1 7 8 D O C U M E N T 9 9 N O V E M B E R 1 9 2 5 [1]Dated from the reference to Max Born’s departure for the United States, on 26 October 1926 (see Greenspan 2005, p. 128). [2]The paper is Jordan 1924, which Einstein had criticized in Einstein 1925o (Vol. 14, Doc. 425). [3]In the final section (chap. 4, sec. 3) of Born, Heisenberg, and Jordan 1926 (known as the “Dreimännerarbeit”) a formula is derived for the mean square energy fluctuations in a narrow fre- quency range in a small segment of a continuous string fixed at both ends. As suggested in the present document, and emphasized, for instance, in a letter from Jordan to Bartel L. van der Waerden of 10 April 1962 (AHQP M/f 1419-006, p. 604 see also Lehner 2011, p. 275) this work was due to Jor- dan. The continuous string can be replaced by an infinite set of uncoupled harmonic oscillators, one for each mode of the string. As the simplest application of matrix mechanics, the harmonic oscillator was first studied in Heisenberg 1925 and later in more detail in Born and Jordan 1925b and Born, Heisenberg, and Jordan 1926. The result of Jordan’s calculation is the sum of two terms: a particle term and a wave term, as is also indicated in the present document under point b). The string served as a model (first introduced in Ehrenfest 1925a) for a container with perfectly reflecting walls filled with radiation. The purpose of the calculation was to show that, at least for this model, the application of matrix mechanics repro- duces both the wave and the particle term in the formula for mean square energy fluctuations in a nar- row frequency range in a small subvolume of a cavity filled with black-body radiation first derived in Einstein 1909b (Vol. 2, Doc. 56). See also Duncan and Janssen 2008 for an analysis and a critical discussion of Jordan’s derivation. [4]Published as Born, Heisenberg, and Jordan 1926. [5]See Born, Heisenberg, and Jordan 1926, p. 607, for the criticism of Debye 1910. [6]In sec. 5 of Planck 1925b, which appeared 4 September, a combinatorial derivation of a formula of the form of Einstein’s 1909 fluctuation result (see note 3 above) is given. This section was repeated verbatim as sec. 5 of Planck 1925c, received 30 October. The derivation was first presented in Planck 1923. In the introduction to this paper Planck cited Laue 1915 and noted that its author had general- ized Einstein’s formula “through a purely statistical consideration, which completely avoids the use of the notion of temperature” (“durch eine rein statistische, die Benutzung des Temperaturbegriffs ganz vermeidende Betrachtung”). Both Laue and Planck thus recognized that Einstein’s formula was a special case of a more general result. Jordan derived the quantum analogue of this more general result: the temperature does not enter in his derivation either. [7]The second term in the equation for the mean square energy fluctuation is the wave term the first one is the particle term. 99. To Mileva Einstein-Mariü [Berlin,] 1. XI. [1925][1] Liebe Mileva! Ich habe schon einen dringenden Brief an Anschütz[2] geschrieben. Hoffentlich hilft es was. Es bleiben Gonzenbach, Zangger und Zürcher.[3] Gonzenbach hast ja Du schon gesprochen, sodass ich es für besser halte, ich schreibe ihm erst dann, wenn Deine Bemühung bei ihm nicht ganz den gewünschten Erfolg bei ihm gehabt hätte. Man schiebt nicht gleich zu Anfang alle Briquets in den Ofen. Zangger will ich schreiben, glaube aber nicht, dass er grossen Einfluss auf Albert[4] haben kann, weil er so fahrig ist. Zürcher kann mehr ausrichten. Dem schreibe ich nicht gleich, sondern halte ihn in der Reserve. Wenn nämlich zu viel auf einmal erfolgt, so sieht es zu abgekartet aus und wirkt nicht aus diesem Grunde. Δ2