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In footnotes or endnotes please cite AIP interviews like this:
Interview of Philipp Frank by Thomas S. Kuhn on 1962 July 16,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
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 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: Ludwig Edvard Boltzmann, Percy Williams Bridgman, Louis de Broglie, Patrick Browne, James Bryant Conant, Paul Ehrenfest, Albert Einstein, Franz Serafin Exner, Arthur Haas, Fritz Hasenöhrl, Werner Heisenberg, Karl Ferdinand Herzfeld, Victor Francis Hess, Jack Kennedy, Paul Langevin, Ernst Mach, Richard von Mises, Max Planck, Erwin Schrödinger, Kurt Sitte, Arnold Sommerfeld; Deutsche Naturforscher Versammlung (Prague), University of Prague, and Universität Wien.
Tell me, when did you actually begin at Vienna as a student?
I think 1904 or something of this type, or a little less even. I graduated from Vienna I think 1907. And since I studied about four years, it must be 1903. 1907 I graduated. I took the (regular) course through the Ph.D. Of course, at this time Vienna was dominated by Boltzmann. I was of course mostly a student of Boltzmann. You see Mach was already retired at this time. When I was a student, he was no longer teaching. He retired rather early. He was not so very old, but he retired early. I was mostly influenced as a student of Boltzmann. And so, to describe the general climate: I studied with Boltzmann. And the interesting thing about Boltzmann and quantum theory was that there was in Boltzmann’s ideas some anticipation of quantum theory in the conception, which was a fundamental conception of Boltzmann, of the probability of a certain state, the phase space. He had from the beginning the idea that the space of phases must be fundamentally quantized. It must be that the cells in the phase space must be a certain finite size, because otherwise the probability would be zero, or infinite. But anyway, the whole idea of probability really presupposes that there are finite cells in the space of phases. As he liked to express himself, because he was you know a follower of atomistic, he said, “Maybe there are atoms in the time too.” The phase space, of course, contains the time, it is energy times time. And he said always, “Probably there will be atoms in the time too.”
I had not known that.
Yes, it was. And this connected with the idea that the space of phases was just finite cells. This was an idea which was very old, it was an idea which was much of course before Bohr, and it was the time when I was a student. … There were so-called quanta of energy too, of course. It was an idea which was very familiar to him — the possibility at least. Perhaps something which will be in the future. It was one of the ideas which existed at this time. The other thing which was connected with quantum theory was the idea of statistics. Of course, this was partly also connected with Boltzmann. But specifically, there was a man, Exner, Franz Exner, who was a teacher of Schrodinger. For Schrodinger was a student of Boltzmann. I was really a classmate of Schrodinger. … I knew him well. (We were both students of) Boltzmann at this time. … And then there was Thirring, Hans Thirring. And then there was Paul Ehrenfest. There was many people who were in this field. I would say Vienna was a big school of physics. And, therefore, since I am a native of Vienna, I didn’t think even of studying any other place. It was general that everybody who was interested in science studied at Vienna, and therefore I didn’t (choose) to study at another place. I studied a little in another place. I studied mostly with Boltzmann. But Exner. You see, he was a man who by one thing became connected with the basis of quantum theory. By this, lecture — I think it was his inaugural lecture as (Rektor) — about the role of statistics in physics. And there he said one thing which became rather famous. He said that it may be that the basis of physics will be statistical. And it may not be that every statistic law can be derived from dynamic laws. Because generally, of course, the idea was that statistics is only an average. The basic laws are the Newtonian laws. But he said already, Exner did, that maybe this is not true. It may be that the basic laws are statistics. And this was what he said here. It had a great influence on Schrodinger.
I haven’t read the Exner address, but I have read Schrodinger’s essay on this. But was this something that the rest of you also knew? Has that statement become famous only since Schrodinger?
Oh no, no. It was at this time already. It was widely discussed. It was in my time when I was a student. … All the physicists in Vienna were interested in the philosophy of science. And so all things which were connected with philosophy have been widely discussed. Exner was not a man who was particularly interested in the philosophy of science, but in this connection too. Because Exner was really a man who was rather in experimental physics, was not in theoretical. … And in Vienna, the theoretical physics was Boltzmann; and Exner was experimental physics. So he was not in theoretical physics, Exner, in general. I think his field at this time was connected with electricity, atmospheric electricity. It was the beginning of the ion theory — of ions and electrons — at this time. And Schrodinger was also a student at this time. Oh yes, it was very discussed, whether the basic is statistical or not, because everything was connected with philosophy. Boltzmann was much interested in the philosophy of science, of course. And he gave us, sometimes, courses in the philosophy of science, although he knew little. But he had a great interest in it. Ja, he was also good in some special fields in the philosophy of science. He was connected with the problem of statistics and such things. But later, the government gave him the role he should give systematic lectures in the philosophy of science. He was at a great loss with this, because he didn’t know anything. He started to give a course about Schopenhauer. And so we asked him why he was interested in this. But the reason was this, because of all German philosophers, he is the easiest to read.
He said this himself?
Yes, he said it is easy to read, not that he liked him. He disliked him as a matter of fact. It was a very strange (course). It was in my last year as a student, when I heard this course. Somebody gave to the government or to the department of education the idea there must be courses in the philosophy of science. And this was a tradition of Mach of course. Mach you see was teaching in the philosophy and history of science, and the title was, I think, “Theory and History of the Inductive Sciences.” And he was teaching philosophy of physics all these terms. And then people said it should be continued, after Mach was no longer here. Mach retired early. Then some people had the idea, Boltzmann would be the good man. At the beginning I think he didn’t like the idea. But later he became somehow familiar with this ides. The first year was very good. He spoke about his own ideas. And then he started speaking about Schopenhauer, he was not very good and not even interested in it.
There are some other things, if I may, I’d like to ask you somewhat more about this. Did you get to know Boltzmann fairly well?
Yes, fairly well.
Did he talk at all about Planck’s ideas, about the Planck quantum?
Yes, you see, that was —. You know that between Boltzmann and Planck there were peculiar relations. As Planck mentioned also in his autobiography, not very good ones. Boltzmann, who was in general a very logical thinking man — very precise — he regarded Planck as having a rather superficial way of thinking. You see, it is connected with a question which at this time played a very great role, the atomistic and energetic. Planck was regarded, strangely it seems to me now, as the champion of energetics. And Boltzmann made, very often, fun of him. And he didn’t like Planck at all.
And this was true even when you were a student?
Yes, it was true. He didn’t like Planck. Planck was for him somehow connected with Ostwald. I think the first book of Planck, which became also some prize on the law of the conservation of energy, in which he —. He had a rather low opinion of Planck as a theoretical physicist. … The whole problem of Planck’s black-body radiation, this also played a role in Boltzmann too. But I don’t think that he spoke as if Planck’s idea of the quantum would be a great new step in science. It was rather something — a vague idea. Oh, he spoke about it —.
But he definitely did talk about it?
Oh yes, he talked about it, he talked about it. He talked also about Gibbs, there was also the relation between Boltzmann and Gibbs. Oh yes, he talked about Gibbs. He was quite an admirer of Gibbs, Boltzmann was. Not at all as a kind of rival. Particularly he gave a seminar about the book of Gibbs on vector analysis. See at this time the vector analysis was not so very much used. Boltzmann didn’t use it at all. In all his writing he didn’t use the vector analysis. And Gibbs had at this time published a book which was on vector analysis. And this book was used, the book of Gibbs. And Boltzmann always said, “You see I would like somebody to tell me an easy way to distinguish what is called the outer product and inner product.” And then we had this one man who always knew such sort of things, this was Paul Ehrenfest. And Boltzmann said to him, “I know you can always make such distinctions. Tell me, give me a rule, a simple rule, how can I remember what is called the external product and the internal product?” And then Boltzmann said, “If you can do this, then I want to ask you another question. How can I distinguish between Ehrenfest and Ehrenhaft?” You see, we had also another student, Ehrenhaft, who was famous by his fight against Millikan, and the experimental proof for the electrical quantum. He was also a student of Boltzmann, Ehrenhaft. He was a little older, but not much. All part of the same group. And Boltzmann said to him, “Can you tell me how I can distinguish Ehrenfest and Ehrenhaft?” And both were very offended about it, because they despised the other one; Ehrenfest despised the other, and Ehrenhaft despised Ehrenfest. But Boltzmann couldn’t really distinguish them. No, I don’t think that Boltzmann gave this Planck quantum a very great importance, that there would be a great change in science from it. He had not an idea.
Did anyone else that you remember in Vienna have the feeling?
I don’t remember that anybody who could have. The only man of course was Ehrenfest, of course, yes, Ehrenfest. He had this idea.
Did he have it already then? …
Oh yes, surely. Ehrenfest always looked for queer things, already. That he wasn’t so much in Vienna, Ehrenfest. He lived in Russia and came only occasionally. I remember that he was regarded as crazy, generally — Ehrenfest.
Was Hasenohrl in Vienna when you were a student?
Yes, but at this time when I was a student he was what one calls a Privatdozent. He was a student essentially of Lorentz, and he was studying electromagnetic theory and similar things. He was mostly influenced by H.A. Lorentz, and (following) his line really. And later of course he was to a certain degree (in) relativity theory. But also more in the sense of Lorentz. He died very young, Hasenohrl. He was killed in the war, the first World War. You see, he became famous because of the Einsteinian formula E equals mc2, which he took in the sense of Lorentz. But he developed it in this line. He was quite an interesting man, but not a particular genius. He could have said something about this Planck, but I don’t remember either that he did. Because he was in general also a man who was interested in Planck and these things. I remember that Ehrenfest came once to Vienna, and he said to Hasenohrl, “I want you to do something.” — Hasenohrl knew a brother of Ehrenfest in Geneva — Ehrenfest said, “Tell him that I am not crazy. It will be a great help to me if you tell him you don’t believe that I am crazy.” Then Hasenohrl said, “You see, this wouldn’t help anything. The only result would be that he would believe I am crazy.” And at this time, of course a student who was a little younger was Karl Herzfeld. But then you see, what was (also) connected with what you asked me there, I should tell in what way Einstein was (brought to Prague.) Whom, what influence. What was really the reason.
Before I let you get there, still back in this period, had the influence of Mach vanished in so far as skepticism about the atom was concerned?
No, it did not vanish. There was always this interesting point: what was the relation between Mach and Boltzmann. This is a point which of course plays a great role, and has become rather of a political issue in the Russian literature. In the Russian literature we find it sometimes said that Mach was responsible for the suicide of Boltzmann. It is said that Boltzmann was so desperate about the —. As a matter of fact this was not the case, because Boltzmann was himself, philosophically speaking, rather a follower of Mach. Boltzmann said once to me, “You see, it doesn’t make any difference to me if I say that all the atoms are only a picture. I don’t mind, this. I don’t require that they are absolute (rules). I don’t say this.” “‘An economic, description,’ Mach said. Maybe the atoms are an economic description. This doesn’t hurt me very much. From the viewpoint of the physicist this doesn’t make a difference.” Boltzmann had a philosophical viewpoint. He did not put any particular attention that you believe in the real existence of the atoms. And there wasn’t I would say, any opposition from the viewpoint of Mach. This opposition existed only, so to speak, in the philosophical realm. Ja, it did exist then. Also, strange as it was, in Vienna the physicists were all followers of Mach and followers of Boltzmann. It wasn’t the case that the people, would hold against Boltzmann’s theory of atoms any antipathy because of Mach. And I don’t even think that Mach had any antipathy. It wasn’t such a case, this doesn’t play such a role, I would say. Also, I always was interested in this whole problem, but it never came to my mind that because of the theories of Mach one shouldn’t pursue the theories of Boltzmann, the atomic theories.
Yet Mach was one who thoroughly believed in a phenomenological interpretation of thermodynamics as against the statistical.
This is to say too much. I wouldn’t say this. I don’t think that he was against the statistical interpretation, I wouldn’t say this. I mean the views of Mach are in the American philosophy presented by Stallo. That’s about similar. He believed that there are no sufficient proofs really, I mean experimental proofs, for the assumption of atoms. And from the philosophical viewpoint, he rejected them for reasons which are similar really to the reasons of Stallo. His chief argument I think has always been that if the atom would be a mechanical object, … then you cannot explain for instance the great complication of spectral lines. This is not known of the mechanics of this time. And therefore it must be something else there. And Schrodinger has always been, his whole life, influenced in a certain way by Mach. And I had always the idea that this atom of Schrodinger is not the atom in the sense of the atomistic theory.
It isn’t, it is not a theory of the atom. And the theory was in the because you see — of course —. Individually and privately there could be cases in which people were somehow diverted from the study of atomistic theory because they believed in the phenomenalistic theory, but I don’t think that it played a great role among the productive physicists, a particular role. All were more or less followers of Mach in the philosophical sense. It is the same thing, you see, you saw in his young years with Einstein. You see the same thing. He was also a follower of Mach. You see this with Heisenberg, and probably also with Bohr. It is strange it didn’t play such a role. There were some people of course, who interpreted it in this sense, and were for this reason opponents of Mach in the philosophical sense.
Was Mach much discussed among the physicists in Vienna?
Oh yes. He was very much discussed among the physicists. He played a very great role, Mach, of course. Boltzmann spoke always about Mach.
And which of Mach’s works were particularly widely read by the physicists?
Most of course in mechanics; and the theory of sensations; and the book which does not exist in English, Erkenntnis und Irrtum. That was also widely read. For instance, Erkenntnis und Irrtum is little known. For instance, Conant had the completely wrong idea about Mach. I discussed often with him. Later he recognized this. Conant liked to say that Mach said that physics is only a description and therefore it is not a method for the progress of physics. But that is not at all the case. In the Erkenntnis und Irrtum it is discussed again and again that what he calls a simple description is just such a description which is productive for the progress. And it is true that in some earlier books he didn’t emphasize this so much. And also Conant recognized it later. Of course I discussed with Einstein often these problems. Einstein was very strong. That is also one thing which I think is interesting for the quantum theory. There was a certain theory existing about the relation between physics and philosophy. The relativity theory is based on the metaphysical interpretations of science, while the quantum theory is based on the positivistic interpretation of science. The first to notice this was a theoretical physicist, Lanczos. And he said bluntly this, that the philosophical positivism of physics is quantum theory, and the metaphysical is relativity theory. When I read this first I was astonished of course, because originally the relativity theory was based on positivism. But the interesting thing is that it had a certain truth in it, which one sees in the whole history of Einstein
. It was Einstein who later more or less abandoned the positivistic conception of science. And this is one of the reasons I think why we have this antipathy against the Bohr theory, the Copenhagen school. The Copenhagen school he regarded as something which is radically connected with positivism. And in the relativity theory later Einstein emphasized another viewpoint. Of course originally, and in his first obituary of Mach, which Einstein wrote, he says still bluntly the relativity theory is based on the ideas of Mach. But he said that the later development of relativity theory, which is based more on the field theory, and not so much based on the physical interpretation of measurements as it was originally. It looked a little from a different way. But so to say in the birth, in the history of birth, the relativity has always been connected with positivism. And the same the quantum theory. That is not so very very known, but it is also a point which I think is interesting. I speak now of the later development with Heisenberg. In his first paper, in which he introduced his matrix theory and the matrices, he says the basis of the theory, so to speak, the philosophical basis, also the physical basis, is that we no longer look now by what kind of motions the spectral lines are produced. Do not try to reduce this to mechanics. But we investigate the radiation by itself, the frequencies, and try to give a mathematical presentation of these frequencies. And then comes the Heisenberg theory. Heisenberg also mentions there explicitly in the first paper that this is an application of the general ideas of Mach on physics. That the quantum theory is also connected there with it somehow. See Sommerfeld, as many German physicists are, was a great opponent of the so-called Machian theories, and believed in the real existence of things — and I remember a speech at a meeting in which Sommerfeld said, “The whole theory of Mach, the metaphysical theory of Mach, that physics is only a description, is completely sterile. It is only hostile to the progress in physics.” Then I said to him, “There are only two exceptions. The relativity theory and quantum theory.”
When did Sommerfeld say this?
He said this in a meeting in Prague. In Germany, all German speaking countries, there was always the Deutsche Naturforscher Versammlung, this was like here the A.A.A.S. And there was also a special meeting later of physicists. One of these meetings was in Prague, and I was the chairman of this meeting. And also Sommerfeld gave a speech in this. And there was a special meeting, which was one of the first meetings of the Unity of Sciences Congresses about the epistemological basis of physics. And in this I gave a speech, which is also reproduced in my book. It is discussed in this that Sommerfeld said this and I answered this. You see that was the thing. In this book is the relation of Mach to the American philosophy. I mean the pragmatism. And then Sommerfeld said, “You see, I really didn’t know anything about the American philosophy by now, but I always believed it must be something disgusting, and now I am sure it is!” Sommerfeld was a man with a very (???) character. Of course he was a very good physicist in many respects. And he was as a teacher, I think, the best theoretical physicist in Germany. But he had always this thing. He had this terribly nationalistic idea, regarding the German philosophy as something which belongs to the German nation, and if you don’t believe in Kant you are just a traitor. He had this idea, this religious idea of philosophy, or nationalistic idea.
So to a certain degree Planck had it even, not to such a degree, but also a certain idea of a vague Kantianism. Planck I knew a little. From the scientific viewpoint the most I ever had to do with him was about this book which he wrote and which got a certain prize, about the first law of thermodynamics. His main point was this: he wanted to refute the ideas of Poincare, and similar authors, that the first law of thermodynamics is tautological. He formulated it in such a way that it is not tautological. And this I rejected, you see, this way of thinking. His main argument was — this is also often repeated — that the first law of thermodynamics has to be formulated in such a way that if you have a cyclic process, then by a cyclic process no weight can be lifted. And I said this is not sufficient, because what is a cyclic process? A cyclic process will restore the same state. And what is the same state? It is the same state if no weight has been lifted. And this is also tautological. Planck was a great enemy of Mach. He wrote a special pamphlet even about Mach. I don’t think that it was really right, what Planck said in this respect. He was a strange character, Planck. Although he was man who discovered some good ideas, he was a man who was not very logical in thinking. He was certainly influenced in some way by a rather popular way of presentation. He was a good writer, Planck. As a popular writer he was good. He was always writing in the Berliner Tageblatt, and he was a good writer. Ehrenfest liked him also very much because of his way of writing.
Of course all his writing has many mistakes — all those (holes) in radiation theory and all these things are not very precise, though he had good ideas. But he confused other people very often, by his arguments, which were not correct. Also in thermodynamics, which he had in the radiation theory, the statistics. And there was between him and Boltzmann a certain rift. And in this last book of Planck’s, which he published at the age of, I think, more than 90 years, Scientific Autobiography, he says, “This was always something which I regretted in my life, that I was never recognized by Boltzmann.” I think it was also wrong partly by Boltzmann. I didn’t like the philosophy of Planck. I think it was very superficial and purely nationalistic. I spoke very often with Einstein about Planck, of course he was a good friend of Planck. “With Planck you see it is this,” he said, “you have always to prove to him that something is correct. If you don’t prove it in a simple way, he will deny it.” For instance, it was the time when we were in the Nazi government. He said, “You have always to prove to Planck that some acts of Hitler are not in agreement with Kant. If you prove to him this, he will say, “All right, this proves it.” He had not the instinct that it does not agree. Einstein said of Planck that it was one of his characteristics that things which he didn’t like he just regarded as non-existent, he didn’t pay any attention to them. The Hitler government at the beginning, for a long time Planck just ignored it. He did as if it wouldn’t exist. But of course in the moment his son was hanged by Hitler — the son of Planck was hanged by him — he noticed it. It was no longer possible to ignore it. He had a sort of aesthetic thinking, Planck. He wrote in a way which is nice, and didn’t go too deep. He was a strange man, Planck. He was a very pleasant man, I would say, Planck.
He was a very pleasant man, but he was naive in some respects. He was most famous in Berlin, everybody spoke about him and his meeting with Hitler. He was supposed to convince Hitler about his behavior to science. And then he came, Planck, and he said, “I was very astonished that it was impossible to speak to him, because he spoke to me as if he were speaking to 40,000 people. He didn’t address me like a single person. He shouted at me as if I were 40,000 people. And how can you make a discussion?” Well, he was of course concerned with quantum theory. He was always instinctively an opponent of the whole quantum idea, Planck, he was. People said in Berlin that for this reason he appointed Schrodinger as his successor, because he believed Schrodinger would ruin the quantum theory. He had the idea that Schrodinger would refute the quantum theory and introduce again the classical concepts. As you would say, it is not completely wrong. He is not completely wrong. The instinct in Schrodinger was also against the so-called Copenhagen school. Schrodinger had not so very different ideas about quantum theory than Einstein had. He didn’t like it either.
Can you remember Schrodinger from his student days?
Oh yes, I remember very well. I remember that Schrodinger, when the quantum theory started, had the idea to abandon physics, and go to another field, because he thought that the development of physics was an illogical one. And it was a certain time he said, “I will abandon physics.” And what did he do at this time? Into the theory of colors. He wrote one paper at this time, I don’t remember this paper, about colors, in a phenomenological sense. This was the reason that he did this, that he didn’t like the whole way in which physics was going.
Did you talk with him at that time about the way physics was going?
Yes. In particular (no), in a general, yes. He didn’t think physics was going in a rational way. One should rather not occupy oneself with it now. But then he introduced this idea of de Broglie, and he came to this. He believed, you know, in the beginning that it was much more similar to classical physics than it turned out later. But he had this idea of course; he had this idea, there is no doubt. However, he never went so far as some people. For instance, I remember him when I saw him the last time, Schrodinger, at the end of his life. I met him once in Austria. There was a meeting there, the (Altbach) meeting, you have heard about this, to which Schrodinger always went. And so I spoke with him about the relativity theory and about quantum theory. He never really reconciled himself to the idea that everything should be based on statistics. He never reconciled himself to this. But Einstein you see in the later years of his life, Einstein said, “As a matter of fact I must admit I have no good objection against the quantum theory. I have no good objection. I believe it isn’t true, but I have not an objection. I think only this,” he said, “that there are not sufficient arguments in order to make such a step,” — in order to reject the causality, he means. “There are not sufficient arguments for quantum theory,” he said. “It is still vague.” For instance, he said always, “Nobody can give you really a precise formulation of the complementarity principle.” I met him once at (???) and he said, “Tell me sincerely, completely sincerely, tell me, can you formulate the complementarity principle?” He said, “Nobody can really formulate — all they do is give you examples. But what is really the complementarity principle? Nobody can formulate.” “So far,” he said, “there are things which are completely vague, which I don’t recognize you can base on it. I feel that many things are good, and they will remain.” All the things will remain which have any relation with experiments, but the principles are not correct. It doesn’t follow from it that they are correct.
You first knew Einstein when? When you went to replace him at Prague?
No. The first I came in connection with Einstein was as a matter of fact on the philosophical side. In the first paper which I wrote — which is also in this book — the first paper which is printed — “Causality and Experience,” — I try and go on the line of Poincare in a very radical way. I said that the principle of causality is all a tautology. It is all a tautology, a convention. This was a paper which I wrote when I was perhaps 20 or so years old, very young. I had just not even graduated. And there I got a letter from Einstein about this paper, in which he said it is very good, and from the logical viewpoint, he said it was very true. “I like this paper very much,” he said. And then he said what one can say against it. And this was also similar to the argument which Einstein always made and which (I make also), i.e., it is also a question of simplicity. But its effect is this, that the world is like this, that we can give simple causal laws in some cases. These are the only ones of which physics consists. We cannot say more, that is true. If we want to go so general a way, philosophical ways, then of course Poincare is very right, and you are very right, this is only a tautology. But the question is this, whether there are examples in nature that you can do this in a simple way. This was also the idea of Bridgman, which went into his thermodynamics. Bridgman accepted also this whole theory, that the first law was only tautological, and only if you give special cases in which your work can be described by a simple number of states, you can do something. In this paper I expressed this theory in a radical way, so to speak, and not in connection with the practical world. Einstein wrote me a really long letter about this, and from this time I was always in connection with him.
Do you still have that letter?
I think I have that letter. (It was almost a lost letter), but I think that this letter — I hope I have it. Some of the Einstein letters I brought from Prague, but most letters I lost, really, on the way.
No, any letters of this sort that you still do have ought really to be copied.
Yes, yes, it is true. I may (still have it), because this is a letter which has played a role, and from this time I was always connected with him. And personally, I think, I met him first in Salzburg, in the lecture at the physics congress in Salzburg. I think this was the first time when I met him personally. But when I came to Prague as his successor of course I did not meet him, because he was no longer there. But I met him once in Prague. I visited him once in Prague, yes, when I was in Vienna still.
Tell me about that meeting in 1909, the Salzburg meeting.
I know really very little about it. I couldn’t say anything. Einstein you see gave at this time a speech on the field theory. It was nothing with quantum theory connected, no.
I know you talked a lot on many occasions with Einstein about relativity.
We talked lots about quantum theory also, about the relation of quantum theory to causality, of course. We talked about that often. Oh yes, very often I would say. And then in Princeton when I was here. Also in Berlin when we were there. Of course, at the beginning everyone was astonished that Einstein took this stand against quantum theory. They believed always he is the founder of the quantum theory. And as he said, “One shouldn’t repeat too much a successful joke.” Because many of the ideas which he later attacked in the quantum theory he said first.
One thing I’ve been curious about. Today, when one speaks of the problem of quantum and causality, one means the things that have happened since the uncertainty principle. But here and there, there is reference apparently to older worries about causality. And I wonder where in your own experience those first begin to come?
Of course there were. No, for instance, I think the first was Bohr’s theory — the original Bohr theory — the theory of the production of spectral lines by jumping from one orbit to the other one. One electron jumps from one orbit to the other one. And some people said at this time it is a proof for the free will. The electron jumps when it wills. Ja, this type of argument. Of course the Bohr theory from the beginning is against causality. And it existed also of course in this Einstein fluctuation theory too, because in the fluctuation theory it was from the beginning, the statistical law is the primary law. Of course, it was the law which Einstein started with.
Well, how did he feel in those early discussions about causality and quantum?
Well, take his first paper on the photon, 1905. It has a strange name, “Heuristic Principles.” And then Einstein told me, “The misunderstanding comes from this, that people didn’t take seriously that I called them the heuristic principles. I meant the heuristic principles. I didn’t believe this is a fundamental principle of science. I meant with the heuristic principles you can from these principles derive some laws, but I wouldn’t say that this is a fundamental law of science. And, therefore, I called these heuristic principles. As a matter of fact one could express it in this way, I didn’t take this as seriously as it was taken later.” He was not taking it so seriously.
And you think he still felt that way about it in 1909?
And in 1916 when he does the probabilistic derivation?
Yes, yes. I would say yes. I would say yes. I would think that it wasn’t satisfactory for him, because his idea was — in quantum theory — what he understood really as his goal in the field theory. His goals were this, he wanted to have a system of differential equations in which the electron and the photon are two special solutions, and both are solutions of the electromagnetic field.
But that’s a relatively late idea. It’s doubtful that he was thinking in those terms in 1910.
Well, more of it was probably later, but it was always the line in which he was going, it should be reduced to an electromagnetic field. This was, of course, expressed later, but this was always his line. Therefore he never liked, for instance, the whole way in which the causality is defended, for instance now by Bohm and who’s the other? (Vigier). He never accepted these things. He was never a follower of this line, no because you see, there must be much greater change. These people who want to reduce it to classical science, they have something which is much too near to the classical science. There must be much more fundamental changes. He didn’t think that it can be reduced to classical science, but he believed that there must be a new science which is far from that. And in his last time, you see, he said, “I admit that all my attempts to bring about such laws, they have failed. But it doesn’t follow that they will always fail. And I don’t say,” he always said, “that the Copenhagen school and these whole things, are logically absurd, and that they are not admissible in science. That would be too much to say. I say they are such a great change, which there is not yet reason to admit.” It is a strange thing, that if you take the whole discussion between Einstein and Bohr, in the last resort these arguments are purely philosophical and not physical. From the physical viewpoint there are no arguments at all. They are all philosophical. They come really from the idea what the people believe is the real world. It is nothing else, Einstein and Bohr, it is purely this. In the last there is no argument which from the viewpoint of physics really that is decisive, none is decisive. You can read these whole arguments between Einstein and Bohr in the Schilpp. In everything that they did, even in this famous paper, Einstein, Podolsky, and Rosen, there is no physical argument. It is a purely philosophical argument, and one must call this philosophical. If you will speak with Wittgenstein, a verbal argument…
You knew Schrodinger during the time when you were a student with him?
Of course, during the time. He was about two or three years younger than I. We were both students of Boltzmann, yes.
Was he at all remarkable as a student?
Yes, he was remarkable. What was remarkable was that he as a student mostly worked on relativity theory, on general relativity theory. This was at the first time that the general relativity existed. He was regarded among the students as a man who was an expert in the relativity theory. Oh yes, he was remarkable certainly, from the beginning. He was remarkable.
And he was working on general relativity while you were still in Vienna?
Yes, yes. At the beginning, he was…
In what way would you say in retrospect doing science in Vienna was different from, say, if you had done it at Berlin or if you’d done it at Munich or Gottingen, in those days?
I would say (???) one of the things, it was more interested in the philosophy of science, in my Vienna years, it was more, definitely more. It was very strong. It was at this time probably the influence of Mach, and probably also other people of the same line. Ja, it was strong. It was very strong, I would say.
Was there any effect on your own education, or on the education at the University because of the interest in radioactivity at Vienna?
Yes, yes, of course radioactivity was connected with statistics, the statistical interpretation of science. Exner, for instance, his main field was radioactivity. And so, of course, it was connected with radioactivity. And the radioactivity in Vienna comes really from a purely, I would say, geographic reason, of the St. Joachimsthal, which is, of course, where Madame Curie got really the ores. Ja, it was a kind of patriotic occupation, the radioactivity. With Boltzmann himself, he wasn’t so particular, but the general idea that this is connected, the radioactivity with the statistical interpretation of science, was, of course, familiar also to him. Oh yes, it was very strict, very connected.
I ask this particularly because in Germany there was not much influence from radioactivity.
Yes, yes, there was less. Oh ja, in Vienna from the beginning it was strong, and it came partly from this. And then there was also the whole field of atmospheric electricity. It was connected with the idea of mountain climbing. ... The inventor of cosmic rays, Hess, was at this time in Austria, and was also a student of Boltzmann at this same time here. This whole line of cosmic rays this comes from it, and was very much cultivated. Hess, of course, discovered the origin. He became later, by strange coincidences, a professor in America at Fordham University. I think he is retired now. But he was until his retirement a professor at Fordham University in New York. And somehow due to the political circumstances he was shifted to this Catholic university. Also he wasn’t any Catholic by belief. He was a Catholic by origin, but not by belief. He didn’t believe anything. I remember once, since I have been in America, I visited him at Fordham University. And he said also to me, “You will be astonished to see me a Catholic. I don’t believe these people. I think these people are just fools at Fordham University.”
What about Herzfeld, who is also at a Catholic university?
Oh Herzfeld, that’s a story by itself. That is a little different situation. You see, he was a young man, a little younger than I. Of course he was even my student in Vienna. In Vienna I was only teaching two years. I was Privatdozent in Vienna for two years. And during these years my student was Herzfeld. I always joke about Herzfeld, because Herzfeld was, of course, of Jewish origin, as the name says, and his father was a professor of gynecology in Vienna. But he somehow became interested in the Catholic religion, it was in Catholic philosophy. He was also a follower of Thomas.
Even then. No, it is not on this occasion that he became a Catholic, he was a Catholic already in Vienna, he was known as a Catholic, well-known. And I remember, this story with Hasenohrl. You see, one of the few rites that the Catholics keep is that they don’t eat meat on Friday. But in Vienna neither most of the people nor the professors at the University — hardly anybody — could keep this. And one day there was a dinner given at the house of Hasenohrl. And the next day Hasenohrl asked me, “Do tell me, what happened to this Herzfeld? When we served him the meat he didn’t take it. Is he a vegetarian?” Because at the same time Ehrenfest, who came also to this dinner, and didn’t take the meat either, was a vegetarian. Ehrenfest always did sort of crazy things, and one of these was being a vegetarian. And Hasenohrl said, “Well what’s with Herzfeld? Now you say Herzfeld is not a vegetarian?” And I said, “Don’t you know the Catholics don’t eat meat on Friday?” “Oh,” said Hasenohrl, “I should really know this.” Because Hasenohrl is an old Catholic family. “Of course, I should know this, but I didn’t think that Herzfeld would place so much on this.” … “But I never thought of this,” he said, “I know the family Herzfeld according to my experience are Jews.” I said, “This may be, but he is not.” And Herzfeld became later a professor at the Catholic University in America. … But he was one of my first students which I had, Herzfeld. He was regarded as a very young boy as a Wunderkind. He was a very gifted boy. That’s what one called a Wunderkind, a prodigy child, like Norbert Wiener almost, not as much. But he had always this Catholic leaning, and he was also a follower of the Thomistic philosophy. Yes, he studied it.
I take it Schrodinger did not have any of the same reputation as a Wunderkind?
No, I wouldn’t say he was a Wunderkind. Schrodinger, as a matter of fact, is a Protestant. He is not a Catholic. Like in Vienna, it is an interesting thing. In Vienna, many people of the intelligentsia dropped Catholicism and became Protestants. Schrodinger, who was of Catholic origin, changed. Also Boltzmann, and Thirring, and many people, you see, of the intelligentsia dropped Catholicism. It is a lot more a political antipathy. It is more a reason of political antipathy. They were regarded as reactionary, the Catholics, and of the whole intelligentsia, many people were Protestants. For instance, ja, Schrodinger was. Schrodinger you see was a long time in Dublin, in which he really should become again a Catholic. I once gave lectures in Dublin. And there I met also the boss of Schrodinger, so to speak. I met the Director of the Institute of Advanced Studies, who was a Jesuit and a priest, Monsignor Patrick Browne. Schrodinger and I were sitting there and this Monsignor said, “You see, but we are liberal people, (and this always was the Irish.) We give a job to Schrodinger. We know Schrodinger is not a friend of the Catholics. He has also not a very (loyal) behavior. We know this very well, but we keep him because he is a good scientist. Therefore you should not say that we Irish are not tolerant. We are very tolerant.” He was the director in Dublin for a time. He is no longer there, this Monsignor Browne. He wanted to give me a job also there. And not only there. He wanted to give me an appointment in Galway college. Galway college is an Irish college, a very strict Catholic college in which he was the President. But I didn’t take this. It was too much. The Catholics, when they are not in places in which they are governing, they are (more illiberal.) For instance, Schrodinger said, he had much more trouble in Oxford than he had in Dublin.
Schrodinger himself said that?
Yes, he said that he had much more trouble in —.
Let me ask you then, in what respects was Prague different from Vienna? What sort of a change was that?
The German University of Prague was really no different. It was just a branch of Vienna. It was different by the fact that Einstein was there, but that was casual circumstance. The man you see who called him to Prague, who was there the professor of experimental physics, was a man called Lampa. He was not particularly interesting. But the interesting point in it is why he came to the idea to call Einstein. And that is connected also with a certain philosophy of science, because Lampa was a student of Mach, and a great admirer of him. The reason why Einstein was here called, was just this, the relation of his doctrine to Mach. That was what connected him with Prague. You see Mach was from Prague. Mach as you know was a professor in Prague before he came to Vienna. He was a professor of experimental physics in Prague. You see, Mach was an experimental physicist. And he was in Prague. And I remember still from our Physics Institute all the apparatus of Mach which he had there. All this apparatus from the history of mechanics, he had this there, and in Prague. And this is the reason also why I don’t think that Prague was any particular interesting point there. It was not different from Vienna. It was mostly people from Vienna who were there, in the German University. Of course, in Prague as you know there are two universities, the German University and the Czech University, now called (Charles) University. But I don’t think that this University was either very different. They were both, I think, mostly the same spirit as Vienna. Only they had less money, and they had, therefore, not so good people. But it was in general the same.
By the time you got there, you taught relativity there yourself?
Was there any quantum theory taught there?
I taught also quantum theory; and I taught also theoretical physics. Except that I taught quantum theory, there wasn’t anybody who taught quantum theory. It was as a part of theoretical physics. We had of course later also a Privatdozent who was teaching, who was more specialized in it, and this was I think Sitte. He had later very strange things happen to him. He later became a professor, strange as it is, in Israel, although he was not a Jew. But by a strange coincidence he became a professor in Israel. And then he not only became a professor in Israel, he was arrested as a Russian spy. He was a very gifted man, a very gifted man. He was, so to speak, what one calls a (Sudeten) in German. But he was not a Nazi. He was the opposite, he was an anti-Nazi. And the Nazis put him into the concentration camp in Buchenwald. He was a long time there. And after he was out of the concentration camp in Buchenwald, he was arrested as a Communist. It is a terrible thing. Then we had in Prague also another man who was a student of mine, and who has a great relation to relativity. That was Peter Bergmann, who is now in America.
Now you were at Prague when the Bohr atom came out?
Yes. When the Bohr atom came out, I was in Prague, course.
Do you remember when you first heard of that?
Oh yes, I remember. I heard it during the World War. And I heard it from a lecture which Paul Ehrenfest gave about it. He gave a special lecture there, in Vienna. It was just at the time when I was again in Vienna.
How did he then feel about the idea?
Certainly he, as far as I remember, found it if anything more rational than Planck’s theory of the quantum. He saw in it a more rational approach, to introduction, because he had the idea that the quantum somehow connected with the action. But I heard several lectures about it by Ehrenfest at this time. It was during an interruption during which I was in Vienna, and there were lectures on it. … Of course, 1914 was the beginning of the World War. You see if you live through all this, you see always the things connected. The connection between physics and war. What is the connection between physics and war.
How did you feel about the Bohr atom yourself?
Oh, I felt rather sympathetic, I think. The Bohr atom, this is a modification of the Rutherford atom.
Had you known about the Rutherford atom?
Because many people had not known about it.
Oh yes, I had heard, I had known about it. Oh yes, in Vienna there were also discussions on the Rutherford atom. The Rutherford atom; the J.J. Thomson atom; the differences in the Rutherford atom.
Were there other models also discussed there besides those two?
Ja, there is one problem which you mention also which I wanted to say. You ask about the model of Herzfeld, whether there was an atom model. Yes, of course, there was one model, this was the one of Haas, Arthur Haas. I think this is even more important, I would say, than Herzfeld. Arthur Haas. But you know Arthur Haas he worked also on the history of physics. He wrote also several papers on the history of physics, Arthur Haas.
I don’t know those papers.
He was a professor in Vienna, later in Vienna. And then in the Nazi time he emigrated to America. He was also of those people who became Catholic. He was in the University of Notre Dame. And he published one paper which was a very interesting one in the sense how to introduce the quantum into the Rutherford atom. This was a paper which became known to some people. It was for instance known by H.A. Lorentz. He mentioned it.
Coming just briefly then into the later period, the ‘20’s, before the coming of wave mechanics, then you were still at Prague. Do you remember either there or at meetings conversations about the de Broglie papers, for example?
On the de Broglie papers I would say something which is rather interesting. Soon after the first World War I came for the first time to Paris, and I met there Langevin. And I got introduced by Einstein, he was acquainted with Langevin. And Langevin told me, he said, “I tell you one thing. I’ve got now a paper for presentation in the Academy by the brother of de Broglie.” By the brother of de Broglie he means what one calls now de Broglie, but at this time only Maurice de Broglie was known. “...the brother of de Broglie. He gave me a paper which is completely crazy,” Langevin said. It was the paper, the famous paper in which he used first his relation. And Langevin said to me, “This paper seems to me completely crazy, but,” he said, “on the other hand, the quantum theory itself is crazy. Therefore if somebody makes an invention which brings somehow light into it, whatever we believe of it, one should advocate this.” And it was really at this time that the French Academy published this paper of de Broglie. Also Langevin presented it without believing it, but he said only because the whole quantum theory is crazy. And Langevin belonged also to the people who never accepted Bohr’s — who believed always in causality.
What about the Stern-Gerlach experiment?
Stern-Gerlach. I know it of course, but I couldn’t say anything about it.
And what about then the matrix mechanics itself?
The matrix mechanics. Well, I would say that the matrix mechanics…, it was as I said also once, it was introduced in the connection that it gives a direct description of facts by mathematical formula. In this sense many people approved it.
Do you remember seeing those papers, or talking to people about them? The papers of Heisenberg, and Born, and Jordan.
Oh yes, I saw them.
Were they very strange?
Had you done work with matrices yourself?
No, I never did with matrices. I don’t think so, no. Yes, they were strange, but we were accustomed already to the strange. I remember when I came to Prague, the dean of the department of natural science said to me, “If you become here professor of theoretical physics the only thing which we require of you is to be mentally normal.” I asked him how he got this idea that theoretical physicists are crazy. And he said, “You would not make me believe that your predecessor was not crazy.” He meant Einstein. And then, of course, he said, that everybody knew that Boltzmann was crazy. And he said that, therefore, in theoretical physics one was accustomed to everything at this time. Before the matrix it was particularly bad. One had already relativity theory, general relativity theory, and all those things. It was not as in the time when I became a student. In this time theoretical physics (consisted) of people who said physics has to be restricted so as not to apply calculus. They only wanted to apply elementary mathematics to physics.
Who said that sort of thing?
Oh, it was when I came to Vienna as a student, there was a man who was at this time a professor of experimental physics. His name was Victor von (Latten). I don’t know whether you know his name. And he published a textbook of theoretical physics. And it was completely based on elementary mathematics. The general idea was, that what comes is always regarded as absurd changes. The Einstein theory — this whole theory of invariance, covariance — they regarded also as absurd, crazy. There also was a time when these people regarded vector analysis as something crazy. It was solely a matter of evolution, what one regards mathematically. At the time of the matrices of Heisenberg, people were very hard-boiled. It was something which was absurd.
Just one other question on this mathematics topic. Tell me about the origin of Frank and Von Mises.
What is the origin of the book with Von Mises. I think that comes from the original book Rieman-Weber. And I think it comes clear from the fact that Von Mises was professor in Strasbourg after the first World War. And there he met the widow of Mr. Weber, and Mrs. Weber told him, “You could perhaps make a new edition, because it is already obsolete now.” This of Weber was a popular book, not popular in the sense of popular, but a very used book on differential equations. And then Mises asked me whether I would help him in this. And that was how it started originally. And then came the book which was originally called Rieman-Weber edited by Frank-Mises, and then Rieman-Weber was omitted. It has a certain history here in America, because during the war-time it was confiscated. It was taken into the American government, confiscated as alien property. Then it was here published in a kind of reprint in German, and we had a great quarrel, because they didn’t want to pay royalty for it. I had a great quarrel with the alien property office about it. Later you see both Von Mises and I were American citizens. We protested completely against this method to confiscate the money by the government. There was a long quarrel. Later we won the whole long quarrel, and that was the first time I found the great genius of Mr. Kennedy. The alien property office wouldn’t recognize that they were wrong, and therefore to pay the royalties back. And then someone told me that what you have to do to convince the (???) office is to ask a senator. And then I had the idea that I should write a letter to Mr. Kennedy, who was a senator at this time from Massachusetts. I wrote him a letter, and in two days we had everything. I saw this man must become a president. Because he treated his constituents in such a way. I wrote him a letter in which I said, “Two of your constituents have been gravely harmed by the government.” And in two days we had an answer, “You will be recovered.” Ja, he must be very good.
When you did the new edition, you did a great deal of rewriting.
Yes, many people are rewriting on it, yes. I myself, I was only writing the chapter on electrodynamics, that was what I was writing. But we had a man, I think in the quantum theory or wave mechanics, Guido Beck, he wrote it. It appears now new. It is strange that recently this year Mr. Vieweg (has recovered) and wanted to give a new edition too.