4 8 6 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
Published in Königlich Preußische Akademie der Wissenschaften (Berlin). Sitzungsberichte (1921):
882–883. Submitted 8 December 1921, published 5 January 1922. A two-page signed manuscript
(GyB Autogr. I/1360) with printer’s marks is preserved. The first page is written on the verso of a
typed letter from the law office of Meinhardt, Danziger, and Nünninghoff to Einstein, 2 December
1921; the second page is on the verso of p. 3 of a typewritten proposal for financial help to German
pacifist organizations. One variation from the printed version is noted.
[1]Since early in 1921, Einstein had been thinking about possible experiments to decide between a
wave theory and a quantum theory of light. In several letters from January 1921 he described an exper-
iment based on the Stark effect on atoms in a radiation field (e.g., Einstein to Arnold Sommerfeld,
4 January 1921, GyMDM, HS 1977-28/A,78). In Einstein to Max Born, 22 August 1921, GyB
Nachlass Born, no. 188. p. 18, he mentioned that “I have thought up a very interesting and rather sim-
ple experiment about the nature of light emission” (“Ich habe mir ein sehr interessantes und ziemlich
simples Experiment über die Natur der Lichtemission ausgedacht”). He described the experiment dis-
cussed in the present document in Einstein to Arnold Sommerfeld, 27 September 1921, GyMDM, HS
1977-28/A,78, and remarked that he was performing it with Hans W. Geiger (1882–1945), Director
of the Radium Research Laboratory at the Physikalisch-Technische Reichsanstalt. In Einstein to
Arnold Sommerfeld, 9 October 1921, GyMDM, HS 1977-28/A,78, Einstein explained the ideas
behind the experiment: “I absolutely do not know what to expect. I am convinced that some kind of
spherical wave is emitted besides the directional energetic process, because of the possibility of inter-
ference for large aperture angles. But since the period of emission is small compared to the period of
coherence for interference with path differences I am not convinced that, whatever is emitted, has
immediate oscillatory character. The mechanism could be more indirect and completely different
from what we usually presuppose, since the conflict between quantum and wave theory persists after
all with unabated vehemence” (“Meine Erwartung ist total unsicher. Dass neben dem gerichteten
energetischen Prozess eine Art Kugelwelle abgeht, das bin ich überzeugt wegen der Interferenz-
Fähigkeit für grosse Öffnungswinkel. Aber da die Emissionszeit klein ist gegen die Kohärenzzeit für
Interferenz mit Gangunterschieden, bin ich nicht überzeugt, dass dasjenige, was ausgesendet wird,
unmittelbar oszillatorischen Charakter hat. Der Mechanismus könnte indirekter und ein total anderer
sein, als wir vorauszusetzen gewohnt sind, zumal eben der Widerstreit zwischen Quanten- und Undu-
lationstheorie mit unverminderter Heftigkeit fortbesteht”).
A letter by Lorentz (Hendrik A. Lorentz to Einstein, 13 November 1921) clarifies this statement.
It shows that, at the time, Einstein was weighing a dual theory of light that incorporated both a light
quantum with particle nature (which he called “energy radiation”) and an unobservable “guiding”
wave (“interference radiation”) that determines the possible trajectories of the quantum. In this letter
Lorentz summarized explanations that Einstein had given to him and Ehrenfest during Einstein’s stay
in Leyden a few days earlier. After introducing the two types of radiation, Lorentz continues: “Al-
ready in a single process of emission the full interference radiation is produced, but only a single
quantum is emitted, which hence can only reach a single spot on the receiving screen. But the elemen-
tary process is repeated innumerably many times with almost identical interference radiation (the
same pattern). The various quanta now are distributed statistically across the pattern, in the sense that
their average number at each point of the screen is proportional to the intensity of the interference
radiation arriving at that point. In this way the observed interference phenomenon corresponding to
the classical theory is produced” (“Bei einem einzelnen Strahlungsakt entsteht schon die volle Inter-
ferenzstrahlung aber es wird nur ein einzelnes Quantum ausgestrahlt, das also auch nur an eine Stelle
eines auffallenden Schirmes gelangen kann. Es wiederholt sich aber der Elementarakt unzählig viele
Male, mit so gut wie gleicher Interferenzstrahlung (das gleiche Muster). Die verschiedenen Quanten
verteilen sich nun statistisch über das Muster, in dem Sinne, dass die mittlere Anzahl derselben in je-
dem Punkte des Schirms proportional der Intensität der daselbst ankommenden Interferenzstrahlung
ist. In dieser Weise entsteht die beobachtete, der klassischen Theorie entsprechende Interferenzer-
scheinung”). Lorentz explored Einstein’s ideas further in Lorentz 1927, pp. 156–165. The letter and
Einstein’s speculations are discussed in Stachel 1986.
[2]The manuscript has “das Licht” instead of “ein Licht.”
[3]“ ” in eq. (3) should be “ .” The mistake is noted in Walther Bothe to Einstein, 7 Decem-
ber 1921.
dn dn n ⁄