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In footnotes or endnotes please cite AIP interviews like this:
Interview of Harold Urey by John L. Heilbron on 1964 March 24,
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 ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Bichowsky, Raymond Thayer Birge, Walker Bleakney, Niels Henrik David Bohr, Ferdinand Graft Brickwedde, Compton, Albert Einstein, Samuel Abraham Goudsmit, Werner Heisenberg, Richard Jesse, Edwin Crawford Kemble, Hendrik Anthony Kramers, Ralph de Laer Kronig, Gilbert Newton Lewis, Hendrik Antoon Lorentz, Robert Sanderson Mulliken, J. Robert Oppenheimer, Linus Pauling, Arthur Edward Ruark, John Clarke Slater, Arnold Sommerfeld, George Eugene Uhlenbeck, Robert Williams Wood; Columbia University, Johns Hopkins University, Kobenhavns Universitet, University of California at Berkeley, and University of Montana.
Now, let’s see, where had we arrived?
Here’s something about the Heitler-London theory of binding [p. 5, outline]. Well now, you know what the Heitler-London theory of binding has been. It was picked up really by Pauling, and Pauling has made the whole story really something worthwhile, but I would just warn you that Robert Mulliken of Chicago has the idea of the orbital method of approach for the structure of molecules, and I believe the orbital approach is proving to be far more useful among chemists than is Pauling’s. But you probably will want to talk to Pauling, and you’ll probably want to talk to Robert Mulliken.
We’ve talked with Mulliken. I haven’t seen the transcripts yet, but presumably the matter came up.
Yes, but I haven’t tried to follow either one, really, in recent years. You say, “What sort of explanation did you then have of the apparent agreement of the doublet separation with the relativistic formula?” I don’t know any more. You see, I don’t remember.
In connection with the spinning electron, I was struck by how much your presentation emphasized the nearly overwhelming difficulty of the idea as well as the difficulties with relativistic problems and various other questions.
Oh, I don’t know; I suppose it’s a matter of temperament. Well, that is perhaps just a matter of personal habit; for example, I do the same thing today in discussing the problems of the origin of the moon and. the solar system and so on. Other people talk with enormous confidence about these things, end I try to present what the difficulties are and point out that you can hardly expect anyone’s idea as to be 100 percent correct, including my own; and people just don’t understand this, I guess. I don’t see that it does anything for science to overstate your conclusions. Well now you ask me here about Sugiura [p. 6, outline]. Well, I’ve told you about that. Sugiura and I never met. I guess we’re about up to Columbia. You ask about the effect this new physics had on chemistry. Well during the years since then, this baa had an enormous effect on chemistry. We go to a chemistry seminar today; and if you are discussing anything about structure of molecules the quantum mechanical picture is immediately introduced mostly in the form of Mulliken’s ideas, of orbitals. We’re always talking about the “a” and the “p” orbitals and things of that sort. Well, Pauling doesn’t neglect this subject. In fact most of his calculations were made on the Heitler-London idea. I really think that Mulliken’s ideas have had a bigger effect upon the thinking of chemists than have Pauling’s ideas. This is after some 30 years, but I think that this is true.
What about at the time?
At the time I think Pauling’s ideas were much — that is, the Heitler-London idea was — more favorably thought of. The reason for it was this, that the Heitler-London theory attempted to do a great deal more for the ideas of valence than did the orbital theory. The orbital theory only tried to tell us what the state, say of the hydrogen molecule or some simple molecule was, do you see? Whereas, the hydrogen molecule is rather a minor part of chemistry; and such a thing as the diatomic molecule OH, trying to explain what its electrons are doing, isn’t much concern to chemists. And just because it started with fragments of chemical molecules, I think it less popular at the time. This is my point of view about it. But you see at this time — now I was worrying about this in ‘29, when as you will remember, we discovered heavy hydrogen in the fall of ‘3l. Now this changed everything I did, because from that time on I began to work on deuterium, its chemical properties and how it could be separated and concentrated and things like this. During the thirties it my entire concern, do you see? And then after the war I’ve gone into other things, but this terminated a period with the discovery of heavy hydrogen. Also at that time I was working on isotope effects in spectra. We published a paper in which we determined the relative proportions of the nitrogen 14 and nitrogen 15 from spectra, things like that.
When did you get into nuclear physics? At about the time you vent to Columbia?
Yes, very close to that time.
Was there any connection, particularly?
No, the only connection was that we discovered heavy hydrogen!
Did your interest in nuclear physics bring about your change to Columbia.
No, no, I just felt that it was a better job. I felt things were developing at Hopkins, from a purely organizational point of view of the department, that were going to make it unfavorable for me to stay there. And at that time, just in the few years there, I left, Herzfeld left, James Franck left, Joe Mayer and his wife [Maria G. Mayer] left, a man vent to General Electric –- I’ve forgotten his name now. The place broke up, and I think it was bad political planning on the part of the authorities at Hopkins that did this. I think all of us left because we felt we were not going to be able to advance at Hopkins in the way we would like to, strictly professional advancement at the university. That’s what I think happened.
What happened to the chemistry department, then?
Well, they’ve drifted along all these years, and it’s been a rather second-rate department most of the time. You hardly ever hear of anything from Johns Hopkins University.
And with the physics department —.
Well, you see the physics department — the university got this man as president, a geographer — what is his name now? This man destroyed the university. They lost [F. D.] Murnaghan, the mathematician. They just had no use for people that most of us regard as outstanding.
What was the trouble, do you know?
Well, just bad ideas about what a university is and what people are to be advanced. If Hopkins could have kept the outstanding men that it had there, it would have been an enormously better institution in the last thirty years than it has been. It’s very sad.
What about Columbia? What was its —
Well, Columbia, Columbia. Don’t tell people I say this, but Columbia has been a kind of a dead dull place, in chemistry particularly; and it has been that way ever since the turn of the century. Somehow a university gets a certain tradition, and you just can’t change that. So you do consulting work for industry, and uninspired — a great deal of personal jealousy between people in the department instead of friendly boosting of each other. At Columbia I had very few friends. They weren’t friendly to each other. It wasn’t just a matter of the outsider coming in from Hopkins, being the ugly duckling that everybody picked on. It wasn’t that. They weren’t friendly to each other, those that were there. Or the people who have been there since.
Was the physics department better?
Physics was much better — some of that in physics, but not nearly so much.
You were there for a long time, then, considering the circumstances.
About 15 years, yes. A lot of it during the war. The war was a very hectic time. I was most unhappy during the war. I had bosses in Washington who didn’t like me, and I had people working for me who didn’t like me. Imagine a more miserable situation — where you can’t resign, but nobody wants you around! About the worst situation you can get in. When the war was over I got out. I was very close to a nervous breakdown during the war. Old General Groves would send his physician around to look me over — Warren. What’s his first name? He’s up here at Los Angeles, a medical man. After the war he saw how perked up I was and so forth, he wondered what had happened to me. “Well,” I said, “I have good bosses, that’s all.” [Laughter] Now let’s see, what else do you say here? The Dirac equation [p. 6, outline]. The Dirac equation –- “How great a flaw were the negative energy states at first thought to be?” What do you think of the hole theory? Do you think the hole theory is a decent description?
Of the present status, or of the status at the time?
Yea; I never liked it myself. I thought it was pure mathematical fiction.
The hole theory, yes. Before the positron I think that most people did consider it a considerable blemish.
Not only that — you’ve got the positron, you could just turn Nature upside-down and now electrons are holes in the continuum. The positron is the real particle, and the electron becomes the hole. It’s completely symmetrical. So far as the behavior of these objects is concerned it’s symmetrical. We’re in a part of the world where matter exists. If it were anti-matter, you wouldn’t be able to distinguish it at all as far as I can see. I wonder if anti-matter doesn’t exist, somehow.
Well, I think that most physicists in their off moments would agree that it does, don’t you?
Well, at least, they certainly wouldn’t question as to whether it isn’t true. Dirac’s identification of the holes as protons: I don’t think anyone took it very seriously. Then the positron came along, and there wasn’t much room for argument about the holes being protons. Positrons and neutrons — of course the discovery of the neutron goes back to ‘30, ‘31; doesn’t it?
Yes, I think it was about ‘32.
Well, let me point out what happened at New Orleans — or was it Baton Rouge. There was a meeting of the American Association for the Advancement of Science, I think at Christmas vacation, 1931, at which we presented our paper on the discovery of the deuteron; and I talked to Robert Oppenheimer at this time. Robert and I were talking about the structure of this, and we mentioned that it probably consisted of a proton and a neutron. Almost immediately thereafter Chadwick discovered the neutron. You see, the idea that neutrons were present and they were building up nuclei was a little bit in the air. You’ll find that our paper on the discovery of deuterium, however, plotted protons against electrons; do you see? It was just the turn-over point! At the time we drew this, we were questioning as to whether neutrons didn’t exist.
What did you take a neutron to be?
A neutron was a neutral particle of about the mass of the proton; that’s all we had. Or it was a close combination in some way of the electron and proton.
Yes, so there was the view, though, that the neutron existed by itself.
Yes, or at least, I don’t know whether Robert would remember it or not; but Robert and I just tossed it off — whether it might not be a neutron and proton, forming the nucleus of the deuteron. It was that clone, anyway.
Was there any trouble, do you remember, about the acceptance of these discoveries? For instance, apparently Bohr was not convinced at all by, anyway, Blackett’s work.
I didn’t know that.
Perhaps it was [C. D.] Anderson. I don’t know which announcement Bohr saw first. I think probably Anderson’s, but anyway he was antagonistic.
Oh, I didn’t know that. When Anderson announced the positron, I think everyone said ‘that’s the hole that Dirac’s been talking about.’
Really? Everyone was familiar with Dirac’s theory then?
At least I thought so. It was in my neighborhood; among my friends we were talking about it.
Anderson apparently didn’t know about the theory; nor did Blackett.
That’s odd. “Statistical interpretation” — yes, Bohr and Einstein [p. 6, outline]. This argument went on for years, and I never did understand what it was about. Einstein didn’t like the statistical interpretation. He wanted cause and effect to bold throughout; and yet he introduced —
Yes, you’re referring to the probability coefficient for transitions.
Yes, the probability coefficient.
It evidently amazed Bohr.
Yea, I’ve seen Bohr after a conference with Einstein, and he’d just hold up his hands and say, “He just doesn’t understand what the argument is about.”
Well, now that’s interesting. Do you have any idea what he meant by that?
No. Well, maybe I’m misquoting him. You see, I’m only stating what the impression was that I gained — that Bohr was very puzzled about why there should be any argument between him and Einstein on this. But you see as I’ve always told you — I’ve told you this before — that Bohr was always a person very hard for me to understand exactly what he meant. So I’m just giving you what was impression of him, not necessarily what he said.
Tell me, do you think this was a real effect [p. 6, outline], that about 1930 or so there was a marked drift into nuclear physics?
Yea, there was. Up until the late twenties, the attitude of physicists was very much that only ‘queer ducks’, in a way, bothered about radioactivity. The really interesting thing was in the quantum mechanics of electrons and atoms. And if you look at the Physical Review, you will find that nearly all the publications were in that field; or look at physics journals all over the world and you will find that that was very much the case. Only the Rutherford school and a few ‘odd ducks’ in the United States worked on this subject. Then of course, with the electron problem solved in principle — because it was solved completely in principle by the quantum mechanics — there was nothing more to do. And then they immediately began talking about — well, at the time that Ruark and I wrote our book, which reflects pretty much what was being talked about at that time —. Let’s see if I can’t find a quote on that. [Pause] No, I don’t think so; I don’t think what I’m thinking of is here. But it was about that time that we began to see that energy levels exist in nuclei, and that you can make an energy-level diagram of nuclei. It was about that time that this became clear. Somewhere in the late 20’s as I remember it. And so of course as soon as you see this, you begin to say, well, quantum mechanics applies to the nucleus also.
What of the celebrated difficulty of keeping the electron in the nucleus? Was that discussed?
That came around at about that time, too. Just about that time.
Was it always considered bothersome?
That I don’t know; it was first brought to my attention somewhere about that time. But I can’t remember anything more definite about it. Now you say here, “The discovery of heavy hydrogen” [p. 6, outline]. We were very fortunate about heavy hydrogen. We got this evidence, we published it. I remember I gave a talk on it to Johns Hopkins and R. W. Wood said, “I rather think you’re right about it; I think it’s real.” But Walker Bleakney at Princeton had a mass spectrometer; and he took our samples and demonstrated its existence by mass spectrometry so that there was a confirmation of our work almost immediately. And that probably saved us a lot of argument in regard to it. Otherwise, someone would have doubted our spectra, or said we had an impurity, or something, something, something, do you see? A ghost of the grating or something.
Why should it have been such a difficult isotope to accept?
I don’t think it has difficult to accept. It was just a matter of people being critical of the experimental demonstration. Lots of people wish to be terribly, terribly critical of anything that comes out; they pride themselves on being very, very, critical about things.
If it were a question of another isotope of some heavier element, would it have received as much attention?
No, they wouldn’t pay any attention to that, but hydrogen — they hardly expected an isotope from hydrogen.
You argue, as I remember, in your paper that from your analysis of the symmetry properties and distribution of the nuclei you were led to infer that it exists.
That’s right; we thought there ought to be one there. But I don’t believe that any of the arguments of that kind bad any real validity. It was just sort of an empirical hint to us that there ought to be one there.
You quote a remark by someone — I think Birge — who had suggested that some difficulties could be explained by a certain —.
It comes this way — that Aston had measured the atomic weight of hydrogen relative to oxygen by mass spectrograph. Of course he was comparing O16 to H1. Then, Giauque of California discovered that oxygen had isotopes. And that means of course that the chemical standard is not the same as the physical standard; yet the chemical atomic weight agreed with Aston’s measurement. Now, this meant immediately according to Birge and Menzel that there should be heavy hydrogen present. That was their prediction. But it turned out that Aston had made a mistake, and their prediction therefore was not valid because of this mistake. Aston made very few mistakes, but he had made a mistake of some kind. Then nothing agreed, do you see? And what is true is that there was also a mistake in the chemical atomic weight. One of the reasons for the mistake in the chemical atomic weight is that the isotopes of hydrogen can be fractionated in en electrolytic fractionation process. The chemists, to get pure hydrogen, had electrolyzed water and combined it with oxygen. They’d been fractionating the isotopes both of oxygen and hydrogen without knowing it all the time they were doing this, so the chemical atomic weight was wrong and Aston’s atomic weight was wrong. So everything was wrong. I want to quote to you what I said in my Nobel dissertation. [Urey shows Heilbron a passage from his Nobel dissertation]
That’s really a wonderful story. So in fact the whole business was based on an accident.
Based, on an accident. You know, in case anybody gets conceited, I always tell them this story. Just suppose the heavy hydrogen hadn’t been there. You can do all the work just as intelligently, you will work just as hard and everything. It’s not there. Nature didn’t put it there. It would have made a lot of difference in life if that had been the case, you see. It had nothing to do with me whatever. [Laughter]
What is the relative concentration of H2? As I remember on the basis of their arguments, Birge estimated 1 in 5000, and the actual number was 1 in 6500.
I think that’s the present figure.
So, in other words they come exactly.
Right square. But there is another interesting story about it. You see, I didn’t think we could detect this faint isotope so we worked out that there ought to be a difference in vapor pressures in liquid hydrogen. So I got in touch with Brickwedde and asked him if he wouldn’t distill some hydrogen and give us the residue. So he had his apparatus down cleaning his electrolytic cells with which he prepared his hydrogen. Finally when he got them all cleaned and everything he filled them up with new electrolytes and started up again; but of course the first hydrogen that came off was depleted by a factor 5 or 6 in heavy hydrogen. So we took very much depleted liquid hydrogen and distilled it, and concentrated the deuterium by about a factor of 30, and then found that we had an increase of about 5 in the abundance, which enabled us to detect it. But if he hadn’t cleaned his cells and used the old liquid —. The deuterium builds up until the composition of hydrogen coming off is equal to the composition of hydrogen going in — which is the normal abundance. And then if we had increased it by 30, we would have detected the line without any difficulty at all. So here was another case in which we worked against ourselves, you see.
That’s an extraordinary story.
Well I hope I’ve been able to be useful to you.
You certainly have, and I thank you.