D O C . 4 8 4 O N T H E T H E O R Y O F L I G H T 4 8 1
of the atomic shell. The energy needed for this is extracted from the quantum and
expresses itself, according to the principles of quantum theory, as a frequency re-
duction of the scattered radiation as compared to the incident X-radiation. This
phenomenon, securely demonstrated qualitatively and quantitatively by experi-
ment, is called the “Compton effect.”
In order to understand this experiment according to the theory of Bohr, Kramers,
and Slater, one must interpret the scattering of radiation as a continuous process, in
which all the atoms of the scattering substance participate, whereas the electron
emissions have the character of individual events following only statistical laws.
According to the theory of light quanta, the scattering of light must also have the
character of an event, and every time that the scattered radiation generates a sec-
ondary effect on the struck matter, an electron ejected in a áentirelyñ definite direc-
tion must be present. According to the theory of light quanta, therefore, there is a
statistical dependence between the scattered radiation in Compton’s meaning and
the electron emission, a statistical dependence that would have to be missing ac-
cording to the theoretical interpretation by the above-named authors.
In order to check how it works in reality, one must have an instrument to detect
a single elementary process of absorption or a single emitted electron. This appa-
ratus is available as the electrically charged needle that captures a single electron
and produces a measurable instantaneous discharge by the formation of secondary
ions. With two such suitably arranged needles, Geiger and Bothe manage to verify
the important question of the statistical dependence or independence of the men-
tioned secondary processes.
At the time of my departure from Europe, these experiments were not yet con-
cluded. Based on the findings up to now, however, statistical dependence does seem
to exist. Should this be confirmed, it would provide a new, important argument for
the reality of light quanta.
A. Einstein 7 May 1925.