Oral History Transcript — Dr. Norman Ramsey
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Pavlish:Today is Wednesday, January 3, 2007. Professor Ramsey, this is our fourth meeting. In our first discussion, you described your work in Rabi’s molecular beam laboratory at Columbia University in the late 1930s. In our second session, we talked about the effect of World War II on your research, and you gave a vivid account of the genesis of your paper with Purcell, entitled, ‘On the Possibility of Electric Dipole Moments for Elementary Particles and Nuclei.’ In our third interview last time we met, you spoke about your most recent book, entitled Spectroscopy with Coherent Radiation which contains forty of your most important—of nearly 500 papers, and how you invented the Separated Oscillatory Field Method. Professor Ramsey, these discussions have been very informative and absorbing for me as an aspiring historian of science, as I wish to understand how your experiments and theories were conceived, planned, and carried out; as well as the stories of particular instruments; and your intellectual interactions. Today, I would like to talk about three things, in addition to discussing submission of these interviews, possibly to Professor Rigden’s journal ‘Physics in Perspective’ which you have already written for as well as a potential film project. First, I would like to discuss the background methods, experiments and ideas you used for your papers explicating theories of Nuclear Magnetic Shielding and NMR Chemical Shifts, which according to one source “provided a sound basis for interpreting chemical shifts and still provides the basic framework for all theoretical treatment of chemical shifts” (Encyclopedia of NMR). I used your book as reference. You have a section here, section 4, on this work. The section includes several papers. I started with the first paper, which is entitled, “Magnetic Shielding of Nuclei in Molecules.” This paper is quite mathematical. As I was trying to follow it, I wondered, how does a paper like this come together? You mention that it is one of your most cited papers. Do you think this way, when you’re by yourself?
Ramsey:It’s various things combined. In the first place, the idea for worrying about it started with a big disappointment to me. I had invented the Separated Oscillatory Field Method, which enabled me to measure the magnetic resonance frequency of magnetic moments of nuclei in a molecule to much greater accuracy than before. But the value of this achievement was reduced by the fact that the nucleus that I was measuring was in a molecule and the molecule had electrons. So, when I turned on the magnetic field, the electrons would be induced to circulate around it, and produce a different magnetic field at the nucleus. The magnetic fields at the nuclei were approximately the same, but slightly different. For atoms, Willis Lamb had developed an easy way to calculate the correction. In an atom, there’s the simple electric charge of the nucleus and the induced circulation of the electron is just circular which made the calculation easy. Since we were working with molecules, not with atoms; that theory wouldn’t work. We were still getting good measurements, but we weren’t getting the magnetic moment accuracy that was characteristic of the method. I worried about this, and worried about doing better than Lamb had done. I realized that with a molecule it was more complicated. The trouble is, that there are two protons in the hydrogen molecule. So one is the nucleus that the electron is going to circulate around. The answer is: neither or both. Well, I didn’t know quite what to do. Then I remembered somewhere that I had studied a different but related method that Van Vleck had calculated. His book is on…
Pavlish:You cite it in the paper, I believe, “Electric and Magnetic Susceptibility” from 1932.
Ramsey:The problem Van Vleck solved was: when you put a magnetic field on a molecule, what kind of a field does the molecule produce? One part of that magnetic susceptibility is the part of that due to the electron motion that’s induced.
Pavlish:He calculated that theoretically?
Ramsey:He calculated the total magnetic field theoretically. Then, it occurred to me that actually what I wanted was closely related to what I wanted, but it was not the same thing. What we wanted was the magnetic field at the nucleus due to the circulation of the electrons. What he wanted for his essentially something proportional to the square of the electron’s distance. What we needed was one inversely proportional to the cube of the distance. I realized that the calculation should be similar so I studied his calculation a little more and basically, my paper followed what Van Vleck had done.
Pavlish:If we could go back a little bit, circle back. So, you were in the process of doing this experiment. What was the situation like in the lab? Were you already getting data?
Ramsey:We were already getting data. We were getting results of the resonance frequency. But we really weren’t getting the nuclear magnetic moment accurately because you didn’t know what that correction was.
Pavlish:I can imagine in doing an experiment like that, you might think that maybe it’s some imprecision in the apparatus. When did it click for you that it was a theoretical problem?
Ramsey:In fact, I realized it to begin with that there was going to be a theoretical problem. In the case of atoms, Willis Lamb had worked out the theory for a simple charge. That was necessary for getting the nuclear magnetic moment from an atom. But for a molecule, I had a different calculation because the electrons did not circulate in simple circles around one nucleus. What I tried to figure out was what the analogue to the things that Van Vleck had worried about — second order paramagnetism. Namely, that the electrons had magnetic moments, so they could be induced to circulate. The detailed calculations were very similar to those Van Vleck did. So that made it easy for me.
Pavlish:You had studied his book at Cambridge?
Ramsey:I had studied his book some at Cambridge, and I had studied it more when I was at Columbia in Rabi’s lab. I was mostly working on experiments there. I principally studied it at Cambridge, and particularly after I was doing my experiments at Harvard.
Pavlish:You might have known him also?
Ramsey:I knew him very well personally.
Pavlish:You knew his book first?
Ramsey:I knew his book first, yes. I first came in contact with it at Cambridge. It is published by Oxford Press.
Pavlish:It wouldn’t be fair to say that when setting up the experiment and using H2, you were already thinking: well, maybe we can find some experimental measurements that will help theory along?
Ramsey:No, when I was doing that, I wasn’t thinking about that. I didn’t know that I would be able to solve that problem. I was getting the best you could get, just treating it as an atom.
Pavlish:It was the disappointment with the oscillatory field method?
Ramsey:It was a disappointment that I couldn’t do better than that. And then, it was a recognition that if I could do that calculation, not the same calculation Van Vleck did, but somewhat in the same spirit of the calculation, I would be able to get that correction. It is interesting to know that originally it was that disappointment that I could not get the magnetic moment more accurately. Later the correction turned out to be very important. I did not anticipate that the first time. In fact, it even shows up in the titles of my papers. I think the title of my first paper on this topic was just, ‘Magnetic Shielding of Nuclei in Molecules,’ which is only a moderately interesting subject. But then I realized that the magnetic shielding would be different for the same atom in different molecules, and for the same atom in a different location in a molecule. I realized that this was a way of calculating that. That turns out to be important, because chemical shifts—what they are called—were observed experimentally about the same time.
Ramsey:Experiments were being done. They were in several laboratories; in particular Felix Bloch’s.
Pavlish:Oh yes, and you cite here in a footnote in your paper, “private communication” from Felix Bloch. You reference him. Now, this is a little off topic, but it is something that I wanted to get to later in the interview. Here you mention that you received information by private communication; near the end of the paper, you mention that you’re grateful to Van Vleck and Purcell for discussions in the course of the calculations. Then, in the introduction to a subsequent paper you mention that Felix Bloch in jest called you his “Haus Theoretiker,” is that house theorist?
Ramsey:Yes, that the German name for a house theorist.
Pavlish:“One day he came to my office with glee that he had experimental disproof of my chemical shift theory”. I was a little confused, because I thought Bloch was at Stanford. How could he have just walked into your office here at Harvard?
Ramsey:He came to visit. He was interested and familiar in the work we were doing and wanted to see our work we were doing. Similarly I visited him at Stanford where I went frequently.
Pavlish:Your ‘private communication’ would have been at one of those visits? Were you also writing letters?
Ramsey:It’s both. Some of them are letters. I think with Bloch it was mostly private talks. He was a good-natured talker. Was this the paper on…?
Pavlish:“Temperature Dependent Magnetic Shielding in Ethyl Alcohol.”
Ramsey:Yes, that’s right.
Pavlish:You had the formula for ethyl alcohol on the board, and Liddell came over and asked you if you were drinking it or studying it!
Ramsey:Yes. He [Bloch] had gotten this result, of temperature dependence. One of the characteristics of my theory was that it was that the chemical shift should be temperature independent. It just depended upon the configuration of the molecule and the circulation of the electrons. But Bloch had observed this temperature dependence, which had me worried. It was a good-natured visit. He sort of said, “See, I gotcha. It’s wrong.” I had the formula on the blackboard while I was still thinking about it. That’s when Liddell came to my office and said, “Are you drinking it or studying it?” I said, “I’m studying it.” He said, “You know, that’s the thing I studied for my PhD thesis. I was studying that molecule’s molecular association.” Namely, that in liquid ethyl alcohol, the molecules tend to stick a little bit together. They stick by having the common bond of hydrogen. Well, as soon as he said that, I realized that molecular association was the temperature dependence. Temperature would determine how much they would stick together and how much they would pull apart. Liddell and I did the calculation the same morning. We mailed in the paper for publication that afternoon.
Pavlish:That’s the paper you said took you six hours, the shortest interval of time between conception and completion.
Pavlish:Now, about this intellectual interaction between physicists. In a previous interview, you mentioned how you and Purcell were best friends at the time of the paper on parity. Then you were communicating with Bloch. Purcell’s office was right next door to yours. You were also communicating with Van Vleck. What was the atmosphere like here at Harvard?
Ramsey:It was very friendly. It varied with different people. I think with myself, Van Vleck, Purcell, and most others were quite frank in talking about what we were working on. We would discuss our problems together and sometimes give hints to help the other person. Sometimes ending up by saying, as in the case with Purcell, on parity, “we should do an experiment together on the basis of that.” There are two ways you can do research. Some people are very secretive about their ideas until they’ve worked them out fully. It turns out they have all sorts of arguments with people. Most of who it can be an argument over priority. Most of us, if you have a very hot, brand new idea, you just thought of, you probably want to initially talk to only a couple of people you have great confidence in, who will be helpful. If they make a good suggestion to you, maybe you’ll do it together as far the one on parity. I’m the one who raised the initial question, but we ended up publishing a paper together with our names in alphabetical order. Some people have wondered if I asked the first question, why it was in that order. Most of our publications were in alphabetical order. Something the first author, who did the most important work or if it was a student who did this for his PhD thesis, then his name usually was first.
Pavlish:When you say at the end of this paper, that you appreciate the discussions — when would those discussions have turned into a whole publication with co-authorship?
Ramsey:In the first place, appreciation of discussions means that it was helpful.
Pavlish:They might have checked your calculations just to make sure they were correct?
Ramsey:We would often talk it over, as I was first thinking about it. Very frequently you’d talk something over with somebody and you yourself would get more ideas, which is not necessarily due to the other person, but it was partly due to the discussion. If, on the other hand, the other person suggests the key idea, then you’d publish it with that person’s name first even if you started the discussion.
Pavlish:I’d like to follow up with a question about mathematics. My advisor here in the History of Science department, Peter Galison, wrote an interesting article about Dirac in which he describes how Dirac was very sparse in person. Dirac was very algebraic and didn’t use many words when he was talking about physics, but then in private, looking through Dirac’s notes, Galison found that Dirac used geometric reasoning a lot.
Ramsey:Yes, that’s right.
Pavlish:My question is not about Dirac specifically, but about you.
Ramsey:There are different ways that people can do it [reason about physical problems]. Rabi, I, and many others, initially think more physically, usually means a little more classically, but then convert to quantum mechanics. I think Schwinger thought in terms of quantum mechanics right away. That’s the way he envisioned what was going to happen. I think Rabi, tended to think of classical analogues. I like to be able explain things in that way, although I’m a full believer that quantum mechanics is the best way of doing it.
Pavlish:How about some of the others of your colleagues: Zacharias, Kusch, Purcell, Bloch?
Ramsey:I think all of that group, maybe not Bloch, tended to do it in the more physical sense.
Pavlish:Is that Rabi’s influence?
Ramsey:Partly Rabi’s influence, yes. But it’s also that it’s an easy way thinking about many problems. Fermi, who’s an interesting case, tended to think in elementary terms, but, he was very quantum mechanical. His books are always easy to understand; Dirac were difficult.
Pavlish:When you work as a scientist, do you present the material differently to different audiences? You must have a specific way of presenting material to students, a specific style in a paper, a specific style when speaking with collaborators, maybe giving a colloquium. Are there distinguishing characteristics of the different methods of communication?
Ramsey:Yes. Probably in the colloquia, I will tend to describe them in physical semi-classical terms, quantized but in a way easy to picture. In a lecture, I’d give the (right) way of calculating quantum mechanical, which I would work up in more detail.
Pavlish:That’s more of a particle physics talk?
Ramsey:That’s right. A particle physics talk and a magnetic resonance talk. We’re looking for an electric dipole moment of the neutron. Our neutron is a particle but the electric dipole is a molecular calculation.
Pavlish:That’s an experiment at LHC?
Ramsey:No, that’s not an experiment at LHC. It’s a continuation of our parity experiment, which we’ve already discussed. It has the interesting circumstance that the first five or ten years were almost ignored by theorists and others because they “knew” there couldn’t be parity because of the parity argument.
Pavlish:You mention in your book that the experiment was before its time.
Ramsey:Yes, that’s right. The theorists began to recognize that there ought to be a very big one [?]. It would take what’s called ‘fine tuning’ to somehow make the electric dipole moment small. There have been calculations on how this could be used as a test of Super-symmetry. If you adopt the Super-symmetry theory in its simple, there should be an electric dipole moment, although very small. In fact, in a range that’s bigger than what our limit is, but it also could go down and include our limit. Another factor of one hundred, which we hope to get eventually should make it if Super-symmetry is correct. Or if there’s some fine-tuning — there’s a paper by R.D. Peccei and H. R. Quinn. Both were theoretical physicist; Helen Quinn at Harvard. Their first paper pointed out that to say it’s so small that you can forget about it is not great. You have to also say why it’s small.
Pavlish:I look forward to your upcoming colloquium. My apologies, I’m taking you back to the 1950s again. In 1952 and 1953, you and Purcell published two papers in The Physical Review (which are not included in your 40 most important collected papers, but were cited in the historical section Encyclopedia of Nuclear Magnetic Resonance) in which you developed the theoretical explanation for the ‘indirect spin-spin interaction’ or the ‘scalar-coupling.’ You described it as a ‘slight polarization by the nuclear spins of the electron spins, which are tightly coupled by the exchange interactions in a chemical bond.’ Would you please describe this, one of several collaborations with Purcell: how did the idea come to you and how did you decide to collaborate?
Ramsey:Yes. Purcell was the one who called attention to there being a problem and other people had observed experimentally that there are spin-spin interactions that do not average to zero. I’ll first discuss it for HD. You have to make a correction for the magnetic effect of one nucleus on another in HD. The spin-spin interaction; I, I2 does not average to zero and in surprisingly strong, so it could not be just the direct magnetic interaction of I and I2. What Purcell and I pointed out — I can’t say who did exactly what since we were very collaborative on that problem — is that you could get an effect. One nucleus could interact with a nearby electron, causing it to want to spin up at a given interaction energy. The other nucleus could interact with a nearby electron. Then, there was a requirement that you had to have, in that molecule, no net electron magnetic moment. You couldn’t re-orient one electron or the other. The presence of the two kept you from reorienting the electron. But there was an interaction between the two protons because of this mutual interaction.
Pavlish:So this proton is interacting with the electrons from this atom, and the other one is interacting with the electrons from the other atom.
Ramsey:Yes. Most molecules are so-called ‘singlet-sigma molecules.’ Those are molecules whose lowest energy state has no net electron angular momentum. All the electrons somehow jointly manage to neutralize each other. That means, that if you do something trying to affect what the electron will do here and over there then something has to come in, which is a basic symmetry property of the molecule [hand motion], that somehow there has to be an interaction that enables the two electrons to cancel each other. We called this an “electron-coupled spin-spin interaction. That effect has a back interaction on the proton or the nucleus that interacted with electron-one and the nucleus that interacted with electron-two.
Pavlish:That reminds me, I’ve noticed in some of my reading, that some physicists personify atoms. Erwin Hahn said something like, “If you treat atoms like people, then they’ll do what you want them to.” Rabi, in Rigden’s biography, says that magnetic resonance goes back to when he was walking to the lab at Columbia and was thinking about nuclei kinesthetically, with his body. Is that something common? It seems a little odd to me.
Ramsey:The source of new ideas is very important and extremely varied. Sometimes it comes from physical intuition when it can be understood classically say, sometimes from an appreciation of a property that the wave-functions have, and sometimes from a totally unrelated anomaly, even from how you’re feeling that day.
Pavlish:I read that Bloch’s ideas of magnetic resonance go back to when he was listening to a concert performance.
Ramsey:Mhmm, that was probably true.
Pavlish:Do you have any interesting memories like that?
Ramsey:Yes, frequently at concerts, even when I’m listening and enjoying them, I do tend to think sometimes about physics problems. Basically, if you have a problem that you’re stuck with, worrying about, trying to understand, it comes in and out of your mind. It probably pays to be a bit relaxed and you get ideas that way. But you also can get incorrect ideas that way, and you would have to sort them out. I’d say, most of the ideas I’ve had from something completely extraneous have usually been wrong, but not always. The occasional good ones are what really counts. You don’t want to discourage thinking about false analogies.
Pavlish:Would it be to press you for the one that wasn’t wrong?
Ramsey:Let’s see, I tend to forget those.
Pavlish:One person would be particularly good at one aspect; in theory as well as in experiment? You’re theoretically very gifted, but also experimentally — in molecular beams, and particle physics.
Ramsey:Different fields can be very supportive. A good example being my search for a neutron electron dipole moment. You can think of it either as a nuclear physics experiment, as an angular momentum experiment, a quantum mechanical experiment, or an atomic physics experiment affecting nuclei. That experiment is a mixture between atomic and particle physics. If I did not know any particle physics, I probably would not have thought of the experiment and if I did not know any atomic physics, I would not have thought of it. Aha, that is a good example: If had known a little more of particle physics, to recognize the distinction between the weak and the strong force at a time when most people didn’t distinguish between the weak and the strong nuclear forces. We would have done the first experiment on parity violation. As I describe in my book, as soon as I heard Lee and Yang (who were still at the time very closely collaborating with each other) give the report on the weak interaction, I immediately wanted to do the experiment on the weak force but the Oak Ridge management, on the advice of the theorists there, postponed our experiment for two years.
Pavlish:I have a few concrete questions, which relate to your work but aren’t directly about it. This one is fun. I was at the Physics Research Library taking out your book and I found this, it says “Gift of Norman F. Ramsey.” It was published in 1935. [written by S. Tolansky]. I don’t know if it looks familiar to you.
Ramsey:Oh yes, it is one I studied when I was a student in Cambridge England. That’s my handwriting.
Pavlish:I marked some of the pages that contain the handwriting, because I was wondering if it was all yours.
Ramsey:This is [pg 7]. In fact, it is not so different from now, a little clearer than my handwriting is now.
Pavlish:These highlights are also yours?
Ramsey:Yes, the highlights are all mine.
Pavlish:Let’s see, here is that also you writing in or is that somebody else?
Pavlish:It looks like you have a special notation; sometimes you write in the paper references, I notice.
Pavlish:I was wondering how this book fits in with your trajectory.
Ramsey:This is a book that I bought when I was in Cambridge. I studied it. I had to take exams and this is one of the books I learned from.
Pavlish:Was it recommended to you by someone. Did you just stop by a bookshop?
Ramsey:It’s a whole series. I have some more of them. I think I gave this one to the library because by that time, I knew most of it. I have a note here, “For simple proof, see Rabi’s notes for Physics 207.” That’s a course I took.
Pavlish:At Columbia? Taught it maybe?
Pavlish:This is one of your textbooks from your Cambridge days. How much did you use books and how much did you use published papers?
Pavlish:Here’s a book, “Proceedings from the International Conference on Radiofrequency Spectroscopy” in 1950. You have two contributed papers. Here’s a picture of the attendees. Was this the first conference on the subject? Or, rather, do you remember going to a lot of conferences in the late 1940s, 1950s?
Ramsey:I did go to quite a few.
Pavlish:Gorter gives the opening remarks at this one.
Ramsey:This is probably one of the quite early ones, yes.
Pavlish:It looks like you were in attendance, and Purcell, and Bloch, and Van Vleck. I didn’t notice anything about this being the first one. I notice some women in the picture, I wonder if they were scientists. I’ve noticed there weren’t many distinguished women scientists in this field.
Ramsey:That’s true, but there were some. There’s probably a list of attendants.
Pavlish:There’s a list of contributors in the beginning. I don’t know if there’s a full list of everyone in attendance. Looking through the papers in there, I noticed that some of the papers use oscillograph photographs of the trace. In the early Rabi papers, I noticed that it was just points plotted and connected as a curve. I was wondering if there was a certain time at which people started including those photographs in their papers.
Ramsey:I don’t know if there was an abrupt use of it. In all of our early graphs, we plotted out the points taken. (The original ones were less frequently oscilloscope traces actually.)
Pavlish:That’s because of the technological level of development?
Ramsey:Yes, basically. For intensity measurements you had a galvanometer: you put a mirror on a delicate magnetometer, a current would move and we would measure it. It was more complicated and expensive at that time, to convert that to an oscilloscope. Basically, people were not so hot on electronics. When they were doing delicate measurements they were trying to get the power for radio signals. I would say that this was, in fact, quite an early one; in March, 1951. The field of magnetic resonance, except for the molecular beam experiments ones that we did in 1939, there were only molecular beam magnetic resonance experiments essentially from then until the end of the war. The earliest papers on NMR with condensed matter were by Purcell, Bloch and their associates in 1946. The war was over in 1945 and their first publication about NMR were in 1946.
Pavlish:January and February of 1946 is when they published their first, brief notes. I’ll look at it in more detail, but you actually have two papers in here, and it seems like everyone else has one. You have one very developed paper; here’s one on pg 388.
Ramsey:What is the title?
Pavlish:“Radiofrequency Spectra of H2 and D2 by a New Radiofrequency Method.” If you wouldn’t mind meeting again for another interview to discuss this further. Another question, I notice in my reading about radar, that a lot of the instruments were called ‘Mark’-something. Mark ‘number.’ In my earlier research on the discovery of the psi particle over at Berkeley, they were using the Mark-II. Here in the Science center there’s a computer called the Mark. I wonder why it’s called ‘Mark’?
Ramsey:It’s a carry-over from people having done a fair amount for the military during the war. It is standard terminology in the development of military equipment.
Pavlish:Does ‘Mark’ stand for something?
Ramsey:‘Model’ or ‘Edition’.
Pavlish:It’s a lingo, not a formal acronym like ‘LIGO’ and ‘LISA.’
Ramsey:Yes, the term had much meaning. I’ve seen papers with that used Mark 5 going back before World War II.
Pavlish:So it’s like a take one, take two.
Ramsey:Model 1, Model 2. It’s a production model. Mark 1 is a particular style. Model 1 could mean that you just made one of them. Model is used somewhat ambiguously: sometimes it means a single model, and sometimes it means a particular design. Whereas, a Mark would mean a design of a product that was produced for production.
Pavlish:As a follow-up to that, did you ever name any of your experiments?
Pavlish:I read in a recent Physics Today a historical sketch about Fermi naming his instruments after characters from Winnie the Pooh.
Ramsey:Are there any other papers that you ought to ask about?
Pavlish:I would like to ask you sometime about one of the final papers in your book, about your ventures into philosophy of physics, philosophy of quantum mechanics. The paper is called, “Significance of Potentials in Quantum Theory,” on pg. 400 and your preface is on pg 399. It details your reaction to Bohm’s thought experiments.
Ramsey:That was not so much a philosophy, as a new discovery. That’s a very interesting phenomenon. I can tell you the history of that one; which also shows the ways of collaborating. In 1959, Aharonov and Bohm had a publication in The Physical Review, which probably eighty percent of the physicists thought was nonsense, and about ten percent thought, “of course, we knew it all along.” Then there was a very small group who didn’t believe it, but felt they didn’t understand it well enough not to believe it, and tried to understand it. Wendell Furry and I frequently ate lunch together and talked. We were initially in the category of not believing the Aharonov-Bohm paper but not fully understanding it. Historically, electric forces were due to Electric fields and Electric Potentials were introduced as a means of calculating the Electric Field. You could take …
Pavlish:That’s still done today, isn’t it?
Ramsey:Oh, yes. That’s still done today, but with a slightly different point of view. At the time, it was introduced as something that you could differentiate and get the Electric Field. It wasn’t the Electric Field but it meant that you could calculate with this potential and differentiate to get the Electric Field, which then was thought to be the physical phenomenon. Bohm and Aharonov pointed out an experiment that could be done which would depend upon the potential and not just the electric field.
Pavlish:So the potential was more than just a theoretical, mathematical…
Ramsey:More than a theoretical friction to deal with. It’s something you could do experiments to measure. There were two forms as I remember. One you could have an electron go through a shielded region. You raise the potential while it’s in it, and drop it down at the end so that the electron never feels an electric field. The other was an interference experiment with an electron, a quantum mechanical experiment. Almost everybody disbelieved it. Furry and I wanted to understand it and we figured out that yes…
Pavlish:Sorry to interrupt, but the Electric Potentials, did you learn those as a student at Cambridge?
Ramsey:Yes. They’d been used for years, before I came around. I used potentials to calculate things at Columbia when I first did physics there.
Pavlish:You didn’t think of it as a measurable quantity.
Ramsey:It turns out, that it is only observable with Quantum Mechanics.
Ramsey:Yes. Without Quantum Mechanics could not be explained. With Quantum Mechanics you get interference between the two phases, two beams of different waves with the phase being shifted and nothing else being different. You still get a difference. It’s a quantum mechanical difference. There’s a basic thing in Quantum Mechanics. You can do an interference experiment for an electron that has a choice of two paths. You cannot get an interference pattern if you do an experiment to tell you which path the electron went on. Furry and I pointed out that one could do that impossible experiment, if it were not for the Bohm-Aharanov Effect. You could have an electron go through, measure the potential, have the paths then go together, at no time is the electron affected by any Electric Field, anything like that. You could use the potential difference to tell which path the electron was going through. Therefore, you could do the thing that’s forbidden in Quantum Mechanics., which is to get an interference pattern and know which way the electrons went. Therefore, the Bohm-Aharanov Effect must be there to prevent this evolution of quantum mechanics. The same is true for the Magnetic Potential.
Pavlish:Their paper showed why you can’t do that? I guess I’m just confused, this is like the Stern-Gerlach experiment where you send a particle through and it goes up or down.
Ramsey:No, it’s not like that. In the Stern-Gerlach experiment, the electron feels an Electric Field. An Electric Field will accelerate an electron.
Pavlish:In this one, it doesn’t feel a field.
Ramsey:It’s like doing a Stern-Gerlach experiment but not from an Electric Field. We point out that without the Bohm-Aharanov effect, the effect of potentials, you could measure the potential when electron was inside this shielded region so you could tell through which path the electron went and still get the interference pattern. But, we pointed out that you don’t get the interference pattern because there is an actual observable effect of the potentials on the electron. It is a quantum mechanical effect that changes the phase of the electron wave function. You only have significance of phases in quantum mechanics, not classical mechanics. Therefore there’s nothing you can do in classical mechanics to tell through which path it went. A few of people said they knew it all the time, probably five percent. Weisskopf was one who did understand it. He said of course, there’s an effect of the potential on the phase. A correct quantum calculation does affect the phase.
Pavlish:That is measurable.
Pavlish:Before I interrupted you, you were going to direct me to what we should focus on in our next interview. I don’t know if you think we should meet again.
Ramsey:I am happy to meet. I will look at what papers we have listed here and I will just comment on the way. The separated oscillatory field, we have discussed. Phase shifts in the molecular beam; that has to do with separated oscillatory fields. Nuclear-audio frequency is paper number 24 is with Bob Pound who in 1950 had a crystal of lithium fluoride, a very pure one, which had the following characteristics. If he put it in a strong magnetic field and waited for ten minutes or so, then the nuclei would be aligned and he could do magnetic resonance with it and when he pulled the crystal out of the magnetic field, and then put it back within ten seconds, then it was immediately aligned again. But then, if he waited for a while it lost its alignment. The problem was, what was the property that helped it remember to go back immediately to the original alignment after being out for ten seconds. Bob consulted with both Purcell and myself. He invited us to his lab. We did some experiments and found it was a scalar property, not a crystal effect, it was not an alignment effect, because you could rotate the crystal and it still would go back to the same magnetization. We eventually realized that it might be something we had heard some of the Dutch physicists talk about before NMR was invented: spin temperatures.
Pavlish:Would this have been after a conference?
Ramsey:It might have been after a conference.
Pavlish:You might have heard them at the conference?
Ramsey:We might have heard about it at the conference. Well, it should have been. When did we do that experiment? 1950.
Pavlish:The conference proceedings were published in 1951, and the conference was in September, 1950.
Ramsey:Their work was known at that time. Let me see if they said anything about it. I don’t think so. I think it was too early to have been reported. If there was a discussion of spin temperatures, we did not understand nor believe at the time. But then there was some scalar property to give the effect. We eventually realized that the phenomenon Bob Pound was worried about and on which we did experiments that could be explained in the following way. If we put the strong magnetic field on and we go from zero magnetic field to strong magnetic field, and have a very low initial magnetization before it reaches equilibrium it corresponds to a very high spin temperature. When we turned on the magnetic field, we heated the spin system. Then, when we pulled it out the spin system was super cold. That is true in general of anything that is magnetized. When it cools down to room temperature, (while it is in that high field). Now, when we pull it out of the magnetic field, you have the spin system aligned. That corresponds to the spin temperature being very high. Then, we wait a while and it cools down to room temperature. But, if we only wait a short time and put it back in the strong field, we immediately get it all aligned. What is really relaxing is the spin temperature.
Pavlish:You don’t actually feel the hot temperature do you?
Ramsey:No, you don’t feel it, it is the spin temperature, that is not in equilibrium with the crystal. I mentioned these first experiments of Gorter in which he tried to detect resonance by heating of the crystal. It did not work because the relaxation times were too long for the crystal to get in equilibrium with the spin system.
Pavlish:So he could not measure the temperature?
Ramsey:Right. That is why Bob Pound’s experiment had this phenomenon because the spin system was in very poor equilibrium with the lattice. So the high temperature of the spin system did not leak off into the vibration of the atoms in the overall system.
Pavlish:For that reason he was having trouble doing the NMR measurement?
Ramsey:No, he was not having any trouble. He was just very puzzled by the anomaly. The problem really was that he was having too much success. In general when you put a system in a strong magnetic field you had to wait, say, five minutes before it was polarized enough to give a resonance. Here, he had a system which immediately gave a resonance. That was puzzling. Using the physical model, in some sense you can even ask, how did the crystal know what was the property it had to know that it had been in the strong magnetic field. How did it remember? Eventually, we realized that the answer was in the spin temperature. The spin temperature was high, and then it cooled down. With that apparatus I invented a way to do NMR at very very low fields. That had considerable use later on. I put the crystal that I was studying in a strong magnetic field, left it there, then pulled it out for a few seconds long enough to do an NMR transition and then put it back to do an NMR transition in a strong field to see if a transition had occurred in the weak field.
Pavlish:You would put it in Bob Pound’s experiment and then you would put it in your molecular beam?
Ramsey:No, no. It was all done in Bob Pound’s lab. He did not have a molecular beam experiment. He had an NMR experiment. I put the sample in a strong magnetic field and let it get to equilibrium and then pulled it out to do a transition in the low field to do NMR experiments with the system. Then I put it in again and did an NMR experiment to see if a transition had occurred at the very low field. This was the only way to do NMR at very low fields. The theory of resonance transitions induced by perturbations at two or more different frequencies proved to be quite important.
Pavlish:We are talking about paper 1.5 in your book, Spectroscopy with Coherent Radiation?
Ramsey:No, paper 1.6. (Paper 1.5 is a theoretical paper showing a simple way of calculating magnetic resonance by going to rotating coordinates.) Paper 1.6 is “Resonant Transitions induced by perturbations at two or more different frequencies.” I initially got interested in this because at the time I wrote this paper we were trying to make uniform magnetic fields. To get a sharp magnetic resonance, you want to do the observation for a long time. Which means, with the beam, you want a long path. You have particles going through. You also have to worry about irregularities in the magnetic field. I did a calculation on the effect of these irregular magnetic fields. It is like having two frequencies. There is the frequency of the oscillator, and there is the frequency induced by the atom going from one field, one frequency, to another. This gives a net pulling of the resonance frequency. It throws the experiment off, if you did not calculate this effect. This turns out to be very important in determining how uniform the field needs to be. It also induced me to invent the separated oscillatory field. Paper 1.7 is the one for which in part I got the Nobel Prize; the separated oscillatory field method which we have already discussed. 1.8 is a more general form of 1.6; namely, oscillatory fields at non-uniform amplitudes. 1.9 is a paper on all of the shapes of the molecular beam resonances and how that can distort the phase and give wrong results if you do not make corrections.
Pavlish:I see. That paper also has some extended mathematical calculations.
Pavlish:Did you keep notebooks for these calculations?
Ramsey:Yes. Two things. In the first place, for the calculations that I did are here [a shelf of notebooks in Professor Ramsey’s office]. These are where new ideas came in. If you want to see my original notebook in which I did my first calculations of the separated oscillatory field it is in one of these. Second, the data are also kept in separate student notebooks.
Pavlish:The Experimental data?
Ramsey:Yes, the experimental data.
Pavlish:Some of these are in the archive?
Ramsey:Yes. I do use them, if I want to go back when someone asks “how did you do such and such?”
Pavlish:With nearly five hundred papers, it would not be humanly possible to remember every detail.
Ramsey:In any case, the “Shapes of Molecular Beam Resonances” was important paper but was published in the wrong journal. I was asked to write a paper in honor of Otto Stern. The first time I was asked to write one under that circumstance. So, I wrote an important one but it is published in an academic press, and is not easily available to those who need it. It calculates what happens if you have separated oscillatory field, three oscillatory fields, oscillatory fields of different amplitudes, and different magnetic fields. Paper 1.10 is the atomic hydrogen maser, which we have not talked much about, have we? We should. The first hydrogen maser was built by Dan Kleppner and me in 1960. It is still extensively used because of its high short term stability. The time signals that the National Institute of Standards and Technology (NIST) send out are from a hydrogen maser with its frequency slightly pulled to make it match a cesium beam tube standard. There are four papers on that, that you can see. We did the same thing with deuterium as well as with hydrogen, from which you obtained a new get a measurement of the quadrupole moment of the deuteron.
Pavlish:This work of yours directly influenced Townes?
Ramsey:We were more or less in parallel. He succeeded me at Columbia when I came here [to Harvard]. We were and are very good friends. I was doing things with radio frequency, for a time, and he was doing things with microwave spectroscopy. He was concentrating on many measurements with different molecules, whereas I was looking for fewer but particularly interesting studies. Paper 1.15, which is a paper I wrote for the Nobel people, that you should pay special attention to. That was my summary in 1989 of what I thought were my best papers in the field of magnetic resonance. Of course I had to limit the number of them. Paper 1.16 is basically extending my work with separated oscillatory fields to optical frequencies. The ones that are in [section] 2 are results of microwave spectroscopy measurements. For example, 2.1 is on the electron quadrupole moment of the deuteron and also the radiofrequency spectra of H2 and D2, which we have already talked about. 2.2 is the rotational magnetic moments of H2, D2, and HD molecules, the basis of my Ph.D. thesis.
Pavlish:Does that distinction between high and low magnetic fields go all the way back to the beginnings of this technique?
Ramsey:We did the first ones with high magnetic fields and then gradually did it at even higher static magnetic fields. Used them in paper 2.4 also went to very low fields.
Pavlish:That reminds me, in a previous interview you mentioned that you worked with waveguides at the MIT Radiation Laboratory. Did you use waveguides in your post-war experiments?
Ramsey:Yes, in some. Most of my fundamental research I was working at lower frequencies. I was very successful for quite a while in arguing that I liked to do things at lower frequencies as opposed to the same measurement at higher frequencies. It is analogous to measuring the width of second street by measuring the width from one side of the street to the other, as opposed to measuring from one side of the street to somewhere on one side of one hundred and fiftieth street, and from the other side of second street to the same side of one hundred and fiftieth street. It enabled us to get more accurate measurements than other people could get them. Microwave and optical is now spectroscopy; so good that they give high accuracy. Atomic hydrogen was fundamental and the hydrogen maser was very accurate. We did it with small boxes and big boxes to get increased accuracy. Sometimes we did the experiments for the interest of the substance, sometimes for measuring a different property, like measuring the rotational magnetic moments of lithium hydride and deuteride. That also was a good way of getting the quadrupole moment of the deuteron.
Pavlish:At this time, were the methods you were using migrating to chemistry?
Ramsey:Oh yes, they started migrating to chemistry quite rapidly. I would say, they were beginning to be of interest to chemists when we observed six resonances in H2. Then the interest increased with my theory of the chemical shifts in NMR, with the…
Ramsey:…chemical shift, yes.
Pavlish:Do you remember interactions with them? Were there chemists coming to colloquia?
Ramsey:Oh yes. That in fact is something we should talk about. Bill Klemperer who is in the Harvard chemistry department and Dudley Herschbach, who won a Nobel prize in chemistry, were very good friends of mine. [Corey?]
Pavlish:They were coming to the physics colloquia?
Ramsey:Not only the physics colloquia, I had a molecular beam seminar with my graduate students that met once a week. The chemist did not have many graduate students initially, so they joined our seminar. We also lent them equipment; there was a lot of cooperation there. For about two years we had a joint seminar on molecular beams, that was the official title. Then, they eventually had enough students to hold their own seminars.
Pavlish:Was that a one-way street, that you were only benefiting them, or did they also come to you with new ideas?
Ramsey:We benefited in all ways. It is probably true that initially we had more that we could give them. We could lend or give them equipment. We are still good friends, particularly with Bill Klemperer, whom I see quite a lot. [cell phone goes off]
Pavlish:I just got this new phone and did not think to turn it to silent. Is this cell phone technology surprising to you?
Ramsey:Oh, no. I’m used to it. Everybody has these different tones. In any case, the tour of some of the various applications you can see by title. Particularly of interest are the series of measurements on the neutron magnetic moment which of special interest to nuclear and particle physicists. Then there are the experiments on parity, time reversal symmetry and electric dipole moments. The first paper in this field was 3.1 by Purcell and me. I continued to do a series of experiments in that region with greater and greater accuracy. Our present limit of the neutron electric dipole moment, dn is |dn|<3.0x10-26 ecm. Because of its very low value it is complementary to some of the expected results with the large hodron collider, LHC.
Pavlish:Your labs were here in this building?
Ramsey:Yes, they were on the ground floor of this building and in the basement. The neutron experiments were at the I.L.L. neutron source in Grenoble, France. “Parity, time reversal, and the electric dipole moment”: those are important papers. They are still of interest. “Theories of nuclear magnetic shielding and electron chemical shifts”: I’ve already spoken about that.
Pavlish:We discussed that today.
Ramsey:There is quite a bit on theories of nuclear interactions in molecules: we have already discussed that to some extent. Vibrational and centrifugal effects, for example, are calculated. In paper 5.3, I describe how we were able to measure the distribution of the electrons in hydrogen. We could actually get a measurement of that by virtue of the resonance method.
Pavlish:These were of relevance to chemists as well, especially the papers in section 4 and section 5?
Ramsey:Right. This paper that you spoke about is, “Interactions of nuclear spins in molecules.” Two nuclear spins can interact by coupling to two nuclear spins in between. Publication 6.1 is a very important paper that was mostly ignored at the time, on thermodynamics and statistical mechanics at negative absolute temperatures.
Pavlish:That is related to Hahn’s work with the spin echo?
Ramsey:No. It is related to Charlie Townes’ later work on the maser. In fact, Townes and I have had recent discussions on this. It turns out that this paper and the one that I mentioned with Bob Pound on negative absolute temperatures are the first experimental observations of amplification by stimulated emission of radiation. Pound, Purcell, and I did this. I described it in terms of temperatures. Namely, that negative temperatures give amplifications. Townes wrote a book, a couple of years ago, called, How the Laser Happened. I read it, and was startled by one thing. He gives reference to our work, then says that we had done some work but that our levels were too low to get significant amplification. That was wrong. We did get amplification. We had an exchange of correspondence about a year ago in which we finally agreed. He agreed that we did have amplification by stimulated emission of radiation. But, we did not call it that; we called it “Effects at negative temperatures”. The curve that is published particularly the one published by Pound and Purcell, shows amplification. Publication 6.1 is a theoretical paper based on that and some other things I found.
Pavlish:That is another interesting case of how language affects science; that what you call a scientific finding matters.
Ramsey:That’s right, we called it something different. We did not call it “amplification of stimulated emission of radiation” because that term had never been used. We called it negative absolute temperatures and showed our observations were due to amplification by stimulated emission.
Pavlish:I found in looking at magnetic resonance in the 1930s that it was possibly used in accelerators, in California, that they would do experiments in accelerators and call it magnetic resonance?
Ramsey:Magnetic resonance? I think it was more electric resonance. The acceleration is usually done with oscillating electric fields. Part 6 includes this paper I mentioned, with Furry. Also my paper on thermodynamics and statistical mechanics at negative absolute temperatures, a paper I wrote before the maser. Publication 6.3 describes a fundamental search for possible new particle forces, done by molecular beam measurement of a molecule; you can see what the interaction is in between two protons. It turned out to be very accurate due to the magnetic moments, which we had also measure.
Pavlish:That is 6.3?
Ramsey:Yes. That kind of magnetic interaction is called a tensor force. This publication says that there is a limit to any tensor force that might contribute other than magnetic. It could be an even smaller. It is the only limit that exists so far.
Pavlish:That is theoretical or experimental?
Ramsey:That is an experimental paper with a theoretical interpretation.
Pavlish:May I test out an idea?
Pavlish:People often talk of experiment and theory in physics. It seems that someone like you defies this categorization: you are both a theorist and an experimentalist. You call yourself more an experimentalist?
Ramsey:I call myself an experimentalist who is very interested in theory, particularly theories relating to my experiments.
Pavlish:What do you think of talking about physics in terms of four categories: calculation, characterization, measurement, and demonstration?
Ramsey:All are important, especially having the first new idea, which can come from any of the phases or elsewhere. Usually a simple calculation comes early along with planning the experiment. Then making the measurement and reporting it.
Pavlish:Yes, calculation, characterization…what the atom looks like, physical reasoning.
Pavlish:Demonstration: to show, for instance how you were passing the magnet around in the lab.
Ramsey:I would tend to think of it more as a combination of in the first place, ideas, ideas of what to do and then some theoretical work as part of the ideas. Then, there are the experiments and finally, the interpretation of the experiments.
Pavlish:So, ideas, theory, experiment, and interpretation.
Ramsey:The sequence is often having the idea, some theory, experiment, more theory and interpretation.
Pavlish:Would someone like Rabi have agreed with you on that, do you think?
Ramsey:I think so, yes.
Pavlish:In another interview you mentioned that people did very good work under Rabi for one of three reasons: either because they loved him, or because his ideas were so good, or because they were annoyed at him.
Pavlish:You experienced all three of these?
Ramsey:I experienced the three of them; the annoyance, rather less than most.
Pavlish:Your students or collaborators had similar experiences with you?
Ramsey:I don’t think they had much annoyance with me. We did share ideas with each other. So we were primarily collaborators. I tended to encourage them to have their own ideas for experiments, as well as doing them. I do not think that any of my students were particularly afraid of me. I was not afraid of Rabi either, but some people were. Other people were often annoyed by him; they usually got over it. After Rabi died, CERN, the place with the Large Hadron Collider (LHC), had a special commemoration for Rabi, in appreciation for what he had done in getting CERN started. Rabi and I had been involved in getting Brookhaven started. And then, Rabi was on an international committee [
Pavlish:UNESCO?] on physics, particularly in Europe. After that meeting (I was not at the meeting) friends of mine, said, “What is this American chauvinist Rabi all about?” Brookhaven was already getting big accelerators here but the Europeans said they could not do so because they were from different countries. Rabi said that it is just as easy to get different countries to collaborate, as it is to get different universities to collaborate. He said that it was the Europeans fault if they could not do something about it. This got them so mad that they did get together and they did get CERN started. In retrospect, after a few years, they recognized that their success was largely because they were mad at Rabi.
Pavlish:Meanwhile, you moved on with Fermilab.
Pavlish:How about any of your teachers at Cambridge? Were you afraid of any of them? Thomson? Rutherford?
Ramsey:I guess, a little of Dirac. I was not afraid of him, but I had trouble understanding the way he spoke at that time. He essentially read his book in class. We did not get much supplement, any information Later he shifted to being much more physical in his description. No, I was not afraid of him.
Pavlish:You seemed to register recognition when I mentioned Peter Galison’s article about how Dirac would do the geometrical reasoning in private and the mathematical in public. Did you actually see some of that?
Ramsey:If I can remember exactly what it was, I have to think about it. Oh yes. The best example I know of is when he became enamored of a demonstration of why it is that in the case of spins of particles, you have to do a 720 degree rotation to get it back to the original state. He could demonstrate that by tying some strings to an object using scissors. In any case, he showed that if you rotate it once, 360 degrees and have strings tied to it, the strings get snarled, but if he did it 720 degrees he could unsnarl it.
Pavlish:He actually did that demonstration?
Ramsey:Oh, he loved that demonstration. He used it to demonstrate the properties of spins.
Pavlish:He had a physical piece of paper and strings, or he just talked about it?
Ramsey:He had a scissors, string, and demonstrated.
Pavlish:You thought physically but you never actually constructed geometrical paper models?
Ramsey:I did not usually use paper models, but I did a lot to describe vectors using my hands.
Pavlish:I noticed that in our interviews as well, you are very clear about the spins. That helps for understanding.
Ramsey:Yes. I describe the spins using my fingers.
Thank you very much it is always an honor.