Otto Stern

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Interviewed by
Thomas S. Kuhn
Interview dates
May 29 and 30, 1962
Stern's home, Berkeley, California
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In footnotes or endnotes please cite AIP interviews like this:

Interview of Otto Stern by Thomas S. Kuhn on 1962 May 29 and 30, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA,

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This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Paul Ehrenfest, Albert Einstein, James Franck, Rudolf Walther Ladenburg, Otto Lummer, Walther Nernst, Wolfgang Pauli, Max Planck, Ernest Pringsheim, Sackur, Erwin Schrodinger, Tetrode, John Joseph Thomson; and Universitat Breslau.


Stern was totally unwilling to have a recorder used. In part, he is somewhat skeptical about the project. Perhaps more important, there is now a tape of his on deposit in Zurich, and he rather regrets having made it. From remarks of his relayed by Minkowski, I have the impression that that tape contains a number of caustic remarks about some of his colleagues. Apparently, he does not wish to be burned again.

In addition, when describing his reservations about the project, Stern made some remarks which are revealing of his attitude towards Pauli. He thinks it would be wonderful to have a “true” account of the development of the ideas of quantum physics. He also feels, however, that such a record would have to be produced by someone who understands the ideas, who was present during their development, and who has a wonderful memory. The man, and apparently the only one, who he believes combined these qualifications was Pauli. Stern had previously tried to persuade Pauli to produce such a record, but Pauli brushed the suggestion aside, indicating only that he might do it when he was older. Stern now does not really believe it can be done. In any case, he doubts that compiling mountains of data is any way to go about it.

Since these reservations to the recorder could not be broken through, the following remarks are dictated by T. S. Kuhn from notes that he took during the course of a two hour conversation. Ironically, Stern was quite willing to talk, and he talked well, also, he had no objections to notes being taken. What follows is a reconstruction made immediately after the close of the conversation and slightly amended in the light of subsequent criticisms from Stern. All remarks in quotations are a close approximation to a verbatim transcription. I have, however, made little attempt to preserve the original order of the material, since the discussion wandered a good deal.

I. Education: Stern’s formal scientific education began at the University of Freiburg i. B. During the summer of 1906, his interest in science having been established early at school and particularly at home. However, very little was learned at that time, most of the term being given to excursions and to sampling the local wines. After this Stern went to Breslau and stayed there until he took his degree in 1912, with the exception of one term, around his fifth, which was spent at Munich. The Munich term, again, Stern describes as largely devoted to excursions and beer. He did, however, try to listen to Sommerfeld’s lectures but found them entirely over his head. In addition, hoping to learn some mathematics, a subject in which he felt and still feels very weak, Stern went to Lindemann’s lectures on integral calculus. He says that, they were simply awful. Lindemann always made mistakes in partial integration; then stroked his beard and said that he would get the matter right next time. Stern indicates that he found Kneser at Breslau a good teacher of mathematics, but that, nevertheless, most of the mathematics he knows was learned from books. At Segre’s suggestion, Stern confirmed that Rieman-Weber would have been one of these, though the indication was that he had never studied it with any great care. Determinants are the one part of mathematics that Stern feels he ever learned thoroughly, and these he taught to himself since he needed them for one of the papers that he wrote in l9l6.

Stern thinks that his education in physics at Breslau was “unfortunate.” To a very great extent he blames this on the faculty, for he was much interested in physics and was free to take as many lectures in the subject as he chose. The principal professor of physics was Otto Lummer for whom Stern obviously has very little respect, though recognizing his experimental genius. Stern did go to his lectures, and also talked with him before the examination in physics which he faced for his degree. On the second occasion Lummer was much impressed by Stern’s command of the field and assured him he would have no difficulty with the exam. But in the examination itself Lummer asked questions like: “When was the steamboat invented?” and “When was condenser discharge discovered?” For questions like these, Stern was entirely unprepared. Also, Lummer asked a few questions about relativity theory (the implication is that these were questions which displayed little sympathy or understanding for the subject), a topic about which Stern knew very little then or later. Stern remembers that Lummer was deeply convinced of the existence of a physical ether and was much distressed by the incompatibility of that belief with relativity theory. Apparently, he said things like: What is it that bears energy to us from the stars?

In this same connection, Minkowski told a story about his own physics examination, which was also conducted by Lummer. The latter asked him: What happens to energy when it goes from a higher to a lower level. Knowing how Lummer’s mind worked, Minkowski promptly answered “entropy.” Lummer nodded in pleased agreement, while Schrodinger, who was also present, suddenly went white.

Stern seems to have seen very little of the other physicists at Breslau. He has no recollections of Ladenburg from that time; Pringsheim was there, but Stern does not remember going to any of his lectures; of Schaefer, he seems to have cordial recollections, and he may have attended some lectures by him - - on the other hand, he points out that Schaefer had very little concern with problems of modern physics and stayed almost entirely in the classical tradition. (Stern has no recollections of having known Born at Breslau. Apparently Born was there too early in Stern’s academic career.)

Under these circumstances, Stern indicates that, as with mathematics, he learned most of his physics from books. Unfortunately, not many of these were mentioned by name, but the following remarks were revealing. He read Boltzmann a great deal, and knew it thoroughly. (In Breslau, where the physical chemists were “liberal,” this was not, Stern thinks, at all unusual, and it stood him in very good stead when he joined Einstein who was also devoted to Boltzmann.) There was no doubt in his mind or in that of his professors about the existence of atoms. On the other hand, when Stern was a Privatdozent in Zurich, the professor of physical chemistry said to him that he could never have passed any of the physical chemistry exams at Zurich because he was a believer in atoms. In addition, Stern read the original Clausius papers with great admiration. Also, Nernst’s book on thermochemistry was a bible. (Segre describes this as the book which never mentions entropy. [On rereading these notes, Stern says that Nernst’s book was not a bible for him. Nernst, he says, was a genius but also so irresponsible as to be a cheater. One proof in the book, for example, was simply wrong, Stern remembers, but when this was pointed out to Nernst, he replied, “Of course, but it’s much simpler this way.” Similarly, though Nernst was a brilliant designer of experiments, he was also quite uncritical of the results if they fitted his theories. Also, he was quite selective of the data he did report.] Stern made no use at all of Planck’s works on physics. He indicates that he now likes them somewhat better than he used to, but that he found them far too formal. For example, he remembers being terribly upset by a remark that Planck somewhere makes to the effect that the thermodynamic properties of dilute solutions result purely from the properties of mathematical functions (presumably their continuity and the continuity of their derivatives). To Stern it seemed entirely apparent that no physical or chemical phenomenon should ever be accounted for in this way. In the same connection, Stern says that Schottky once told him that he had heard Planck’s lectures on electromagnetic theory and had later been terribly surprised to find out that quantities like electromagnetic force could actually be measured in the laboratory. Apparently, Planck treated them as pure abstractions.

The situation in physical chemistry at Breslau seemed better to Stern than the situation in physics. In particular, Stern thought very highly of Sackur, who seems to have been a primary formative influence. Also, Stern speaks well Abegg, though he does not give evidence of having seen a great deal of him. Asked about the relationship between fields, particularly between physics and physical chemistry, Stern immediately replies by saying that there was academic freedom: a student had to go to a few lecture courses because the man giving them would be his examiner; beyond this, however, he could follow whatever lectures interested him. On the other hand, though unfortunately only when pressed on the point, Stern does indicate that at Breslau most physical chemists probably thought of themselves more as chemists than as physicists. In this respect he thinks he may himself have been somewhat unusual. He is less sure, however that this would have been true in Berlin. Nernst, was a good deal of a physicist, and when Stern was at Nernst’s lab during the war he found people there to be more like the physicists than those at Breslau. Also, he thinks this same identification with physicists would be true of Volmer (I think this must be the same man with whom Stern later worked in Berlin during the war -- if not, the spelling may be wrong) and of his student Tetrode. Incidentally, when talking on this subject Stern gratuitously remarked on how very, very able Tetrode was.

Stern indicated, when asked, that he had heard very little about the quantum during his days at Breslau. He immediately, and somewhat irrelevantly, added that he still rather blamed Schaefer and Ladenburg (note the conflict with the doubts that Stern expressed about having known Ladenburg during the Breslau days) for not having told him anything at all about Rutherford’s work on the deflection of alpha particles and the resulting discovery of the nucleus. (There was appreciable feeling at this point, and it seemed somewhat strange. In the first place, the answer was somewhat off the point. In any case, that discovery came close to the end of Stern’s irk at Breslau. It is not clear that he could reasonably expect to have been told about it. One gets a sense that he may feel he missed an opportunity at just this point.) Pushed further on this subject, Stern indicates that naturally there was very little discussion of radio-activity at all: “It wasn’t so interesting for us. We couldn’t understand it.” Talking on this same subject, Stern indicates that though he knew nothing of the nuclear atom, he was very well acquainted with the Thomson model It was relatively standard at Breslau. Schaefer was working on the spectral frequencies to be expected on this model, and he apparently taught it also.

Apparently, because Lummer and Pringsheim were there, black body radiation was much talked of at Breslau. However, the quantum was treated only in a very primitive form during discussions of the law. The question as to whether it was a fundamental innovation or not was still very much open. Stern emphasizes that Planck himself tried hard to reconcile it with classical theory. Stern says, “One just didn’t know there was great confusion. I learned only when I came to Einstein that it was fundamental.” [On rereading, Stern wants to say that what he learned from Einstein was the irreducible conflict between the quantum and classical physics. This had not previously been clear to him.]

Stern also indicates that, at least at Breslau, the physical chemists were not at all concerned with the photo-electric effect, and thinks that if anyone cared, it must have been the physicists. Also, Stern does not remember that Einstein’s work on specific heats was discussed at Breslau. Apparently, he is not at all certain about this, but imagines, in any case, that if there was any discussion it would have been extremely superficial.

Stern also says that during his Breslau days Nernst’s heat theorem was by no means generally accepted although Sackur was doing work based upon it. This whole question led Stern to a series of remarks that run through his whole life. He points out, for example, that Einstein himself did not believe that the Nernst theorem applied to the case of mixtures, but adds that he was convinced by Stern’s paper in 1916. Somehow word that Einstein had been brought around reached Nernst who made a very considerable point of it at some meeting, saying something like: Einstein has crowned me with the victor’s laurels. Ehrenfest, too, was very much interested in knowing whether the theorem was really confirmed by the experimental evidence. Nernst insisted that it was, but he was known not to be a very responsible or moral reporter of experiments. Since Ehrenfest himself lacked the experimental know-how to evaluate the existing literature or to do experiments on the subject himself, he sent a student, van de Sande Bakhuyzen to Stern at Frankfurt to investigate the subject through a critical survey of the literature. They found that in the case of the experiments which looked most trustworthy, the law did hold up very well. On the other hand, there were a number of cases, which Nernst either doctored or dismissed, in which the evidence was far more dubious. All of this was reported n Bakhuyzen’s thesis, and it annoyed Nernst very greatly. He then told Bakhuyzen that unless all copies of the thesis were destroyed, he, Nernst, would destroy Bakhuyzen’s career as he had destroyed that of Von Laar. Stern adds that it was not Nernst who destroyed Von Laar’s career. He would not have had one anyway. Again, when Stern was a visiting lecturer at Berkeley around 1930, he found himself having to defend the Nernst theorem against the attack of G. N. Lewis and his students. Lewis was one of the great opponents of the theorem. Stern then points out that the Nernst theorem, or, so-called, third law of thermodynamics, is a thread running through his entire life and unifying much of his work. He has just, he says, finished a paper for the Helvetica Acta in which he proposes to base quantum mechanics upon the third law. This whole effort, he says, has been recurrent since his two papers in 1916 (not the one on solid solutions, which I know). He has always felt that there was some much deeper meaning in that paper than appears on the surface and has repeatedly tried to discover what it is. Trying to specify that meaning, Stern suggests that he has hoped by the method there used to discover what the “individual particle” is. Presumably, he means something more like what it is that constitutes the individuality of the particle. Stern refers to this particular paper as the Lomsha paper (that being the place of its composition during the war) and indicates that the name comes from Pauli who always referred to it that way.

Finally, Stern made a couple of other remarks about his student days. Quite early in his university career, at a time when he knew Clausius’ equations relating the change of vapor pressure with temperature to the heat of vaporization, he had an experience that he has remembered ever since. During a walk in a local park he suddenly saw how to derive Clausius’ equation on a molecular basis. At that point, Stern indicates, he did not know that his derivation was old stuff or that it was somewhat too simplistic. However, even when he found that out, he retained some of his feelings about the episode. It got the main point. My suggestion that his own later work often recapitulates this pattern -- i.e., doing duplicate derivations on a pure thermodynamic and a statistical mechanical basis drew very little response.

Stern also talked a bit about his thesis, which he did for Sackur. The problem was one he had chosen for himself, the osmotic pressure of concentrated solutions. Sackur however suggested the experiments which attempted to find the law by measurements on solutions of dissolved carbon dioxide. Stern then worked both on the theory and the experiments simultaneously. At this point, Stern says that he was experimentally very naive. As illustration he points out that he knew some glass blowing and tried to make a movable joint, first blowing it and then grinding with emery. Sackur found him at it and said: “This is the sort of thing one buys, one does not make it.”

II. Zurich and Einstein: Stern joined Einstein in Prague immediately after finishing his thesis during the spring of 1912. He was there for only one semester. Then Einstein was called to Zurich, and the two were together there from August 1912 until the spring of 1914. At that point, Einstein went to Berlin and Stern spent the early summer in Zurich, visiting Einstein again in Berlin in the fall.

Asked why he went to Einstein rather than to Nernst or Haber, Stern says he does not quite know. In 1912, he says, he had not yet met Einstein but knew that he was a great man. This knowledge, he insists, was quite general among those who were really up on what was going on. Stern then talked to Sackur about the possibility of joining Einstein and Sackur concurred. Sackur in turn talked to Haber, who knew Einstein, and Haber then talked to Einstein about it. Einstein was willing so Stern went.

In speaking of Einstein during this period, Stern repeatedly insists that Einstein was the very first man to recognize that the quantum was fundamental. In addition, he points out again and again, that it was only after joining Einstein that he himself came to realize what was going on in contemporary physics. Apparently they talked together a good deal about these problems. In Prague Einstein seems to have had no one at all to talk to until Stern got there. This despite the fact that there were four local institutions at a university level, each with a full professor of physics. All of these men were, however, doing classical problems and had no use for the concerns which were on Einstein’s mind. Even Einstein’s close friend in Prague, the mathematician (Pick), repeatedly laughed at Einstein and Stern for talking about atoms. This Stern finds somewhat understandable, because (Pick) in his younger years had been an assistant to Mach; after that he had become a mathematician; and his physics was frozen at an earlier level. Despite their differences in background and experience, Einstein and Stern apparently found plenty to talk about. Both were profound admirers of Boltzmann’s. In addition, Stern says that Einstein was one of the very, very few contemporary theoretical physicists who really knew thoroughly and valued thermodynamics. Furthermore, Einstein insisted that the subject was absolutely fundamental, the very few parts of physics that could not be changed. Stern also indicates, however, that though Einstein often talked to him about relativity theory, including general relativity which was Einstein’s main concern during the days at Prague and Zurich, Stern himself never came to feel at all at home with the subject.

Even though Einstein’s main concern when Stern knew him was with relativity, the problems of quantum mechanics continued to bother him a good deal. In this connection, Stern quotes Einstein as having said to him repeatedly, “On quantum theory I use up more brain grease (rough translation of German idiom) than on relativity.” Stern insists this quotation comes from the Zurich period. As to what the concerns were, Stern indicates that they grew out of the attempts to reconcile the particle and the wave nature of the photon, both of which seemed to Einstein absolutely fundamental and quite indubitable. He also speaks of the Zurich period as one in which many, many physicists were trying in all sorts of ways to “divide the quantum” (it is not clear from the context that this remark was meant to include Einstein). In response to a question about this, he indicated, however, that this did not mean that the Nernst-Lindemann half-quantum had been taken seriously by responsible physicists. It was, he said, far too obvious that Einstein’s specific heat formula was an approximation and that the restriction to a simple frequency only was introduced to test the basic idea.

On the subject of Einstein’s teaching, Stern indicates that while in Prague and Zurich his main, and perhaps only, formal teaching was in the standard general physics course. [Einstein’s real teaching at Zurich was done, Stern later adds, in his colloquium which was great. At various times von Laue and Ehrenfest participated with Einstein for extended periods. Also a group of good students: Stern, Herzfeld, and a man called Kern. The latter was a student of Debye’s, a rich young man who had done many things including exhibition horseback riding. He was killed in the war.] Einstein’s lectures, Stern says, were good but not for elementary students. Einstein did not prepare them in advance but just talked at the board. As a result, there was, some lack of organization, though the better students could see his mind at work. In particular, Stern tells the following story. Once in a lecture on classical mechanics, Einstein had difficulty making one of the standard transformations, perhaps from the Lagrangian to the Hamiltonian form. After several false starts and periods of staring at the board, Einstein finally said, “I’ll tell you tomorrow.” Then he stopped short and stared at the board again. Suddenly he burst out, “No! That is against my honor.” At that point Einstein attacked the problem again and carried it through. This story provided Stern with the occasion to emphasize that Einstein always told people never to hesitate to admit their mistakes. On many occasions, Einstein himself did so with relish. Stern also spoke of his prolonged observations of Einstein’s changing attitude toward mathematics. When Stern first knew him Einstein said that, when he first finished his thesis, “I thought any theory needing more than sines and cosines is no good.” By the Zurich period Einstein had somewhat relented, but still felt that to lean on mathematics was bad. As time progressed this attitude became still milder, and, when Stern saw Einstein at Princeton in 1933, he told him he was becoming a formalist. Stern particularly asked where the clever experiments like the elevator experiment were now. Einstein said that he would like very much to have such experiments but could not find them anymore. After the Second World War however, in another conversation, Einstein said to Stern, “Our salvation lies in mathematics.” Apparently, Einstein felt that this was the way of working quantum mechanics and relativity together. One incidental personal remark about Einstein also emerged. Apparently, during his days in Italy before his job in the patent office at Bern, Einstein and his family were often genuinely hungry. The Bern position was necessary in order to keep them alive.

Stern has very fond recollections of a long visit which Ehrenfest made to Einstein in Zurich around the fall of 1913. There was a great deal of discussion extending over several months. Stern thought very well of Ehrenfest, and apparently Ehrenfest sent him a reprint of the volume on Statistical Mechanics with a dedication. It is from conversations in this period that the quotation at the start of Stern’s article on gas dissociation comes. Stern, remarks incidentally that Ehrenfest hated Vienna which had been his home town.

About this same paper, Stern indicates that it was done in complete independence of Einstein and that the latter knew nothing about it. However, when Einstein went to Berlin in the spring of 1914, Nernst assumed that Einstein had had to do with the paper and bothered him a good deal about it.

III. The war and Berlin: During much of the war Stern was a corporal at a meteorological station at Lomsha in Russian Poland. It was, he says, sometimes disagreeable usually tedious work, since the first observations had to be made before sun-up when it was often 40°centigrade below zero and when it was necessary to stand around following meteorological balloons into the clear air for as much as thirty minutes. There was, however, much time left over between observations for work on papers.

Late in the war (perhaps early 1918 and not before) Nernst succeeded in getting Stern to his laboratory in Berlin where he was put to work on producing thick oil from thin oil. At this time he saw Nernst every day, but Nernst never asked about the progress of the war work but instead discussed general political questions. It was at this point that Stern got to know Volmer who was teaching at an officer’s school in Berlin. They worked together on two papers in their spare time of which there was a good deal.

During this period Stern also saw a good deal of Franck. He, Volmer and Franck always took long walks together on Sunday. Stern remembers some repeated friction between Franck who was quite conservative and Volmer who leaned quite far to the left.