Gregor Wentzel

Brief Note:

The texts which follow were dictated by T. S. Kuhn from notes made during interviews with Gregor Wentzel on the afternoons of February 3, 4, and 5, 1964, in the Research Building of the University of Chicago.

Thomas S. Kuhn:

Before the first interview Professor Wentzel indicated great reluctance to have the discussions tape-recorded. Some of the things on his mind would, he insisted, have to be withheld if the recorder were used. The interviewer therefore reluctantly agreed to work from notes. As a result the conversation was less rich than it otherwise would have been, and the account of what was said is significantly barer and more schematic.

The record of each interview was dictated shortly after the interview itself—usually the next morning. In each case some attempt was made to reorder the material so that related items would be near each other in the transcript, but this resulted In significant changes of structure only in the first Interview.

Speaking of his family background, Professor Wentzel said that his father had been a lawyer working in the bank of one of the provinces of Prussia, a quasi-governmental position. He was a man of broad intellect with many and varied interests, among which music and literature were particularly prominent. Wentzel's mother, about whom he said somewhat less, came from an old intellectual family whose men were usually either doctors or priests. education he described as relatively standard. He attended a gymnasium of the intermediate sort: neither a real humanistic nor a Realschule. For example, although Wentzel studied both Latin and French, he never studied Greek.

He had the full range of usual science courses, including mathematics through differential calculus. Physics began for him in the 8th year of the school but was not generally at all well taught. There was, however, one important year when he was taught by a young man, a candidate for a teaching credential, who was extraordinarily enthusiastic about the sciences, particularly about the science of astronomy. Wentzel found this enthusiasm infectious and himself built a small telescope and looked at the moon, etc. It was really this year which aroused his interest in science, an interest that he thinks had scarcely preexisted. Though it was the only influence of the sort that occurred during his school days, it did stick with him and turned him towards his ultimate career.

Wentzel indicates that before he went to the university his choice of either science or mathematics for a career was quite clear. For this choice, he got very little support from home, though his family apparently did not exert a great deal of negative pressure either. The point was that a scientific career pretty clearly meant a career in high school teaching, a career of less prestige than the fully had previously known. His father apparently said to him when he left for the university, "All right, you study your science for a while, but take jurisprudence at the same time so that you will have another career open and available to you." At the very start, Wentzel says, he did take lectures on jurisprudence, but he dropped these comparatively early in his university career.

Wentzel's early university career was varied and also interrupted. It began at Freiburg in 1916, but initially continued there only for one-and-ahalf semesters. After that, during virtually all of 1917 and 1918, Wentzel was called for military service. Fortunately, at the end of the war, Wentzel's unit was returned, still in uniform, to the neighborhood of Freiburg, and he reentered the university, still in uniform spending another one-and-a-half semesters there. After that Wentzel went to Greifswald in the autumn of 1919 and continued there until he went to Munich around November 1920.

Asked why he had picked freiburg and Greifswald as places to study, Wentzel said that in fact, he had known very little about the strengths and weaknesses of the different German universities in the sciences, and had, at his father's advice, therefore picked places which would take him far from home and introduce him to other parts of Germany. The idea of studying abroad was not at that point at all prevalent in Germany. Apparently, at this point in his career, Bonn would have been the university nearest to home, and the decision and the decision was not to go there but to go further away.

Both at Freiburg and Greifswald, Tentzel studied mostly mathematics. At Freiburg he particularly remembers having courses with Loewy in analytic geometry, and with Bolza in functions of a complex variable. During this period he had, however, no "mathematician's mathematics." That first came when he got to Greifswald and studied with Hausdorff on set theory. He liked the course and remembers it but was not really at all enthusiastic about this sort of mathematics. Wentzel is vague about other courses taken in these early years, but is quite certain that he did take the regular elemtary course in experimental physics at Freiburg and thinks he may have had a bit of chemistry there, though without lab. At Greifswald, he took no more physics except a seminar, for which see below.

He felt he had already learned, by reading, as much as he could get from the elementary theoretical physics lectures, so he did not take them. Asked what he had read by this time, Wentzel indicated that he had been through Haas, Introduction to Theoretical Physics. Then he had gone on to Weyls Relativity, a book which he read carefully : found fascinating. He n.hasizes that in this period he had had no introduction at all to problems of atomic theory. The one physics course that Wentzel did take at Greifswald was a seminar with the f- ous Stark assisted by Seeliger, the son of the astronomer. Wentzel's recollections of it are vague, except that Stark constantly sneered at atomic theory. Apparently the Bohr atom was the particular butt of Stark's negativism. To him it was absurd in physical principle. For example, first the electron is here, and then suddenly it is there. What happens to it?

I asked whether Seeliger had made any effort to undercut this attitude, as was told that in seminar, at least, he had not done so, but had simply been quite quiet. Nevertheless, Seeliger did play a significant role in Wentzel's career later at Greifswald when he had to take his candidate's exam for the teaching credential and, to write a minor thesis, a pure work of compilation with no original research in it. Seeliger was one of the examiners and asked Wentzel about Maxwell theory. Apparently, he was very much impressed with Wentzel's response, and took him aside to ask what he meant to do. Wentzel indicated that he was still quite uncertain about it. To this point, he had oscillated between mathematics and the sciences without any very clear career notions. Seeliger immediately said, "Oh, then of course you must go to Sommerfeld." This advice was welcome to Wentzell who actually still knew nothing at all of Sommerfeld. In fact, it was simply this advice which took him to Munich, where he began studies during November 1920.

On first arriving at Munich, Wentzel went to see Sommerfeld, was admitted to the seminar, and was given a quick assignment for a seminar talk. s This was to be a report on a paper of Debye about dispersion due to permanent electric dipoles. Apparently this was a purely classical treatment with no quantum mechanics at all. Wentzel, with his minimal background, really had very little idea what was going on, and went to Pauli, Sommerfeld's assistant, whom he got to know well in these years, and asked what he ought to do. Pauli said, "Read the Bible." Meaning, of course, Atombau. Wentzel got at it right away. Apparently, his seminar talk itself was quite successful, for he insisted on deriving the same result Debye had managed by a different method, and this impressed Sommerfeld a good deal. Wentzel was then rapidly accepted as a Ph.D. student.

Wentzel is not at all sure, under questioning, what else went on in the seminar during that semester or later ones. He thinks, though he is not really sure, that at some point Sommerfeld asked him to do a report on alpha and beta scattering, and suggests that this was probably the reason he took up the beta scattering problem for his Habilitationsschrift. In January 1921, however, Sommerfeld gave Wentzel an experimental problem, that, I gather, was initially thought of as a possible Ph.D. problem. There was some existing X-ray spectra work on the rare earths in water solution, and Sommerfeld thought the results must be wrong, since they did not fit at all his fine structure formula.

Wentzel repeated the experiment with the equipment in the basement of Sommerfeld's institute, and entirely confirmed previous measurements. As a result, this problem had to be considered as a failure, and. Sommerfeld began to look for another problem. Wentzel here remarks, parenthetically, that he believes Sommerfeld somewhat naively over- applied the fine structure formula to X-ray problems without thinking very hard about the physics of the situation. In the case of the rare earths in water solution, he failed to see that ionization, the water medium, etc., would affect the results to be expected. Apparently, though, this is retrospective on Wentzel's part. He did not, that is, attempt to explain the divergences from theory himself at the time. Rather, he just presented the experimental results.

At about this time, a blind or very nearly blind dutch mathematician, by the name of Ver Sluis, a senior man of perhaps 40 or 50 years, was looking for someone who would go to Locarno with him and read mathematics aloud during the morning. Sommerfeld mentioned this possibility to Wentzel, who accepted the job, went to Locarno, and read to Ver Sluis for a couple of months. Before he left however, Sommerfeld said to him that he had thought of a problem that might be worth Wentzel's trying. X-ray spectroscopy was, Sommerfeld said, now in a mess that he found himself unable to work out. "I am prejudiced," Sommerfeld said, "and cannot do anything with it. Perhaps you as a younger man may be able to make sense out of all these lines that increasing experimental work has produced." When completed, the problem turned out to be that of the discrete X-ray spectrum, but Wentzel thinks that at the time both line spectra and the continuous spectrum were involved in the assignment.

Sommerfeld gave Wentzel some rather speculative ideas that Smekal had advanced in an article and also the measurements by Coster and a few others. Wentzel was to take these with him to Locarno, to think the whole problem over, and to see whether he could get anywhere with it. At Locarno Wentzel feels he did not really do a great deal of work on the problem. In the mornings he read to Ver Sluis, and in the afternoons the weather was so splendid that he did not do very much work.

But the basic idea came to him quite rapidly, and Wentzel clearly remembers a rainy Easter Sunday afternoon when he realized that the answer must work out in a particular way. At that point he made some prerliminary efforts and found that his idea worked out quite rapidly and well. The problem, however, then became that of reinterpreting what had been said about existing X-ray energy levels. A number of these had been misattributed, and he had to go back and do a good deal of recomputation and revise level assignments. He may have done a bit of this at Locarno, but most of the main work on levels was done when he got back to Munich. That work actually started in April, when he began putting nmubers in. By May he was ready to write the results up, and he received his degree at the end of that month.

In answer to a questionWentzel indicates that the basic insight that he had during the afternoon at Locarno was the realization that there must be three L levels, five M levels, and so on, this being quite different from the number of levels that Sommerfeld had worked with. Also, if there were to be this number of levels, then there had to be an extra quantum number.

As soon as Sommerfeld saw how the results came out, he was greatly impressed and wrote very quickly to Coster, who was, however, furious about it, because he had just found the same explanation himself. Sommerfeld felt that, because Coster had supplied the data, he was entitled to every consideration, and he arranged to hold up the publication of Wentzel's paper, so that it would appear in the same issue of the Zeitschrift as Coster's. Ultimately the two did come out together. In exploring the background for Wentzel's success with the X-ray spectra, I asked about the role of Hertz's work on irregular doublets, and this Wentzel did not remember though he, of course, did not deny that it must have played a role. On the other hand, he did suggest that a very considerable role had been played for him by Schrödinger's paper on penetrating orbits which enabled him to work out the first quantum numbers that he needed for his identifications.

Other, though older, parts of the background needed, were Kossel's ideas on closed shells, which were widely known at Munich and basic to work there. Also, obviously, Sommerfeld's fine structure formula, was essential. Asked about the relation between the two types of doublets, relativistic and screening, that appear in his work, Wentzel is sure that he had absolutely no ideas on this subject. He did not know at all what the physical significance of the Grundquantenzahl was to be and thinks that really there were not such ideas. Reminded that Sommerfeld had suggested that the inner quantum number was probably something to do with a magnetic interaction, and that he himself had suggested that the inner quantum number and the Grundquantenzahl might be closely related, Wentzel brusquely asserts ignorance and memory failure. He does say however, that Sommerfeld was rather unhappy about, the name "Grundquantenzahl" because he thought it rather overambitious. "Grund actually means fundamental," Wentzel states.

Sommerfeld however did not want to interfere with the work and let the nomenclature stand. More generally, Wentzel insists repeatedly and urgently in discussing all this work, that the spectroscopy, X-ray and optical, of this period, was always quasi-empirical—the search for empirical laws which were never themselves at all understood. There was never, he says, an ything like a theory in this area, and there could not have been because spin was unknown. I suggested that the availability of spin was an unfair because retrospective, criterion, but this had little effect on Wentzel.

He insists that, however Sommerfeld may have felt, he himself was never at all satisfied that they were doing anything but getting particular empirical rules that things never did seem satisfactory until the new quantum theory appeared, despite the fact that people were making obvious progress. I tried to get further along this line by asking Wentzel about some of the theoretical generalizations which did exist and played a role in spectroscopy in this period, and his general tendency was simply to shove them aside as things he had never paid much attention to. I asked him, for example , about the role that had been played by Ellipsenverein, cubes, and so on.

He insisted that these had never made much difference to him and that, by the same token, the Bohr theory of building up the periodic table through closed shells constructed of unsymmetric orbits had not changed the situation much. I then showed him the opening sentence of the published version of his thesis) which produced laughter, and a tacit admission that he must be wrong, but which did not at all bring out recollections about the manner in which theory had been used in the period. Wentzel simply insisted that it had always been clear to co on sense that the cubical atoms, etc., could not be right, but he could not say why they had seemed nonsensical. Another theory on which Wentzel commented. negatively was the Rumpf model.

He was not involved with it, he said, nor did it bother him very much. Furthermore, it is his impression that people around him scarcely took it seriously, but were instead looking for empirical laws. Again, Wentzel insists that the whole work was semi-empirical with little role for theory and no theory actually in existence. that is why) he says, people were generally simply amused by Epstein's insistence that he could solve the helium atom with 22 contact transformations.

The only exception that Wentzel will allow was the constant guidance that people received from the Correspondence Principle, a principle that remained helpful throughout but that was never either entirely satisfactory or unambiguously formulated. This remark about the Correspondence Principle led me to ask about the development of Sommerfeld's views on this subject and about the differences between Bohr and Sommerfeld. Beyond admitting that the Correspondence Principle does play a more and more prominent role in successive editions of atombau, wentzel did not think that there was much change in Sommerfeld's attitutde. Furthermore he said that he had scarcely noticed any differences between Munich and Copenhagen, even after he had visited Copenhagen during 1925 or 1926.

This whole area and type of discussion leaves Wentzel rather cold. As talk now continues, Wentzel reminds me that throughout this period he still knew very little physics and was constantly studying it by himself, in occasional courses by Sommerfeld and others, and in long and frequent conversations with Fault, of whom he saw a good deal, and to whom he became very close. What these discussions were about, he does not for the most remember. Looking at our questionnaire, he singles out the Stern- Gerlach experiment as one that was discussed, and he adds that the whole problem of the existence of two rather than three beams was a very real puzzle for the group.

I reminded him that there had been a proof by Sommerfeld that the electron would fall into the core with in equal 0, but Wentzel said that this had been thought to be extremely shaky, and that the problem remained. The whole subject had continued to pose problems, though what they were is less clear to . In connection with Pauli, Wentzel remembers that Pauli was greatly discouraged at the end of his work on the hydrogen molecule ion, and did not really want to submit it for a dissertation. Despite my reports that the contradiction with experiment scarcely shows in Pauli's published paper, Wentzel feels quite certain that the problem which bothered Pauli was that disagreement.

He has promised to look at the paper again to see whether he can reconstruct more fully. Also, Wentzel says that he was in Hamburg for a couple of months from November 1925 [in the second interview this visit is described as being for four months, from Nov. 1925 to Feb. 1926.], just the period when Pauli was belatedly converted to the idea of electron spin by Thomas' proof of the relativistic factor of two. Wentzel says that the reason for Pauli's resistance prior to this time was the feeling that the idea of s intrinsic angular momentum for a point particle was simply too crude and therefore unacceptable. Wentzel does confirm that Pauli threw the entire great weight of his authority into the resistance to the idea. After doing his thesis, Wentzel still had a few odds and ends to clear up with respect to X-rays. In particular, there were still the few unexplained lines which he first accounted for in his paper on the e Funkenlinien.

In discussing the treatment of these, Wentzel emphasizes that he was quite wrong in details, since he misidentified a number of the actual processes involved, but that the general idea of multiple excitation has held up extremely well. Also, in answer to a question, Wentzel does say that once these problems were out of the way, the whole treatment of the subject of X-ray line spectrum did seem entirely secure. It was not clear that there were any other problems still to do. This brought us to the fall of 1921, at which time Wentzel became assistant at the Munich Institute. One of his main duties in this job was as reader for Sommerfeld, which meant he had to correct the student problem assignments in Sommerfeld's large lecture course.

He would, he said, have liked to go to the course, since he would have learned a good deal that he needed to know. Actually, he did go to two lectures but had to say to Sommerfeld afterwards that he had really not understood what as going on. Sommerfeld had blushed, said, "Oh, but it was all wrong, you must not come again, and there after prevented his coming. In addition to these lectures, Sommerfeld giving a number of special lectures and colloquia, which Wentzel did attend. The special lectures dealt with problems that Sommerfeld was actually working on himself at the time.

Many of these grew out of the work for the fourth edition of Atombaul and one of Wentzel's duties was also to help with the composition of parts of this work. Simultaneously, Wentzel went to work on his Habilitationsschrift, for which, as previously indicated, he chose the problem of beta scattering. One part of that work, the criterion for distinguishing single from multiple scattering, has been more influential, Wentzel suggests, than anything else he has ever done. At least, it is cited again and again and again. On the other hand, Wentzel indicates that the work itself has never interested him very much and that he has e no attempt to continue with it.

I asked whether there had been any real resistance to taking up the idea of multiple scattering again after Rutherford's success with single scattering, and Wentzel suggests why there was no such problem. The multiple scattering he was defending was scattering by successive atoms, whereas Thomson's multiple scattering had been multiple collisions within a single atom. The ideas were different enough so that the defeat of one would not necessarily lead to resistance to the other.

Wentzel had a number of interesting things to say in response to my question about consciousness of the Compton effect and the photon problem at Munich. The Compton effect was known very early at Munich, Wentzel says, because Sommerfeld always got reprints, and therefore knew of the work quickly from Compton and Debye. Everyone was very much excited about the experiments and their interpretation, since this was the really binding proof of the corpuscular nature of light. People had, of course, been quite aware of Einstein's photo-electric effect before.

There had, however, been a general tendency to see the hv of a photon as nothing more than the hv which occurred in Bohr's frequency condition. The former did not, in any sense that was clear to people, have to be a corpuscular momentum. Therefore prior to the Compton effect there was a good deal of confusion about and playing down of the problem. Though it was clearly directed to the problems presented by the Compton effect, people at Munich felt very strongly that the Bohr-Kramers-Slater-paper was simply a faux pas, a bad step and in a bad direction. On the other hand, they were, Wentzel feels quite certain, considerably impressed by the de Broglie paper, though they did not attempt to do much of anything with it. Sommerfeld had heard of the paper, Wentzel thinks, from Einstein, and Einstein's great reputation therefore helped to bring them to it.

Wentzel is quite sure it must have been discussed in seminar, and he also remembers that he himself reviewed the paper for, perhaps, Physikalische Berichte. He no longer remembers what he says, but thinks his review was favorable, and knows that he was impressed. Initially, Wentzel thought that it was perhaps the de Broglie paper that had led him to his "Quantenoptik" paper, but the dates, he admits on looking at them make this highly unlikely.

About the "Quantenoptik paper Wentzel has very little to say, nor can he say much about the two papers which are associated with it, one on Bremsstrahlung and the other on dispersion. He does remember that almost no one paid any attention whatsoever to this work, though he is quite clear that it was the only attempt at a fundamental treatment of the difficulties which he himself attempted during this period. Apparently among the very few exceptions was von Laue, who did write Wentzel about these papers, and was apparently generally sympathetic, both with the approach and with this sort of work in general.

In addition, Wentzel indicates that he remembers these papers as being somehow concerned or related to the general idea of guiding waves, and that the idea of guiding waves was extremely common in that period, it being quite usual to suppose that Maxwell's equations must describe a field which was somehow a statistical field, or something of the sort.

As to the tie to the Bremsstrahlung paper, Wentzel insists, as Laporte did, that this was always a very lively subject of interest at Munich, dating back to Sommerfeld's early classical theory on the subject. wentzel also remembers that he did not at all like Kramers' treatment of the subject, and wanted very much to try to improve upon it . To what extent the problem of aperiodic phenomena at large was also a significant one at Munich Wentzel is less sure, but at this point he himself notes what is said on the subject at the beginning of his paper. In an attempt to get more material on this subject, I pointed out to Wentzel that these three papers strike me as being unlike almost all of those which appear from Munich, and rather more like papers from Copenhagen and Göttingen.

He thinks this is probably true, but has nothing whatsoever to say on the subject. One last remark, referring back to the reception of the de Broglie papers, Wentzel supposes that de Broglie's supplying a derivation of Sommerfeld's integral conditions was probably also very important in making people take the paper seriously at Munich. The first interview closed at this point. Wentzel had promised on the preceding day to look over his reprint of Pauli's paper on the hydrogen molecule ion in order to determine, if he could, what aspect of Pauli's treatment had seemed so very unsatisfactory.

Having done so, Wentzel feels that the crux of the matter occurs on page 236, though he thinks there are other similar examples elsewhere. On that page Pauli shows that the lowest energy orbit discovered by application of the quantum conditions is actually unstable therefore ruled out. In its place Pauli uses the next higher energy level as the likely ground state of the ion. All of this seemed extraordinarily fishy and unsatisfactory to him. It was a mess even though the mess did not derive from the conflict between theory and experiment. "All of this went to show," said Wentzel, "that you can't make models even in this very simple case."

I suggested that perhaps this particular sort of conclusion was typical for Munich and that others would perhaps have said instead that the whole approach was wrong. To this Wentzel responded that perhaps all he had meant was that these models were wrong rather than that you could not make models at all. Continuing the discussion of models, Wentzel said that until reminded by me, and by reading his papers, he had forgotten all about the Rumpf and about the possibility of attributing to it angular momenta and so on. Rereading a few of his old papers has brought it back, but he neveletheless remembers that he was always very skeptical about model developments of this sort.

What he had faith in, and what he thought was solid were the laws which pointed to some simpler sort of physics that was to be found underlying them. Wentzel's suggestion that there were clearly underlying laws to be found led me to raise with him the question about the extent, in Munich, of the realization of the existence of a crisis in quantum theory. He answered that of course they all knew that things were wrong and would need to be straightened out.

On the other hand, he thinks that there was very little sense, as there surely was at Göttingen and perhaps also at Copenhagen, of the rapid approach of a new theory. In evidence, Wentzel suggests that he himself did not attach nearly enough weight to the Kramers' dispersion formula. He knew of it, but did not see it as a really radical new step pointing to other things that might now be achieved. He was, he suggests, more aware of some of the quantitative spectral regularities—the interval rule, the intensity rules, and so on. These were not, however, for him, examples of "the extended Correspondence Principle" and therefore did not promise a rapid breakthrough. Instead they were additional particular successes in the nature of semi -empirical laws of the sort that had often enough been discovered before.

It was quite clear that progress was being made, but there was no sense that either the tune of the progress or the nature of the obstacles were different in kind from those experienced before. Wentzel's remark about the Kramers' formula led me to ask him how seriously the dispersion problem had been taken in Munich in the period before the formula appears. He is entirely sure that it was discussed and remembers vaguely that there was discussion early in his time at Munich of the Ladenburg Formula.

He is not, however, at all clear that the problem seemed to have a very special importance or that the contradiction raised by the frequencies of anomalous dispersion was particularly clearly recognized. I asked him especially whether he could supply any background for Heisenberg's contribution in correlating the problem of dispersion with that of the quadratic Stark effect in 1922, and Wentzel felt he had no recollections of - prior discussions that would be relevant.

He himself was not at Göttingen for the Bohr meeting. In fact, he occasionally has boasted to people from, Göttingen that he spent only a day there in his entire life. I once more pursued the topic with Wentzel of Einstein's influence at Munich, Wentzel reiterates that it was considerable. In particular, he states that everyone at Munich regularly did learn the Einstein fluctuation arguments considered them a strong case for the existence of photons. Here again he indicates that the photon hypothesis was taken considerably more seriously at munich than elsewhere. (I queried this, reminding Wentzel that many people speak of Sommerfeld as less concerned with statistics and less good at it than he was at a number of other fields.)

Wentzel thinks that Sommerfeld was really better at statistics than other people have thought, but adds that part of the reason the topic of the fluctuation formulae figured so regularly at Munich may be a residue of Debye's influence there. Debye had of course, been very much concerned with these questions. Wentzel, quite unlike Uhlenbeck, speaks of the 1911 Debye paper as the first satisfactory treatment of the Hohlraum. Also, Wentzel remembers that the Einstein-Bose gas papers were discussed at Munich, or at least that be heard about them there. On the other hand, he has no recollection of the context of the discussion or of any impression that these papers were more isolated oddities. He has no sense that they seemed to fit with other things then going on.

As to discussions of the Einstein-Ehrenfest paradox for the Stern-Gerlach experiment, Wentzel has no recollections on this subject at all. Finally, in the preliminary portion of the interview, Wentzel described his paper on the Compton effect as a pure review paper with nothing new in it. He cannot at all remember either why he was asked to do it or by whom, nor does he know how he happened to have so very large a knowledge of the literature on the problem. Perhaps, he suggests, he had reported to the Munich seminar on this subject. He has, however, no recollection of any such talk.

I next tried to obtain from Wentzel some additio info uation about his actual education in the sciences at Munich.The Indication is that he act took very little course work. In particular he never did go to the lectures in Sommerfeld's physics course, except the two lectures attended when he was Sommerfeld's assistant . On the other hand, he went regularly to the special lectures on current research problems that Somme-rfeld offered twice a week. These, however, Wentzel says, were not at all well given. Sommerfeld considered his graduate students to be colleagues and therefore saw no need in doing the work to really put things in clear and logical form for them. Presumably, Wentzel suggests, he went to lectures given by some of the other physicists in the vicinity: Herzfeld, Lenz, and Ewald. He is not, however, perfectly sure that he did so, and is entirely unclear about what the lectures might have had for their subject. also, though he is sure that he went regu larly to Sommerfeld's seminar, he is vague about the existence of a colloquium, much less has he recollections of his participation in one.

I asked Wentzel to say a bit more about a subject mentioned briefly after the end of the previous interview, that is, about the relative change in Wien's attitude towards sommerfeld's theoretical students during the time that Wentzel was in Munich. Wentzel had described himself as "the first" to take the examination that Heisenberg ultimately very nearly flunked. In the beginning, Wentzel stated, Wien was well satisfied. Wentzel himself was given a summa. On the other hand, Wien gradually got rougher on the students. Wentzel suggests that he thinks Pauli got only magna, and Heisenberg himself almost did not pass.

Asked what he means in describing himself as "the first" of the group, Wentzel thinks his probable intent is to indicate that he was the first actually examined by Wien. This discussion of education and local conditions at Munich led on to discussion of Pauli and of Wentzel's relations with him and with Heisenberg. Wentzel indicates that he was genuinely close to Pauli and remained so, but that for various reasons not easy to specify he was never at all close to Heisenberg. Their temperaments were rather different. Furthermore, Wentzel insists that Pauli was never personally close to Heisenberg either, and that he shared wentzel's reservations about bim from the start, though in both their cases these were further intensified by the war.

Apparently, Pauli referred to Heisenberg as the "Pfadfinderseele" and continued to do so from time to time in later life also.Wentzel translates this word "scout" and indicates that as used by Pauli it had the implication of naivete and simplicity in human Wentzel also insists that throughout their lives Heisenberg sought out Pauli as a collaborator, but that the relationship never went in the other direction. He adds that the feelings were intensified during the war, and that Pauli was very much hurt by Heisenberg's wartime behavior and by the fact that never in the post-war period did Pauli get from Heisenberg even a hint of an apology for things that happened during the war.

Talking more directly about Pauli, Wentzel indicates, as others have, that he was a man of deep and stubborn prejudices. Often it was necessary to work very hard to talk him out of them . On the other hand, once he was converted, he was not ashamed or bothered by having made a mistake. On a number of occasions these prejudices against certain sorts of physics grew out of personal resistance to the man concerned. If Pauli did not like someone, perhaps just for the shape of his nose, he would not like his physics either. A number of Pauli's assistants during the Zürich period suffered badly from such obscure personal causes. Wentzel suggests that Peierls was one of these.

I told Wentzel of Laporte's suggestion that these problems and prejudices of Pauli's had developed in the post-Munich period but not existed at that time. Wentzel simply says that this is a mistake and that Pauli was relatively uniform throughout. It is true, Wentzel indicates, that in the very early '30s Pauli's acute personal problems left him in a state of fairly steady depression. His first marriage very recently broken up, and quickly. The second marriage was not yet in sight. During this period Pauli also drank a good deal too much though in the early '20s when Wentzel first knew him, he had been decidedly 'anti-alcoholic. The transition to even normal use of alcohol only begun in hamburg when a colleague who owned a vineyard introduced Pauli to wine and campagne and taught him something about it.

Wentzel points out that the term 'anti-alcoholic' used here has no moral overtones. Pauli simply knew nothing about alcohol and saw no point in getting started with it. From this point wentze1 proceeded for a time to look briefly at a number of reprints -which he had also had a look at during the morning. About each one of them he bad a few words to say, though these were usually not very illuminating historically.

Looking at paper No.20 "Über gestrichene und verschebene Spektralterme," Zeitschrift für Physik, 31 [1925], 335-338, Wentzel stated that he no longer at all remembered the debate with Landé, but was pleased to notice that he and Laporte had been right, Landé wrong. They actually had the data, while Landé was working from preconceptions. From that paper Wentzel turned to No. 21, "Über die Komplexstruktur der Röntgenfunkenspektren," Zs. f. Phys. 31 [1925], 445-52, and immediately indicated how much he still found 'himself liking it, though he had almost forgotten it in the interim.

Here, as elsewhere, it is clear that Wentzel's in standard throughout is the question: does a given paper still stand up by modern stndards? In this case, Wentzel not only liked the X-ray theory, but was particularly glad to notice that he had stated here that a hole in a closed shell is like an electron outside of a shell. Wentzel now passes with enthusiasm to paper No. 22, "Termprobleme der Dublettspektren, etc.," Annalen der Physik, 76, [1925], 803-828. He is particularly impressed with the computation of screening constants and the results gained in this way. On the other hand, he has scarcely looked at the second part of the paper which deals with relativistic doublets. This part, he says is nonsense.

He is somewhat surprised, though still not very inter ested, to discover that it is this aspect of his work that is most typical of the period, and that directly influenced Goudsmit and Uhlenbeck on the road to electron spin. [Paper] No. 24, "Zum Problem der relativistischen Feinstruktur," Zs. f. Ph.,33 [1925], 849-854, brings out the same type of reaction even more forcefully. Wentzel refuses to attach much significance to this paper and insists that it doesn't make much sense. However, in a conversation about it, he adds that he himself never believed in magnetic interactions and thought that multiplicity must be explained throughout as a relativistic effect.

I asked, in connection with this paper, how one was to understand the complete failure to cite Pauli's argument against the relativistic explanation of doublets. Wentzel found it odd, and was unable to explain it. It was also clear, however, that he did not even quite remember what the Pauli argument had been. He tried, that is, to assimilate it entirely to the exclusion principle which follows it, though only briefly, in time. This completed the discussion of developments prior to the new quantum theory, and Wentzel now turned to a few remarks about reactions to that theory when it came.

About spin, he stated that he had no recollections whatsoever of when and how he first heard of it, or of Just how he felt about it. His only recollections, which he has already really provided, is that he was present at the time of Pauli's conversion which must have occured during the four months he spent in Hamburg, November 1925 to February 1926. He was there to replace Lenz, who was ill , and had gone to adler in Vienna for treatment.

He describes this whole episode by saying, at this time everyone but Pauli had been converted, and Pauli was converted too when he discovered the Thomas factor of two.

This led me to ask why he had himself, still later, published a paper that criticized spin, and what his attitude at that point had been? He indicates that though he presumably accepted the spin theory at that time, it still needed to be checked. Wentzel describes the first of the Heisenberg first "Matrix mechanics" papers as one which was "received as a curious new approach." Real confidence in Matrix mechanics came, he now thinks, only after the reception assimilation of wave mechanics. He himself is not sure how he worked on the new matrix methods—he clearly did some work for he published a paper in the area— and states that he was very much relieved when the Schrödinger formulation appeared. Then he and Sommerfeld were able to use a more familiar sort of mathematics. Asked about the interpretation of Schrödinger's equation, Wentzel feels sure that none of, the people vho had been seriously concerurad with quantum mechanics during the '20s were at all tempted to follow Schrödinger's continuum explanation. Past experience with the photon and other similar phenomina showed that it could not be done. Wentzel's strong impression is that the need for a statistical or guiding wave explanation was taken for granted from the start. Thus, when Born's paper on scattering appeared, people were genuinely inter- ested in the treatment of the scattering problem, but found nothing particularly novel or surprising in the statistical interpretation itself.

In this connection, but with relevance not altogether clear to me, Wetzel pointed out that Schrödinger's paper on the Doppler effect was an extremely important one in this period, though one that has now been largely forgotten. It was this paper, Wentzel insists, in which Schrödinger came closest to taking the photon seriously. Wentzel also insists that he thought not only of wave functions but also of matrix elements as probabilities, and that he thought this about matrix elements even before the Schrödinger function appeared. As he says, it is not an unnatural thing to do for anvone who has previously been treating the Fourier coefficients in the expansion of the classical polarization as related via the Correspondence Principle to the Einstein coefficients. Nevertheless, it is not at all clear that Born, Heisenberg et al. did think of them at all in that way Wentzel next made a few remarks about his own first work on the new quantum theory, stating that both his paper on multiply periodic systems, and his paper on a generalization of the quantum conditions had as an object the attempt to discover how far the old and familiar methods of quantum theory could be recaptured, rediscovered within the new matrix and wave mechanical methods.

He was not, he indicates, very happy with the outcome of the first of the papers. It is not clear to me that that unhappiness may not be largely retrospective, since Wentzel is clearly not very happy with that paper now. Obviously he is a good deal happier with the second paper, which contains his own contribution to the W.K.B. method. About it he also has an interesting remark or two to make. In the first place, wentzel says that the W.K.B. paper -was widely criticized at the time by the Munich Mathematicians who kept asking how can you do all These contour integrals in view of the fact that there is an essential singularity at infinity. Sommerfeld also was very unhappy about this point and quite uncertain whether the method was right. Wentzel tried quite hard to get rid of the essential singularity or to justify proceeding in its presence, and was unable to do so. Nevertheless, he was entirely sure that his result was correct, simply because it came out so very well. In particular he was just delighted with the computation of the second order Stark effect even though it did not agree with the older result by a constant factor. Deriving that constant factor of 19, he now regards as one of his real triumphs.

The implicit contradiction in saying that he was sure of the method because it came out so well and simultaneously that it failed to give the anticipated result is one that he cannot now altogether resolve. Wentzel also insists that his work on this subject was entirely independent of Brillouin's and Brillouin's of his, even though he had already gotten to know Brillouin quite well in Paris and saw him from time to time.

Apparently, during the years from about 1925 Wentzel had a girl friend in Paris and visited there frequently during vacations. While he was there, he also went to seminars and other meetings of scientists where he met Auger, Perrin, Brillouin, and others. Brillouin, in particular, used to take him around a good deal and show him Paris. At this point, the interview departed markedly from chronological order, leaving ground that must be retraced in the final interview.

I asked Wentzel about his views of the interpretation of wave mechanics: the Dirac-Jordan transformation theory, the Lisenberg Uncertainty Principle, and Bohr's Complementarity. On none of these questions had he any special recollections or opinions to contribute. This sort of problem had, he said, never really worried or concerned him at all. One had wave mechanics to work with, and that the thing to go ahead on. His attitude seems to have been very close to that of Sommerfeld.

Asked also whether Sommerfeld had yet become involved, with the theory of metals before he himself left for Leipzig, Wentzel indicates that Sommerfeld's first real involvement with the theory came only after he had gone. Finally, Wentzel remembers that he first heard. Dirac's radiation theory when be visited Göttingen briefly in 1927, and heard Dirac lecture on the subject. That lecture made a great impression on him, particularly since he had been generally quite dissatisfied with the Correspondence Principle treatment of radiation up to this point. In particular, he was very much struck by the fact that at last one could get spontaneous emission naturally into the quantum theory without some forced Correspondence Principle argument. Though this fondness for the Dirac theory does not show up in Wentzel's work until 1929 and after, Wentzel insists that he was Intimately acquainted with the various follow-ups on the Dirac article. In particular, he remembers this literature a great deal better than he remembers other literature of the same period: the Jordan-Klein, Jordan-Wigner, Heisenberg-Pauli, and other relevant papers come immediately to mind, and he speaks with an enthusiasm I have encountered nowhere else about Jordan's closing section of the Born-Heisenberg-Jordan paper which he thinks the very best thing in that paper. Clearly, he began to read and think about the whole set of problems of field theory from a very early date, even though his own publications in the field scarcely start until about 1933.

Asked why he delayed as long as he did in contributing directly to this line of theoretical development, Wentzel gives two sets of reasons. In the first place, his own concern in his work on the photo effect and in Compton's scattering was not the general Compton effect but as the interaction of light with bound electrons. Since, as he puts it, the better Dirac treatment justified the use of the Correspondence Principle in its most usual applications, there was no need to apply the newer and more eleborate theory to his problems until the potentialities of the older method were exhausted.

His concern was not with utilizing the very best methods on an older problem, but with a brand new problem, that of the bound electron. In addition, Wentzel points out that his own work was considerably delayed after 1928 for he then went to Zürich and had to develop courses for the first time. Also, this is just the time at which he got Married. Both of these delayed continuation of his own research.

Nevertheless, Wentzel continues to insist on his enthusiasm for the Whole approach, and gives one further piece of evidence for it which may have some importance for the way the field developed in the United States. When Wentzel came to the United States in 1930 for a visit, he took the Dirac radiation theory as his special topic for lectures and, as he says, propagandized for it all over the United States. In contrast to the radiation theory, the Dirac electron theory did not please Wentzel very much even though he was entirely conscious of its great beauty.

The negative energy state bothered him as did the infinities and hole theory. Wentzel is not really able to take apart these various different problems and to talk about the points at which they came to seem fundamental or irremediable. In talking generally about the problems presented by the Dirac electron, Wentzel insists that Pauli very much shared his unhappiness with that whole approach and that it was not until about 1935, when vacuum polarization and other such approaches became available that either of them began to take the Dirac equation and its correlates at all seriously.

The final talk opened with remarks by Wentzel about his transition from Munich to Leipzig which took place in the autumn of 1926. As the record indicates, what took Wentzel to Leipzig was the offer of a job, in this case an extraordinary professorship. At the time there were rumors that this job, which had been held by a man called Jaffé, had been offered to both Heisenberg and Pauli and that they had both declined it. They had better possibilities in hand, but Wentzel was only a Privatdozent at the time.

In 1926 the two professorships of physics at Leipzig were held by Wiener and Des Coudres. But Des Coudres died even before Wentzel's arrival in Leipzig, and wiener died just a bit later. It was these two almost simultaneous deaths that made possible the sudden and complete rejuvenation of Leipzig as a center for research in physics.

At the time when Wentzel was there, Leipzig in the physical sciences was a relatively old-fashioned and not at all lively institution. The single exceptions were in the area of applied physics, particularly electro-technics, led by a man called Karolus to whom Wentzel was quite close, and also in the group working on hydrodynamics under a man called Schiller. The very fact that the tradition in the sciences was weak proved uable to Wentzel, for it meant that the teaching was easy and undemanding. He taught only an elementary mathematics course for physicists and probably a few special seminars which did not draw much audience. As a result, he had a great deal of time to work on his own problems.

During the course of the Year 1926-27, the chair in experimental physics was offered to Debye who accepted it and rapidly came to Leipzig. He had the decisive voice in selecting the man to fill the chair in theoretical physics, though others were in on the decision also. The choice of Heisenberg seems to have been unanimous, perhaps he was placed even ahead of Pauli. In addition to the role in picking the theoretician, it is likely that Debye had special promises of funds, reorganization, and so on, though Wentzel's recollections on this score are quite vague. Heisenberg arrived in the fall of '27 or thereabouts; a theoretical institute was then started too; and everything was suddenly very very different. Students began to stream in, including Bloch, and there was a very very lively group already during Wentzel's second year at Leipzig. Hund was not yet there, however. He came the following year to replace Wentzel who had, gone on to Zürich.

Looking back, Wentzel insists that these two years were perhaps the most important in his scientific life. It is rather difficult for him apparently to say just why, but it appears to have been largely a combination of the state of the field—good problems to get started on—and the fact that he had a significant amount of time for himself. When he went to Zürich he had important teaching to do and for six years, he says, it took a great deal of his time. This is because he had to develop a standard three year sequence in theoretical physics at Zürich. He did try to develop a new technique to modernize the whole curriculum, giving in particular one semester to quantum mechanics. As a result, he was writing lectures all the time during the second three years.

The overall curriculum at Zürich was the traditional six semester course, given four hours a week. Wentzel himself offered two semesters of mechanics including hydrodynamics, one semester of thermodynamics and kinetic theory, one semester of elecomagnetic theory, one semester of optics, and one semester of quantum mechanics. In addition, he gave two hours per week of special more advanced courses on topics directed to the survey course of the previous semester. As already suggested, these lectures took a great deal of preparation not only the first time they were given, but really again the second. That, combined with his marriage, slowed Wentzel's research significantly.

At Zürich Wentzel's principal colleagues were Pauli,Scherrer, Edgar Meyer, Richard Bär. Some of these men were at the university, some at the Technishe Hochschule. At the university the students came mostly from Switzerland itself, though there were some foreigners. In spite of this, however, most of Wentzel s graduate students there were from abroad. Fierz was actually the first of Wentzel's Swiss graduate students. Earlier gragraduate students were Fischer, a Czech; Bargman, from Berlin; and N. Kemmer,whose background was international in the extreme. Returning now to the period at Leipzig, Wentzel mentioned that he had actually had other offers besides the Zürich offer during the time he there. was there. In particular, he was offered a professorship in Halle. Leipzig, to keep him, offered him a "Personliches Ordinarius," 'that is, an appointment which gave him the title but not the chair of the usual German professorship.

He accepted the Leipzig raise, but then the correspondence with Zürich started and he accepted the job there during the summer of 1928. It was not at all an easy decision. The university job was not so good as an equivalent job at Leipzig. But this was not the only consideration. At Zürich he would have the chair. More important, perhaps, he would be back in the immediate vicinity of his old and admired friend Pauli, with whom he was far closer than he was with Heisenberg. Finally, he could not help being somewhat impressed by the constant reminders from Zürich of the famous tradition created by his predecessors in the Zürich chair : Einstein, Debye, von Laue, and Schrödinger.

Nevertheless it was not easy to leave Leipzig, where the dominating trait for any theorist was Heisenberg's presence and leadership. At this time, however, Heisenberg was Almost excluively concerned with ferro-magnetism. etism. Not nuch, if at all, involved during this year with problems like the Dirac radiation theory and other such problems which were beginning to interest Wentzel considerably. Wentzel now returned to an article-by-article discussion of his own writing. From the Leipzig period, he first took up paper No. 30, "Zwei Bemerkungen Über die Zerstreunng korpuskularer Strahlen als Beugungserscheinung," Zs. f. Phys, 40 [1926-27], 590-593. Though received at the Same time as paper No. 29, this one is actually the earlier in conception and composition. Wentzel describes it as representing the last of the papers in which his fundamental object was to discover a way of preserving older results with the new quantum theory.

The more important part of this paper is actually the one which utilizes a Coulomb force in the Born approximation and thus succeeds in deriving the Rutherford formula , the first time this had been done. Wentzel had a letter from Born about it, saying that he had tried to do the same problem but had been bothered by the divergence of the integrals that resulted when the Coulomb field was introduced. Wentzel had easily gotten around this problem by saying that all atomic forces are ultimately screened and a minimum of screening force is sufficient to make the interval converge.

At this point Wentzel began to try to solve problems that existed but been unsolvable in the old quantum theory. Particularly, he turned to the problems which had been presented by the continuous spectrum. In the development of this work paper No. 29, "Zur Theorie des photoelektrischen Effekts," Zs. f. Phys,40 [1926-27] 574-589, is particularly important, Wentzel feels. It contains the germ of many later methods for handling scattering and conversion problems. By utilizing Sommerfeld's old technique of deforming a contour integral until it gives only an outgoing or only an incoming wave, the paper gives just the solution desired in forms that are now standard for all textbooks though their source is rarely if ever cited. Clearly Wentzel has particular feeling for this paper. No. 31, "Über die Richtungsverteilung der Photoelektronen," Zs. f. Phys,41 [1927], 828-832, Wentzel simply dismisses as "Unimportant and wrong."

Nos. 32 and 34 are the papers "Zur Theorie des Comptoneffekts I," Zs. f. Phys.,43 [1927], 1-8, 779-787. Here characteristics of the photo effect are derived from the Kramers' dispersion formula and the Correspondence Principle treatment. Wentzel feels that this was entirely justified since Dirac had already derived the dispersion formula. Under those circumstances there was no need to utilize the more elaborate theory developed by Dirac himself. In this application it was not needed.

Paper No. 33 is "Über strahlungslose Quantensprünge," Zs. f. Phys.,43 [1927], 524-530, and it deals with the Auger effect. One likely source of it is Wentzel's knowledge of Auger himself in Paris. Having discussed the effect often, he saw that it was a transition from the discrete spectrum to the continuum and must therefore be treatable by methods he had already developed. For Wentzel, this paper is one of the very most important he has ever done. In it are derived formulas now used for all aperiodic phenomena in quantum mechanics. There is one fornrula in particular which Fermi always referred to as "the Golden Rule Number 2" of quantum mechanics. In calling it that, Fermi and others had no notion that Wentzel had first derived it. Dirac also got the same formula utilizing time-dependent perturbation theory rather than, as Wentzel, time-independent theory. Actually Dirac's method is a bit neater, but Wentzel's came first. 

I asked Wentzel at this point in the discussion whether he bad ever worked on the older problem much discussed in Munich of the continuous X-ray spectrum. He said he had certainly not touched it at this point, or thought he had not, because it was just too difficult, and demanded radiation theory. His impression is that it was actually done by Sommerfeld in the early '30s for the nonrelativistic case. 

Paper No. 35 is the application of previous work on photoeffect to the electron gas, "Photoeffekt an Metallen" in.the Festschrift volume Prob. d. Mod. Phys. A. Sommerfeld 60. Geburts. [1928]. Wentzel now speaks of it as "not very fundamental," because it omits all considerations like those of lattice potential. In fact, Wentzel says, he was not really very much up on the work done by Sommerfeld's group on metal theory. He has, however, a very clear recollection of visiting Munich and talking about the theory. At that time he was very deeply impressed by Houston's remark that the mean free path of an electron gas becomes finite only when the nuclei in the lattice have a thermal motion. In the absence of thermal motion one gets only Laue-Bragg scattering which does not produce a limitation on path length. 

Wentzel next dismisses papers 36 and 38 as review articles with nothing new in them. Paper No. 37 is "Über den Rückstoss beim Comptoneffekt am Wasserstoffatom," Zs. f. Phys., 58 [1929], 348-367. It is of interest because here Wentzel for the very first time actually utilizes the Dirac radiation theory. Again, he insists that he had always known it and been interested in it. However, this was the first paper in which he had any need to use it. Given his concern here with the recoil of the atom, he could not have handled the problem by his older techniques. 

Finally, Wentzel points out that there is one paper of his not mentioned in our bibliography. It is called "Sur la predissociation" and is printed in a volume of the conference, Réunion Internationale de Chimie Physique, held in October 1928. This paper utilizes just the methods developed for the Auger effect. Wentzel seys that he knew of the problem from Victor Henri who was at Zürich at that time. However, this may not be the case since Wentzel had scarcely gotten to Zürich in October of 1928. 

This was as far as the conversation went. Wentzel suggested that from these problems his research had gradually turned to electrodynamics which is really another story.