368 THE QUANTUM THEORYbeen concluded ... [and which] reproduces our observations [from —188° to
1500°C] within the limits of error' [Rl].
Kirchhoff had moved from Heidelberg to Berlin to take the chair in theoretical
physics. After his death, this position was offered to Boltzmann, who declined.
Then Heinrich Hertz was approached; he also declined. The next candidate was
Planck, to whom the offer of extraordinarius (associate professor) was made.
Planck accepted and was soon promoted to full professor. His new position
brought him close to the experimental developments outlined above. This nearness
was to be one of the decisive factors in the destiny of this most unusual man.
Planck most probably* discovered his law in the early evening of Sunday,
October 7. Rubens and his wife had called on the Plancks on the afternoon of that
day. In the course of the conversation, Rubens mentioned to Planck that he had
found p(v, 7) to be proportional to T for small v. Planck went to work after the
visitors left and found an interpolation between this result and Wien's law, Eq.
19.5. He communicated his formula by postcard to Rubens that same evening and
stated it publicly [P3] in a discussion remark on October 19, following the pre-
sentation of a paper by Kurlbaum. Expressed in notations introduced by Planck
two months later, he proposed thatwhich is indeed correct in the quantum regime hv/kT 3> 1, a condition that is
well satisfied in Paschen's experiment mentioned earlier (hv/kT ~ 15 for T =
1000 K and X = 1 /urn). Strange as it may sound, the quantum theory was dis-
covered only after classical deviations from the quantum regime had been observed
in the far infrared.
It would do grave injustice to Planck if I left the reader with the impression
that Planck's discovery was exclusively the result of interpolating experimental
data. For years, it had been his ambition to derive the correct radiation law from
first principles. Thus the rapidity of his response to Ruben's remark is less sur-
prising than the correctness of his answer. I must refrain from discussing Planck's
earlier research (cf. [K4]), nor shall I describe how he made his guess. However,
it is very important for an understanding of Einstein's starting point in 1905 and
of the subsequent reactions to the light-quantum hypothesis to give a brief account
of Planck's activities from October to December 1900, the heroic period of his life.
*Here I rely on the obituary of Rubens by Gerhard Hettner [HI], himself an experimental expert
on blackbody radiation. Hettner's account differs slightly from the recollections that Planck himself
wrote in his late eighties [P2].(19.6)
(19.7)
Equation 19.6 contains Wien's law of 1896: