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Still Shrouded in Mystery: The Photon in 1925
Richard A. Campos
Department of Physics and Astronomy
Lehman College, the City University of New York
Bronx, New York, 10468-1589, USA
(15 February 2004)
We present a translation of Albert Einstein’s Rio de Janeiro manuscript on
light quanta. In it, Einstein evaluates the Bohr-Kramers-Slater refutation of
light quanta, which wasconcurrently the subject of intense empirical scrutiny
on two continents. Written shortly before Heisenberg’s discovery of
quantum mechanics, the manuscript likely represents Einstein’s last
published remark on the constitution of light in the historical period known
as the old quantum theory. It crystallizes the fact that by 1925 the lightquantum
concept was still insufficient, even as thecorpuscular properties of
light gained decisive empirical confirmation.
1. Introduction
As we approach the centennial of Einstein’s introduction of light quanta [1], it
is well worth reconsidering just how long it takes for some very good ideas to
make their way into the scientific schema. The journey can be tortuous, and in
the case of the photon [2], it is great fun to recollect thelegendary remarks about
Einstein’s “reckless” hypothesis [3], which should not be “held against him” for
having “missed the target” [4]. In the particular case of Einstein’s photons, two
decades would pass before the conspiracy of theory and experiment left little
option but to incorporate them into physical theory. But this is far from the
whole story, because the 1905 light-quantum could notexplain the wave
qualities of radiation well known long before Einstein’s arrival on the scene. The
purpose of this work is to draw further attention to this point by way of a unique
manuscript recently uncovered from the Einstein archives.
We remember the history of quantum mechanics as an encyclopedia with two
great volumes. The first, the “old” quantum theory, begins with Max Planck’sintroduction in December 1900 of the energy quantum for material oscillators
and the fundamental constant now bearing his name [5]; the second, the modern
quantum theory, begins with Werner Heisenberg’s discovery of quantum
mechanics in June 1925 [6]. Both of these great advances resulted from intensely
concentrated intellectual efforts, in Planck’s case an “act of desperation” to
resolve aninconsistency borne out of measured hot-body spectra, and in
Heisenberg’s case an inspiration “from the fog” of remote Helgoland island to
resolve the insufficiency of classical mechanics to describe atomic phenomena.
For both discoveries, the accent rests on the application of quantum principles to
Between these two landmarks, Einstein championed a quantum hypothesis
for light against ascientific community fixated on the wave picture of classical
physics. The turning point for acceptance of the light quantum arrived in the
early 1920’s with Arthur Compton’s analysis of frequency shifts for X-ray
scattering from matter, modeled after a collision between a photon and a nearly
free electron [7]. The additional confirmation of spatial correlations between the
scattered light andthe recoil electrons in 1925 finally put to rest those theoretical
programs, such as that of Bohr, Kramers and Slater, which rejected light quanta
outright. But the view of a photon as an independent particle bearing energy
and momentum could still not account for optical interference phenomena. It is
at this critical junction that Einstein packed his bags for a lecture circuit of SouthAmerica, focused almost entirely on the relativistic theories. There is one
remaining document which shows that Einstein’s longstanding preoccupation
with the photon extended well into the South American tour.
The manuscript of interest, “Remarks on the present situation of the theory of
light,” was penned by Einstein on May 7, 1925 for the express occasion of his
visit to the Brazilian Academy...
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