Oral History Transcript — Dr. Alfred Lande
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Alfred Lande; March 8, 1962
ABSTRACT: 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: Ernst Back, Niels Henrik David Bohr, Max Born, Constantin Caratheodory, Peter Josef William Debye, Paul Ehrenfest, Albert Einstein, Walther Gerlach, Samuel Abraham Goudsmit, Werner Heisenberg, Heinrich Mathias Konen, Peter Lertes, Fritz London, Erwin Madelung, Wolfgang Pauli, Max Planck, Erwin Schrodinger, Arnold Sommerfeld, Otto Stern, George Eugene Uhlenbeck; Artillerie Prufungs-Kommission, Universitat Gottingen, Universitat Marburg, Universitat Munchen, Universitat Tubingen.
Kuhn:One of the things we haven’t talked about at all is the problem of X-ray spectra.
Lande:The X-ray spectra were treated with exactly the same methods as the optical spectra, so there was hardly any brand new problem turning up. The only place perhaps was this. After Sommerfeld had shown that the hydrogen spectrum was of a relativistic origin, the suspicion was that maybe the doublets in other spectra were of the same origin. Maybe the relativistic explanation did not only apply to the hydrogen, but also to other spectra. But the possibility also arose that it was the other way around; that the various orientations, with one angular momentum was in the last resort also responsible for the hydrogen spectrum. As in fact it turned out later, Sommerfeld’s calculations were correct only because two mistakes cancelled, and in fact it is due to a difference in spin orientation. This was always in our minds. There are papers by Millikan and Bowen -- and, papers about their papers -- in which this problem was discussed. I think I myself also wrote a paper on it…
Heilbron:Yes. You argued that the doublets were caused by different orientations with respect to angular momentum. And in that connection, I wonder if you recall that the orbit of the outer electron is to be arranged with respect to an inner core. The inner core is taken as the innermost ring, and is given an angular momentum of one. All succeeding closed shells are given an angular momentum of zero. The innermost one is given an angular momentum of one; although it is known that there are two electrons in that inner ring. This strikes one as being a bit arbitrary, and I was wondering if there is some background to that selection of the inner ring as unbalanced, whereas all the succeeding ones are regarded as balanced with respect to angular momentum?
Lande:I think there was discussion of that, certainly, and as it later turned out, these inner K shells -- doublets -- are of the same nature as those of the hydrogen. Now I am not quite sure. It can only be the ionized K shell in which then one electron is left in the inner shell which can be compared with the hydrogen electron.
Heilbron:Yes, but I think in this article, when you talk of ionization of outer shells, not of the K shell. If an L shell -- electron is removed, a rearrangement in the L shell takes place so that most of the angular momentum is cancelled out. The one electron left uncompensated is then enabled to take two different positions with respect to an already existing angular momentum in the core.
Lande:Of course there was a great deal of discussion going on at this time, and as always each of us had a different idea about that… There is one item which might interest you, in connection with all these spectral formulas. There is always, instead of j2 there is j times j plus 1, and r times r plus 1. Now this of course was absolutely mysterious on the grounds of the Bohr orbits. There were always two quantum numbers, j and j plus 1, k and k plus 1. I remember having spent a lot of thinking on a very fundamental change of the whole set-up. According to Bohr, a spectral line is the combination of two terms, two energy levels. Now this j and j plus 1 seemed to indicate that even a Bohr energy level is a combination of two things, with a sum of one unit. I made long walks and thought and thought, but of course nothing came out of it. But at that time it was really a problem. I do not know whether other people also had this idea, but I definitely remember… This was even as late as l924 and ’25… The spectroscopists were happy to have a formula which worked, and people who worked on the foundations of quantum mechanics, and doubted Bohr’s scheme, were very few. Bohr himself and a few people in Copenhagen, some in Munchen. So it simply was accepted. In order to stress the very strangeness of this for everybody to hear, I gave this little talk about the difficulties of present-day results, even within the quantum theory.
Kuhn:You write j2 - 1/4 rather than what is obviously the arithmetic or algebraic equivalent, j + 1/2 times j - 1/2.
Lande:If you have this plus 1/2 or minus 1/2, or you use quantum numbers 1/2 elevated, then it reads j times j plus 1. And this is always written in the literature, because if you do it with one quantum number, you must do it with the other quantum number also… This was the most natural thing to do, to symmetrize this j2 minus 1/4 in a reasonable way. And furthermore, the same quantum numbers j plus 1/2 and j minus 1/2, occur again in the interval rule, so this was quite natural.
Kuhn:When writing them in the factored form, and noticing the integral difference, which of course as you say does immediately suggest two quantum numbers since they differ by an integer, did this suggest changing the values of the quantum numbers themselves?
Lande:This was always a suggestion, but a very superficial one, because everybody realizes that you can count. Let’s say the oscillator levels are 0,1,2,3, and then have n as integral numbers. You can just as well count them as n plus 1/2 -- if you count n itself as 1/2 numbers. Sommerfeld and myself and whoever worked with this, choose the normalization of the quantum numbers more or less according to his taste. I mean, there is nothing central involved here.
Kuhn:I’m very much interested in this particular story you told us about worrying about the possibility of multiplicity in the Bohr levels themselves, as indicated by this. And it is just exactly the thing we will never discover by reading the literature.
Lande:All these great difficulties of the Bohr model were investigated in various directions. This idea that maybe the Bohr level is already a combination of two of them, plus 1/2 and minus 1/2, was a dead alley. And the approach of Schrodinger and Heisenberg simply led to something.
Kuhn:Do you remember if you talked to any people about this idea?
Lande:Not to any of those who mattered. I talked to people in Tubingen, but not to others.
Kuhn:We got into this by the X-ray problem. We’ve said nothing at all about the intensity problem.
Lande:I wrote one paper in which the intensities of the Zeeman patterns was also treated, and it is derived qualitatively from the correspondence principle. But quantitative calculations about intensity were first done, as far as I remember, by Pauli. He had quantitative intensity rules, also dominated by integral numbers, with some rules and things like that. And also by Kronig. Kronig and someone else -- I really don’t remember who -- wrote an extensive paper about the exact intensities to be expected in each Zeeman component. These were very complicated formulas which he derived from certain principles of symmetry, together with some rules, e.g. the total intensity should be one, like that of the original line. All of these formulas were later exactly confirmed by quantum mechanics. But the qualitative look of a Zeeman pattern -- either it’s more intense in the middle or more intense on the sides -- this can be found in a paper of mine.
Kuhn:You speak in your book of the problem of reconciling the correspondence principle treatment of intensity with the observed integral values of the intensity ratios. Was this a particularly bothersome problem?
Lande:I didn’t give too much thought to it.
Kuhn:Going back for a minute to the time when Duane’s paper was written, was there contemporary discussion of it then?
Lande:At that time yes, but of course only about light. It was taken as a very interesting sidelight to Einstein’s heuristic working hypothesis; this hypothesis even worked in this respect, although everybody knew of course that Maxwell’s theory of continuous waves is at the basis of optical phenomena… Ehrenfest and Epstein applied the same quantum mechanical principles on the behavior of photons, also on any distribution, not only a periodic one. They worked out the same for say a screen with a hole. I don’t think they worked it out in detail for a screen with two holes, but obviously it would work in those cases also. It should have been the obvious thing to remember Duane’s paper also in the case of the diffraction of matter through crystals, and in this way save a unitary quantum mechanics, instead of this mysterious back-and-forth transformation and double manifestation.
Kuhn:Did you talk with Born at all about the Duane paper?
Lande:No, I did not. At that time for years I didn’t see Born, during this period. And I myself didn’t take this Duane paper all too seriously. I think that if people would have been more conscious of Duane’s quantum mechanics, which he applied to light, and had applied the same thing, as he should have done, to matter, I think 30 years of dualism would never have arisen. We have a unitary theory, and to demonstrate the deviousness of this double manifestation, I always take this example. Here is a wall, and here comes a particle at a given angle. And this particle is reflected at the same angle. Why is it reflected at the same angle? Because of the conservation laws of momentum and energy. Now the dualist, however, if he doesn’t want to desert his cause under fire, would argue as follows: ‘Here comes a particle. Near the wall it transforms itself into a wave covering the whole wall. According to the Huygens principle a plane wave hitting the wall in this direction is reflected only in the one direction at the same angle. The Huygens principle asserts there are secondary centers sending out circular waves, which however cancel each other except in this one direction. So, the angular law of reflection is due to a wave action. After these waves have done their job of interference they retransform themselves into the form of particle again.’ Now no physicist in his senses would accept this heuristic theory for the reflection of the particle from a plane wall. What is the difference between a plane wall and a crystal? A crystal is periodic, it has only certain definite periodicities, L, L/2, L/3. Consequently, it can give out momenta with conservation of total momentum and energy only in certain quantized amounts, and this is the reason for the selected reflection. An ordinary wall however does not contain only one L as periodicities, but it is unperiodic -- it contains every length, L. Consequently, it is capable of reflecting under the laws of mechanics the particle coming from any direction into the corresponding reflected direction. And so this is a demonstration that things can be done in a unitary fashion. We do not need this whole doctrine of double manifestation. Everything goes on in a natural way.
Kuhn:Do you know whether anyone in the early ‘20’s, after the Duane paper and the attempts to extend it, considered attempting to apply it to the Ramsauer effect?
Lande:What is the Ramsauer effect? … I don’t remember any discussion about that. But let me only add a few words to this. The quantum mechanics of 1926, a combination of Schrodinger, Heisenberg, Born, Jordan, is a mechanics of particles and probabilistic interaction between particles and bodies -- that means systems of particles. In this quantum mechanics there occur these waves which are interpreted as probabilities for the location and behavior of particles. So, it afterwards turned out that Duane’s mechanical theory of reaction leading to the interference maxima and minima, is just a special case of a more general quantum mechanics. Today this is nothing revolutionary at all but is the accepted quantum mechanics. But, as I always express it, it is accepted only on weekdays; whereas on Sundays the dualistic talk goes on and on. And I think this is very illogical. If we have a unitary quantum mechanics, then this whole dualism is completely antiquated. This idea that the pictures had no physical reality, even for the physicist, had very great consequences. It has led to the whole subjectivistic world view of Copenhagen and of Heisenberg. It appears in this still-persisting Sunday talk that there is a double manifestation of matter. Actually every physicist knows that this isn’t so. I am rather convinced that under the impact of writings, in particular of Popper and Einstein -- who never gave in to Bohr -- and now my own writings, there will come a time when even the physicists see that they cannot talk differently on Sunday from what they act on weekdays. So because I foresee the time in which dualism will be abandoned in favor of a realistic view of quantum theory as a completely objective science in which objective data given by instruments are connected by probabilistic rules, because I think this time is not very far away -- I think it would be interesting also in this connection to see where are the roots. And the roots are already contained in this paper of Duane of 1923.
Heilbron:May we talk a little bit more about the year in which Duane’s paper was published? You said that it was generally regarded as being a sideline or a curiosity. It seems that at the same time as the Compton effect was discovered, it should have been taken a little bit more seriously than it might have been a few years earlier. And so one wonders whether or not the Duane interpretation was coupled with the Compton effect.
Lande:In some way of course it was coupled with it. Duane was an X-ray spectroscopist, and Compton was very close to all that too. And certainly these two men discussed this idea also. Duane’s paper was certainly known in Europe. I had a letter from Max Born, which he wrote me in answer to one of my recent papers where I complain that people simply have ignored this very important paper of Duane. At that time Duane was certainly taken very seriously and discussed, but he is never quoted again. This happens always -- only the real big, great men are quoted. That means Duane doesn’t belong to the real great men. And isn’t it on this postal card of Sommerfeld which you found among these papers of mine, that he wrote that Duane’s paper was regarded as a kind of curiosity? … Anyway, it was considered, just as you said, as a curiosity, and not pursued. Because at that time nobody thought of matter diffraction -- in 1923, 1924. When 1927 came -- it was almost at the same time as Schrodinger’s wave equation came out -- then it was the natural thing to identify these two things. The Schrodinger waves are the same agents which are obvious in the Davisson-Germer experiment.
Heilbron:Well, even in 1927 or 1926, weren’t there continuous undertones of dissatisfaction with the interpretations that were offered just after the new mechanics came in? When would you say that the Copenhagen interpretation was consolidated and firmly established?
Lande:Bohr came out with his principle of complementarity for the first time for a larger audience at this Volta Congress, which I gave you the pictures of. And Heisenberg immediately took it up, and so it dates from that time.
Heilbron:And you would say that compelled immediate adherence?
Kuhn:Do you remember discussions there?
Lande:No… It’s a very fascinating idea, but according to the statistical interpretation, it was an absolutely illogical idea. How can a particle be complementary to one of its own properties, its statistical tendency of distribution in space.
Kuhn:How did you yourself feel about the idea at the time?
Lande:How I felt? This is really hard to say. Everybody found a certain beauty in this symmetry of waves and particles, without giving too much attention whether it is logically tenable or not. If there were people in opposition they didn’t make their opposition public, because, you see, physicists are not so much interested in ideologies, but rather in working theories and formulas. And then if one of them, such a prominent man as Bohr, comes and also gives them the Sunday words to sanction their ideology, then most people are quite satisfied. Particularly physicists who are realists and don’t care so very much. Except a few individuals. Einstein simply objected to this whole thing, and to everything connected with it, to the idea of two pictures instead of one reality; to the transformation of Heisenberg’s uncertainty principle of prediction into an indeterminacy of being. Einstein simply didn’t accept this, and at one place he said, “If we have the statistical interpretation, then this whole egg-walking about existence and non-existence is completely superfluous.” He used the German word “Eiertanz”. He never gave in. Schrodinger certainly was always convinced that a unitary theory is necessary to satisfy ideological demands of consistency. But the great majority didn’t care. And as for myself, for 20 years I tried to imbue my students with the Copenhagen spirit, only having more and more bad conscience.
Heilbron:But that’s in its more exotic formulations. For instance, isn’t it true that Einstein, after the series of arguments with Bohr, agreed that at least in Bohr’s formulation of complementarity, there was no intrinsic logical problem?
Lande:I don’t think that he conceded anything like that. The whole discussion went back and forth over the years. Einstein never yielded and Bohr never yielded, and if you look up the Schilpp Einstein volume, and see Einstein’s reply, he was very dissatisfied.
Heilbron:Oh yes, very dissatisfied. But in all these little experiments that Einstein designed evidently Bohr succeeded in persuading Einstein that his proposed experiments did not violate or invalidate the logical consistency of at least Bohr’s interpretation of complementarity.
Lande:Now thought experiments are always a very dangerous way of proving something, and Einstein may have conceded that this is, but I don’t think that he finally consented anything. And in fact, Bohr’s big guns to convince Einstein, rested on Einstein’s own general relativity, which he needed in order to prove the indeterminacy of energy and time. Now this is not my remark but the remark of a pupil of Popper’s, Dr. J. Agassi: “If you need general relativity to prove quantum mechanics, then it ought to be the other way around also. From quantum mechanics you ought to be able to derive general relativity.” I mean if you need an argument of general relativity to confirm quantum mechanics, the two are closely connected -- one depends on the other. If general relativity were different, then this proof of quantum mechanics would not be valid, then quantum theory would have to be different than it is…
Kuhn:Now there has to be some time structure to this. This has gone on over a period of time, with changing opinions and so on, and one would like to recapture some of that.
Lande:It is a matter of social psychology -- it doesn’t belong in physics -- how people react to a great man when he utters ideas which are in fact out of his own field, and when a physicist becomes a philosopher… As I said, the main opposition was Einstein and a few others of his school. One of them in particular was Popper. But Popper was a preacher in the desert, because the physicists simply said, “What does this man understand of physics?” “He is just a philosopher. He knows everything only second hand. He never has succeeded to understand our great Bohr.” This is the attitude of most people, because in fact they don’t care. They are satisfied to have a ready-made ideology, produced by a few of the great names in physics. And why worry? And in fact it hasn’t hampered the progress of physics the least bit that people on Sunday talk of this or that. The research in physics has gone on without interruption, and furthermore, the idea of dualism is of great heuristic value. You always can imagine that these waves are waves, and follow wave equations. You carry through your calculations and in the last moment remember they are probability functions for particles. So it is of great value… The research in actual physics has never been hampered by this idea of dualism -- that matter has a dual nature -- because everybody applies the statistical interpretation, the pure particle interpretation anyway. But what it has hampered is further research into deeper foundations of quantum mechanics. Quantum rules are simply accepted. During the first decade after Planck’s discovery, practically every young man in physics tried to solve the quantum riddle. I told you about this, about Sommerfeld’s seminar. I was one of them, and several others also tried to solve the quantum riddle by trying to find some gap in the equi-partition theorem of the energy. Then gradually the interest in solving the quantum riddle died down completely, and the quantum rules were simply regarded as something very fundamental, and that’s all. But of course one can ask whether they cannot be reduced to something more fundamental. It is exactly the same as relativity. Assume that the formula e = mc2 or the dependence of the mass on the velocity has been found. First the great shock that mass is not constant. Gradually people got used to it and accepted it. But Einstein has shown that not only the mass dependence, but also e = mc2 and many other things, can be derived from simple principles of invariance and symmetry. Then if you have these principles, then you understand the facts better. And this same procedure in quantum theory has been delayed about 30 years by the blind belief in duality as fundamental. Hence there must be wave-line functions, and so on. So ideologies have a certain influence on the course of science. We haven’t spoken about the exclusion principle at all. Of course first comes the spin, which is Goudsmit’s department -- or the exclusion principle came first?
Kuhn:How did you first hear of the spin?
Lande:I think I first heard about it from hearsay. Someone told me, “Did you hear the latest this and that of the spin.” And then I looked up the paper in Naturwissenschaften and saw it. But who told me I don’t remember.
Kuhn:How did you react? Did it seem to you then a definitely great idea?
Lande:Everybody immediately accepted this as the simple solution of the whole problem. In particular after the work of L. H. Thomas.
Heilbron:Professor Lande, I think you said the first day we talked that the strangest idea of all the ideas -- models -- which were advanced to account for the anomalous Zeeman effect and others was spin…
Lande:Because when a charge e of mass m and radius r = 10-12 cm spins so fast as to yield a magnetic moment of one magneton, it yields a spin of h rather than 1/2 h, and there are incredibly high centrifugal forces. It’s just as strange as suddenly to be told, the earth doesn’t stand still but turns around. It’s obvious to everybody, the electron is a little sphere, and it revolves; and numerous other people always assumed it was a little sphere. Maybe it is compressed into an ellipsoid, when it travels fast and so on. Nobody ever thought of anything like that, and if he thought of it, then he immediately came into difficulties: what keeps it together against the centrifugal force? And furthermore, at that time already it was thought not elegant to think about the internal structure of the electron at all. Because what keeps the charge together is already an unanswerable question.
Kuhn:But your impression still is that as soon as the idea came out, it solved so many problems, it was immediately accepted?
Lande:It was immediately accepted, because with one stroke it solved all the difficulties, in particular in connection with the exclusion principle -- the fourth quantum number. Everything suddenly fell into place. Except the mechanics of the spinning electron itself… My only recollection regarding the exclusion principle is that Pauli came to Tubingen and found there was a certain line missing in the spectrum of lead which gave apparently the last confirmation of his principle being correct… Pauli told me at that time that there is a fourth quantum number, and all that. But this story of the exclusion principle is much better represented in this article by Van der Waerden especially dealing with it. And then comes another very interesting article by Goudsmit about the nucleus. About that I also can only say that Back always showed me this strange multiplicity of spectral lines of lead and other elements, and I always thought, “Well this will clear up in the course of time, probably these are isotopes.” But of course it couldn’t be isotopes because some spectral lines were quintets, and others were triplets, and some were single. I didn’t give much attention to it, because I was busy with working out the multiplet structure of complicated elements like neon and argon and so on. Prime terms and, double prime terms, and things like that.
Kuhn:Was Back, as an experimental spectroscopist at Tubingen, himself closely interested in this development of the theoretical models, and so on, or was he simply most interested in pinning down the spectroscopic data?
Lande:He was very much interested in the models also, only he was not a schooled theorist, and so all his material had to go through the hand of someone who had, more time to arrange it in order.
Kuhn:Now I wondered whether Back’s more fundamental effort of direction had been towards the elucidation of the lines. To what extent did he look especially for things that you or other people might speak to him about, or write him about, that on the basis of theory might have been expected? How closely was his own work guided by suggestions from the more theoretically oriented members of that group?
Lande:I am certain that Back sometimes sat down with all of his material and tried to put some general sense into it. But first of all, he didn’t know very much about quantum theory. At that time I don’t think he ever applied the vector model, but his attitude was finding out a more general formula. He would have been quite satisfied with finding the g formula without any model sense in it. This is the usual approach of the experimentalist. But on the other hand ho was very insistent on his experiments being absolutely reliable and true.
Heilbron:Those lead lines you spoke of a little earlier, that’s the hyperfine structure?
Lande:Yes. Back always worried about it, because the first idea of the spectroscopist, if he sees such lines, is to think that they are ghosts produced by imperfections in his apparatus. But they were absolutely real. But it’s a blessing that there are so many physicists -- that one finds this and another finds that…
Heilbron:I wonder if we can change ground completely and if I may ask you how the change to American educational standards and systems affected you. How this differed say from what was true at Tubingen?
Lande:My main impression was that the education here is much more thorough and precise than anything in Germany. In Germany there were lectures, and at best you could come after the lecture and very bashfully ask a question. The man looked at you with big eyes -- it wasn’t encouraging at all. Whereas here in America these constant quiz sections keep everyone going and immediately realizing whether he is understanding it or not. I think the system here is very, very much better. Of course how much better it is depends on the individual teacher, whether he teaches superficial techniques or once in a while alludes to ideas.
Kuhn:How did you find research in this country? One thing that occurs to us is that the years since you came to the United States have been years of an immense pick-up in American physics.
Lande:Well, it’s just as you said. When I came there wasn’t too much research going on. Things just began to pick up. My first year here -- I think it was in 1950 -- there was a Physical Society meeting in Chicago, and at the dinner were perhaps 30 or 40 people. Now there are 3000 people -- an enormous change. But as to the methods of teaching, I think they are much more thorough than over there.
Heilbron:On the other hand, it seems that with this cafe society approach, and so on, that students were discussing contemporary problems in Germany; whereas here one must be a graduate student at some advanced level before one is even conscious of what is going on.
Well you must remember that of the students in Germany at that time there was a very, very small percentage of people really wanting to go into scientific careers. The physics students were only a small group who had a particular interest in that field. Here selection is complicated and different.