Peter Debye - Session III

Uhlenbeck:

You said that you didn’t collect these papers?

Debye:

[With reference to the volume of his papers] I did not collect them you see. They were collected for me by the boys there. I had no influence on selection.

Uhlenbeck:

I think the ordering is infuriating.

Debye:

Well, that’s possible. They were just given to me.... There are several which are not in there. You see, there is this what’s his name? Decker, Decker, you know. The Dutchman. He is the Inter-science man. He came, you see, and wanted to do that. He said, “Oh, we will take care of everything.” It is useful for me because I have no reprints left. All my whole library and everything vanished during the war• The Russians took everything...

Uhlenbeck:

Why don’t we talk further about this past. You have been in so many places. Where did you like it best?

Debye:

Where did I like it best? I must say in Zurich.

Uhlenbeck:

In Zurich. The second time.

Debye:

Well, no. I really had a good feeling about Zurich from the first • It was so nice and you could do everything, there was no trouble. I could have what I wanted. Really the first time was the best time.

Uhlenbeck:

And the scientific atmosphere?

Debye:

Well, there was no scientific atmosphere, but I did, not need it. Later on we created a scientific atmosphere. That was the second time. The first time I had several ideas which I wanted to do, you know. And nobody interfered. And I had not much to do, and what I had to do with the laboratory I wanted to do. So it was quite nice. My most dismal time was in Utrecht.

Uhlenbeck:

But that was short.

Debye:

It was short, yes, yes.

Kuhn:

If you had been longer at Göttingen -- if it had not been for the war and so on at the time you were there -- might that have been ‘as agreeable a place as Zurich?

Debye:

Yes, oh yes. That was quite nice. With Hibert there and connections... I was there still at the time when Felix Klein left. That was nice, because you see Felix Klein didn’t want to give any courses any more. He was already very old, but he got me and Caratheodory and Prandtl in his room once a week to give historical talks about mathematics in France in the 18th century

Uhlenbeck:

That book he wanted to do -

Debye:

He wanted to write a book about it? I did not know that. Well, anyway, we sat there every week for an afternoon - - for a few hours with him, and he was talking about that, which was quite interesting, very interesting.

Kuhn:

Had Klein been as close to the physicists as Hilbert was?

Debye:

No, no. Klein was more organizing this business, between technical things and university -- apart from his mathematics, hyper-geometric function and these things. But he was typically the opposite of Landau. He was the geometrical man. And Landau was the numerical man, analytical.

Kuhn:

I had one very little question left over from yesterday. In connection with the photoelectric effect... What about the photo-effect measurements? Was the linear dependence of kinetic energy of photoelectrons on frequency clear by 1910, say?

Debye:

Well, I don’t know, but I remember that in Munich we talked about it. And I remember very vividly how I talked about it with Jaffé.... Jaffé came down every vacation to work with Roentgen. And they were doing experiments on electrical conductivity in (calcite), where you see the, ions are not discharged as electrodes but built up so then you have no (Loeb’s) law anymore. And so he had been measuring those things with Roentgen, and then was always complaining. He could not get Roentgen out for publication. It was lying on his desk a whole year and then he still was not very confident that they really had something to show. He was very, very troubled by his reputation. He did not want to publish anything.

Uhlenbeck:

But Jaffé was also for a while assistant of Roentgen.

Debye:

Oh yes, but I think that was before my time, because I remember now very clearly that he came every vacation to work on this (calcite) business. And I remember very clearly that he was talking about the photo-effect. I don’t know at what time that was, but it must have been during the time he was in Munich.

Uhlenbeck:

And it was so to say an experimental critique. Was he doubtful that the experiments were right?

Debye:

No. My remembrance is that he said, “Well, it is a very clear thing of showing that the quantum hv- that is, not h, but hv -- exists.

Kuhn:

Well, when you and Sommerfeld worked on the photo-electric papers, did you both feel, “We’ve got to get the hv as an end result”?

Debye:

Oh yes, no question about that, no question about that. It was not a question of making another law.... But the question was really, “Can you find any kind of a classical picture for an energy quantum?” That was the point. It was really impossible.

Uhlenbeck:

Apparently also Landé in this Munich period worked on such prob1ems still. That must have been after you.

Debye:

Oh yes. Well after all, if you talk to De Broglie now, he is still thinking that way. And if you talk to Einstein, he was thinking that way.

Kuhn:

But by the time Land é was there, he says he and many other people were still trying to do it -- but not Sommerfeld, any more.

Uhlenbeck:

Sommerfeld apparently gave it up • And you too, apparently.

Debye:

Oh yes, I gave it up very early. I had long discussions with Sommerfeld about it, and he was still (grumpy) at the beginning. (But when he stopped to think, why did we publish? We should never have published,) because we were absolutely convinced it was not.

Kuhn:

And he by the end of it was convinced?

Debye:

He was positive of it, yes.

Kuhn:

Do you think it’s that as much as anything else that convinced him?

Debye:

I don’t know. Maybe he put so much effort in it, that after all he put a lot of effort in it. If you put so much effort in, and it does not work, then you get more and more convinced that it is no good.

Kuhn:

Can you remember more about these conversations with Sommerfeld over the fundamentalness of the quantum theory?

Debye:

Well, that would be hard, that would be reconstructing now something which is my own feeling at the moment....

Uhlenbeck:

It would also be nice to know, if you could tell a little bit about what kind of ways you came to successive problems. Because you have always jumped from one to another.

Debye:

Well, which one do you want? I told you about electrolytes, how I came to that. Just out of the blue you see, because I heard this talk, and said it is nonsense. And then I had to do something about it.

Uhlenbeck:

Yes, but for instance the specific heat.

Debye:

Well, that has certainly in my feeling connection with this thing about the 1910 paper. Well, I got a little bit angry with ‘Nernst and Lindemann. I did not want the half-quantum. Maybe that can be a thing which induced me to do something about it.

Kuhn:

Did they insist that that v over two term was a half-quantum rather than another frequency?

Debye:

Yes, oh yes. It wrecked my paper. Then I tried to show that in some way this is a kind of an approximation. So I was very much concerned about it, and I was concerned about showing there was some sense in it. Also that there is some sense in getting another frequency in it. Einstein had said that too, you see.

Uhlenbeck:

Yes, but still the main stuff is that you think of the eigen vibrations of a lattice, that was the great step.

Debye:

Yes, of course. And there I wasted some time, because this is not so easy to calculate, and so that is why I talked to Haar about this thing and started that way. Then later on I got the idea, “Well I don’ t need to do all this trouble• I can take the continuous thing to get an approximation,” you see.... And the cut-off was right from the beginning. It had to be.

Uhlenbeck:

That must have been somehow a kind of illumination, that you could do it like that. Did you worry about how good an approximation?

Debye:

No, no. I said to myself, it is good at high temperatures; it is good at low temperatures; now the intermediate one cannot be very far off.

Uhlenbeck:

I asked you the question because of course after you too many people worried about it.

Debye:

Yes I know. No, I knew at that time, I knew one-dimensionally the whole thing - - which you can do very easily. I knew from the one dimensional thing that you don’t have ‘a continuous spectrum but another one which comes up • I knew all these things for the linear thing, but in order to make it in three dimensions, it was much too difficult.

Kuhn:

You never felt the desire to go back and go on with that.

Debye:

Oh, well Born was doing that, you see• So Born has been doing that for years and years. There was no sense in it. No, you see I in general am like this as soon as I have one part of a thing, and I am convinced that that is the right direction,then I am very glad if other people do it. I don’t want to stick to one thing though. It is really true, I had a lot of things where I could have developed further, but I did not do it. Because I was not so interested in that.

Uhlenbeck:

And your interests became more and more towards the experimental situation.

Debye:

Oh yes, yes. In this sense, that I wanted just to be making some crucial experiment. But I needed to experiment. I needed it for myself, to do something about it• Not just waitng until somebody else wanted to do something.

Uhlenbeck:

But did you do experiments yourself?

Debye:

Oh yes, yes. For instance, in Zurich I tried well, I did experiments about the temperature dependence of the dielectric constant. And that was not so very good. Then I got Jona in G öttingen, and Jona did the experiments much better. So I always did experiments, but the experiments were rather qualitative. Then I got the boys to develop it, that was the point. Just as in the case with the X-rays. I did the experiments and then Seherrer came in, and then he took over. So I always did some experiments, but never so they could be published.

Uhlenbeck:

But that was surely the reason why you always had to have a laboratory.

Debye:

Yes • I was not happy without one.... I have always five post-doctoral men now, you see, which are supported by foundations and so forth. And I take them just for their experimental skill, by doing experiments, and by doing what I tell them to do.. When I was in Munich I had very good contact with the people in the laboratory of Roentgen with Peter Paul Koch and Wagner - - and people who were his assistants. Experimentalists, at that time at least, they did not want to know anything about theory. Except if you could make the theory understandable to them not with x, y, z • That was a very nice time, because that induced me, when I had an idea, or when they came with questions, to think about that inner physical way and to translate mathematical formulation into something which they could understand • And you still find a lot of people like that around, although they may not want to confess it.

Uhlenbeck:

But you were originally much more mathematical?

Debye:

Well, really that was the influence of Sommerfeld. Because Sommerfeld is purely -- well, not purely -- mathematical. I learned my mathematics from Sommerfeld, not from courses. And I tried once to follow a course in Munich, a course of Lindemann, who showed that pi is transcendental. Well, I could - not understand.

Kuhn:

But in a sense you learned your physics also from Sommerfeld, except that you taught it to yourself.

Debye:

Well, the physics I did not learn from Sommerfeld, but the mathematical formulation of the physics, that I learned from Sommerfeld. I learned physics by myself. I told you yesterday I think that one of the books which was really a big influence was this optics of Drude.

Uhlenbeck:

But you didn’t study such things as Kirchoff’s lectures on theoretical physics, that kind of thing.

Debye:

No, no.

Kuhn:

And Boltzmann?

Debye:

I looked at that occasionally, but I did not study it. Well, I read the gas theory, but it was not a big influence, because there were only the principles which were important.....

Uhlenbeck:

I would Iike especially to ask about the polar molecules.

Debye:

Well, I told you how the idea came. In the laboratory of Kleiner, all the experiments about dielectrics were done• That was his field. But it was more engineering. The first period in Zurich, at the University there, when I was successor of Einstein. Kleiner was very glad that I had shown an interest in the lab. And I was very glad that I could show an interest in the lab. And so I had to talk to those people about dielectrics, and loses and all these things. At the same time there was some contact with magnetism -- magnetism and dipoles. So it was very clear that this temperature dependence, Curie’s law, should be possible. There should be molecules that were plus -- minus. The real crucial thing was to show that this was at the same time an explanation for the anomalous dispersion which Drude had found.

Uhlenbeck:

There came then somehow a connection also with the theory of molecular forces.

Debye:

Oh yes. Well this theory of molecular forces, this was the business. I can tell you what the point was. They must be electrical. Now if they have to be electrical, and you are going to talk about the interaction of systems which are noncharged and consist of plus minus, then they cannot be rigid molecules. Because if there are two rigid molecules then they will have a potential energy with respect to each other which depends on the orientation. And if there is no preference of orientation, if you are at high temperatures, then there is no attraction. And if you look at the tables which Keesom for instance published in the Enzyklopadie der Mathematisschen Wissenschaften you see that according to these tables, experimentally that there is still Van der Waals attraction, even at infinite temperatures. So then the first question was, how can that be connected?

Kuhn:

When you say, if there are forces, they must be electrical, these are the attractive forces only

Debye:

Well, they must all be electrical. We did at that time not look at repulsion.

Uhlenbeck:

Although you had a short paper about it• But that’s not in here.

Debye:

I know, I know. No, that’s not in there • No....

Kuhn:

How early was that paper?...

Debye:

The same time you see. Let me go on now. So you have to talk about systems which are not rigid. That they are not rigid is clear because there is a dielectric constant or an index of refraction, And so as soon as you have that you can say that if some of these particles with polarizability get into a field E, then the potential energy is with the negative sign. So even if the electrical field in the average is zero, you still have a potential energy. And as long as alpha is positive there is an attraction. So that was the clear thing. Then you said, “All right, now I am through, now I have a field around the atom. It may be a dipole or a quadrapole or anything else. You will always have a field so there will always be attraction. And there is where I talked about quadrapoles. But then there 265 was trouble with the rare gases • And then you have to say, “How about the rare gases?” Now I never dared to say that the field around rare gasses, the instantaneous field, is something different from the, average field. Because then it emits energy. You cannot use Maxwell. And that is bow London came in, because what London does is say, the instantaneous field is a dipole field, the field is one over r to the third, so the square of the field is I over r to the sixth, and there you have the attraction. Now he put that in a mathematical form and put it with quantum theory, but it really has nothing to do with quantum theory in the first thing.... To say it in a physical way then you really have nothing else than that. Now you can, then talk that the systems which are nearer together have other levels. Then you can calculate the levels and find out that the levels are lower than the average, and so on. Like he did then in a special case for the vibrators.,,.

Uhlenbeck:

You say, “That’ s the place where London came in.” Did he talk with you about these things

Debye:

Oh yes, later on when he came. You see we had then these conferences in Leipzig. And so he came to these conferences and then presented what he had done, and we agreed that it .. would be the explanation for the rare gases. -

Kuhn:

From the very beginning, was it clear that all of the inner-molecular forces must be electrical fields?

Debye:

Well, I think to everybody. It was also clear to Keesom. He only made the mistake that he thought how molecules were dipoles.

Uhlenbeck:

Well he had also quadrapoles.

Debye:

Yes, but that was no good, because that gets zero at infinite temperature, and that is against the experiment.

Uhlenbeck:

Not at finite temperatures.

Debye:

Oh no, of course at finite temperatures it is there. And all these finite forces are there still, you know, on top of what you call the tendon forces. The question is only, “how big?” In the case of water, for instance, the interaction of the dipoles is much bigger than the London interaction....

Uhlenbeck:

There is one point there which always bothered me. London made such a big point of the additively. Because with your effect it is not additive.

Debye:

No, no, of course not. It is not additive.

Uhlenbeck:

But with his effect its additive.

Debye:

No, it is not additive. Well, now I am going too far for you as an historian. If now you talk about Van der Waals’ forces and you are convinced that they are electrical,then you are going to find: a) you cannot get interaction between two bodies by just adding up the molecules. Because if it is an electromagnetic effect then it is going to depend on the form and so on. In other words, there must be a shielding from the Waals’ forces. And not only that, Van der Waals’ forces must be dependent on the temperature. And so it is intrinsically wrong if you say, I have a gas and I know what the molecular force between molecules is, and I apply this force to a liquid. It is intrinsically wrong.... This is the modern trend. Because this begins really with Casmir -- what Casmir calculated. And this is now connected with quantum theory too. If you talk about Casmir, then he says, “I have two mirrors, and I calculate what is the force between these two mirrors that does this.” He has this cavity, and he has the mirrors at a certain small distance, and he takes all the proper frequencies of this cavity, and he gives to each of then an energy hv divided by two • And he calculates that a little bit better in taking care of the fact that they are discrete levels. Now his energy is infinite, so he has really to determine infinite minus infinite. Two calculations, one continuous and one (discrete). And t1is difference between the two is Van der Waals’ force, This is really the clear case, that Van der Waals’ forces are very characteristically connected with quantum theory and with very deep things in the quantum theory. Really with the deviations from Maxwell’ s equations - - with quantum electro-dynamics. Because you should not calculate it as infinite minus infinite.

Uhlenbeck:

Well he did it of course then differently too, by perturbation theory.

Debye:

But that was between the particles, and there, still, he has to take the infinite part. He, with this other man (Bolder) has also the same trouble. There he has to have the cavity, and put two particles in. And then he has just exactly the same trouble, only then it is more hidden there. Were as in the case of - -

Uhlenbeck:

That is more unique, how you have to subtract the infinity.

Debye:

Yes, yes, yes. Now the Russians now, this Landau and Lifschitz, they have done the same kind of thing in another form again, for a case in which you have media with a dielectric constant, then you have to know the dependence of the real and imaginary part of the frequency in order to calculate. Now the queer thing about this whole business is that the value calculated by Casmir, which does not contain anything else than h and c, is really good experimentally.

Kuhn:

I still want to go back to the early days. I talked a bit with Polanyi about electrical theories of the atom. While in many circles these were apparently quite popular, there were other people who simply put their foot down and said, “How can you have diatonic hydrogen molecules?”

Debye:

I never had anything like that. I never had anything like that. At least the people I talked about.

Kuhn:

This was Nernst in particular.

Debye:

Oh Nernst’ Nernst Did I tell you what Nernst said to me when I had done this specific heat business? I visited him. He wanted me to come and talk to him about it. Do you know what he said to me? “Why do you want to be a physicist? You can get much more money when you are a banker.” That’s what Nernst said to me, “Why don’t you go --“.,. He thought I was wasted in physics.

Uhlenbeck:

Nernst must have been the absolute incorporation of the Bonze. And that’s where Lindemann must have gotten it from.

Debye:

I knew Lindemann. I visited him several times at Oxford. He was quite nice to me, but was not nice to other people....

Uhlenbeck:

But now to come back to the repulsive forces. Why didn’t you follow this thing up?

Debye:

Well, I thought you see that when London had done his things, then there was not much space.

Uhlenbeck:

But also London did not worry about the repulsive forces.

Debye:

No, no. But there were so many people who were going to do things about that, so there was no point in crowding.

Uhlenbeck:

Well, I mean the repulsive force is much harder to get from electrical -

Debye:

Yes, yes.

Kuhn:

But your first work on electrical repulsive forces was also back in 1913?

Debye:

No, that was later, that was the time when I -- well, I had to fight a little bit with Keesom about this polarizability, that that was necessary, but that was never appearing really.

Kuhn:

Was the long wave dispersion problem on your mind from the start with the dipoles, or did you think there were dipoles and then look for dispersion?

Debye:

Well, in connection with electrical things, and connection with Drude, who had made these measurements. They showed this anomalous dispersion and other things don’t show it, that the dielectric constant goes down with increasing frequency, and then it dawned to me that that was what you would expect. But then there was the question of formulating that in a mathematical way, so I had to talk about relaxation times..,.

Uhlenbeck:

Now with all this dipole work, the thing which is striking I find is that you, at least in the papers, never connected these with molecular models explicitly. The molecular models were there.

Debye:

Yes. Well, I say a few words. You know I wrote this little booklet - it is the only one I ever wrote -- “Polar Molecules.” And there I did something about that. You see it meant very clearly that water was not one linear thing. That it has to be a thing like that. That NH3 has to be N with three H's below and so on, that was very clear. That hasn’t been worked out very much more. But then the connection of using dipole moments for parts of molecules and putting them together as vectors.

Uhlenbeck:

But I mean you didn’t connect it with the Bohr models, something like that?

Debye:

The only thing which was done there was this. If you take HC1 for instance just let me explain in an example - then you know your H is positive, one positive charge, and Cl is one negative charge. And you put this thing at a distance which is the HC1 distance, which you get from infra-red for instance. Then you get a moment which is six times too big. And so you have to see that if you get this H in the neighborhood of the chlorine, then you get a distortion of the electronic atmosphere which is going to compensate for five-sixths of the effect. And in this way it has been calculated. And it then appears in the literature. But then you need only the polarizability, so to say, of the atom. Did you calculate that?

Debye:

, Yes, yes. And it is also calculated in the case of water. And Heisenberg made a little calculation once, and so on.... It was later then. it was later when you begin to talk about what is the dipole moment, how big is it, how many units, and so on,... U: Well maybe now we can go back to the period that you liked best, the Zurich period. That was also on the one hand such a crucial period in physics. Could you tell us a little bit more about Schrödinger? What kind of a fellow was he?

Debye:

Oh, he was quite a nice man.

Uhlenbeck:

I never met him, so I don’t know at all.

Debye:

Yes, oh, he was quite a nice man. He was not too healthy..,. Even then, yes. For instance, he always had to smoke a pipe in the morning before breakfast so that his digestion was all right

Kuhn:

Does this mean he took particular care of himself?

Debye:

Oh yes. And he had to sleep with cotton wool in his ears so that he was not disturbed.

Kuhn:

Did he sleep nights, or did he sleep days?

Debye:

He slept nights, yes. He slept nights if he could.... But he complained about it, that he could not sleep without that. So he was rather nervous, but not nervous of the type of Zermelo. On the other hand he was quite more le a nice Austrian,

Kuhn:

Was this something that went on all the time, or did he have periods of --?

Debye:

No, no, all the time.

Kuhn:

Then he. was never particularly sick or particularly well?

Debye:

No,. not sick, not really sick, but very sensitive.

Uhlenbeck:

But apparently he was not an Austrian in the sense that he was very voluble.

Debye:

No, no, he was not. Was it. not, his mother or his father his mother I think was English? One of the two was English. He had quite a nice wife, but they never had children,, for instance, which is also a little bit out of the order.

Uhlenbeck:

But how were your contacts with him? Did you see each other?

Debye:

Oh, occasionally, we saw each other, were very friendly with each other. It was not so that it was an intimate relationship.

Uhlenbeck:

And not very much scientific contact?

Debye:

No, no. That came about because we were in two different places. He was at the University, and I was at the Technische Hochschule.

Uhlenbeck:

How far apart are they actually?

Debye:

Oh, (R ) strasse goes, down to the lake,... If you come up this way from the lake, then on this side there is a small building, and that was the Physics Institute of the University. Opposite that was the Chemistry Institute where (Werner) did his things rotation of the plane of polarization, and other things, than just carbon atoms connected. And then you go up farther, and higher up here, there is the w1ole complex of the Technical. And, then you go farther up on the Gloriastrasse, and you come to the Physics Institute of the - - rather far away. I would say, if you walk, well you could better walk down the hill than up the hill • But if it was flat it would be a distance of perhaps a kilometer or so.

Kuhn:

You could have gone to Schrödinger, but he could not easily come to you. Did he quickly have quite a few students right away?

Debye:

Well , as far as I know he had students there, yes.

Uhlenbeck:

I understand that in that period also Heitler was there, do you know?

Debye:

I don’t know. I have met Heitler several times, but I don’t know whether he was there in this period. He may have worked with Schrodinger. He certainly did not work with us • We in turn did this other part -- I myself and Scherrer.

Uhlenbeck:

And who were your students in that period?

Debye:

Oh, several people. They are still partly professors in Zurich, technical people. More people who do technical things, not in fundamental physics. And then later on you see I did not do anything about nuclear things. That was introduced by Scherrer later. After I left.

Kuhn:

But you really then had very little contact with Schrödinger?

Debye:

Very little scientific contact. You see I Learned about his things from the experimental physicist who told me about what was not yet published, but he knew about it; Because he met him every day of course -- they were in the same building. IJHL: But then you told yesterday that you got De Broglie’ s thesis?

Debye:

Yes, yes, but that was before, that was before.

Uhlenbeck:

That would have been about ‘24 too.

Debye:

Yes, well maybe a few months before that • You get these things, and then you say, “Well this is a thing for our colloquium, we have to talk about it.”

Uhlenbeck:

Was that a publication? Or De Broglie sent you his thesis?

Debye:

No, no. It was a publication, it was printed. I know that it was printed....

Kuhn:

A very long paper, about 100 pages.

Debye:

And so, “We must look at it,” “we must understand it.” “And I have no time to do that, lt and. “that fellow at the University has nothing to do.”

Kuhn:

There is a story going around that Schrödinger did in fact present this to the colloquium, or to a colloquium. He talked about the idea that there were to be waves correlated with matter, and you said that for waves, there must be a wave equation.

Debye:

No, I don’ t remember that.. I don’ t remember that. I had. no influence. Certainly not. [Time out for phone call.]

Debye:

It was really not a big surprise. Because you see there is this connection. Ii’ you take the Hamiltonian mechanics, then you have an equation which resembles the equation of rays which you can get out of the differential equations of Maxwell ‘by making a very simple.... Well I once made a paper about that. So you have only to go a little in to reverse • So that was very familiar to me • There is this eikonal business,,.. You see, once I did something which was just the opposite. Take Maxwell, and then get to the eikonal from Maxwell • Now what Schrödinger does is just the opposite, going from the eikonal within mechanics to the differentials. I only want to say that I was not surprised, but I had no influence in this, no question about that.

Kuhn:

Except the one involved in having given him the De Broglie paper

Debye:

Yes, well that was absolutely .-, just

Kuhn:

How did you feel about the wave-like character of matter?

Debye:

Oh, that was a natural generlization to me, just from this point of view, you see • We are just talking about eikonal and I say, that was not a big surprise to me, when Schrödinger made that. Because I once had talked about the derivation of the eikonal rays out of Maxwell’s equations. And so what he is doing is just the opposite But that was a natural generalization.

Uhlenbeck:

Well the great point was of course the derivation of the Balmar formula.

Debye:

Oh yes. That came out then, but the generalization itself, which is the fundamental part of it, that looked very natural to me....

Kuhn:

Were you involved in any of the debates about the probabilistic interpretation?

Debye:

No, I was not involved in that..,. It is a little bit silly, this whole discussion.

Uhlenbeck:

But I noticed also here again, looking through your papers, that in this book there is really only two papers in which you use the wave mechanics.

Debye:

it is possible.

Uhlenbeck:

One o them with Miss Manneback on the symmetrical top.

Debye:

Now, that. really goes farther back, because that was the, question of the dispersion of a polar gas.

Uhlenbeck:

Yes, but then, this is this paper on the - - I think Manneback carried it further’ the energy levels of symmetrical rotator.

Debye:

Yes, but that is really in this little booklet about polar molecules.

Uhlenbeck:

There is also a paper.

Debye:

Yes, there is a paper there, but that is rather early.... You see, you take a polar gas. What is then the dispersion of the polar gas? And in order to do that you have to do quantum theory. So I calculated out what Are the levels of the polar molecule which can rotate and so on, And how is the influence that ins all in this polar molecule thing. Because that then is different from the polar liquid, because the polar liquid IS all that relaxation business • But if the molecules are independent of each other then you get all these absorption lines. And that you can only do in the case of quantum theory. I even made a few remarks about how you have to do it in this little booklet.

Uhlenbeck:

There is here this paper on the basic laws of “Grundgesetze der Elektrischen und Magnetisehen Erregg. v. Standp. d. Quantentheorie.” That was one of the few papers where you really put in discrete energy, and show how one has to compute it by -

Debye:

Yes, yes. You don’t have then Langevin anymore, which makes it exponential instead of l/T.

Uhlenbeck:

Part of that could have been done much earlier of course.

Debye:

Oh yes, yes.

Uhlenbeck:

But was this influenced by the deve1opment of the quantum mechanics?

Debye:

Well, it was influenced in this way, that you can say, you had now a method where you can calculate it. Whereas before that you had to guess how to do it.

Uhlenbeck:

Well, you could say after Stern and Gerlach there must be the

Debye:

Oh yes, yes, but still you could not make dispersion theory at that time. No, this was more the Sommerfeld direction you see. Now you know the mathematics and then -- [Break for phone) Well, I helped getting him the Gibbs’ medal. And it was all there when he got this. Well, you know that.

Kuhn:

I said I wanted to go back to the Van der Waals’ forces once more.... You’ve really told us about how you got into the problem of the polar molecules. I’ d wondered if you could tell us the same sort of thing for the inner-molecular forces. DEB; No, that was no outside influence. It was just a question that I said, “Well I have to understand the molecular forces, and they have to be electrical. And so what does it mean?” That was the whole reason.

Kuhn:

The work you had done in the interim, had been the X-ray work really. You got further and further into the X-ray problem.

Debye:

Yes, yes. Well, the X-ray work, that always came back you see, this question of interference, and what can you do with interference about structure. That has been coming back the whole time. It came back again in Leipzig. It came back again when this man of the Bell Telephone Company during the war wanted me to be interested in rubber. I told him I don’t know anything about rubber, and he says, Well these are just the people we want” Well, what am I interested in now-- critical (???). This is all the same business.

Kuhn:

It was really then through X-ray structure to the question of molecu1ar forces that determine the structure the X-rays were unraveling that got you back to the question of the inner-molecular forces?

Debye:

You can say that. I would. not know; I would not know • It was just a question to understand molecular forces from an electrical point of view, you see, as electrical forces, was all

Uhlenbeck:

Ja, but when did you start to think about it? You can’t simply say one morning, “Now I have these electrical forces, now I am going to make a theory of it.

Debye:

It happens like that, you know’.

Uhlenbeck:

Well, there must be some kind of stimulation in order that you suddenly say, “There is something which is a problem.”

Debye:

Well, you talk about a lot of things, and you have to give courses and during the courses you -- You see, I had to give this experimental course in Zurich.., I had to find different people - I told you yesterday. Scherrer had to give a part, and I had to give a part, and so on. And if you talk about these things you go through the whole business • I had to show critical phenomena and so on. And then all at once you said, “Well, I really don’t understand how all these’ things act on each other.” I must say that just in giving general courses like that, you go over the whole field you see, and then you find some places you really don’t understand what it is. And I would not be able to pin-point the moment I got interested in it....

Uhlenbeck:

Yes, but there are so many things which one doesn’t understand.

Debye:

Ah yes, of course there are so many. And then you think a little bit about it, and you come to the result you still don’t understand it. You don’t know how to attack it • But in other things, when you think a little bit about them, you think oh yes, there might be a possibility of doing something like this, you see. And it is not a question of choice of the problem. It’s just that there is a problem’.

Uhlenbeck:

Ja, but there are many problems, and the’ selection’ of it is of course -- you say it often came for you because of having to teach it.

Debye:

Yes, yes. Having to teach a general course. You see with this general course in physics, you have to go over all the things in physics. And. so here and there you pick, like a hand picking up a stone.

Kuhn:

We had a two year introductory course • But we never got to a problem that was close enough to the forefront of research to have any idea of what was going on. ‘

Debye:

Oh, but the level of this physics course. Well, in Leipzig for instance, the level was so that it had to be understood by medical people who were going to be medical students.

Uhlenbeck:

And in Zurich too?

Debye:

In Zurich it was for engineers. There it was on a higher level.

Uhlenbeck:

Did you use calculus or something like that?

Debye:

Oh, well what I used were a lot of experiments.

Uhlenbeck:

Ja, but I mean you certainly talked also about theoretical pictures and so on. Did you calculate a little bit?

Debye:

Oh, a little bit. But as little as possible. I always am against this using the blackboard in these lectures • That should not be just problems like they do in the United States, problems and paper and pencil work • it should really be something that they understand the whole business, that they have a feeling for the whole business. And in order to do that, there should be used more, many more experiments than they do here. That was the critical thing always.

Uhlenbeck:

But this was certainly not the way Sommerfeld did it? ... He used the blackboard all right?

Debye:

Yes, yes • And that was just the thing which I did not like so much. Although I learned my mathematics from him. And it was all right. I have the feeling that in all the lectures the fundamental understanding, the fundamental picture is not stressed enough.

Kuhn:

Often you write much more personally than it is now customary to do, and I suspect somewhat more than even your contemporaries did.

Debye:

If you want a man who writes personally, then you take Schrödinger.

Uhlenbeck:

Ehrenfest.

Debye:

Or Ehrenfest, yes.

Kuhn:

But that has become over the years harder and harder to do. There is much more emphasis upon compression and upon depersonalizing papers.

Debye:

I always say there is much more emphasis on hiding how you get to it’. And I don’t like that.

Kuhn:

I don’t like it either, but I think part of what it comes from is just the cost of paper.

Debye:

I am not so certain. I think it is a bad habit’....

Kuhn:

Have you seen that happening, seen this change in the acceptability of a more personal style? DEB; Not so much at that time. What all people had against me in editing the was always that it took too much. I had always the impression -- and I defended that -- if I see a thing is wrong, but if there is also a good part in it, then I’d rather publish it because another man is going to find what is wrong and have an idea about it. That is my defense always. And that of course you cannot do nowadays. At that time I could take what I wanted. You had not to send the paper to somebody else.... There was no referee with those.

Uhlenbeck:

Did you reject once in a while?

Debye:

Oh yes, yes. All nonsense, that I rejected. But if it was half nonsense, I would not reject it.... That was my own responsibility.There was no referee system like here.

Uhlenbeck:

Of course certainly the style in physics changed radically with the quantum theory. Then you got these long papers which have long calculations. And I know so well that Ehrenfest suffered so much from this because he, like you, always wanted to understand with his fingers. Did you suffer from it?

Debye:

Suffer?

Uhlenbeck:

I mean did you dislike this whole 1’

Debye:

Oh, dislike; yes, I dislike it very strongly.

Uhlenbeck:

I mean also these long calculations.

Debye:

Oh, long, long calculations. What is the end of It? Weil, it is 3.6 instead of 3.2. There’s some point in it

Uhlenbeck:

One has machinery now, this quantum mechanical machinery, and then you just have to put it in.

Debye:

Well I’ll tell you, as a member of the advisory board of the (Welsh) Foundation we support a lot of things. We support all of these things of (Matson), you know, Although I have the feeling it is not of much use • He gets a lot of money from us. I support it, but I am not convinced that it is good to support it. You see, you calculate years and years and years to get the structure of lithium hydride. And what do you get? Is it worth this effort? We are not going to learn anything new, because you know the principles. You see, veil, the way I talk for instance to C???). I tell him, “You always want to be busy, you are an American. You always want to be busy.” And if they have no idea, then they begin to calculate, or they use the calculating machine. Instead of being lazy and waiting until they have a good idea’.

Kuhn:

I want to come back to this question as to how much personal can one put into an article, and how much does one have to condense it and get out all the history. You spoke of more and more emphasis on keeping out bow you got the idea and how it developed. Did that happen quite suddenly?

Debye:

No, no, no. It is also the result of the pressure from the editors to make the things shorter. They could, however, be made shorter in another way. But it is not only that. It is also this feeling of the author himself that he does not want to stand there naked....

Uhlenbeck:

The change in style is very clear, because there are very few people who write like you now. Fermi did still

Debye:

Yes, Fermi did. Yes.

Uhlenbeck:

Perhaps we can now talk a little bit about Leipzig, because I am also very interested about your relations with Heisenberg.

Debye:

You see in Leipzig there was Des Coudres, and there was also an experimentalist. Then Des Coudres died, and so they offered mc this professorship for theoretical physics, during the tie I was in Zurich. Arid I did not want it. And then very soon after that the experimental physicist died - - he was the one who had made standing light waves • He died, and then they came back, and said, “How we will give you the laboratory if you want to come.” And then I said all right, I would do that, provided you let me get the theoretical physicist. Then Heisenberg was still, well, he was not “habilitiert” and so he was in Copenhagen at that time • So I said I want Heisenberg, after I had talked with Scherrer • Scherrer and I had decided between us, a gentleman's agreement, he would leave Heisenberg to me and he was going to take Pauli. So then I had a lot of trouble with the faculty in Leipzig because they said, “Such a young man,” and “This is an ordinary professorship,” and so on and so on.... It was very queer, you see. But I pushed it through. And then from there on, well, I had connections with him. He was living for the first time in the Physical Institute there, and my place of living was also connected with the Institute, so we came together several times. But then of course his interest was more in the theoretical direction. But still, we decided about conferences which we wanted to have. We bad these Leipziger Vortrage every year. I got the money from. several places.

Kuhn:

Where did one get money for that sort of thing? D Oh, I got money from book editors. I got my money from them, and so on.

Kuhn:

en you said there was trouble with the faculty, was this with the physicists or with the philosophy faculty?

Debye:

With the philosophy faculty. These things had to go to the faculty. And at least in Germany you had to put up a list of three names, and these were first, the second, and the third. Then there is a lot of discussion even from people who do not know anything about it. But the general idea was -- the faculty was all full of Bonzen -- they did not want a young man there at the same--... Heisenberg and I discussed things. For instance, this whole business of quantization of electrodynamics. It’s very clear. You have your cavity and you have your waves in it. And those waves are never going to be in equilibrium as long as you don’t put a little (particle) in it, you see. But that is no good. That is not fundamental quantum theory. And so you should have something where waves are scattering each other. So we talked about that problem very much with each other. And he calculated out how much it was, it’ s so little you know that you cannot do any experiment about it. But these were things we talked about. Now he was not so very much interested in my things. My things were, at that time, magnetic low temperatures, and the experiment with gases, and the interference experiment.

Uhlenbeck:

And the scattering of light on sound waves?

Debye:

Oh, well that came out of the United States.... That was started at M.I.T. That was induced by Brillouin’ s thing. You see Brillouin has waves, and the light goes through and then he gets reflection on the waves. So when I was visiting at M.I.T., I wanted to do some experiments about bow light was reflected on artificial waves. A man of the name of Sears and I set up an experiment. An when we got up this experiment it turned out that instead of the reflections he was looking for, we got this spectrum of the light where the waves were like a lattice. This was the experiment which I did first at M.I.T. when I was visiting there for a short time. And therefore I published also this thing in the National Academy. URL: This was Scars. This was the 1932 paper. But then you had. also this paper in the (???) Berichte.

Debye:

Oh that is possible.

Uhlenbeck:

And also the scattering of light by sound waves again. So you must have continued that in --

Debye:

I may have continued that, yes. But it started at M.I.T. It started because I wanted to see experimentally whether I could reflect a light wave from a sound wave. This is what comes out of Brillouin’s calculations.,.. And so I made artificial sound waves, let the light go through it, and I saw the spectrum....

Uhlenbeck:

Since this happened at M.I.T., it might be of interest to tell me, when did you come first to America?

Debye:

My first impression of the United States was New Year’s Eve on Times Square I was invited to go to M.I.T., it must have been 25 or ‘26. And to talk there. I think they wanted me to talk about the dielectrics. So I came over on a Dutch boat --

Uhlenbeck:

And that was the first time that you were in America?

Debye:

First time, yes. And I arrived in New York on the last day of the year. And so that was my first impression, you see • Then I went the next day, or the day after that, to Boston. I was in Boston for a good deal of the winter. Then Noyes, who was in Pasadena, wanted me to come out to Pasadena, so in February or so I went out to Pasadena. And saw a good deal-of the West toast on that occasion. And that is the time when I met Pauling.

Uhlenbeck:

You wrote a paper with him.

Debye:

Yes, yes. Then I wrote a paper about the effect of the dielectric constant in the neighborhood of the ion. You know even there you can do what you want, just not change the law, in electrolytes. But that was the point. That was what we discussed at that time, because Noyes was interested in electrolytes. So this was my first visit....

Uhlenbeck:

Well, but you said also you were out in Madison.

Debye:

Oh, well since then I have visited the United States several times. Once during a summer time I was in Columbus, Ohio . Once I was in the summer term at Columbia.

Kuhn:

You don’t pick the cool places, do you?

Debye:

No, it was too hot, Columbus, Ohio, was the worst place. Then, oh, I was in Berkeley once. I was once in Madison, Wisconsin. They wanted me. And so they said come over and you can see how it is. And then I did not want to.

Uhlenbeck:

Well I placed that earlier because of the story you, told yesterday, that the Do Brogue thing was discussed.

Debye:

Yes, that was during the time when I was in Madison, Wisconsin.

Uhlenbeck:

That must be then ‘27

Debye:

Well, maybe the next year or so. I don’t know, but that was later. The first time was M.I.T.

Uhlenbeck:

: Well maybe M.I.T. was earlier than you say. D Well, I don’t know. I know in Madison, Wisconsin, we lived for a good deal of the winter, and there we had a house, because one of the men who had a sabbatical year was in Europe, and we rented his house, with the maid and with the student who took care of the first... So I have been here several times, you see. And so I knew the United States when I was still a foreigner. And this story you know, this business with the Nazis.

Uhlenbeck:

That was when you were in Berlin? V

Debye:

When I was in Berlin. The Dutch Queen had given me permission to take over that, because otherwise if a Dutchman takes over a position of another state, then by that fact alone he loses his Dutch citizenship.

Uhlenbeck:

But was Leipzig not in possession?

Debye:

Well, for Leipzig I had also the permission. And in order to be certain I got the permission for Berlin • Which by the way was built with Rockefeller money. That was built with Rockefeller money, the whole Institute. You see the Kaiser Wilhelm Gesellschaft at that time bad no institute for physics. So they came to me, and they wanted me to do that. And the money came from Rockefeller. The only thing Planck wanted was that Laue should also have a part in it, so we made Laue the vice-director. And he had his rooms in there and could do something. At the same time now -- you see that was only one part. The other part was this: Van’t Hoff, when he became a professor in Germany, by that fact alone he became a German citizen. So I knew that. So I got a script from the German government signed by (Rust), who was the Minister of Education at that time, that I was not going to get German citizenship by taking over this. Because they made me at the same time a Professor at the Berlin University. That was done in order that I could have an opportunity if there was a doctor candidate, I could do something about. that. So I had both these scripts, you see. and one Saturday, after I had built the whole Institute and was just beginning - - and that was quite nice, you see. Because they had built this thing that had 3 million volts and the whole current behind it, and I bad the whole temperature business, including the magnetic temperatures. It was just that thing, was all Then one morning the administrator comes in and tells me he was very sorry but I could not go in the Institute any more if I did not become a German citizen And then I said to him -- and I had to appear at the Ministry -- and the representative at the Ministry said, “Oh, why don’t you stay home? You know you can write a book.” And I said, “No, 1 have an invitation to go to Cornell to give the Baker lectures. I’ d rather go there and give the Baker ii lectures, That was the beginning. 3 that is what I did.

Uhlenbeck:

And you never came back?

Debye:

Never came back • I had trouble getting my wife over, and so forth. But that was all before they had invaded Holland • I heard about the invasion of Holland. on the train from Ithaca to New York, in the next spring. So that was the year before.

Kuhn:

Then you actually went to Berlin in what year?

Debye:

Oh, I don’ t know exactly the year, because we started building this thing, at the time when I was still in Leipzig. I sent over some of ray assistants and then I looked over the maps and so on, you know. So it was going on for a year or two while I was still in Leipzig. It was a pity because I had put in a lot of work and I had really a good outfit, you see. And at that tine 3 million volts with any current in back of it was a real good thing. I built the thing anyway with a kind of cylinder at the end. t was an empty cylinder in order to get large distances for the right tensions.

Uhlenbeck:

Was the idea really to get into nuclear physics, then?

Debye:

Well, I wanted to investigate high voltages, and shoot things onto each other.

Kuhn:

I think I’m out of questions. (Recorder on again after break)

Kuhn:

Well you’ve seen a good deal of that change in the differences between the situation in this country and abroad.

Debye:

Well, there was this tine though. A lot of people had the feeling they had to go to Europe in order to get real education. And it was true. Now my general impression, if you want a general impression, and I have said that many times is this: In Europe if you look at the universities, the universities are research institutes; and on that you have grafted some teaching. Whereas in the United States the universities are teaching institutions, built as such, and you are grafting on -- and they are doing it more and more now -- some research. That is the fundamental difference. And there is not enough fundamental research going on. There is a lot of research going on that is called research. But it is the application of all the old principle.

Uhlenbeck:

Yes, but there is of course in this country the undergraduate teaching, which in Europe never existed. And that made, by necessity, everything o large.

Debye:

Yes, yes. And it has all to do with the fact that the high schools are so bad.

Uhlenbeck:

They used to be bad.

Debye:

Yes, well they’re getting better now, but they are still no good. I always put it this way: “The first two years of the university are there to repair the damage of the high schools.”.,. In Europe they were very much better prepared. There was also a selection. Because the selection cane in the years of the gymnasium. My place was a small city at that time, a city of about 30,000 or 32,000 when I was a boy. Not very many boys went to the high school. We started with about 30 or 32. In the fifth course, there were only 11 left. So there is a selection going on there.

Uhlenbeck:

Of those 11, how many went to the university? D2: Several. Perhaps five, half of that you see.

Kuhn:

Now in your own case, by the time you went to the university ... how much mathematics and how much physics had you had?

Debye:

Well, a Good deal of physics. A good deal of chemistry. And before all, a lot of languages.

Kuhn:

You said yesterday you’d had no calculus.

Debye:

No calculus and no analytic geometry....

Kuhn:

Then how much physics and what sort of physics?

Debye:

Oh, physics? Well, we knew about electrical waves, for instance; and wave motion. And we knew some organic chemistry, a good deal of inorganic chemistry1 More than the boys do here when they come from high school. I could watch that with grandsons, you see. That was because I went to a so-called “higher citizens” school. This is the opposite to what they call the gymnasium1 The one has old languages, Latin and Greek, in the other there is no Latin and Greek.

Uhlenbeck:

More science.

Kuhn:

More modern languages.

Debye:

More science, yes. Yes. Now I must say I did not learn very much in physics in the course in Aachen, and I did not learn very much in chemistry in the course in chemistry in Aachen. In order to learn something I had to take special courses.

Kuhn:

And most of the basic physics that you. were expected to know, you were expected to bring to Aachen with you?

Debye:

Yes, yes. Although there of course the physics course was on another level, you see. It was a level where you had to repeat a lot of these things.

Uhlenbeck:

Ja, that was also true in Leiden. The first years were so to say a repetition of what you had learned in high school.

Kuhn:

When you talk about lack of concern with basic problems in American universities now, in spite of the amount of research, is Europe still quite different In this respect?

Debye:

Well, I would not be the right man to ask that, because I don’t know too well how it is in Europe now. You see I have been away 20 or 22 years. The only thing I can say is this: There are good people around here in the United States.