382 THE QUANTUM THEORYexperiments and a summary of his beautiful results: Eq. 19.24 holds very well
and 'Planck's h has been photoelectrically determined with a precision of about
0.5% and is found to have the value h = 6.57 X 10~^27 .'
The Volta effect also confirmed the heuristic principle. This evidence came
from X-ray experiments performed in 1915 at Harvard by William Duane and
his assistant Franklin Hunt [Dl]. (Duane was one of the first biophysicists in
America. His interest in X-rays was due largely to the role they play in cancer
therapy.) Working with an X-ray tube operated at a constant potential V, they
found that the X-ray frequencies produced have a sharp upper limit v given by
eV = hv, as had been predicted by Einstein in 1906. This limiting frequency is
now called the Duane-Hunt limit. They also obtained the respectable value h
= 6.39 X 10~^27.
In Section 18a, I mentioned some of Millikan's reactions to these developments.
Duane and Hunt did not quote Einstein at all in their paper. I turn next to a
more systematic review of the responses to the light-quantum idea.19f. Reactions to the Light-Quantum Hypothesis
Comments by Planck, Nernst, Rubens, and Warburg written in 1913 when they
proposed Einstein for membership in the Prussian Academy will set the right tone
for what follows next. Their recommendation, which expressed the highest praise
for his achievements, concludes as follows. 'In sum, one can say that there is hardly
one among the great problems in which modern physics is so rich to which Ein-
stein has not made a icmarkable contribution. That he may sometimes have
missed the target in his speculations, as, for example, in his hypothesis of light-
quanta, cannot really be held too much against him, for it is not possible to intro-
duce really new ideas even in the most exact sciences without sometimes taking a
risk' [K5].
/. Einstein's Caution. Einstein's letters provide a rich source of his insights
into physics and people. His struggles with the quantum theory in general and
with the light-quantum hypothesis in particular are a recurring theme. In 1951
he wrote to Besso, 'Die ganzen 50 Jahre bewusster Grubelei haben mich der
Antwort der Frage "Was sind Lichtquanten" nicht naher gebracht' [E13].*
Throughout his scientific career, quantum physics remained a crisis phenome-
non to Einstein. His views on the nature of the crisis would change, but the crisis
would not go away. This led him to approach quantum problems with great cau-
tion in his writings—a caution already evident in the way the title of his March
paper was phrased. In the earliest years following his light-quantum proposal,
Einstein had good reasons to regard it as provisional. He could formulate it clearly
only in the domain hv/kT^>\, where Wien's blackbody radiation law holds. Also,
*A11 these fifty years of pondering have not brought me any closer to answering the question, What
are light quanta?