D O C . 6 8 E X P E R I M E N T O N L I G H T E M I S S I O N 4 8 7
Stark 1905. From a passage in the letter by Lorentz cited in note 1, it can be reconstructed how,
in Einstein’s view, the assumption that the frequency of the emitted radiation is independent of the
direction could be reconciled with the observed Doppler effect in canal rays. Apparently, Einstein
thought it possible for the interference radiation to show no Doppler effect, since it is determined by
Bohr’s quantum condition, while at the same time the observed energy radiation does show the effect.
Lorentz criticized this assumption by pointing out that it is the interference radiation that prepares the
way of the energy radiation. If the interference radiation would not show the Doppler effect, then it
would not be understandable why the Doppler effect can be seen in a spectrometric observation such
as Stark’s. If light emitted by positive rays passes through a prism, the Doppler effect causes the angle
of refraction—and thus the light path—to change with the changing angle of emission. But since it is
the interference radiation that determines the path of the energy radiation in Einstein’s model, the
interference radiation has to exhibit a Doppler effect as well. Lorentz concluded that the deflection
predicted by the wave theory will be present in both cases considered by Einstein. Lorentz continued
his letter with an argument for the consistency of the wave and particle descriptions of optical phe-
nomena. He also referred to Emden 1921 as containing similar ideas.
The experiment was performed by Hans Geiger and Walther W. Bothe (1891–1957), Regie-
rungsrat at the Physikalisch-Technische Reichsanstalt. It is discussed in Hans Geiger to Einstein, 7
November 1921, and Walther Bothe to Einstein, 7 December 1921. Einstein presented the result that
no deflection was observed together with Geiger and Bothe at the Prussian Academy of Sciences on
19 January 1922 (a title and summary have been published in Königlich Preußische Akademie der
Wissenschaften [Berlin]. Sitzungsberichte , p. 2; parts of the manuscript are preserved [2 086]).
Einstein’s experiment was criticized by Paul Ehrenfest (Paul Ehrenfest to Einstein, 19 January and
26 January 1922) and Max von Laue (Einstein to Paul Ehrenfest, 21 January 1922) on theoretical
grounds: both argued that even according to a classical wave theory there should be no deflection in
the path of the emitted ray. After another attempt at deriving his postulated result (see manuscript for
presentation at the Prussian Academy on 19 January 1922 [2 086]), Einstein had to admit that he had
been mistaken (Einstein to Paul Ehrenfest, 30 January 1922). He presented a derivation of the correct
result at the Academy on 2 February 1922 (Einstein 1922f). The experiment and the ensuing discus-
sion are treated in Klein, M. 1970b, pp. 8–13.