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MAX LUDWIG HENNING DELBRÃCK 76 THE BERLIN YEARS (1932-37) Max's appointment as assistant to Lise Meitner, who was collaborating with Otto Hahn on the results of irradiating uranium with neutrons, was, in effect, to be a consultant on theoretical physics. During this period he did write a few papers, one of which turned out to be an important contribution on the scattering of gamma rays by a Coulomb field due to polarization of the vacuum produced by that field (1933). His conclusion proved to be theoretically sound but inapplicable to the case in point, but 20 years later Hans Bethe confirmed the phenomenon and named it "Delbrück scattering." A second seminal paper with Gert Molière, which Max referred to retrospectively as "very learned" (1), attempted to apply quantum mechanics to resolve the paradox of irreversibility in statistical mechanics (1936c). Not long after the beginning of Max's Berlin period, which coincided with Hitler's rise to power, he organized a private group of five or six theoretical physicists to join in fairly regular discussions among themselves, often at his mother's house. At his suggestion some biochemists and biologists also joined the group. Among these were K. G. Zimmer whose interest was the dose effect of ionizing radiation on biological systems, and, most significantly for Max's future, N. W. Timoféeff-Ressovsky, a Russian geneticist from the Kaiser Wilhelm Institute for Brain Research who had been collaborating with Zimmer on the genetic effects of radiation for some 2 years before contact with Max was established. Timofeeff-Ressovsky's experimental organism was Drosophila, the fruit fly, which was then, and still is, very popular with geneticists because of its short generation time and the large populations that can be raised in the laboratory.
MAX LUDWIG HENNING DELBRÃCK 77 Zimmer records that he remembers vividly the discussion that followed: ''Two or three times a week we met, mostly in Timoféeff-Ressovsky's home in Berlin, where we talked for ten hours or more without a break, taking some food during the session. There is no way of judging who learned most by this exchange of ideas, knowledge and experience, but it is a fact that after some months Delbrück was so deeply interested in quantitative biology, and particularly in genetics, that he stayed in this field permanently" (2, p. 33). The upshot of all these discussions was a paper by Timoféeff-Ressovsky, Zimmer, and Delbrück (1935b) on the nature of gene mutation and gene structure, in which Max was mainly responsible for the theoretical interpretation. He supposed that the molecules from which genes are made must have a very unusual atomic constitution, since they show such remarkable stability in a cellular environment otherwise subject to constant chemical change. This stability suggested that each atom of the gene molecule is fixed in its mean position and electronic state by being sunk in "energy wells," so that discontinuous changes in their state, expressed as mutations, could arise only by the acquisition of very high energies such as ionizing radiations would impose (18, p. 26). It is difficult to say how much interest this paper aroused at the time. Max reported that it got "a funeral first class" (1) since it was published in a little- known Göttingen journal, but Timoféeff-Ressovsky must have sent reprints to many geneticists although it is unlikely that they would have known enough physics to understand it. It was not until ten years later that the paper became famous through the publication in 1945 of Erwin Schrödinger's little book, What Is Life?, in which he maintained that Delbrück's model of the gene was the only possible one, and went on to put
MAX LUDWIG HENNING DELBRÃCK 78 forward the romantic and paradoxical idea, first proposed by Bohr, that "from Delbrück's picture of the hereditary substance it emerges that living matter, while not eluding the 'laws of physics' as established up to date, is likely to involve hitherto unknown 'other laws of physics' which, however, once they have been revealed, will form just as integral a part of this science as the former" (16). Max, of course, was already long embarked on his quest for this Holy Grail, but Schrödinger's book was influential in attracting into biology many physicists, curious to solve the paradox (see Stent, 2, p. 3). Meanwhile, when Max was spending all this time immersed in biophysics, Hahn and Meitner's work on the irradiation of uranium with neutrons was revealing the emission of many characterizable transuranium products that were interpreted as elements, but their number then became so large that they were assumed to be isomers of transuraniums, and Max went along with this. As he admitted (1), ". . . this was really immensely stupid of me; I should have guessed what was really going on, namely fission, but I, like everybody else, lacked imagination to see that . . . it was something any experimental physicist could easily have figured out . . . all you needed to know was that there was excess energy there; the neutron enters and there is enough energy there to blow the nucleus to pieces. You needed to just be able to add and subtract . . . and it didn't occur to anybody until they were literally forced to this conclusion only the year after I left." Max's decision to visit the U.S.A. was prompted by three circumstances. One was his now dominant interest in quantitative biology and especially in Drosophila genetics, which he wished to experience first hand. Then, a few years after becoming Lise Meitner's assistant, he had considered a future as a lecturer at the university, but, apart from aca
