Mr. Chairman, ladies and gentlemen, Nobel Prize-Träger, civilians and students, I am very sorry that I cannot give my talk in German.

I spent two years in Germany, let's just say in German-speaking countries, if you can count Bavaria, as was suggested,

and Switzerland, somewhat more doubtful, and I don't know what people think of Hamburg, but this was my introduction to German, a time which I learned a great deal, enjoyed very much.

After my first year, I first came over on a Columbia University fellowship, and then when I learned that I was going to stay another year and had another fellowship, I said to Professor Otto Stern, now I must really learn German.

Ach, he said, das wäre reiner Luxus. So taking him at his word, the best I could do was a language we contrived in Hamburg called molecularstrahldeutsch.

When I spoke from this podium three years ago, I spoke of a general subject of education, the future, the meaning of life, or what direction to go, the meaning of life, the role that science must play in education and so on.

It apparently made a very great impression on the students, because just at that time the serious troubles with students began in the United States and Germany and all over. So this time I thought I would pick a more innocuous subject,

namely 60 years of molecular beings. I'm delighted to be following Professor Mossbauer, because he illustrated as well as he could in his faultless Bavaria, which I'm proud to say understood every word. I don't know why.

How difficult science is. It has been said by some great man, das Leben ist zeitraubend und kostbelig. And now the same can be said of science. And I like this picture of a student spending a hundred years

getting various points and so on. I think the time may come. As it ensures back to Methuselah that a hundred years would be very easy. The subject of molecular beings started in 1911

with an experiment by Dinweye, in which he showed an apparatus for which I make no slide, but it would be clear from the words I say, that when he had a source of atoms, and I think in that case it was sodium, and interposed certain slits, he got sharp shadows

if he had a suitable vacuum which these atoms could traverse. And in this sense he showed, for the first time really, that this was a stream of particles, not a hydrodynamic stream, but a stream of particles.

Now look that from this distance sixty years later it seems to be an experiment that was hardly worthwhile doing. But you must remember even the mighty Mach, Ernst Mach, died, I think in the twenties,

without believing in the existence of atoms. This was a live subject at that time. We go on for another ten years and we come to the career of Otto Stein. And by the way I did not hear it mentioned

that Otto Stein died about a year ago and I hope you will all remember the name and his works. And the reason I'm giving this talk about sixty years of molecular beings is to a great degree of remembrance

of this very great German physicist who unfortunately had to leave his country and its culture and went to America, where he never did succeed in entering a new culture.

And I regard it as a great personal tragedy for this great man and an enormous loss for science. Because beginning when he entered the field of molecular beings after a rather distinguished career as a theoretical physicist, he built a new technique,

a new way of approaching basic questions in physics. And not only through a technique, but applying the technique to basic discoveries. I will talk about them for a while and also from what followed.

But I thought at this particular time it is well to remember some German scientists who may have been forgotten in their own countries and not sufficiently remembered in the countries to which they went. And another such personality I'd like to mention at this time,

who was here I believe at the last meeting of physicists, and that's James Franck, who had this great career in Göttingen and then in the United States. To come back to my subject and an illustration of the theme,

which was started by Professor Mosbach. In 1920 Outerstein with a genius apparatus was able to measure and demonstrate the distribution of velocities in the stream of atoms. Now the Maxwell distribution of velocities

had been known for more than a half a century. But enormous calculations had gone on in theory about velocity distribution. But it was never really demonstrated. It was long through a long chain of reasoning and it could have been otherwise.

Well, he showed this velocity distribution and started the subject on this way. Right early in his career in developing molecular beams, he together with Gerlach did the famous Stern-Gerlach experiment. I'd like to wax a little bit philosophical at this point,

because I want to be understood from my point of view, and I'm not talking to my colleagues of the Nobel Prize, but to the students. From my point of view, space quantization was discovered in this experiment.

It had been suggested earlier, but all sorts of suggestions are made in physics, as we know. Most of them wrong. The discovery is an actual one. It was assumed after the earth was round

that there could be other land masses, but until Columbus was there and found it, it was a discovery. I think Stern-Gerlach discovered the space quantization. Furthermore, and it wasn't very much noted at the time, it was contrary to the predictions

which were made by Sommerfeld and others. There was no central line. Now let me, before I get away from it and speak to the civilians a bit, I should say first what is a molecular beam and secondly how these discoveries were made.

A molecular beam to begin with is a stream of atoms confined in a narrow space moving in the same direction and not interacting. It's the simplest possible case in which you can think of a physical system.

They're moving in vacuum, not interacting. Under such circumstances, one can subject these atoms to various forces and get results which could be unique and simple. The only problem in the technique,

and Professor Maspar will appreciate this, would be in the first place to have a source of sufficiently high intensity and secondly to have a detector of sufficient sensitivity. And now we're right back to the previous talk.

Actually, this is the basis of all physical advance. Whenever you make any study of a physical situation, first of all you must have a source of the material which you're going to work with and secondly you must have a detector.

Now, when you have a stream of atoms, they're not energetic. They're just the same energy as the ordinary temperature. It might be room temperature or a few hundred degrees and that's all that's to it. Well, an atom of that sort doesn't do very much. So when you set up your experiment

and you subject the atom to various forces, how do you know what happened? That's the problem of detection. Well, the earliest experiments were done with detection by deposition. These atoms would fall on some surface, discolor it or make a chemical reaction

and you observe this with a microscope or if you can take a picture and put it through a densitometer and try to get the distribution of atoms. These were the first experiments and then this was the experiment which Stan did to get the velocity distribution

and later on Stan and Gerlach to demonstrate space quantization and to show the magnitude of the magnetic moment of the atom. The results were quite astonishing but the element of astonishment there

was lost in the rapid surge of theoretical ideas because as we see it now, long before the concept of the spinning electron, they really had discovered that the spin of the electron, at least of the silver atom, is one half because they had those two lines

and actually that there was space quantization and indeed had the magnitude of this effect. I'm talking to you now of 50 years ago and those of you who have a taste for history, if you read back to the literature of that period,

the unit of magnetism was not the Bohr magneton that we talk about now, but the Weiss magneton, which was something quite different and enormous researches had been done over a period of years to determine that unit of magnetism, the Weiss magneton.

And this was substantiated in its magnitude the suggestion that had been made from Bohr's theory that the unit of magnetism for an atomic electron would be the magnetism of a circuit which had a unit of angular momentum. The circuit of electrons had a unit of angular momentum.

So you can see at this point it was a momentous discovery, a really momentous discovery. Space quantization, the magnitude of the electron and basically the spin, which had to be one half, because before it would be, if it were one,

there would be three states, one, zero, minus one. They would have had three lines. Instead of that, there were only two lines with nothing in the middle. There was one other thing which impressed me and that's why I like to talk about this. Shortly after this discovery, I was in the position of most of the people back there.

I was a graduate student. And the quantum theory, not quantum mechanics, the quantum theory was very young and very strange. And at Cornell University, where I was studying at the time, people found it very hard to believe this strange theory, this strange combination of classical theory

and some selection that was put in to pick the stationary states. And no reasonable person could really believe it, because it was contradictory. And I've always described this as the state of the human mind where you can compartmentalize and be very expert in classical mechanics and then say, well, it's no good anyway,

and we'll select for arbitrary reasons certain states according to certain principles. And many people very ingeniously tried to construct classical models of the atom. But when one saw space quantization in this experiment of Stern and Gerlach,

that ends such speculation and shows the full miracle of nature, because here are atoms coming out of a source, which is just some sort of oven, higgly-piggly, colliding and so on, and they come out. And you put an external force, a magnet, that pulls them to one side.

Those that are oriented in one direction go to one side. Those oriented in the other direction go to the other side. But how did they get themselves oriented to begin with? Because they came out quite disorganized. This experiment, more than almost anyone, displays for you the miracle of the laws of nature,

of now we can call it quantum mechanics, that somehow or other in all this the separation occurs. Now, I think Einstein first raised the question, how does this happen?

Atoms coming out of a source, colliding every which way, and yet, when subjected to this field, are oriented either one way or another. There certainly was not enough time to radiate away their energy or to receive energy from the completely random directions which they had.

And this was only solved later on through the event of quantum mechanics. And the authors are here, Heisenberg and Dirac. Bohr, unfortunately, is not. To show that these states originally were oriented in the sense that they were specific states,

of all the possible states, they were specific states, and had to come out this way. What I want to describe to the students now is the impact on such an experiment on me as a graduate student looking forward to make a career, being struck all at once with the force of this experiment,

that there is no point in being ingenious classically to find the inside of the atom, because here is something that was so simple, so overwhelming, that you felt you had to face a new world. I'll go on and describe some other work

of Stan and his laboratory. I'll describe it first by making a suggestion to you. The work of this laboratory, with the exception of the first experiments, is published in the Zeitschrift für Sieg under the symbol UZM series,

Untersuchungen zur Molekularstrahlung. And they go from 1 to 30, at which time Otto Stern had to leave Germany. I had the honor of being number 12, and an American colleague, John Taylor,

was number 9 in this series of Untersuchungen. And it's one of the most brilliant series of papers which has occurred in the history of experimental physics. I've just described this measurement of the magnetic moment of atoms.

Another beautiful experiment, which he initiated in his school, immediately after the De Broglie suggestion, was to get the refraction, the scattering of not electrons, which had been done by Davies and Germer,

but atoms. And they succeeded most brilliantly in showing the wave nature of the hydrogen atom, the hydrogen molecule, and the helium molecule. Now, once you can scatter and diffract

and show the wave nature of hydrogen, then of course it holds for all matter. You could just hope someday to, if you're foolish enough, to scatter a grand piano off a grating. Again, now this is a rather obvious thing.

But at that time you could have a specific property of the electron, which made it wave-like, but doesn't hold for aggregates and doesn't hold for something where you have a center of mass. These became so accurate that they used a velocity selector, tooth wheel and so on, velocity selector.

The measurements were so accurate that one time when they did not match with the De Broglie relationship, they looked back and they had counted number of teeth and the velocity selector were wrong by one. I'll say the most important thing for Otto Stein. He never in his life did an experiment

because it was doable. Every one of his experiments was an important experiment, important in the sense that it would make a great difference in his way of thought, one way or the other, so that he was always on the forefront of these matters. I'll go on to...

I won't give any more fuller descriptions of the problem that when an atom is quantized in a certain direction, what changes its direction, the form which it had to change its direction, this direction. I called it in my experiments flopping, but he had a much better word.

Umklappen was the German word for this. Perhaps the most important experiment from the point of view of modern physics of particle physics was the last experiment he did before he left Germany. And that was a measurement

of the magnetic moment of the nucleus. The electron magnetic moment had been measured in his laboratory. And the magnetic moment of the electron as had been predicted, and really quite accurately by Dirac, was given by Eh over 4mc.

But this is the electron mass. Now he wanted to measure the magnetic moment of the proton. And almost any theorist he spoke to will say this is not a terribly useful measurement, because we know what that should be.

It should be Eh over 4m, the proton, c4 pi. Well, nevertheless he did measure it. It was an extraordinary measurement, and difficult. And to everybody's great surprise,

well, it was even consternation, because nobody tried to explain it for quite a while, it turned out three times as large, approximately three times as large as had been expected from basic theory, which showed that the proton was far from being elementary

in the sense that the electron is, but must have a structure of some mysterious kind. Later than that I can make the experiment and measure it, and the proton has a positive charge. But of course his experiment being symmetrical did not determine the sign.

I later on found a way to determine the sign. And people first thought it would be a foolish experiment. But then I said it's anomalous anyway. You don't know what causes the extra moment. It could just as well be negative.

This was about the last of this series of experiments which were done there. I won't give the whole list, but these were done over a period from 1926 to 1933.

This was a heroic period, and the molecular beam method was established and also shown to be able to tackle and illuminate problems of the most fundamental nature. Since I'm talking in general terms,

I'll perhaps give some personal reminiscences of how I happened to come to this laboratory. And I did carry on some of the work that was started by Atul Stern. After I got my degree from Columbia University,

I had a small fellowship. And having been fascinated by the developments of the ideas of physics, I had read with tremendous excitement the theoretical papers which were coming out at that time. De Broglie, Heisenberg, Schrodinger, Dirac.

I felt I had to come to Europe, which was the source of all this knowledge and inspiration. Because in the United States, and I'm talking now about the period in 1927, at my university, Columbia University, the only people who had read the papers of quantum mechanics were the graduate students,

none of the professors. It's quite different from now, but a little sense of history. So I had to come to Germany and to Europe in general, in Germany in particular,

to come at the source of this knowledge. By the way, talking about the first papers in quantum mechanics, I advise all of you to read Professor Heisenberg's first paper on quantum mechanics to show what tremendous advances

can be made if one does not try to be logical and does not know too much mathematics. That paper is an absolute classic and nothing gives me more joy than to look back on it from time to time. My daughter was thinking

of going into science and I told her she'd better go into law because her mind was too logical. Well, I had to come to Europe and I had a small fellowship, very small, and I went around first to Zurich,

where Schrodinger was. He left the day after I arrived. I had nothing to do with one another. Then I went to Schrodinger, had a wonderful time there, and saw something of the organization of German teaching in theoretical physics.

Professor Sommerfeld was a man of enormous dignity, although he was short. But his dignity was not in proportion to his height. And he had this nice large office. Next to him was the office of the assistant. This was Herb Beckert. And then there was a small room

in which the graduate students sat and the visitors, like myself. The journals were all in Professor Sommerfeld's room. So there was quite a ceremony. If you wanted to consult a journal, you had to go and ask Herb Beckert. He'd knock on the door and so on. You'd come in and find your journals.

Amongst the people at that time who were graduate students, by the way, were none less than Hans Bethe, who was a particularly brilliant graduate student, Professor Piles, Rudolf Piles, and Unsold, now the theoretical astronomer

was an assistant there with Sommerfeld. Unsold just had a fellowship to go to Pasadena. And he was very meticulous in true German fashion of what should he wear, what should he take, how should he act, and so on. It was complete. And finally Sommerfeld said to him,

Nehmen Sie das nicht so ernst. Das Leben in America ist gar nicht so schwer. Da wird jeder jüngere Mann ein assistant professor. I don't know what it... It cheered him up. Amongst the Americans there was Professor Condon

and whose books you must all have seen and H.P. Robertson, who died a few years ago and was a great expert on relativity. Well, from Sommerfeld I went to Copenhagen and just to show you how beautiful things were at one time. I had not written.

We came to Copenhagen. I checked my bag, bought a map. My wife and I walked to the institute. I rang the bell. The secretary answered and I said, my name is Robbie. I've come to work here. She gave me a key to the institute, suggested a panzeon, left my wife at the panzeon and came back and started to work.

There was no soul present. Nobody. They were all on vacation. This was in September. Finally some people came along. But the upshot of it was and this is the part Fortune plays in men's lives. When Bohr came back he was very tired and was arranged that Nishina the Japanese physicist the Klein-Nishina formula

and I go to Hamburg to work with Poway. Now, this was the most fortunate thing in my life because at Hamburg there were Poway and of course Otto Stern. Professor Lenz he was the ordinarious and one of the most brilliant minds

I've ever counted. He's not well known in the history of physics but there was no question about his brilliance. A younger man by the name of Walter Gordon the Klein-Gordon equation and an American Englishman who were working with Otto Stern. Well I worked along with

Poway but I had a fundamental necessity to talk English so we spoke to my American and British friend Ronald Fraser John Taylor and at that time I had some idea of an experiment to do and Professor Stern invited me to do this experiment.

Well I didn't come to do experiments but I was told this was a great honor. Being a fresh Ph.D. with no job and a very precarious fellowship I was no position to refuse an honor and that's how I got to work in molecular beams.

But I want to describe the informality and in a certain sense the brotherhood which existed amongst the physicists. I had the good fortune there that Stern Poway and Lenz were all bachelors. So for about a year I lunched

with them every weekday. It was a tremendous education not only in physics but in the whole social question besetzungsfragen and so on in German universities things of that sort. And after I had done my experiment I went straight to Professor Heisenberg

in Leipzig and joined his theoretical seminar. I was not terribly proficient in ping pong so I don't think I made a permanent impression there on the social community. But on the other hand I made some impression on Professor Heisenberg because he got me my job at Columbia

for which I'll be ever grateful. And from Heisenberg left for America and from there I went on to Pauli who by this time had transferred to Zurich. I'm taking this time to give you the story of a wandering American

not known at all in the centers of learning how easy it was to come in with just on your own with your name, with your interest and join the community. I think it's probably very different from what it is now and it's a great loss.

Well I came back to New York and with this job in theoretical physics which Professor Heisenberg got me. And I taught almost all the theoretical physics courses but I discovered that when I sat down to do basic theoretical physics all the ideas I had

were in a solid state. I was not interested in a solid state then to tell you the truth now. So I decided that I can do experiments and started again on this molecular beam technique which I had learned in Hamburg. Now I resume the thread of

the story and my time is almost up. A great advance in technique had been made by Taylor in Stern's laboratory at a suggestion of Langmuir. And that is the phenomenon of surface ionization that some metals particularly alkaline metals striking a proper surface

at the proper temperature let's say tungsten wire will re-evaporate as ions. This was a fairly quantitative affair. So all at once the sensitivity of the whole technique was increased by an enormous factor instead of having a visible

deposition you could now count almost atom for atom for the techniques which were prevalent at that time. Once you can do that it broadens the whole situation and you can do very sensitive experiments and to some degree your problem of

intensity is very much ameliorated because you can use lower and lower intensities. Is this machine working? Or is this my antenna? I see. You can use lower and lower intensities. And with these means we could greatly extend the

method and ask other questions. For example the atom in the atom you had the electron spin, the spin and the angular momentum of the extra nuclear things but in addition to that the nucleus was shown by various arguments which I wouldn't give now

that the nucleus could have an angular momentum and then which would result in hyperfine structure. Someone can take up this whole field of the studying of hyperfine structure and even get to the point where nuclear moments themselves could be measured. And this we did

through a long series of experiments of many nuclei and things of that sort. And in this way one could keep with this technique starting with the most gentle sort of atoms moving along with the own velocities, one could keep up with the very rapidly moving and fascinating field

of nuclear physics which was at that time the high road in which physics went. But in addition one could further stand initial experiments on the moments of nuclei, the moments of the proton, the moment of the deuteron

and questions of that sort. Reviving an old idea because we could having these atoms in their simple states moving along and not interacting with others, one could influence them very sensitively by radio waves and there again

since I'm going a long history I won't go into details but it was quite possible to measure some of these moments, nuclear moments, so hyperfine separations in the nuclear moments to a higher degree of accuracy than you could measure an ordinary magnet with its own

magnetic moment, I mean accuracies of ten to the sixth, ten to the seventh could be achieved. And in this way there was measured the magnetic moment of the of the proton and the magnetic moment of deuterium

and through the hydrogen atom and through the measurement of the deuterium atom. And a discrepancy was found in this and that discrepancy led Schwinger to suggest that the magnetic moment of the electron

was not a simple Bohr magneton as had been expected but was actually influenced by the properties of the vacuum which were not simple properties of the vacuum. At the same time using the molecular beam technique Willis Lamb

and Rutherford through a brilliant experiment, brilliant both in experimental execution and theoretical insight demonstrated in another way that the levels of hydrogen again are not what would come from straight Dirac theory of the hydrogen atom

but were somewhat different and he could measure this difference and from that you had the whole growth of the quantum electrodynamics extension of the quantum electrodynamics which was originally started by Heisenberg and

Pauli and Dirac to take full account of the vacuum and extended the power of field theory vastly beyond what it had been before. So I'm only about halfway through the 60 years of molecular

beams but to show that the growth of a technique in source and in sensitivity and so on made it possible to explore these very profound questions. While all this was going on, other people were using accelerators with

millions of volts to bombard the nuclei and learning a great deal about nuclei. I described the two as one system which sort of very gently observed the Taj Mahal and the other which bombarded it and then tried to construct it from the pieces which came off from the cannon balls.

All this time there were most interesting chemical questions to be discussed of elementary chemical questions just the structure of molecules and others. We tried to do some of these but at that time somehow the chemists

were not interested. This is a chemical question like firing one beam of molecules or atoms at another getting the cross section or getting what comes off. Some of this was done in our laboratory at Columbia but it had to wait until

after the war for the subject to be tackled in a really serious way. And this is a rather philosophical point. What war does, we all know that there are important scientific advances that come as a result of war and

high energy physics. And the reason is this, the military asked for the impossible but they're willing to pay for it. And the result is great advances in technique and the methods. The same is true of high energy physics also recently. They asked for the impossible in the way

of sensitivity, counting, discrimination and so on. And again they were able to pay for it. So we've had in recent years, last 20 years an enormous increase in the possibility of observation. And to some extent you can say that particle physics, present-day particle physics, is to some degree

a byproduct of this fantastic development of observations, which has been possible. As an example of this, in our experiments at Columbia we'd buy a vacuum pump for eight and a half dollars. Now it could be a hundred times that or two hundred times that. Of course it's

much better vacuum. They're much more sophisticated. And again all sorts of things can be done. For example a molecular beam traversing a vacuum can be of such weak intensity that there's only one atom in the beam in the apparatus at any instant. And all the other atoms are there to

distinguish one from the other by characteristics. But now you can increase a vacuum of 10 to the minus 10th. Not terribly unusual. Which makes possible all sorts of experiments where you can bombard one molecular species with another molecular or atomic species and

get the results of what comes off. So I come back to the point I made earlier. The Wissenschaft ist kuschspielig. These new methods cost a lot of money. And I'm sorry the president of Bavaria wasn't here for me to explain to him that

inherent in the actual progress of physics is the supply of funds. I once spoke to Powell who discovered the mesons other than the mu mesons. And with very simple means he exposed plates on top of a mountain.

And so he was very proud of being able to do so very cheaply. And I asked him, suppose you had to construct a mountain. The price would have been different. And initially, initial experiments are always very cheap. Otherwise you wouldn't have found it. It's what follows later on when you want to get more

precise information. For right now it may be that the use of atomic and molecular beams to go further in these fundamental questions of physics and particle physics may indeed be over. But a new era has started in the application to physical chemistry and the nature

of reactions. I have with me a paper, a review paper, molecular beams and chemical reactions. It's a long paper. And I will just read the conclusion to you to show that this is a subject which is very much alive. And it's a 1971

subject. And I read the conclusion. Molecular beam experiments, the interpretation of these experiments and the theoretical studies of reactive collisions are bringing new information to chemical kinetics. For example, for sample reactions we can now list

values of such detailed quantities as the total cross-section parameters for the interaction potential for the reactants, the threshold energy, distance and impact parameter, the probability of reaction as it varies with B and V sub r, I won't explain the terms,

the reaction cross-sections as it varies with energy and orientation and the partitioning of energy between the internal and the translational energy of the products. This is a start at looking inside the thermal distributions of kinetics to discover more of what happens in

reactive collisions. I finish this way with chemistry because in a sense this has been the history of physics. It starts with physics and more or less ends with chemistry as far as technology is concerned. It starts with physics in one direction and ends

with philosophy in another direction. Physics to my mind is the central core of our knowledge of the universe and also provides both the techniques, the instrumentation and the tools

for investigation of all the sciences. And I just finish by commending this thought to the students who are present and wish you careers of happy hunting. Thank you.