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Oral History Transcript — Dr. Norman Ramsey

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Interview with Dr. Norman Ramsey
By Ursula Pavlish
At his office in Lyman Hall, Harvard University
November 27, 2006

Transcript

Session I | Session II | Session III | Session IV | Session V

Ramsey:

Two suggestions for next time. That is, when you type up our interview, if you would make double or even triple spaces. Itíll make it easier for you and for me. Itís very hard to write in suggested changes in the space available. In two places there were sufficiently large changes that I actually typed out a replacement, one of which is called Insert A, the other is Insert B.

Pavlish:

Iíll make the changes and Iíll send it back to you with triple spaces.

Ramsey:

Iíve made a fair number of changes. So Iíll give you this back. One general comment: Different people have different views on interviews. Iíve noticed that all along, Iíve had some very successful interviews and some very unsuccessful ones. Basically the ones that have been the most successful have been when the person whoís been transcribing it feels free to make grammatical and other corrections. That doesnít bother me in the least. Iíll go over it and make sure itís still mine. Thatíll help me; I wonít have to make as many such corrections. So feel free to do that, and particularly do the double-spacing, and then Iíll go over it carefully.

Pavlish:

Yes. Iím sorry I should have done the double spaces.

Ramsey:

Thatís all right, thatís one of the things we learn.

Pavlish:

While weíre on administrative things, I went to the Harvard Archives at the library. They have a collection of your papers. Itís not for public view, even for University students. So they asked me to get your signature for your permission and they said, even if I get your permission, I might not be able to see them.

Ramsey:

Iíd be happy to have you see all of my papers. I donít worry about that.

Pavlish:

I donít know exactly what they have on file. Do you recall?

Ramsey:

I donít either. I have the papers that I wrote up to 1980, roughly, I have bound together in five volumes. Iíd be happy to have you look at that. One or two of them may be around here.

Pavlish:

Thank you. Do you have old lab notebooks still?

Ramsey:

Yes, I have old lab notebooks. Some of them have gone to the Harvard Archives. Since much of my research has been done jointly with my í84 Ph.D. students and their Ph.D. thesis are locked in the Physics Library, these thesis provide an excellent review of much of our research. Why donít we also do the following, you may borrow these two first volumes of my collected papers until next time. Iíll see what I can do about the other volumes.

Pavlish:

Thank you. Now for the formal interview, I prepared a summary of what we talked about last time and what I would like to talk about today. Today is November 27, 2006. I am here to interview Nobel Laureate in Physics Professor Norman F. Ramsey. We sit in his office in Lyman Hall, Harvard University. My name is Ursula Pavlish. Professor Ramsey, during our last interview we talked about your time in I. I. Rabiís lab as a graduate student at Columbia University. If youíll bear with me, Iíd like to recap the results from that interview, to be sure that Iíve got the facts right and also to allow you to expand on what you described if you wish. You described Gorterís visit to the lab, and your collaboration with Kellogg, Zacharias, and Rabi himself immediately after Rabi had invented the first successful magnetic resonance experiment with molecular beams. As I understand, you started with the H2 molecule, then moved onto HD for which you found an impressive resonance signal.

Ramsey:

For H2 we found an initially puzzling resonance signal, and thatís usually interesting.

Pavlish:

Your personal project after this was to investigate what looked like noise in the H2 signal for your PhD.

Ramsey:

Correct.

Pavlish:

In the process of this experimenting, you found six sharp resonancesónot noise at all, but the effect of the magnetic field of the proton.

Ramsey:

Yes, of the other proton in the molecule H2. You look at the resonance of one of the protons but there are two protons there, and thereís a magnetic field from the other proton in addition to the external magnetic field. This is the first observation in magnetic resonance of an internal molecular interaction giving a shift, which later became very important.

Pavlish:

This, you mentioned when pressed, had not been theorized before.

Ramsey:

Thatís right.

Pavlish:

You did the first experiment observing the quadrupole moment of the deuteron and a new nuclear force (which Schwinger later characterized).

Ramsey:

Yes, which implied a new nuclear force.

Pavlish:

Your thesis became something different from what youíd anticipated. You wrote on the structure of the rotational magnetic moment of H2, D2, and HD.

Ramsey:

Yes. That I should explain a little more. The reason for this change of thesis, is not that the first wasnít a good enough project, it was too good. Three of us had participated in building the apparatus. Columbia had a requirement of only one name on the thesis. What we had thought was the unimportant discovery was going to be my thesis, but it became the most important part of the discovery. So we shared it together. I did a new topic on another phenomenon I had discovered, on the rotational magnetic moment.

Pavlish:

If you were to characterize this topic that eventually became your thesis, was this almost as significant as the paper that you published jointly?

Ramsey:

No, the paper we published jointly was an extremely important paper, it found entirely new forces between elementary particles. The other one was a good paper; it complemented the other work we had done on nuclear resonances and was the final magnetic resonance study of rotational magnetic moments, the fact that the molecule is a charged system; when it rotated, it created a magnetic moment. It was important by far, less important than the experiment, which started out to be my thesis, but became our joint papers on molecular structure and the deuteron quadrupole moment.

Pavlish:

And you recognized that while you were working? What was the trajectory of your emotions? When you were looking for noise were you more likely to come in late to the lab?

Ramsey:

I was coming in late a lot at that time because our method of making the measurements was quite slow. That was the problem with molecular hydrogen. The only way I could detect the beam was by the pressure that built up when we had a Panini detector there and that was a very small pressure difference. I had to have a very good vacuum. I had to wait half a minute for every point I measured.

Pavlish:

You also mentioned that you changed the magnetic field by increments and that was what, a day and a half perÖ

Ramsey:

Per one full sweep. Yes, thatís right.

Pavlish:

Before we go on to what I would like to talk about today, just for my own clarification now, the six sharp resonances that you found were due to the effect of one nucleus on the other. The reason for six sharp resonances was due to the different rotational states?

Ramsey:

Yes and no, it was due to this (now this is a bit of quantum mechanics). Since the two protons were identical particles, as soon as we have identical particles, we have the so-called statistical requirement. Itís either a boson or a fermion. The proton is a fermion.

Pavlish:

Fermi statistics?

Ramsey:

Yes. We had molecular hydrogen, which even at very low temperatures would exist in the first rotational state. It would still be rotating. That was because of the requirement of Fermi statistics. They couldnít all be in the same state. We had to go to a higher rotational state to see them. On the other hand, when you did that with HD, the proton and the deuteron are different, that molecule could exist in the zeroth rotational state. Now in the zeroth rotational state we donít get nearly as elaborate structure. In the first place, if itís in the zeroth rotational state we can almost forget about the nuclear moments interacting. Theyíre in a state where theyíre anti-parallel, they cancel each other, which is what we have with so-called parahydrogen. The other is orthohydrogen, (when theyíre parallel like orthotics for shoes.) When the spins are parallel, itís orthohydrogen. Orthohydrogen can only exist in the first or the third rotational state, it canít exist in the zeroth or the second. Parahydrogen is the opposite. In this case, the molecules were rotating, and that gave the possibility of greater variety. You could observe the interaction between the two protons, and the interaction of the proton with the magnetic field. It was a much richer spectrum. As a result, it was a spectrum instead of having a single frequency at resonance, it had six frequencies which were at resonance. There would be one set of resonances at which the two were parallel, one at which they were anti-parallel, and then they were in different orientations.

Pavlish:

Thank you.

Ramsey:

Youíre welcome.

Pavlish:

Today, I would like to continue chronologically and also pursue another interest of mine so I would like the goal of our discussion to be an understanding of how your physics research before World War II was continuous or discontinuous with your wartime investigations, and how the results of the physics research in World War II contributed to your research and that of your colleagues after 1945.

Ramsey:

Good question.

Pavlish:

Iím interested in this question generally, but also specifically in the connections between war work and magnetic resonance.

Ramsey:

Yes, very good. I could start with a general comment I make which is a little different from what most people make about war work and what it contributes. It does contribute but it also delayed things a lot. In our case, we (and I) lost out a good deal because of the interference of war work. We moved to more specific applications. After the war used many of the techniques we developed during the war, in our research. On the other hand, it almost brought to an end the fundamental physics that we were doing in a brand new field. We in Rabiís lab had the new discoveries we had made beginning in 1937 and í38 and by 1940 our exploitation of them was stopped. We were a unique laboratory — Rabiís particularly — we had a lot of new discoveries to be made. It simply stopped. So that was a big loss.

Pavlish:

Did you feel that at the time?

Ramsey:

Yes, we felt that at the time, very much so. But also, in 1940, World War II looked very bad for the United States, and for the world in general, because the Nazis were winning all their battles and there seemed to be no stopping them. For example in the summer of 1940 they had captured France. In fact, at that time, my wife and I and Jerald Zacharias, were driving to a scientific meeting in Seattle (essentially, my wifeís and my honeymoon). The Nazi forces were going almost as far per day through France as we were going across the United States per day. It really looked very bad. We stopped our basic research and shifted to radar work. It gave me my first experience with microwaves and my first experience, really, with a lot of radiofrequencies which was beneficial later. Itís a mixed bag. Some people tend to think that the major advances that occurred after the war were solely due to the wartime research. Well, some were delayed because of the wartime research.

Pavlish:

In what you were just saying, I thought that during the war you did not only scientific workóboth at the Radiation Lab at MIT and at Los Alamos —you also held leadership positions as an administrator, as a scientific expert. So for you particularly, maybe even more than for other physicists it was an interruption of your looking at atoms and nuclei, in that you were running theÖ

Ramsey:

Yes, thatís right. I think that for or most of us it was a very major interruption because the objective was different. In the case of fundamental science, youíre really looking for new things. You often donít know quite what youíre looking for but youíre looking for them. In the case of the wartime research, it was different; we knew what we wanted to do. For example we wanted to work at higher frequencies than weíd worked before. It was more of a development than an invention of new things. The idea of radar was discovered before World War II but not fully exploited. It was much advanced during the War, particularly by the Radiation Lab and by the British.

Pavlish:

I was just reading a paper about climate change that says that all the technology exists to deal with climate change. Somebody made the analogy, saying that before all of the work at Los Alamos and at the MIT Radiation Lab, all of the technology existed to have all of those new applications, but the unprecedented investment of the scientific community was really needed to fully exploit that potential.

Ramsey:

Thatís right. It concentrated our attention on applications, which is a little less likely to make totally new discoveries but it does make very important applications. New developments in conjunction with the applications can later lead to valuable new discoveries. So itís mixed.

Pavlish:

Was any part of your molecular beam work at Columbia in Rabiís lab directly used at the Radiation Lab or at Los Alamos?

Ramsey:

Yes. In the case of the Radiation Lab, the knowledge that I had learned about radiofrequency oscillators and electronics helped. For my thesis I had to make an oscillator to do magnetic resonance, and all that did contribute.

Pavlish:

You made an oscillator in Rabiís lab for magnetic resonance and then you made an oscillator at the MIT Lab. Did any of the actual scientific apparatuses from Rabiís lab actually go to MIT?

Ramsey:

No, essentially none. That later occurred more in the opposite direction. At the end of the war, a number of the very sophisticated electronic things that we had developed, we were allowed to take back to our laboratories. The difference between fundamental research and applications. The difference is you know your objectives in applied research and you often do not in fundamental research. We knew what we wanted to do at the Radiation Lab. One of my key things was developing radar at a shorter wavelength.

Pavlish:

Radiation at three centimeters, right?

Ramsey:

Yes, at three centimeters.

Pavlish:

I wanted to ask you, was that because of your prewar skills that you were assigned to three centimeters? In a previous interview, you said that when you visited England in 1941, the British were behind the U.S. in three centimeter but ahead in ten centimeter. I wanted to ask you, why three centimeters?

Ramsey:

Why three centimeters is important. It is a shorter wavelength. The angular resolution you get is the ratio of the wavelength to the aperture of the antenna. That means if you have a shorter wavelength for a given antenna, you get better resolution. That was probably known even before I began working at Columbia. In fact it is a fundamental of wave properties.

Pavlish:

You were introduced to this fundamental property through quantum mechanics?

Ramsey:

Yes, through early quantum mechanics and through the theories of light and sound. Thatís why, with light which has a very short wavelength, you get a very detailed image of what you look at. On the other hand, one didnít know how to produce good coherent and intense radiation at light frequencies.

Pavlish:

Before the war, you wouldíve thought of this concept more theoretically, or you were thinking of it in terms of applications? Iím assuming that at the Radiation Lab it was a goal to get down to shorter and shorter wavelengths.

Ramsey:

Let me shift to the actual origin of the Radiation Lab. In the summer of 1940, things really fell apart in Europe. It was a total triumph for the Nazis. France fell, and Britain was expected to be invaded. About the history of radar, Iíll say the following (itís a little interruption but I need to say it). Radar was independently invented by about five or six different nations. I think the United States Navy and the United States Army, independently invented different versions — they didnít tell each other much about what theyíd done at that phase. In fact, before World War II, when France built their super passenger ship ĎThe Normandy,í they installed a radar iceberg detector.

Pavlish:

Not for military purposes?

Ramsey:

Not for military purposes, but proving that they knew the techniques. Likewise, the Germans invented it. But, the British had one huge advantage on developing it. They had a very real problem requiring radar. Even before World War II officially began, it was quite clear that they would be involved, and the Nazis had an extremely strong air force which the British did not, and they were very vulnerable. There were two hopes they had. First, they did develop very good fighter airplanes called ďSpitfiresĒ. Second, even more importantly, they recognized that almost their best hope was to develop good radar. At that time, the wavelength wasnít ten centimeters or three centimeters, it was about half a meter — fifty to one hundred centimeters.

Pavlish:

Iíve seen pictures of the huge towers used to emit and detectÖ

Ramsey:

They decided to go all out with these so-called Chain Stations.

Pavlish:

Chain Home?

Ramsey:

Thatís right. Along their Western shore. So that they could pick up incoming aircrafts. They concentrated hard on developing a good system rather than just detecting an airplane, as done by the other countries. The British developed a system for having these stations record the airplanes and the womenís air corps would plot the airplanes as they came in. The British would leave their fighter planes on the ground, and only send them into the air when they knew where the planes were going. Then they could send their planes into the air and into a favorable attacking position. This really won the Battle of Britain. The British were shooting down planes at such a furious rate that the enemy couldnít maintain their bombing. But that system didnít work when the Germans changed to night bombing, because basically what they did in day time was to put the airplanes in the air at the right position and then they saw where the enemy airplanes were in front of them and then they could shoot at them. This didnít work at night because the resolution was so poor. They could put them in a position such that in good seeing conditions they could find where the airplanes were but could not in darkness or in bad weather. What the British then needed was a way for the airplanes to see, so that when theyíre put roughly in the right position they can identify the enemy. They did a very good job of developing 10 cm radar.

Pavlish:

That would go on the airplane?

Ramsey:

That would go on the airplane, thatís correct. They could use that to identify where the airplanes were. This was working more or less but the resolution wasnít really good enough. Nevertheless, it was what the British had. In 1940, the British realized they needed help on the development and they sent a mission to the U.S.

Pavlish:

The Tizard mission?

Ramsey:

Yes, Air Chief Marshall Tizard and several physicists whoíd been involved in radar and other people.

Pavlish:

Were any of those Physicists ones you knew from your time at Cambridge?

Ramsey:

Some of them were, yes, very much so. The British sent them over. They had meetings in the United States with people from the government and civilians, essentially asking for help. I think the first meeting was with very senior physicists, people like Rabi, Ernest Lawrence, not so much new Ph.D.s like myself. They decided jointly with the United States government, that yes they should do something.

Pavlish:

You didnít hear about this from Rabi did you? It was top secret?

Ramsey:

I did shortly after hear from Rabi. The summer of 1940 was a very discouraging one for me. It looked as if we were losing the war to the Nazis. I think I first heard about radar in September of 1940, more or less simultaneously from three different people whoíd been involved. I donít know whom I heard from first. One was Rabi, one was Wheeler Loomis who was chairman of my physics department at Illinois, when I worked at Illinois. Both of them urged me to go there. They pointed out that Iíd probably be drafted anyway and I might as well do work that would be useful.

Pavlish:

You were one of the first dozen or so physicists?

Ramsey:

I was one of the first, as a result of that. I had just gone to Illinois. I originally had a research appointment at The Carnegie Institution of Washington. That appointment would have lasted for two years, but I felt that things looked so bad that Iíd better get a more permanent position in a university than this one which was for just two year. There were not very many Postdocs positions at that time and I had one of the very few.

Pavlish:

You held a Postdoc at Carnegie?

Ramsey:

Yes, at the Carnegie Institution of Washington. That worked out very well. I did fundamental research experiments there.

Pavlish:

Related to your work in Rabiís lab?

Ramsey:

Yes, somewhat, but some of it was in particle physics and nuclear physics. I did work with Jim van Allen, later known for the so-called van Allen belt in cosmic rays. There were two Postdoc positions. He had one, and I had the other.

Pavlish:

Were there specific elements of your molecular beam research that helped you in particle physics? Iím trying to understandóbecause after the war you also did both, you named Brookhaven! At Harvard you directed a molecular beam laboratory and at the same time you headed the Harvard cyclotron. I think of particle physics as a huge part of postwar, late 20th century physics, and then I think of nuclear, condensed matter, atomic physics.

Ramsey:

What later became particle physics, during the war was nuclear physics because those were the particles we had. There was a little beyond that, but basically the fields then were more nearly cosmic ray physics and nuclear physics because you couldnít artificially produce really high-energy particles.

Pavlish:

Would your molecular beam work have been counted as nuclear physics?

Ramsey:

Some of the results contributed importantly to nuclear and particle physics. Itís true Iíve been involved in two different fields for most of my life. Rabi had the belief that his best students ought to branch out and do something different after their Ph.D.s. Heís the one who persuaded me to take the position with Merle Tuve at the Carnegie Institution of Washington for what was primarily nuclear physics, which is what there was of particle physics at that time. I did experiments with Jim van Allen on neutron cross sections. With their accelerator we produced high-energy neutrons, which is standard nuclear physics. I confused matters by finally deciding Iíd work in both fields even though they were quite different. As I mentioned earlier, I only accepted the Carnegie fellowship for one year. I resigned after one year so that I could get something that looked like it might eventually become a more permanent position. It was a low level position at the University of Illinois known as Associate. You taught for a couple of years and if you did well, youíd become an instructor, then an assistant professor and then you could go on. At present the associate and instructor positions have essentially disappeared. Usually the assistant professorship is what you start off with and then go off to associate professor when youíve done well with teaching and research. Now, you skip the lower grade of associate. In any case, thatís what I had. I had only been at Illinois for about a month when I was drafted into the Radiation Lab. I had no research started yet there.

Pavlish:

What were you anticipating to do?

Ramsey:

I was anticipating doing some things in particle physics. Maybe working part of the time with an electron accelerator and also wanting to continue my molecular beam experiments. I wanted to continue those two quite different fields.

Pavlish:

Did it seem like there were molecular beam labs sprouting up around the country?

Ramsey:

No, not until the end of the war. The field of molecular beams and even of atomic physics, was recognized as important by the people who were doing it, but not so much by the people who werenít doing it. Nuclear physics was the hot subject, and maybe cosmic rays. Solid state wasnít thought much of at that time either; it was thought of as having too much to do with applications. The field of molecular beams was not big at the time. But after World War II, and with the molecular beam people making new discoveries, new labs were established in atomic physics and molecular beams. The development of microwaves at higher frequencies stimulated research in these areas.

Pavlish:

You characterized this two-field focus as somewhat schizophrenic — so there wasnít an overlapping region of research? Did anything you learned in particle physics help you with molecular beams?

Ramsey:

In fundamental theories, it definitely helped. Iíd say that several of my most important experiments in the nuclear physics direction originated from ideas Iíd had in the case of atomic physics and vice versa. For example, my experiments setting limits on the neutron electric dipole moments uses methods of both particle physics and molecular beams. I thought of these because of my involvement in both fields. The method of doing the experiment turned out to be low energy physics, except it uses neutrons, which are produced by nuclear physics so I had to go to a lab that could produce neutrons.

Pavlish:

So when you say something like, you ďthought of it,Ē would you elaborate on that? There are sometimes stories of say, a physicist eating a meal and coming upon a new idea. I donít think there are any of physicists waking up in the middle of the night with new ideas.

Ramsey:

Well, sometimes they do. I think a very important and very interesting subject is: Where do totally new ideas come from? I will give an example from my own experience. In 1949, my second year teaching at Harvard, I was giving a graduate seminar on molecular beams. We had a good magnetic resonance molecular beam apparatus. I was giving a graduate seminar on the subject. And Ed Purcell — who was on the faculty and one of my very best friends — was sitting in on my course. I soon learned the following. First, it was wonderful to have him in my course. After almost every lecture, weíd have interesting discussions. We had a fine time together and we both got new ideas from it. However I also soon learned that if I tried to give a derivation that I did not fully understand, there was a high probability that Ed might ask me an astute question that would show I did not. One day I was planning to discuss nuclear magnetic moments and was preparing to give the then standard proof that one did not have to waste time looking for electric dipole moments since they were known to be impossible by the then universally believed assumption of parity, P, (left hand-right hand) symmetry. I knew the proof well, but then I had the worrisome thought that Ed might ask me for the experimental evidence for the ďuniversal beliefĒ in the parity symmetry assumption for nuclear forces. I looked up all the books I knew and could find no proof. Finally, on the military principle that, if about to be attacked, counter attack, I went to Ed two day before my class and said that I could find no proof in the case of nuclear forces. He said there must be lots of proofs, but he could not find any proof either, so we published a paper saying there was no experimental evidence for the assumption of parity symmetry for nuclear forces and set up an experiment with a graduate student, Jim Smith to find or set a limit to the electric dipole moment of the neutron as a test of parity. We chose the neutron because it would not be accelerated from our apparatus by the applied electric field. Although we did not find an electric dipole moment, seven years later a failure of parity in the weak nuclear force was discovered. With this elimination of the parity argument theorists then said there still could be no electric dipole moment of the neutron because of time reversal, T, symmetry. I then wrote another paper pointing out that there was no experimental evidence for T symmetry in nuclear forces so we continued our search. Several years later other experimentalists found a failure of T symmetry in the decay of the KL0 particle. With the failures of both the P and T, arguments against electric dipole moment searches, a number of different electric dipole moment searches are now in progress. I must admit, our paper originally pointing out that we were going to do an electric dipole moment experiment was pretty thoroughly ignored because everybody knew the answer. It was still sort of fun giving colloquia at the time while we were doing that experiment because you could always count on bright young theorists saying that ďthatís a stupid experiment because one canít have an electric dipole moment because of parityĒ To which we replied, that is what we are testing.

Pavlish:

It hadnít been tested. It was just assumed.

Ramsey:

It was just an assumption and it was thought to be such an obvious assumption that our experiment was thought to be just a waste of time. It turned out a few years later: we were right! Iím a great believer that thereís good luck and bad luck in experiment. Take advantage of your good luck and donít be too discouraged by your bad luck. My worst bit of bad luck was probably this. At that time people talked about nuclear forces lumped together. They did not distinguish between strong nuclear forces, which hold the nuclei together and give rise to an electric dipole moment, and weak nuclear forces, which are associated with Beta ray decay. What we were actually doing, unfortunately, was testing for a parity violation in nuclear forces but the one we tested was the strong one. It turned out some years later that there was a violation in the case of the weak nuclear forces. This violation was first suspected by Lee and Yang, while they were studying a peculiar nuclear phenomenon called the Tau Theta paradox. They said the paradox might be explained by having a parity violation for the weak interaction. Well, I heard Yang give a talk on that at MIT. That was the first time Iíd heard anybody mention that there might be different behavior in the weak and strong forces. I had done by that time several experiments searching for parity violation and Iíd also gotten very comfortable with the idea of parity violation. It seemed to me reasonable that Nature should violate parity. But as soon as I heard that possibility in the weak nuclear force I called up L. Roberts of Oak Ridge, who had polarized significant amounts of nuclei (Cobalt-60 because of its ferromagnetic nature you can get quite a bit of it polarized). Immediately we made a date to do the experiment of looking to see if more decay electrons went one way more than the other.

Pavlish:

To do the experiment with the weak interaction?

Ramsey:

Yes, which we would do by taking the polarized Cobalt-60. It would be his job to polarize it for me. He had already taken his apparatus down, so it would take a while to get it going again. I was going to develop the electronic equipment to tell whether more electrons in radioactive decay went up this way rather than down that way relative to the spin direction, which would be a violation of parity. So we prepared to do this. Then, we had the following bad luck: namely, Roberts made an important discovery which is usually not bad luck. But it is when it interferes with an even more important discovery. He discovered that in the process of nuclear fission the angular relations between the fission particle directions and the (two) neutrons that were produced in fission had a different angular distribution than was believed theoretically. At which point, the theoretical advisors at Oak Ridge told him that he had a real discovery and he should abandon or delay our search for parity violation. Oak Ridge had already supported some of our earlier experiments on parity violation. In any case, the Oak Ridge management decided that thatís Roberts should concentrate on his new discovery for the next two years. By the time they told me, I learned that Madame C. S. Wu, a very good physicist at Columbia, had also started doing the same experiment. We had to get a new source of funds, a new collaborator, and new apparatus, and I realized that it would be a speed race. While originally weíd had a head start, she had the lead if I had to find a new collaborator. Of course, I was disappointed to learn about it; that was my worst bad luck.

Pavlish:

When you learned that she was doing that experiment, you didnít know the result yet?

Ramsey:

I didnít know the result yet.

Pavlish:

But you knew that it would be an important discovery.

Ramsey:

Absolutely, I knew that it would be an important discovery but I knew that she would be there first.

Pavlish:

Did she use the same method you would have used? The Cobalt?

Ramsey:

She used Cobalt-60. I do not know and I never asked whether she had learned of the Cobalt-60 possibility from what we were doing or not. I discussed our experiment with Frank Yang, he knew quite a bit about it. Iíve never asked whether he discussed this Madame Wu or not. Heís a very good friend of mine. Cobalt-60 was a known substance and it was known that it could be polarized, so it was the obvious substance to use.

Pavlish:

I wonder how much that happens in physics that you hear about an experiment or theory through the grapevine. For example, in the simultaneous discovery of magnetic resonance in condensed matter, there was Bloch and his team in California and Purcell and his team here. How is that possible that within a few weeks of each other, them obtained identical results?

Ramsey:

Oh, I can tell you exactly. Itís a different subject. Both of them were very good friends of Rabi and myself during World War II, and so they knew all about what we had done detecting the magnetic resonance by the effect on the molecule. Actually, four teams independently thought of doing the experiment by detecting the effect on the oscillator that induced the transition. The Dutch physicist Gorter, whoíd failed earlier to detect a resonance heating of the crystal tried during WW II to detect a resonance by the effect on the oscillator and published the results as a failure. It is now a mystery as to why he failed since Bloembergen many years later found his original crystal and obtained an NMR signal. Rabi and I, in 1944, calculated that such detection should work but we did not push it. One evening before the end of World War II, we were together (we had decided that weíd devote one evening to what we jokingly called post-war planning, thinking of good experiments to do after the war) and it was a very productive session. We invented two very good experiments. One was to try to detect the resonance transition by the effect on the oscillator, which we calculated should be observable. The other, probably more important, was that we could now measure the magnetic moment of the proton and the magnetic moment of the electron. Theorists knew the wave function of the hydrogen atom, from the Dirac theory, so we should be able to calculate what the interaction of those two should be in the hydrogen atom; the so-called fine structure. So we also thought of an experiment that was very accurate, for measurement of the fine structure.

Pavlish:

This was just you and Rabi doing post-war planning? Purcell wasnít involved?

Ramsey:

No, Purcell wasnít involved. It was the two of us one evening at Rabiís house, in Cambridge. We invented that experiment, which we thought was maybe even a more important experiment than the NMR experiment. But also, we knew about Gorterís experiment in that intervening time and that it had failed. He tried and failed to do NMR. Thereís this distinction, and Iíd better make it clear. NMR is usually what we call it when itís done by detecting the effect on the oscillator. Magnetic resonance is a general term. The particular term we use for a special case of nuclear magnetic resonance, namely the molecular beam magnetic resonance method is where we do it by the effect on the molecule. We did the work on the molecule before World War II but even earlier Gorter tried a different experiment that couldnít have succeeded. Then, during World War II, under extremely difficult conditions, he was in the Netherlands which was occupied by the Nazis, he managed to do the NMR experiment and it failed. He published it again, like the previous one, as a failed experiment. Then, independently of that, Purcell and Pound and Torrey, and also Bloch, Hansen, and Packard, thought of doing that. But then, the following occurred. In the first place, Rabi and I knew of the failed experiment and we decided weíd concentrate on the hyperfine interaction in atomic H, which we thought was both more important and hadnít failed. Purcell and Pound initially had never heard of the Gorter experiment, so they continued quite far in getting their experiment running. But before it was done Rabi and I told them about what had been done by Gorter. They felt badly but theyíd gone so far that they decided theyíd go ahead with it anyway. In the case of Bloch, Hansen, and Packard, they knew about both of Gorterís failed experiments, but they thought they knew what was wrong with his second experiment. They thought he hadnít waited long enough for his system to get into thermal equilibrium when he changed the magnetic field.

Pavlish:

Is it true that the thing that they thought was wrong wasnít actually the thing that was wrong?

Ramsey:

That is correct. The idea of Bloch, Hansen, and Packard, was to put the sample in a low temperature and in a magnetic field and to go skiing for a week before they even made a first measurement. When they came back from the ski trip it turned out that the relaxation time was only a fraction of a second and they could have gotten the resonance right away. I think they still wouldnít have beaten Purcell, Pound, and Torrey, but it would have been close. Purcell, Pound, and Torrey, published their paper about a month before the others. Both groups went on with the experiment. I remember telling Purcell about Gorterís failed second experiment. Purcell was very disappointed and thought they had goofed, but might as well continue now.

Pavlish:

Do you remember what stage the experiment was in? Was that in late autumn?

Ramsey:

I donít remember in time. The apparatus was mostly assembled and they were getting ready to do it.

Pavlish:

They told you about what they were doing?

Ramsey:

Oh yes.

Pavlish:

Presumably you and Rabi would have kept it, not secret, but you wouldnít have told everyone about your ideas for post-war work.

Ramsey:

We knew them pretty well. We could have talked to them about it. Weíd already decided to do the other experiment but I suppose that a long time before their experiment, we didnít talk much about it.

Pavlish:

So it wasnít that you and Rabi gave either of these groups the idea or any hints on how to do the experiment?

Ramsey:

No. Bloch was aware of this work of Gorter. Now it turns out, that thereís an intriguing postlogue. Some years later Nico Bloembergen was visiting the Netherlands. Gorter had by that time died. In Gorterís laboratory Bloembergen found the crystal that Gorter had used. Well, Bloembergen found this crystal and did the NMR experiment and it worked.

Pavlish:

Really? He did the experiment in the Netherlands or he brought the crystal home?

Ramsey:

He did it there in the Netherlands. It must have been that the Gorter technology wasnít quite good enough to detect it. As I said before, thereís good luck and bad luck. Gorter seemed to have had only bad luck.

Pavlish:

That leads me to another question, a side question: how much of the instrumentation survives? In your Nobel lecture you have a picture of your separated oscillating field apparatus, you have a picture, it is pretty big. I was wondering if that survives or if parts of it do?

Ramsey:

No, that didnít survive. We probably cannibalized it for other experiments, but now two of my apparatuses of that kind did survive. One is the first atomic hydrogen maser, which is still used by NIST and remains the standard of the time signal they broadcast, because it is extremely stable. It isnít the definition of the second and they recalibrate it with a cesium atomic.

Pavlish:

This we should definitely talk about another time. The tape runs out after an hour and a half. So you say that that apparatus actually still exists?

Ramsey:

That apparatus actually exists. It was on exhibit for a long time by the Museum of American History.

Pavlish:

Oh, so it is there?

Ramsey:

Yes. They rotate their exhibits and now they have it in storage.

Pavlish:

I think there are other instruments related because they have one of the original cavities from the Purcell and Pound experiment. I wonder how much they have down there.

Ramsey:

I think they should also have the two coils I used for the first separated oscillatory field experiment. But I donít have it, not here at Harvard. Iím somewhat sorry. It wouldíve probably been among the Harvard scientific instruments.

Pavlish:

The Collection of Historical Scientific Instruments.

Ramsey:

I sort of did the Harvard collection in because they were so interested down at the National Museum of American History. It was on exhibit there for about fifteen years, so I canít criticize them for eventually changing it.

Pavlish:

Iím going to rather abruptly end just so we finish the tape. Thank you very much for the fantastically interesting interview.

Ramsey:

(Note added later) Our best separated oscillatory fields molecular beam apparatus was eventually lent to St. Olaf College where it is still actively used in research.

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