet Einstein's apartness did not end even then.
I have already mentioned that Millikan relished his result on the photoeffect
but declared that, even so, the light quantum theory 'seems untenable' [M5]. In