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MAX LUDWIG HENNING DELBRÃCK 73 N.T.)* describes him as a cheerful, outgoing person and one who rapidly established a reputation as a theoretician who was always ready to discuss problems of any kind with experimentalists who needed help and advice. Herzberg also recalled that he "fitted in very well with the group of younger physicists there because of his (then) gregarious ways and the ease with which he made friends." Max published two papers from Bristol in English (1930b; 1932), on topics related to the quantum mechanical theory of homopolar bonding on which he had written his thesis. Copenhagen and ZÃ¼rich Following his Bristol experience, Max obtained a Rockefeller Fellowship (Physics) to study with Niels Bohr in Copenhagen where he spent the spring and summer of 1931, and then spent the last 6 months with another quantum physicist, Wolfgang Pauli, in ZÃ¼rich. In Copenhagen he roomed, and collaborated on a nuclear physics project, with George Gamow (1931) with whom he established a lasting friendship. Also working with Bohr at that time was Victor Weisskopf, a very close friend since their student days together at GÃ¶ttingen; they arrived in the United States almost simultaneously in 1937 (see below) and remained in personal contact until Max's death. Anyone who might infer from all this that life in Copenhagen was a staid and serious business should read Max's lighthearted and facetious account of the gaiety and practical jokes of those days, in his contribution to a George Gamow Memorial Volume (1972c). Weisskopf (pers. comm.) has commented on his wonderful sense of humor: "There was a custom in Copenhagen, at each of the early conferences * The initials in the text that indicate the source of quotations are explained in the acknowlegements at the end of the memoir.
MAX LUDWIG HENNING DELBRÃCK 74 organized by Niels Bohr, to have what we called a session of 'comic physics.' It was always Max who was the most spirited leader in these activities with his humour and intellectual fantasy. You must have heard of his rewriting of Goethe's Faust to make fun of the physics of that time." Max's short visit to Copenhagen became of greater importance to him than he could have imagined, for it marked the turning point in his life that changed not only his career but his philosophical outlook as well. The determining influence was Bohr's formulation of the complementarity concept as a generalized extension of Heisenberg's uncertainty principle. Thus, the propagation of light may be unambiguously defined, in a probabilistic way, either as a continuous motion of electromagnetic waves or as the exchange of individual quanta of energy related to the wavelength of the former by Planck's constant, but not by both at the same time; the two expressions of reality stand in a mutually exclusive but complementary relation to one another. According to Max, Bohr then very vigorously asked the question whether this new dialectic wouldn't be important also in other aspects of science. He talked about that a lot, especially in relation to biology, in discussing the relation between life on the one hand, and physics and chemistry on the otherâwhether there wasn't an experimental mutual exclusion, so that you could look at a living organism either as a living organism or as a jumble of molecules; . . . you could make observations that tell you where the molecules are, or you could make observations that tell you how the animal behaves, but there might well exist a mutually exclusive feature, analogous to the one found in atomic physics . . . in many respects Bohr wasn't sufficiently familiar with the status of the science (biology). So it was intriguing and annoying at the same time. It was sufficiently intriguing for me, though, to decide to look more deeply, specifically into the relation of atomic physics and biologyâand that means learn some biology"(1).
MAX LUDWIG HENNING DELBRÃCK 75 Much has been written of Bohr's profound influence on Max. Thus, Gunther Stent writes, "I think it is fair to say that with Max, Bohr found his most influential philosophical disciple outside the domain of physics, in that through Max, Bohr provided one of the intellectual fountainheads for the development of 20th century biology" (3). Again, Horace Judson said of Max, "His mind and style had been formed by Niels Bohr, the physicist, philosopher, poet and incessant Socratic questioner who made Copenhagen one of the capital cities of science between the wars" (4, p. 50). But Max himself saw more than this in the so-called Copenhagen Spirit, as shown by his reply to a question about the Phage Group: "Well, the phage group wasn't much of a group. I mean it was a group only in the sense that we all communicated with each other. And that the spirit wasâopen. This was copied straight from Copenhagen, and the circle around Bohr, so far as I was concerned. In that the first principle had to be openness. That you tell each other what you are doing and thinking. And that you don't care whoâhas the priority" (4, p. 61). It followed that, after a further 6 months with Lennard Jones at Bristol, Max decided to accept an appointment as assistant to Lise Meitner at the Kaiser Wilhelm Institute for Chemistry in Berlin in the autumn of 1932, because of its proximity to the Kaiser Wilhelm Institute for Biology. But before returning to Berlin he paid a short visit to Copenhagen to hear Bohr deliver his famous address, "Light and Life," to the opening meeting of the International Congress on Light Therapy in August, in which he explicitly stated his views on complementarity in biology (9). Odd though these views may seem to us now, in retrospect, this lecture confirmed Max's decision to turn to biology.