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Interview of Norman Ramsey by Ursula Pavlish on 2006 September 20,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Today is September 20, 2006. I am here to interview Professor Norman Ramsey in Lyman Hall, in his temporary office due to construction. My name is Ursula Pavlish and I am a History of Science graduate student at Harvard University. My primary questions today, Professor Ramsey, have to do with your work in magnetic resonance. You have a very distinguished career in other aspects of physics as well, but my primary interest is in your work in magnetic resonance.
In one write-up, you mention that theoretical ideas about the magnetic resonance method trace back to early quantum theory speculations. In another place, you mention all of the illustrious physicists who you came across when you did your studies at Cambridge University in England. I was wondering, how did the scientists you knew at Cambridge — J.J. Thompson, Rutherford, Chadwick, Eddington, Born, Dirac, and others—set the stage for the developments to follow in magnetic resonance?
They established quantum mechanics and the fundamental theory of the atom. That was key to the subsequent work. There was an initial attempt by the Dutch physicist C. J. Gorter to observe a magnetic resonance by the heating of the sample, but this method not only did not work but it could not work, as we now know, due to the incompatibility of the various thermal relaxation times. Gorter published his results as a failure and his paper was mostly ignored. Independently of Gorter’s work, I. I. Rabi had been successfully measuring nuclear magnetic moments by deflecting atoms moving in inhomogeneous magnetic fields, but it was difficult to obtain high accuracy because of the difficulty of measuring the inhomogeneity of the field. During this time I was a student at Cambridge University; a second time as an undergraduate, taking courses with the people that you mentioned. My tutor there at one period was Maurice Goldhaber, who’s still living, and was director of Brookhaven National Laboratory many years later. He asked me to write a tutorial paper on the subject of the measurement of nuclear magnetic moments, which I did [see Informal Discussion with Professor Ramsey for more on the relationship between himself and Maurice Goldhaber]. I returned to Columbia, to work with Rabi, who let me start immediately doing research.
In Rabi’s lab?
I chose to work in Rabi’s lab; I was going to work on measuring magnetic moments in inhomogeneous magnetic fields. Then I had the great good fortune, after that about two months, Rabi invented the first successful magnetic resonance experiment. This is one in which we used an oscillatory magnetic field, which we swept through the frequency at which the nucleus would be processing in the external field. The technique there was, instead of trying to measure the heating, Rabi proposed looking for a change in the orientation state of the nucleus, which he could detect by the techniques of deflecting the nucleus in an inhomogeneous magnetic field. Immediately, two groups working with him on other problems, including the one in which I was working with Jerry Kellogg, I.I. Rabi, and Jerrold Zacharias, changed research projects. We were working with the hydrogen molecule, H2, so we started working right away to use the new method on this molecule.
Do you remember the atmosphere in the lab at the time? What was it like?
Yes, it was, well, oh it was very exciting. Actually, Gorter was visiting. He came for a one-day visit. He described his method, which didn’t work and he saw that Rabi was doing deflection experiments inhomogeneous fields. I think that both of them may have then independently thought of doing the molecular beam magnetic resonance method. Immediately our group and the group of Rabi, Millman, Kusch and Zacharias started converting our apparatus for the proposed new molecular beam magnetic resonance method. Our group worked with molecular hydrogen (H2) which was fundamentally more interesting but more difficult to detect than LiCl used by the other group. The resonance pattern for LiCl was similar to that expected theoretically, but ours with H2 was initially very disappointing since it was very different than expected. Instead of a clear sharp resonance we found a broad ill defined pattern extended over a wide frequency range. Our primary objective was to measure the nuclear magnetic moments of H and D (deuterium) and we soon discovered we could obtain sharp resonances for these with the molecule HD, because that molecule with two different nuclei could be in the zero rotational state.
This was after you’d got the messier resonance for H2?
Yes, we were getting the beginnings of a messy resonance, which we thought was just junk. What we were interested in doing, was measuring the proton magnetic moment very accurately. Incidentally, Rabi had written a very interesting paper, which gave the fundamental theory of magnetic resonance. But he was really trying to account for the experiments that had been done with static inhomogeneous magnetic fields, which varied from one location to another. He decided that the easiest way to calculate this was to pretend it was an oscillatory magnetic field, so he calculated the fundamental theory of the oscillatory field. But, he then averaged it over the velocities of the molecules, which made the sharp resonance disappear. Rabi published this paper in early 1937, just before I came to the lab. The title was a good one, something like “Magnetic Transitions in a Gyrating Field.” Although he calculated the transitions it for an oscillatory field, his effective frequency depended on the molecular velocity, which was different for different molecules so he didn’t get a sharp resonance. So he didn’t quite propose doing a resonance experiment. Then, after I was working with him [Rabi] for about two months, the Dutch physicist Gorter [pronounced Horter although it is spelled ‘Gorter’], who had done this unsuccessful experiment, visited our lab.
I was familiar with it, but I’m glad to hear you pronouncing it correctly because I had just said Gorter [pronouncing the hard g], but I guess that’s incorrect.
I don’t do it right. A real Dutchman says it differently. In any case, he stopped to visit our lab and was describing his experiment and Rabi described what we had done. My impression is that both people probably independently thought of the idea. In any case, Rabi in his first paper on magnetic resonance gives credit to the visit of Gorter as one of the things that stimulated him. By the time he had written that paper, he actually had done the first magnetic resonance experiment.
Rabi had the theory first. He published that paper.
He had that theory, but he didn’t quite recognize the potential of it. He didn’t recognize that it could give sharp resonances if he actually used an oscillator. We were busy. The laboratory was doing excellent work with the deflection method. You know, that’s one of the problems in a research lab. If you’re busy with a good program, it’s sometimes hard to shift over quickly to a new one. In any case, right after or Gorter’s visit, two groups of us started working, one with lithium, one with the more interesting but more difficult hydrogen, the more difficult hydrogen.
So you built two apparatus, yes?
Well, we had two apparatuses going, measuring things with the deflection technique. So we simply adapted those methods because that was going to be the way that we detected whether there was a change in state. We adapted those apparatuses to be able to put in oscillating fields.
Those were the first working…
Those were the first successful magnetic resonance experiments. The first one was the lithium one, which was actually a team of Rabi, Millman, Zacharias, and Kusch. The first one published was the one with lithium. Ours was later, with Rabi, Kellog, Zacharias, and me. We were initially terribly disappointed with our experimental results with H2 which did not give the expected single resonance. However, when we did the experiment with HD, we found a good resonance for hydrogen and another for deuterium. So we could measure their magnetic moments. We them had to decide what to do next. At that time Columbia had a rule that any PhD thesis could have only one author and he had to submit something like 2,000 copies of his thesis. Since there were then no high quality duplicators, the published article had to have a single author. So there was a problem with supervising a Ph.D. student using an apparatus built by more than one person. What can you give for the PhD student, which on the one hand is sufficiently interesting to be worthy of a PhD degree, but not so interesting that it’s unfair to the other people on the team. It was decided that I was to study this stuff that we thought was noise. We didn’t know from where! And that what I should do for my thesis would be to investigate this noise and then with the others, would publish the magnetic moments of the proton and of the deuteron in joint papers. Well, I started this work over the summer and I soon found that — we had been way overpowering, using much, much too much radio-frequency power. I reduced the power level of the oscillatory field (unfortunately, successively by a factor of only two each time, which meant I had to do a number of these very slow experiments). It took a day and a half to run one experiment and I found that each time I reduced the power the results got more interesting. Finally it became clear that what we actually had was six sharp resonances.
This was with which? Were you working with HD or H2?
What I worked with initially was H2 and found the six sharp resonances. Then, with discussion amongst all ourselves, we decided our observations corresponded to a real phenomenon. It wasn’t noise, and it meant that we were seeing the effects of the molecule, as well as of the external magnetic field. In the first experiment by the other group with lithium chloride the molecule was just thought of as a container of the lithium nucleus. They observed the lithium nucleus and that was all. But then, in the case of H2, we found this more complicated result. And then we realized there were intermolecular problems. There was the magnetic field at the proton due to the external magnetic field, which corresponded to the fundamental resonance we were looking for, but then, there was also the magnetic field due to the other proton, of the two protons in the molecule. And depending on what its orientation was, we would get a different magnetic field. So the resonance would be, it turns out, at six different frequencies.
Was this the first time that physicists discovered that?
Yes, that’s right. This was the first time.
This was discovered as part of my PhD…what was to be my PhD thesis.
Are these the hyperfine splitting, things like that?
No, but these splittings are analogous to hyperfine splittings. The hyperfine is the exact same thing when it’s the electron interacting. We worked on that later. In any case, this was the nuclear, the first time this had been seen in magnetic resonance. And in the case of H2, and lithium chloride also, these were so-called ‘singlet sigma’ molecules, which had no net electron moment. So you didn’t see the electron effect.
Had this been theorized before?
No. Well, the theory that there could be, that there was a magnetic moment? It had never been theorized that there could be an interaction with the molecule that would be big enough to be seen. It had not calculated that we were the first to find it experimentally. What was starting to be my PhD thesis, which was to find the noise, was a very interesting physics. By example, we could measure properties of the molecule: the average value of the distance between the two protons, its [1/R3], because if you know the magnetic moment of the proton, then you know the magnetic field at that location. So, there was good physics to be obtained from it. Well, then I went on. I was to do this whole thing not only with H2 but also with D2 where we had gotten similar confusion. But then I hit the second big surprise, which was even more spectacular. We had thought these were all magnetic effects that we were looking at, and therefore we expected the deuterium resonances would be much closer together, because the deuterium magnetic moment is much smaller than the proton magnetic moment. Therefore, the magnetic field from the other nucleus should be much smaller. In addition, the molecule was heavier and there was another effect; a contribution from the rotation of the molecule. The molecule can rotate, and there’s a magnetic field from the charge rotating. But in the lowest rotational state the deuterium molecule rotates more slowly than the hydrogen.
You saw that in the peaks?
Well what I saw was that the peaks were further apart in deuterium than they were in hydrogen. But they should have been closer together because the magnetic moment of the proton is five times bigger, something like that, and the molecule is heavier. So when it rotates, the effect of the rotating charge, it rotates more slowly, because of its greater mass in the same quantum state. And therefore the magnetic field from rotation also should be less. This was a really big surprise. Rabi at that time was a visiting professor at Stanford. I was doing this work while other people in our group were setting up an apparatus to do a more accurate measurement of the things we had done on the magnetic moment of the proton. With correspondence back and forth and discussion amongst ourselves, we realized there had to be another interaction, which turned out to be a thing which had been speculated about for other nuclei but not for this one; namely, a quadrupole moment of the nucleus. Everyone assumed, up to that time, that the D nucleus was spherical in shape, and therefore it could not have a quadrupole moment, which corresponds to it being cigar shaped rather than spherical. And our result corresponded with there being a quadrupole moment of the deuteron, which had not been expected.
The ground state of the nucleus had to be different from what was thought before. This led to J. Schwinger proposing that it corresponded to a tensor force, a new fundamental nuclear force. That indeed was the case. This was the first experiment observing this quadrupole moment of D and observing a new nuclear force. Well, it became apparent that this was not the uninteresting experiment for a graduate student. We agreed to write joint papers on the deuteron, on the structure of H2 and on the deuteron quadrupole moment. We published this jointly, although the others did include my original experimental curves. The others then in the meantime, made the apparatus longer and better and so Kellogg, Rabi and Zacharias published the H2 and D2 papers jointly. As a result, there was a delay for my Ph.D. Incidentally, it shows how things are done when they’re done right. Columbia also had the rule that after you finished your research, you had to take one more major examination, which was on all the advanced topics in physics — quantum mechanics, etc. As a result of the credit I had achieved by giving up my single discoveries, to make a joint publication, my co-authors agreed to help me some in the process. So, while I was actually taking my exam on Quantum Mechanics, Zacharias was taking some of the data for my thesis. What we chose for my thesis was something that I had also discovered in the process in doing these experiments. It was known that the rotating molecule should also have a magnetic moment and I found we could get a resonance from that. And in fact, it correlated very well because I could also see multiple resonances in the rotational spectrum, so the study of this became my thesis.
Also, we realized we could look for atomic hyperfine structure in an atom if it had an electron magnetic moment, as was done later on others in the lab. I was very fortunate that it turns out that it’s much easier to do the first experiment with a new method than the last experiment with an old method. Lucky or not; It meant that although I had worried, my chosen path (of taking two undergraduate degrees before pursuing research at Columbia, instead of doing a doctorate in England) was beneficial. When I went up to Cambridge I had the choice of being either a graduate student, in which case, due to the different rules they had there, I would have immediately have started doing research there and never have taken the advanced courses in quantum mechanics. Or, I could be an undergraduate and take the advanced courses. For example, Dirac’s Quantum Mechanics was an undergraduate course by their definition. If you were an undergraduate physics major, you only took physics courses. Distribution was supposed to have been done earlier, in preparatory school. So I elected repeating my bachelor’s degree. I’m one of the very few people who have two bachelor’s degrees. One of my best students, Dan Kleppner now a professor at MIT, also did the same thing at Cambridge, and came back and worked with me. In any case, my thesis became the resonance structure of the rotational magnetic moments of H2, D2, and HD. The research was completed rapidly because it is usually much easier to do a final experiment with a new method than the last experiment with an old method.
I’m a little ignorant about the history of the fine structure and the hyperfine structure. Had there been people working on these phenomena?
The fine structure had been observed quite a long time before, between the electrons, fairly early, before quantum mechanics was in anything like its present form. There was the vector model diagram and in the optical spectra, of some of the heavier atoms one could see additional structure which was called the fine structure. Well, the electron interaction was really the fine structure and then there was the hyperfine structure, which was when you could see an interaction with nuclei.
Between electrons and nuclei.
And then the work that you did, that would have been your thesis but became the group paper was the interaction between protons and nuclei.
The nuclei; one proton and the other. And the magnetic interaction with the other proton, and the deuteron quadrupole moment, which had never been seen and never expected. Rabi’s method was invented just to measure the magnetic moment. In fact, this paper I just gave you says something about that. You don’t have to look at it now, but it mentions the fact that the magnetic resonance method was started in the very limited field of just measuring nuclei magnetic moments. Through work which we and others did, it subsequently expanded to many different sciences, including molecular structure, chemistry, biology, MRI, medicine, time keeping, astronomy, and engineering.
I have many more questions, but I feel that that’s a very eloquent way to end if you do have a meeting at 11:30 am.
Ok. Yes, I do.