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In footnotes or endnotes please cite AIP interviews like this:
Interview of Herman Burger by John L. Heilbron on 1962 November 15,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with circa 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: Niels Henrik David Bohr, van Cittert, H. B. Dorgelo, Paul Ehrenfest, Albert Einstein, James Franck, Werner Heisenberg, Hertz, Holst, Hendrik Anthony Kramers, L. S. Ornstein, Wolfgang Pauli, Jean Perrin, Arnold Sommerfeld; and Philips Laboratories (Eindhoven).
I forgot to tell you that in 1919 Bohr visited this laboratory. He came to give a speech on quantum theory to a Congress which met here for three days. Then I met him. He visited the laboratory and showed interest in the problems that were getting along here, especially the intensity measurements that were planned then. There was a group of visitors and Moll’s microphotometer was shown — the instrument to measure intensities inside spectral lines with a high resolving power. Bohr showed some interest. But after some time we didn’t see him any longer he was in another room walking to and fro, and I don’t know what he thought about. But after the demonstration there were questions asked, and what struck us was that Bohr’s question was, “What’s the price of this instrument?” But after some time, he bought an instrument, and measurements of spectral lines were done in Copenhagen. So that was a question that afterwards was understandable, but at that moment we didn’t understand bow a theorist could ask a question like this. This is what I remember of Bohr.
Another point is the yellow helium line. That played some role in our work. The yellow helium line was double, but it should have been a triplet, and hence with the distances in the ratio one to two and the intensities in the ratio one to three to five. Therefore, we tried to resolve it with a high resolving power, van Cittert and I. I think we used a Fabry-Perrot, but I don’t remember exactly. Well, first it was very difficult. There were instrumental difficulties. But then there was another difficulty there was a printing error in the tables. We used the tables of Fowler in that time, which gave a lot of information about spectral lines and there was a printing error in them. We looked it up in another table afterwards, but at first we trusted this table. And then we found the ratio of the intensities was one to eight, so the three and five were coinciding. We saw that the rule for the distances was not true in this case, but the rule for the intensities was. That was an important fact.
And you realized that was a triplet line at that point?
What was the general attitude towards the helium lines, though? They were thought to be doublets at that time.
No, but we had the conviction that it had to be triplet lines at that moment. And, therefore, we thought that the intensity measurements could give the explanation. Therefore, it was a disappointment that we found only two lines, as everybody had found. Afterwards it appeared that three and five were coinciding, and then by cooling with liquid air it was possible to find a trace of resolution of the three and five. I was not involved in that, but it was, I think, done by a man of my name, but he was no relation whatsoever. Then it was found one to three to five, so the intensity rules, we thought, were mightier than the rules about distance. That gave us the conviction that our work was worthwhile; that was a very nice result for us. Without going deeper into theory, but again purely formally, we tried to see what the range of rules was and how far we could extend them.
Another extension of the rules was to more complicated multiplets. For the simple multiplet the sum rule was enough, for higher multiplets it was not, and we tried to find a formula for it. We had tried all kinds of formulas, but none of them fit. Then finally there came the paper by Sommerfeld and Hönl. And they gave very nice formulae, but they were not true for the case we knew, and that was the iron spectrum… It was not yet published, but (???) had measured the intensity of that very complicated iron multiplet. We had gotten the data from Sommerfeld before they were published, and, therefore, we could measure the intensities, and the sum rules were all right — but not those formulae. We couldn’t find the formula; we knew too much at that moment. There was a complication that the coupling was not so simple as was thought at that moment…
The episode you’re talking about was after you’d published your article with Ornstein?
It was certainly later, yes. I’m not quite sure what the year was, but it was certainly later, yes. It proved that sometimes it can be a difficulty when you know too much. I will not say that it doesn’t help in making a theory, but it can be a difficulty in finding an empirical rule. We couldn’t find it because we knew how it really was, as regards the intensities of the spectral lines.
Another problem was the ratio of one to two for the doublets of the alkali metals. We thought it was one to two, but later measurements showed that it was true for sodium, but not for the alkalis of higher atomic weight. The ratio gradually increases; it was measured by Sambursky here. I’m not sure what theory, at that moment, predicted, but I think that later on this was all explained. That proves that that simple extension of a whole number rule was not at all correct. But happily the experiments didn’t give one to two; they gave, I think, the values that could be explained by theory.
Now I remember a general point, since we mention here the name of Sambursky. In those years there were many foreigners here working in this laboratory. Ornstein attracted many people, and we had people from the United States, from Germany, from France, from England, from Sweden, and so on. They were all working on these problems of the intensity of spectral lines. And now the last point; I must mention the measurements of band spectra which was done later. I never did work on this subject, but other people did.
There was Brinkman; perhaps you know the name of Brinkman in this respect. He made his doctor’s thesis on this subject, and others too. But that was a very important problem — temperature measurements with band spectra, but also the band spectra themselves, and the rules of intensity. Now among the things that also came later was the attempt to measure the intensities of the helium lines and explain the way in which the ratio of those helium lines the ratio of intensities changes with current density. We wanted to understand what happens in a capillary discharge tube, both electrically and optically. Well, we didn’t succeed in explaining much… It was too difficult a problem to say much from the theoretical side. The only thing that came out of it was the ratio of helium lines corresponding to transitions originating on one and the same level. Well, these were the points I wanted to mention.
Well, good. Thank you. We were going to begin to talk about the very interesting series of papers you published with Ornstein on the light quantum.
I must say that my first impression when I received your letter was that I was surprised that we had done so much on this subject. I had almost forgotten it. I knew the fact, but I thought that was only one paper, and that we left this subject very soon. But now I saw that we continued it (during two years) or so. Well, you see in this work again this classical approach of the kinetic theory of gases. I think it was simply the application of an old line of thinking to this quantum problem. That is also characteristic of our way of thinking then. That was all a very rough approximation not giving the numerical factor, but only the way in which a quantity depends on others. It was more or less a dimensional way of treating the problem, rather than a real theory. Again, I must say, that Ornstein was very much leading in this. I liked to speak with him about these things, but the initiative was certainly from his side. This was entirely his way of thinking, and his way of approaching a problem. And we thought that we were on to something, because several points came out. Now, afterwards, I must say I have forgotten them all. But I think I had the impression that it was more or less a chance that we struck the right things because there were only dimensional results, and perhaps not much more; but I don’t know.
The arguments were a little rough.
Yes, yes; I think so. I think nobody believed anything of it in those days. It was considered entirely nonsense. I have read the papers you have mentioned, and I have read them with interest, but also as if they were the papers of another man. I saw that in these papers we had the idea of light quanta as spheres, but I remember there was a time when we had an idea of long, thin objects. I think we did publish that (somewhere) because I remember that a physicist spoke about those
Würmer; he was a German. He found it terrible. I don’t remember exactly who it was. I remember that I met a physicist in a streetcar in Innsbruck and we talked about this, but he didn‘t appreciate it very much.
Do you remember what Kramers or others said about it?
No; I fear Kramers said nothing about it because he didn’t believe that it was worthwhile.
What about Ehrenfest? Ehrenfest might have had something interesting to say.
Yes, Ehrenfest would say what he thought, but we had no contact with Ehrenfest, I had contact with Ehrenfest during my Eindhoven time, but he didn’t often come here to Utrecht. It was very seldom that he was here, or never. I had no contact with him, and I didn’t know what his opinion was about this. I had no idea.
There is this idea of introducing the cut-off frequency where the volume of the light quanta equals the volume of the container. Do you remember what people thought of that argument?
I’m very sorry, but I must say that I have read it with interest as though it is the work of another. I’m sorry, but I don’t remember more than these few points. But how did we come to it —? It was, of course, influenced by preceding purely classical thoughts. We had done a lot about all kinds of classical problems of correlations, and gas theory, and so on. They were not published at all. Brownian movement was our interest and I remember also linear problems. We had the problem of one dimensional linear molecules moving along a line and colliding. Then we wanted to find how the velocity frequency distribution is changed when you begin with an arbitrary distribution. There were problems like that. It looked more or less like Flatland. Well, we lived for a time in such a one dimensional world, and there I think we had the idea of the influence of the lengths. That might have influenced our thoughts here with the quanta to take into account the dimensions of these things. So that was not such a new idea; it was a quite classical idea again applied to these quantum problems.
Do you recall if you discussed it with Einstein at all? You mention in one of the papers that you had talked to Einstein.
It must be so, but here my memory fails; no, I don’t know. I know discussions with Einstein on those problems of influence on the level going up and down and of transitions between discrete energy levels on the equilibrium of electrons when this chemical reaction is included. There must have been more talks with Einstein, but I do not remember a talk on this subject with Einstein. No.
Do you remember how you felt about it at the time you were doing these things?
Well, I never thought that it was very important. No, I didn’t like it so much, and as I said it was more Ornstein’s idea, and I just worked with him. I tried to do it as well as possible, but I didn’t like it so much myself. It has never made me very happy to do things like this.
Was Ornstein content with it?
I think so, yes, yes; certainly he was content because he always liked to explain the things classically, and he had the conviction that behind quantum theory was hidden a reality that is classical. This is a purely classical approach, too, you might say. There is the idea of those spherical particles, but then after having put forward this idea, the rest is purely classical. And I think that he was very content to see that things came out that were in accordance with reality; you could explain Planck’s law. You could not really explain it, of course, but you could see in a development of Planck’s law correlations of quanta two, three, and so on. I think it was a great satisfaction for him to see it, but it was not so much to me.
How did one collaborate with Ornstein? I mean, what were the mechanics? Did you write the draft? Did he write the draft? Or did you both write and compare?
No, we did it together. First one sentence and then another. It was a real collaboration and it was quite interwoven. We sat down, and first we made a scheme of the paper, what had to be put into it. Then, by chance, came a sentence from one side or the other, or there was a criticism that it was not good, and it was canceled and another sentence came. This was our way of working… It was quite different from the work with Moll. We have also written several papers and a book together. With Moll, I wrote it in the first instance. Then he corrected this, and it was often quite different. Then I had again my criticisms of his changes. We worked one after the other, but alone. With Ornstein it was together.
What about Dorgelo, or that was —?
It was not so often; it was relatively seldom. I think I have written the papers and he has given his opinions, as far as I remember. That was not such a real collaboration.
For a paper say for the Zeits. f. Phys., would you write it in Dutch, and then translate it into German, or just write it in German?
[???] school as a little boy. German was spoken by one teacher and Dutch by the other, so, more or less, I was educated in a German (manner). Then, afterwards, I went often to Germany, and we used German books, also. During the University, it was all German; almost no English books, or American books. Then the contact with the Germans was greater than with others. That made it so that we were entirely fixed to German, so that we wrote it immediately in German; it was quite normal.
When did things change a bit, and English become more prevalent?
That’s the difference between before the war and after the war. I think before the war there was no sign of change, whatsoever. I don’t remember that English was more prevalent than it had been twenty years before that moment. And then, after the war, it was at once quite different. It was at first because in Germany there was not so much published, but more important, I think, because there was no tendency here to read German works, as you may understand. During the war, well, there was not so much done, but there was the tendency to look more to the English. But during that time there was the transition from one to the other quite distinctly. Before the war we had here perhaps more English than German speaking physicists visiting the laboratory, so we spoke English with them. But our data came from the German side more. I’ve worked, you know, a long time with two Englishmen; that was before the war. We spoke English during the time we worked together, and that gave me some experience, but I read almost exclusively German books and German papers. And these English people, of course, introduced the English things, but they had not so much effect, I think.
I was interested that so much of the Dutch science published abroad was published in German journals, but that the version of the Transactions of the Academy of Sciences that is intended for others to read is in English, and was in English from the very beginning. So there seemed to be a kind of split there.
I don’t know why that was. I had not many papers in the Transactions of the Academy, only two or three or so. That was the influence of the director of the institute here, Julius. He was (most active) in the Academy, and it gave the possibility to have things published very rapidly. So that was the reason why it was probably published there, and it had to be written in English. But I don’t know why that publication was English when for us it was all German. I remember we had an English book for astronomy during the first years that we studied, and it was very strange. Many students didn’t understand English enough; it was taught in school then, just as it is now. People should have read English books, but it wasn’t done; it was considered to be too difficult, and German was much easier. There was that very strong tendency to stick to German. Now, I remember that when we wrote a German paper, we didn’t have it corrected by an expert. Now, when we write a paper in English, we have an expert, who corrects it for us before it is sent to the journal. That wasn’t done with German. Well, I think there was an editor who corrected them a little bit now and then, but not so much.
Do you know what the refereeing policies were in German journals? Did they send out articles to be refereed?
I never hear of it; I think they did not. There were not so many papers, of course; you understand. So, perhaps, it was not necessary. Now and then, very unhappily, papers were refused. I believe the papers of a very famous physicist were refused in the beginning because they thought the work was not true. Then the reverse policy began; papers were accepted that were entirely wrong. In Phys. Zeits. of those times you can find papers that are entirely nonsense. Perhaps my papers with Ornstein belong in this category, but there were others where this is perhaps still more evident — entirely nonsense. And not only nonsense, but even there were papers of a man who has constructed his results. It was the diffraction of an electron beam by a crystal. He calculated how the diffraction rings had to be, and he showed them. He wrote an article, and it was accepted.
What is the policy now in Dutch journals?
Well, you see for many years I have lived in a medical world, the American medical world. So I know how it is in medical periodicals in the United States. And there they are certainly refereed. (They accept only a third, I think.) One of my colleagues has said that if you extrapolate from the last twenty or thirty years, in one-hundred and fifty years the total mass of the Physical Review will be equal to the mass of the earth. You see how necessary it is to reject a lot of papers.
But how it is now in Dutch papers, and in our Dutch periodicals, I don’t know. We have had a Dutch periodical on physics since about ‘21, or so. And that was split in two. One is more popular and one is more, well, in the other direction. That was also long ago. And I had only a very few papers in that popular periodical and none in the other, so I know very little about it.
Well, perhaps you would tell a little bit about how you pursued your career after the work with Ornstein and how you got interested in medical physics.
Well, as I told you, I was interested in medical problems very long ago because I didn’t know how to decide between the study of medicine and physics. Then I had a very strong influence from my brother; I had a brother more than three years younger than I. He studied medicine for during my time of study and a little bit later. He always told a lot about his study, and that gave me the interest in medical problems. Well, it was more or less latent. Then when I came from Philips here, I had the job here to lecture on physics for the medical students. That was my main job. That brought me more and more into the medical problems. At the beginning, it was only a very little, although I had the interest. But gradually my interest grew.
Finally there came a moment that my brother sent me a medical paper with a drawing — it was in ‘38, I think — and he said, “Now here must be something wrong.” And he was quite right; there was something wrong from the
physical side; it was quite nonsense. Then began the time that I abandoned fundamental physics totally, and I went entirely to medical physics. It was this impulse of seeing how medical people were on an entirely wrong track. That gave me the impetus to help them and to put things right. Since then I have stayed on those problems. I am now so far on the medical side that I see patients here every week. Well, I must say that I’m very happy with this change of subject because now I have the feeling that I have two lives. I’ve had one, and that was finished, and I began again. It was not abruptly, but gradually more and more in that direction. Well, I have this past and now I have my present problems, and my present life, so I have experienced a lot in n life I feel quite happy in this way.
Well, we would be delighted for anything else you would care to say about the world of physics or medicine, or the history of physics.
Yes, this is history, but I am not especially interested in historical problems; certainly not history that is loosened from the general ideas. I like more the history of an idea than the history of a man. As a matter of fact, I am now giving a lecture for the older students and for my associates (on electrocardiography.) And I perceive now, more or less with astonishment, that I’m treating this subject historically. I’m treating the subject of how physics developed and how this problem developed in the medical surroundings, quite without any contact between the two. That’s quite strange with the subject of (electrocardiography.) And it developed, well, without any (contact). And then there came a contact, and. from then it was entirely changed. Well, I must say that I feel quite happy that it was possible to change it, but physics came in too late. It serves as a warning that physics must not come in too late, because then the things are settled, and it is so difficult to change. When the new physical method is introduced and immediately, or shortly afterward, physics comes in, then you can direct your way of using the method in the right direction. But when it is left to the medical people, the danger is that they will use it in the wrong way with wrong notions and thoughts, and then it is very difficult to change this.
It’s always difficult to change the medical profession.
Yes, perhaps a little bit more difficult than the physical. On the whole, there’s not such a great difference between the people, but their surroundings differ. One man is different from another in this field because his surroundings are different, and his surroundings are different because every man is different. So there is this coupling. But the important point is that we once thought that in medical circles it was more normal to be more secretive about scientific results. I have talked with several physicians and also with physicists, and I think that the result is that there is not a real difference between the two groups. There are extremes of secrecy and of quite open behavior in both groups. How the average is, I don’t know; I don’t know how to measure it. I think there is a difference, but it is not so great. But it is significant that when patients come, physicians feel obliged to help the patients. There is not one patient, but there is a second and a third and so many more — one after the other, and the physician has no time to think. They have simply to apply their knowledge, and that draws then apart from science. The physicist is different. When a physicist is in practice, say in a research laboratory, or in a University laboratory, then the practice is not only to apply what he knows, but to bring forward new knowledge. That is his job, and it is not the job of the average physician, and that makes a difference. Therefore, I think when a physician has, more or less by chance perhaps, found a thing that is worthwhile, he is very afraid that others will take it away from him. This makes it so that some physicians see this behavior and so react against it that they go quite to the other extreme and say, “All must be entirely open, and everybody can come here, and I will tell him everything, and when he wants to use it for himself, let him do it.” I think that it is in reaction to the average attitude that you get this extreme in the other direction. Perhaps that is quite right. I feel always the necessity of being open and to receive people here and show them what we are doing.
Well, you certainly have been open and very receptive to me.
Well, yes, and. I have never seen a bad thing from it. [Recorder is turned. off here, and comes on again with the following statement from Burger.] Ornstein became ill; he had a heart attack in the thirties, but he recovered. For several years he was quite well again; he came to the lab and enjoyed work and so on, but then came the war in ‘40. The Germans left him alone for a few months, but then he had to go. He was released — simply put away, and that was so bad for him. He stayed at home, and he worked a little bit. Then he died from sorrow and grief; that was the end. So I cannot say that it was exactly a murder.
He died in ’40 or ’41?
It was ’41, January of ’41. The beginning of the war here was in May of ’40, and he remained in his place here, I think, until August. Then he was at home during the rest of that year. He died very suddenly. It was very typical that he worked still at home, but the question was what to do without periodicals, without people to speak with, without instruments. One of the problems he worked on was the optical illusion of two equal lines, one with an arrowhead at each end and the other with an arrow tail at each end. The line with the heads looks shorter than the line with the tails. He investigated that quantitatively. He made the former longer than that of the latter, so that the lines looked equally long, and then you have the measure for the illusion. And this measure was obtained as a function of the angle of the arrowhead. That was his work in his later months. Then he could be busy quietly sitting in his chair without having more than a paper and a lead pencil.
It was very difficult for him not to be able to come here, not to be able to work as he had done during so many years. He had been a professor for 25 years; that’s a long time. The number of theses of what we call promotions, made under his supervision in those 25 years was almost a hundred, 90. I think there was never a professor here with so many theses under his direction as he.
Did he work closely with those students?
Oh, yes; he was always talking with students. I remember him sitting in that room talking with people, and then going around and knowing all of what happened in the lab here. He was an extremely active man, and he worked very hard until May, l940. The only thing that was different after his first heart attack was that before then, he came on his bicycle to the Lab, and after that by a taxi.
How did the laboratory get on after the Second World War? Did the Germans interfere with it curriculum at all?
Happily very little, but it was closed in the last year, (or quarter of a year), because there was no heating and no light. Now and then we came here — there was one room where there was a stove in which we burned peat. We sat around that stove and talked a little bit and went home again. It was that last winter; the winter of ‘44 and the spring of ‘45. During the other time there was a little bit of work done here, but gradually it decreased; in the year ‘40 there was almost full work, and then gradually then it decreased to zero because people disappeared. Students were taken away, and they didn’t dare to come here; only the older people cane here, because it was too dangerous. They were all distributed over different places and put in cellars and so on. I think the only thing that was taken by the Germans were two ammeters or so; it was very little — no really necessary pieces or instruments. So just after the war, we could start again. I remember in the summer of ‘45, in August, we started again. There had to be just a few months to recover again and to make plans. During that year from August ‘45 until August ‘46, we had twice as many courses as otherwise in a full year. We had a thousand students in that time. Before the war, I had — but they were medical students — about a hundred or so. The students had accumulated during all these years, and they had to be helped. So we had to make a course a little bit abbreviated. The number of physical students has also increased enormously. As I told you, we were seven in my first year in physics, mathematics and astronomy together. And now the number is about one hundred and fifty to one hundred and eighty.
Did the Germans make any effort to try to enlist any of the scientific personnel in their war effort?
No, they took people away, but not for scientific work, I think. There were the strangest things; they took away first year medical students, and they had to do the work of physicians, for example. But real scientific work I think was not done by the young people taken away from here. But they didn’t all return; perhaps fifty per cent returned; the rest died there. That was a terrible time. I was too old to be in any danger.