Francis Perrin

Kuhn:

He wants to make a different statement about what happened.

Perrin:

I remember that when de Broglie presented his work for a thesis on the fundamental relation of a wave associated to particles with a connection between the wave length and the momentum of the particles this was a rather shocking proposition for a physicist. At that time I remember that it was Paul Langevin who said, “Well, it is so well founded in the relativity invariance principle and so on that I think it is something valuable.” And it was Langevin who took the responsibility of accepting the thesis of Louis de Broglie, though it was rather shocking. At the soutenance de these my father was a member of the jury — he was always very much interested in the real connection between theory and experiment — at the end he asked Louis de Broglie if there could be some real experimental verification of his ideas. And Louis de Broglie stated at that time that maybe diffraction of particles through very narrow slits could be observed and show the existence of the wave length associated with the particles. And indeed it was not with the slits that it was done; it was with crystals — that was later on. But de Broglie really, already at that time, considered that there could be an experimental verification of associated waves with particles. I think there are the two points for this special very important work of Louis de Broglie which I could add — historical points — on this development of the fundamental ideas of a wave associated with particles.

Kuhn:

Were you at the soutenance de these?

Perrin:

Yes, yes; I was there. I was there. This is a direct recollection of this; the question of my father and the answer of de Brogue and the fact also that I was among those who were shocked by the proposal of Louis de Broglie. I remember very well what Langevin said on this, and he really considered —. At that time, you see, Louis de Broglie had spoken of the fact that when an electron hits a target it gives X-rays and so we are more or less used to this transformation of particles into waves. But it was obvious that the wave length of the X-ray connected with an electron was quite different from the wave length or energy associated by de Broglie to the particle itself. So this argument of this easy production of waves by collisions and transformation of momentum into production of X-rays, or photons in general, was not at all a good connection between waves and matter particles. But Langevin said, “Well, it is not at all this; it is really this intrinsic, invariance relation at, the foundation of relativity which seems to me very coherent and true in showing that it’ s something; that is an interesting argument.”

Kuhn:

This was also something that Langevin said at the soutenance, was it? Or would that have been later?

Perrin:

It was either at the soutenance or before, because this was discussed very little before. But I think that essentially at the soutenance Langevin said how well-founded it was and how logically founded was the relation of de Broglie on the invariance with a Lorentz group.

Kuhn:

Do you remember, did your father talk at all about the thesis, beyond this question?

Perrin:

Well, essentially not. My father tried to see if there was any reality that could be connected with this formula of de Broglie’s. De Broglie was, already at that time, aware that there should be some comportment of the particles which could show this associated wave length by diffraction. This was a thing that really, as with the photons — where if we have very short wave photons at first, they are like particles, but if we have a very narrow slit, we have a diffraction. And the diffraction can be evaluated only by the wave structure of light. De Broglie was quite aware that the association of particles with waves should lead to some really different dynamics for particles when diffraction could appear, with the dimension of the diffracting object coupled to the wave length. I don’t know why he didn’t talk about crystals at that time because for X-rays it was an obvious state of affairs. I remember he mentioned slits, but obviously afterwards Davisson and Germer and G.P. Thomson used crystals with a considerable success. But it was really a continuation of the fundamental idea of de Broglie, like Laue had done for the X-rays. After there had been no success in trying to diffract X-rays by slits, Laue succeeded with crystals, and G.P. Thomson and Davisson and Germer succeeded in diffracting and measuring the wave lengths of the associated waves for electrons at that time.

Kuhn:

Now, I realize there is something just in the formality of the examining that I have no idea about. With so very many people coming up to be examined for degrees, one tends to take it for granted that the only person present who will have read the thesis with care is the person who is the director of the thesis. Would that also have been true in this period? Does one take it for granted that Mauguin, your father, and I guess Cartan was the other member of the jury, would actually have studied the thesis, or would they have glanced at it but would mostly Just be listening to what went on at the examination itself?

Perrin:

I think that Mauguin and my father had looked through the paper before the thesis was expounded, but I’m not sure. In a way Langevin had thoroughly studied it, and probably my father had spoken with Langevin, maybe, already before the day of the exposition of the paper.

Kuhn:

But it would be unlikely that anybody except Langevin had really, with great care, gone over it?

Perrin:

I think so.

Kuhn:

By and large that would be his responsibility; the others might look at it — probably would look at it.

Perrin:

But really it was the responsibility of Langevin, and though we were —. De Broglie had already, I think, spoken of this in a preliminary communication to the Association of Physics or something like that; I don’t remember exactly.

Kuhn:

There are several papers in the Comptes Rendus.

Kahan:

In ‘23.

Perrin:

‘23, yes. And it was about these preliminary papers or the communication to the Association of Physics that we probably had some discussion already. Langevin was really the one who first considered this as a really good work and not as most of the other physicists at that time. To us it was just shocking and had no value.

Kuhn:

Now, I’ll ask you another question along this same line. As M. Kahan says, there were three papers in ‘23, in fact, on the photon without reference to matter waves; but on the photon itself, there are several papers in 1922. Then there is a series of two or three papers in 1923 on matter waves in the Comptes Rendus; the first two of those papers were presented to the Academy by your father.

Perrin:

Yes. Yes. So it was just at that time that my father had already discussed with Louis de Broglie some of the points. It was not only looking through the thesis itself but this preliminary communication. Certainly my father had discussed with Louis de Broglie, and possibly — I can’t remember — he had spoken with Langevin on that. But it’s quite possible because Langevin was not a member of the Academy at that time, so it couldn’t be Langevin who could present this.

Kuhn:

When a member of the Academy presents a paper for someone else, does he actually physically read the paper to the members of the Academy?

Perrin:

No, no. In most cases it is just given to the secretary, but very often the paper is not read. It is practically never read, but very often — and more at that time than now because there were fewer communications — the member of the Academy who presents the work goes to the blackboard and for a few minutes speaks of what is in the communication. Myself, for instance, maybe it will be so for one out of ten communications for the Comptes Rendus that I go to the blackboard to tell the Academy of something which is of general interest in the communication. But at that time it was more frequent because now we have about the same number of hours for presentation to the Academy but maybe three or four times more communications, so the proportion of those which are presented to the Academy —. It could be possible maybe to find in the records of the Academy if my father really spoke in the Academy on the paper of Louis de Broglie. I’m not sure at all about this.

Kahan:

I looked for it, but I didn’t find anything.

Perrin:

You looked in what?

Kahan:

The Comptes Rendus.

Perrin:

Oh, it’s never mentioned in the Comptes Rendus; it could be in the secretariat of the Academy, just in the written minutes of the session.

Kuhn:

It could be in the Procès-verbaux also.

Perrin:

The Procès-verbaux? I’m not quite sure that it is even mentioned in this, but this could be ascertained; I could ask to see the documents of this period. But certainly I remember we had discussion among physicists before the thesis. It was at that time that Langevin was the one who supported this as a serious work and said this of course at the exposition of the thesis itself. I remember that the question of my father on the possible experimental consequence was at the exposition.

Kuhn:

When you point out that people were generally — then I take it that even to some extent Langevin had some skepticism about this, was rather shocked by the idea. Did that go far enough, so far as you know, to lead anybody to wonder whether in fact the thesis should be accepted?

Perrin:

Well, I think that Langevin considered that it should be accepted probably, and my father was ready to accept it only if there was some experimental verification of it.

Kuhn:

You mean some possibility of it.

Perrin:

Some possibility of it. And if there was no real direct possibility of measuring this wave length, my father wasn’t very much interested, in a theory which couldn’t be connected with experiment. He said, “Well, it’s not a real theory if there is nothing new in it which brings some new experimental discovery or verification of something that couldn’t be explained by the previous theories.” But Langevin was much more favorable to the inherent coherence of a theory, for relativity and so on; even before any experimental verification, if the theory was beautiful and coherent, he was much more inclined to think that it had some real value.

Kuhn:

In general in this period, how much work and consciousness of problems involving the quantum was there in France?

Perrin:

Well, not a lot certainly. I told you we were a little shocked by the ideas of de Broglie; then afterwards, when the papers of Schrodinger appeared, they were immediately very much appreciated. The ideas of Heisenberg were less accessible to the French physicist. It was later, well, in ‘26 or ‘27, when the principle of uncertainty and the probability interpretation of wave mechanics appeared, that we had discussion. And after some reluctance, many of us said, “Well, it is really so and it is very satisfactory to have a new physics in which we have not the strict connection between observation and prediction of the future.” After being very shocking for many physicists the statistical connection was accepted and indeed that was satisfactory, just thinking of the connection with living ratter and conscience. What we knew was troublesome was the idea of a strict materialistic determinism in connection with a rational interpretation of life and thinking. And the fact that there was an opening in the new physics in this direction was eventually considered very satisfactory by myself, for instance, after I had been shocked as most of the physicists were —.

Kuhn:

Was that an appeal from the beginning? I mean, granting the initial shock, did people very quickly take hold of this idea that this does give us an opening on the problems of determinism?

Perrin:

Well, it is difficult to say, but I think that in the years, let’s say ‘27 and ‘28 — in this period — there were quite a number of French physicists who were very happy with this development of the fundamental principle of physics. But many were still very reluctant: Louis de Broglie himself, like Einstein, who was always very much shocked and, indeed was against any probability interpretation of quantum mechanics.

Kuhn:

I meant a slightly more concrete question. I didn’t mean how long did it take before people adopted the probability interpretation, but I did mean, how quickly did this larger philosophical problem of the old, long-standing difficulties of determinism play a role in people’s attitudes toward it. Since the developments themselves, there has been a great deal of talk — some of it extraordinarily loose talk — about how one does anything from “get God back into the Universe” to at least “get free will back into the Universe”. I wondered, though, really how much in France that had played a role among physicists? And how quickly, perhaps, that had occurred? It isn’t really how quickly did people come to believe it, but what was the role of this larger philosophical consideration and how quickly did it play a role?

Perrin:

That’s difficult to answer. I don’t think it played a great role in the contribution of French physicists to quantum mechanics. But I remember that among the young physicists at that time certainly it was considered rather quickly as an essential character of the importance of the new physics. Say in the year — the uncertainty principle and all this is about ‘26, ‘27.

Kuhn:

‘27

Perrin:

‘27. And so it is in the year after that, about ‘28, that I was speaking with Auger; I worked very much with him at that time. I think that it penetrated the philosophy of a fraction of the young French physicists rather quickly, though with some reluctance at the beginning. Before that, in the years between ‘24 and ‘27, in the early beginning of quantum mechanics I remember discussing with Auger this experiment on the direction of emission of photo-electrons, and we presented an argument in full agreement with the wave structure of light. I remember a meeting in Oxford, I think, of the British Association in this year — it would be ‘25 or something like that — where I presented this work, and there was another work presented by, I don’t remember who, a German I think, which had measurements showing a very sharp peak of the emission of the photo-electron. After that Ehrenfest said, “Well, Perrin has something to say on this.” I wasn’t on the list of communications, but I was there. And I said that I had just published this paper with Auger and I said, “It is impossible to have such a sharp structure; it is a contradiction to the essential wave structure of light.” And Ehrenfest was, at that time, quite convinced that I was right and that. we had better understood the necessity of considering that any measurement should be in agreement with the wave structure of light, with the quantum only appearing in the discontinuity of the —. But a statistical distribution should be calculated with the wave structure of light.

Kuhn:

What followed on this remark of yours at that time? Was it accepted by the group? Was there further discussion?

Perrin:

Oh, I think that it was really accepted. Ehrenfest said, “Certainly Perrin is right; the measurements of Auger and Perrin are certainly much more satisfactory. It is impossible to have such a sharp peak because it is in contradiction to the fundamental wave structure of light.” But it was so that Ehrenfest at that time was already deeply convinced of this necessity of using waves for calculation of any statistical distribution and photon only for the individual elementary phenomenon.

Kuhn:

When you were at the Ecole Normale, how much did the problems of the quantum enter into education, or to what extent was a good student at the Ecole Normale exposed to this?

Perrin:

Very little, very little. At that time teaching at the Ecole Normale was good on classical physics, but very bad on modern physics. In France it was really Langevin who introduced teaching at the College de France many years before on relativity; but there were seminars also on quanta and spectroscopy and things like that. All the development of quantum physics was rather frowned upon in the official teaching of the University and the Ecole Normale at that time. We had had this great discontinuity of the war from ‘14 to ‘18 which had corresponded to the fact that the French physicists at that time had been completely involved in the practical research and were not much connected with the development of physics during this period, except Langevin who was deeply involved in ultrasonic results at that time. But immediately after the war he introduced general relativity which was born during the war — less in the field of quanta maybe; but nevertheless in his seminars at the College de France there was communication on the development of the atoms of Bohr, of Sommerfeld, and so on, on the quantum physics which was eventually the origin of the new quantum mechanics.

Kahan:

When did Langevin lecture for the first time at the College de France about quantum theory, I mean, a whole series of lectures that he gave on the quantum theory?

Perrin:

The real teaching of Langevin on quantum physics, let us say, was when he lectured about the equilibrium between electrons and photons, considering not only the discontinuity of absorption, but also the fact that you couldn’t have thermal equilibrium between light and matter if, for instance, absorption of light was corresponding to photons giving impulsion to the atoms absorbing the photons and if the emission was isotropic. It showed that there must be also a recoil at the time of the emission if we want to have equilibrium in this. I think that was his personal contribution to this — and it was one in the series of lectures he gave on this equilibrium between the photons and natter and this necessity of having a general description of the absorption and emission in terms of photons as well as waves.

Kahan:

You are referring to his lectures of ‘27?

Perrin:

Maybe it was in ‘27; I don’t remember.

Kahan:

And before ‘27 he never lectured about —

Perrin:

It was only in seminars — where all the young physicists under his direction spoke of papers by Sommerfeld, by Schwarzschild; but Langevin himself during the years following the war was essentially interested in relativity, special relativity and general relativity.

Kahan:

Well, what are your early impressions?

Perrin:

No, I think it was later, after ‘27, he lectured on quantum theory, the Dirac equation, for instance, but at that time I couldn’t follow his lectures. [Telephone call interrupts]

Kahan:

Est-ce que vous savez ce qui s’est passe exactement entre Langevin, Einstein et de Broglie? C’est, je crois, Louis de Broglie qui a envoye son manuscrit a Einstein; mais est-ce par l’intermediaire de Langevin, est-ce sous la suggestion de Langevin, ou est-ce Langevin lui-meme qui a envoye le manuscrit? Est-ce que vous savez?

Perrin:

Je ne sais pas, je ne sais pas, je ne peux pas vous le dire, il faut le demander a Louis de Broglie.

Kahan:

Louis de Broglie dit que c’est lui qui a envoye le manuscrit, mais je ne me rappelle plus s’il avait dit par intermediaire de Langevin ou directement ou sous la suggestion de Langevin. C’est une question qu’on va lui demander.

Perrin:

Oui, je ne peux pas vous dire.

Kuhn:

I’d really like at this moment, before you must run, to ask you a bit really about your father. Partly it is the question of whether you remember anything he said about this earlier atom model of his own, his feelings about it and about what had happened to it, but also more generally. You speak, of course, of Langevin as the man about whom the new physics centered. Where did your father stand with respect to these problems, what was his own interest in them and feeling about them?

Perrin:

Well, he was himself very much interested in the development of the quantum physics, but, let’s say, on the old line of the Planck relation and discontinuity of atomic and molecular structure, connection between frequency and energy and all this. Well, maybe he wasn’t much connected with the development of quantum mechanics itself. He accepted it, but without working himself in the field of the new quantum mechanics. He was quite satisfied with the development of the fundamental principle of uncertainty and things like that, but really he himself didn’t work in this direction and was very loosely connected with the real development of quantum mechanics, of wave mechanics, and —

Kuhn:

Of course, I think also of the older period. To a very great extent from my point of view, almost everything happens then, let us say, between ‘26 and ‘28. But there is this terribly important period from let’ a say the Bohr atom, up to ‘22, ‘23, ‘24 which is preparing the way for this and which is also extremely important.

Perrin:

Well, at that time he worked very much on the connection between the discontinuity of the molecular and atomic states and light, in fluorescence and photo-chemistry and so on. He developed ideas which connected closely the interpretation of fluorescence essentially with these general quantum relations without himself working in the field of spectroscopy. The actual connection between the principle of combination he developed although he was never working in spectroscopy himself. But he showed that this general idea of the discontinuity of the states of molecules and atoms and the connection with the absorption and emission of light according to the Planck relation was essential in the field of kinetics in chemistry, for explaining the old law of Arrhenius, and also in the field of fluorescence and the transfer of energy between molecules and meta-stable states, and so on, and transfer of energy between these activated states, and so on. And this was always in close connection with the fundamental idea of Planck, let us say, and —.

Kuhn:

The Planck ideas and the Einstein ideas equally here then were working tools for him in his experimental work; but was he the sort of person who was bothered by the conflict between this set of ideas on the one hand and classical mechanics and classical electromagnetic theory on the other?

Perrin:

He was worried with this, but not as much as a man like Langevin, who was deeply involved in the old classical theory and who was much more shocked by the idea of radiationless trajectories and things like that.

Kuhn:

Finally, is there something about the atom model that you can —? From the paper, in which your father says very little there about evidence for this idea, it is not at all clear whether this is something that he is deeply convinced about, or whether it’s a model?

Perrin:

Well, let us say, he proposed this model of a more solar system; there was a central positive charge with electrons rotating around it. Maybe because he himself wasn’t so aware of the difficulty of this model — a man like Langevin would have said immediately, “but there will be radiation.” And that’s why a man like Thomson proposed a more static description of the atom in which the electron is imbedded in a continuous distribution of positive electricity in an equilibrium with a possibility of elastic vibrations. My father thought that this was not a good model, and he proposed a model with the idea of showing these electrons going in and out of the atom and thought that it was much more satisfactory to think of the positive electricity also as concentrated in particles. So that led to this idea of a nucleus with an attraction between the electron and this nucleus. The idea was that the positive electricity should be corpuscular in nature and not this continuous distribution of Thomson. He couldn’t see as vividly as Langevin, for instance, the difficulty of this model, and, well, he was right eventually not to be too much worried about this difficulty. But certainly it was much more difficult for a man like Langevin to accept this; on the contrary, the general idea of this corpuscular structure of matter and small corpuscles with forces between them was much more acceptable to my father, and so I think that he proposed this without solving at all the difficulty of radiation.

Kuhn:

Did he ever work with it as a working model, do you know, or continue to be involved with it?

Perrin:

Only maybe very late when at the Solvay Conference in the ‘20’s he was the first to propose the correct interpretation of the transmutation discovered by Rutherford. Rutherford had shown that nitrogen atoms bombarded by an alpha particle will emit a hydrogen atom, and at that time he thought that it was more like an explosion of the atom produced by the impact of the alpha particle. And at one of the Solvay Conferences in ‘22 or ‘23, my father said that, no, the alpha particle must stick to the nucleus and then a projection of the hydrogen atom occurs, and the energy comes from not only the incident energy of the alpha particle, but the energy of the fusion of the alpha particle with the nucleus. So he was the first to write the correct equation for the transmutation giving an oxygen atom. It was only later that Blackett proved it to be so in showing that there was only recoil of the nucleus and an emitted H, and no alpha particle after the collision. But this was not on quantum theory essentially, but on the molecular structure of the nucleus of the atom. He deeply understood the necessity of this association and the compound nucleus, let us say, in such a nuclear reaction.