The Genius of Science: A Portrait Gallery

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Abraham Pais is the author of the definitive biographies of Albert Einstein and Niels Bohr. Indeed, his biography of Einstein was the winner of the 1983 American Book Award and was selected by The New York Times Book Review as one of the Best Books of the Year. As a distinguished theoretical physicist who was a friend and colleague of Einstein and Bohr, Pais is able to blend a sophisticated understanding of physics with a first-hand knowledge of the private individual, offering us insights into both. It is this ...
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Abraham Pais is the author of the definitive biographies of Albert Einstein and Niels Bohr. Indeed, his biography of Einstein was the winner of the 1983 American Book Award and was selected by The New York Times Book Review as one of the Best Books of the Year. As a distinguished theoretical physicist who was a friend and colleague of Einstein and Bohr, Pais is able to blend a sophisticated understanding of physics with a first-hand knowledge of the private individual, offering us insights into both. It is this unique double perspective that makes his work so valuable.
Now, in The Genius of Science, Pais offers us insightful portraits of twelve of our century's most distinguished physicists, all of whom he has known personally. We meet, among others, the famously taciturn Paul Dirac; Max Born, who coined the term 'quantum mechanics'; Wolfgang Pauli, famed for his exclusion principle and known as the conscience of twentieth-century physics; Mitchell Feigenbaum, inventor of chaos theory; and John von Neumann, one of the most influential mathematicians of the century. Other scientists profiled include Res Jost, Isidor Rabi, Viktor Weisskopf, and Eugene Wigner. In addition, because their work is so relevant to the others discussed, Pais has included chapters on Einstein and Bohr, in each case giving the essence of the man's character and scientific achievement.
Throughout the volume, Pais illuminates the personalities and achievements of these stellar scientists. The result is virtually a who's who of 20th-century physics--a superb collection of portraits that sheds light on the physicists, their work, and their lasting influence on science.
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Editorial Reviews

Giovanni F. Bignami
In covering a wide range ot topics, from relativity to chaos theory, Pais' life-and-science stories provide, above all, the staple on which young (and not so young) scientists should feed their ambitions. Alternatively, we can just enjoy and be enriched, both culturally and emotionally.
Publishers Weekly - Publisher's Weekly
Physicist Pais won an American Book Award for his 1983 Einstein bio, Subtle Is the Lord; here he offers short, memorable, avowedly subjective sketches of the lives, accomplishments and personalities of 16 men whose work drove modern physics. "I have known all of them personally," Pais remarks; sometimes, delightfully, he brings his reminiscences to the fore. The very likable Niels Bohr--who helped discover the structure of the atom--kept up friendly professional quarrels with Einstein and struggled, often fruitlessly, to articulate his subtle thought to lay audiences. (Pais has also penned a full-length Bohr bio; Bohr, Copenhagen and Denmark's Bohr Institute provide a sort of center from which Pais draws anecdotes and recollections about several later figures, among them Res Jost and Oskar Klein.) Mitchell Feigenbaum, who helped unfold the mathematics of chaos, attributed his most important discoveries partly to his primitive programmable calculator. The calm and magisterial George Uhlenbeck wanted to be a historian--until he discovered that electrons have spin. Einstein himself turns up for a brief essay, as do game theorist John von Neumann, Wolfgang Pauli of exclusion principle fame and Eugene Wigner, who applied mathematical group theory to quantum mechanics. Pais assumes his readers know at least some of the relevant physics--the volume shouldn't attract, and doesn't seek, an audience of novices. Instead, Pais assembles admiring, enjoyable tales about physicists, just as many previous writers have compiled tales about painters and composers, for aficionados, professionals and students of the discipline. (June) Copyright 2000 Cahners Business Information.|
Library Journal
What makes this book unique is Pais's acquaintance with the 20th-century physicists (e.g., Niels Bohr, Max Born, Paul Dirac, Wolfgang Pauli, Mitchell Feigenbaum, John von Neumann, and Albert Einstein) featured in this collection of short biographies. Unlike standard studies, each of these biographies contains personal reminiscences, giving readers a sense of what the subject was like independent of his scientific achievements. Known for his American Book Award-winning Subtle Is the Lord: The Science and the Life of Albert Einstein, among other works, Pais is a noted physicist in his own right. Recommended for people interested in the history of science, this book is suitable for the general reader at public or academic libraries.--James Olson, Northeastern Illinois Univ., Chicago Copyright 2000 Cahners Business Information.\
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Product Details

  • ISBN-13: 9780198506140
  • Publisher: Oxford University Press, USA
  • Publication date: 4/28/2000
  • Pages: 368
  • Product dimensions: 9.30 (w) x 6.20 (h) x 1.00 (d)

Meet the Author

Abraham Pais is Detlev W. Bronk Professor Emeritus at The Rockefeller University in New York City. He is also the author of 'Subtle is the Lord...' (for which he won the American Book Award), Inward Bound, and Niels Bohr's Times.

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Read an Excerpt

Chapter One

Niels Bohr, the man and his science


Winston Churchill has written in Volume 1 of his History of the English Speaking Peoples:

No one can understand history without continually relating the long periods which are constantly mentioned to the experience of our brief lives. Five years is a lot. Twenty years is the horizon ... Fifty years is antiquity.

    These words fittingly describe the times of Niels Bohr's life, which has spanned revolutionary changes in science itself as well as making a dramatic impact on society. As a first example, consider what Federico Mayor, Director General of UNESCO, wrote only a few years ago:

In October 1988 an immense power of destruction was, perhaps for the first time in human history, being dismantled precisely because of the enormity of the risk it entailed for our species.

Now compare this with the state of affairs at the time of Bohr's birth, when practical applications of atomic energy, for good or evil, were not even visible on the far horizon. In fact, as the curtain rises, the reality of atoms is still under debate, the atomic nucleus is not yet discovered. All that changed during Bohr's life, much of it under his own influence. He was the first to understand how atoms are put together, he played a leading role in the development of the theory of the atomic nucleus, and he was influential in creating nuclear medicine at his Institute. He was also the first to bring to the attentionof leading statesmen the need for openness between West and East, a need resulting from the advent of formidable new weapons developed during and after the Second World War. Time and again he would stress that openness was essential for political world stability.

    Even more profound than new discoveries and perceptions regarding the structure of matter are new physical laws discovered in the same period. Here the key concepts are relativity theory and quantum theory. Bohr was the principal figure in elucidating the revisions of the philosophical foundations of physics for a comprehension of quantum phenomena.

    After this bird's eye view of Bohr's scientific legacy, I turn to an account of Bohr the man and the scientist.

* * *

On the morning of October 24, 1957, Robert Oppenheimer and I took an early train from Princeton to Washington. We were on our way to the Great Hall of the National Academy of Sciences, where, that afternoon, the first Atoms for Peace Award was to be presented to Niels Bohr. It was a festive event. James Killian read the award citation, from which I quote:

Niels Henrik David Bohr, in your chosen field of physics you have explored the structure of the atom and unlocked many of Nature's other secrets. You have given men the basis for greater understanding of matter and energy. You have made contributions to the practical uses of this knowledge. At your Institute for Theoretical Physics at Copenhagen, which has served as an intellectual and spiritual center for scientists, you have given scholars from all parts of the world an opportunity to extend man's knowledge of nuclear phenomena. These scholars have taken from your Institute not only enlarged scientific understanding but also a humane spirit of active concern for the proper utilization of scientific knowledge.
In your public pronouncements and through your world contacts, you have exerted great moral force in behalf of the utilization of atomic energy for peaceful purposes.
In your profession, in your teaching, in your public life, you have shown that the domain of science and the domain of the humanities are in reality of single realm. In all your career you have exemplified the humility, the wisdom, the humaneness, the intellectual splendor which the Atoms for Peace Award would recognize.

    Killian then presented the award (a gold medal and a check for $75 000) to Bohr, while a smiling President Eisenhower looked on. In his brief response Bohr stressed the need for international understanding: `The rapid advance of science and technology in our age ... presents civilization with a most serious challenge. To meet this challenge ... the road is indicated by worldwide cooperation.'

    Next, the President addressed Bohr, calling him `a great man whose mind has explored the mysteries of the inner structure of atoms, and whose spirit has reached into the very heart of man.'

    Killian's citation eloquently describes that combination of qualities we find in Bohr and only in Bohr: creator of science, teacher of science, and spokesman not only for science per se but also for science as a potential source for the common good.

    As a creator he is one of the three men without whom the birth of that uniquely twentieth-century mode of thought, quantum physics, is unthinkable. The three, in order of appearance, are: Max Planck, the reluctant revolutionary, discoverer of the quantum theory, who did not at once understand that his quantum law meant the end of an era in physics now called classical; Albert Einstein, discoverer of the quantum of light, the photon, founder of the quantum theory of solids, who at once realized that classical physics had reached its limits, a situation with which he never could make peace; and Bohr, founder of the quantum theory of the structure of matter, also immediately aware that his theory violated sacred classical concepts, but who at once embarked on the search for links between the old and the new, achieved with a considerable measure of success in his correspondence principle.

    How different their personalities were. Planck, in many ways the conventional university professor, teaching his courses, delivering his PhDs. Einstein, rarely lonely, mostly alone, who did not really care for teaching classes and never delivered a PhD, easily accessible yet so apart, ever so friendly yet so distant. And Bohr, always in need of other physicists, especially young ones, to help him clarify his own thoughts, always generous in helping them clarify theirs, not so much a teacher of courses nor a supervisor of PhDs but forever giving inspiration and guidance to so many engaged in post-doctoral and senior research, father figure extraordinary to physicists belonging to several generations, including this speaker.

    Bohr's researches, his teachings, his endeavors in the political sphere, and his relations with other major figures of his time—these are among the themes to be developed in this paper. But there are more. There is also Bohr the philosopher, the administrator, the fund raiser, the catalyst in promoting physical applications to biology, the helper of political refugees, the co-founder of international physics institutes as well as the nuclear power projects in Denmark, and, last but not least, the devoted family man. A composite picture will emerge of a life so full and dedicated that one wonders how a single individual could have managed so much.

    Bohr's spectrum of activities was broad; the intensity with which he attacked whatever task lay before him was high. All who knew him well were aware of his immense powers of concentration which one could often note simply by looking at him. A story will illustrate this.

    Bohr's aunt, Hanna Adler, once told me of an experience she had long ago when she sat in a Copenhagen streetcar together with Bohr's mother and the two young sons, Harald and Niels. The boys were hanging on their mother's lips as she was telling them a story. Apparently there was something peculiar about these two young faces in concentration, for Miss Adler overheard one lady in the streetcar remark to her neighbour, `Stakkels mor' (that poor mother).

    As I have been told time and again, those who have read recollections and biographies of Bohr often tend to come away with the overall reaction that his life story is too good to be true. I, too, believe that his was a wonderful life and that he was a good man, capable of both bringing and receiving happiness. I do not consider him, however, as an angelic figure to whom struggle, ambition, disappointment, and personal tragedy were alien.

    Which brings me to some personal recollections.

* * *

In January 1946 I arrived in Copenhagen for the first time from my native Holland, the first of the post-Second World War generation to come to Bohr's Institute from abroad for a longer period of study. The morning after my arrival I went to the secretary, Mrs Betty Schultz, who told me to wait in the library where she would call me as soon as Professor Bohr was free to see me. I had sat there reading for a while, when someone knocked at the door. I said come in. The door opened. It was Bohr. My first thought was, what a gloomy face.

    Then he began to speak.

    Later I have often been puzzled about this first impression. It vanished the very moment Bohr started to talk to me that morning, never to return. True, one might correctly describe Bohr's physiognomy as unusually heavy or rugged. Yet his face is remembered by all who knew him for its intense animation and its warm and sunny smile.

    Soon thereafter I had my first opportunity to talk physics with Bohr. I told him of problems in quantum electrodynamics I had worked out during my years in hiding in Holland. While I was telling him what I had done, he smoked his pipe; he looked mainly at the floor and would only rarely look up at the blackboard on which I was enthusiastically writing down various formulae. After I finished, Bohr did not say much, and I felt a bit disheartened with the impression that he could not care less about the whole subject. I did not know him well enough at the time to realize that this was not entirely true. At a later stage I would have known right away that his curiosity was aroused, as he had neither remarked that this was very interesting nor said that we agreed much more than I thought—his favorite ways of expressing that he did not believe what he was told.

    Bohr had in fact become quite interested in what I had told him. One day in May he asked me whether I would be interested in working with him on a daily basis during the coming months. I was thrilled and accepted. The next morning I went to Carlsberg. The first thing Bohr said to me was that it would only be profitable to work with him if I understood that he was a dilettante. The only way I could react to this unexpected statement was with a polite smile of disbelief. But evidently Bohr was serious. He explained how he had to approach every new question from a starting point of total ignorance. It is perhaps better to say that Bohr's strength lay in his formidable intuition and insight rather than in erudition. I thought of his remarks of that morning some years later, when I sat at his side during a colloquium in Princeton. The subject was nuclear isomers. As the speaker went on, Bohr got more and more restless and kept whispering to me that it was all wrong. Finally, he could contain himself no longer and wanted to raise an objection. But after having half-raised himself, he sat down again, looked at me with unhappy bewilderment, and asked `What is an isomer?'

    The first subject of work was the preparation of Bohr's opening address to the International Conference on Fundamental Particles to be held in July 1946 in Cambridge, England. I must admit that in the early stages of our collaboration I did not follow Bohr's line of thinking a good deal of the time and was in fact often quite bewildered. I failed to see the relevance of remarks such as, for example, that Erwin Schrödinger was completely shocked in 1926 when he was told of the probability interpretation of quantum mechanics, or references to some objection by Einstein in 1927 which apparently had no bearing whatever on the subject at hand. It did not take very long before the fog started to lift, however. I began to grasp not only the thread of Bohr's arguments but also their purpose. Just as many sports players go through warming-up exercises before entering the arena, so Bohr would relive the struggles which had taken place before the content of quantum mechanics was understood and accepted. I can say that in Bohr's mind this struggle started afresh every single day. Einstein appeared forever as his leading spiritual sparring partner; even after Einstein's death Bohr would argue with him as if he were still alive.

    Sometime later the Bohr family went to their summer house in Tisvilde. I was invited to stay with them, so that the work could continue. It was a wonderful experience. A good deal of the day was spent working in a separate little pavilion on the grounds. All during this period, his son Aage Bohr joined in as well. We would go for a swim in the afternoon and often work more at night. In fact after Aage and I had retired, Bohr would still come in sometimes, in a shoe and a sock, to impart to us just one further thought that had occurred to him that very minute, and would keep talking for an hour or so.

    Other evenings were spent in the family circle, and sometimes Bohr would read one or more of his favorite poems. He liked especially to quote the following lines by Schiller:

... Wer etwas Treffliches leisten will,
Hätt gern etwas Grosses geboren,
Der sammle still und unerschlafft
Im kleinsten Punkte die höchste Kraft.

These lines have been translated as

Ah! he would achieve the fair,
Or sow the embryo of the great,
Must hoard—to wait the ripening hour—
In the least point the loftiest power.

Like everything Bohr did, large or small, he was able to put his whole being into it and he could convey beautifully how small the point was and how lofty the power.

    Bohr was an indefatigable worker. When he was in need of a break in the discussions, he would go outside and apply himself to the pulling of weeds with what can only be called ferocity. At this point I can contribute a little item to the lore about Bohr the pipe smoker. It is well known that to him the operations of filling a pipe and lighting it were interchangeable but the following situation was even more extreme. One day Bohr was weeding again, his pipe between his teeth. At one point, unnoticed by Bohr, the bowl fell off the stem. Aage and I were lounging in the grass, expectantly awaiting further developments. It is hard to forget Bohr's look of stupefaction when he found himself holding a thoughtfully lit match against a pipe without bowl.

    Bohr devoted tremendous effort and care to the composition of his articles. However, to perform the physical act of writing, pen or chalk in hand, was almost alien to him. He preferred to dictate. On one of the few occasions that I actually did see him write, Bohr performed the most remarkable act of calligraphy I shall ever witness.

    It happened during that summer in the pavilion in Tisvilde, as we were discussing the address Bohr was to give on the occasion of the tercentenary celebration of Newton's birth. Bohr stood in front of the blackboard (wherever he dwelt, a blackboard was never far) and wrote down some general themes to be discussed. One of them had to do with the harmony of something or other. So Bohr wrote down the word harmony. It looked about as follows:


However, as the discussion progressed, Bohr became dissatisfied with the use of harmony. He walked around restlessly. Then he stopped and his face lit up. `Now I've got it. We must change harmony to uniformity.' So he picked up the chalk again, stood there looking for a moment at what he had written before, and then made the single change:


with one triumphant bang of the chalk on the blackboard.

    In the fall of 1946 Bohr and I were in Princeton, where I helped him prepare a lecture. Bohr was not a good public speaker, yet he was a man of the greatest lucidity of thought. Nor was that entirely due to the fact that Bohr's voice did not carry far, which made it impossible to hear him at the back of a large audience. The main reason was that he was in deep thought as he spoke. I remember how that day he had finished part of the argument, then said, `And ... and ...,' then was silent for at most a second, then said, `But ...,' and continued. Between the `and' and the `but' the next point had gone through his mind. However, he simply forgot to say it out loud and went on somewhere further down his road. To me, the story was continuous as I knew precisely how to fill in the gaps Bohr had left open. And so it has come to pass more than once that I have seen an audience leave a talk by Bohr in a mild state of bewilderment, even though he had toiled hard in preparing himself in great detail. Still, when he would come up to me afterwards with the characteristic question: `Jeg håbber det var nogenlunde' (I hope it was tolerable), I could assure him that it was much more than that. In spite of linguistic shortcomings, this unrelenting struggle for truth was a powerful source of inspiration.

    My own first direct experience of the impact of Einstein on Bohr happened in 1948, at the Institute for Advanced Study in Princeton. At that time Bohr was a temporary visitor there, I a permanent member. One day Bohr came to my office. He was in a state of angry despair and kept saying `I am sick of myself,' several times. I was concerned and asked what had happened. He told me he had just been downstairs to see Einstein. As always they had got into an argument about the meaning of quantum mechanics. And, as remained true to the end, Bohr had been unable to convince Einstein of his views. There can be no doubt that Einstein's lack of assent was a very deep frustration to Bohr. It is our good fortune that this led Bohr to keep striving at clarification and better formulation. And not only that: it was Bohr's own good fortune, too.

    Another day Bohr came into my office and started as follows: `Du er så klog ...' (you are so wise). I started to laugh (no formality was called for with him) and said: `All right, I understand.' Bohr wanted me to come down to his office and talk. We went there, and it should be explained that Bohr at that time used Einstein's own office in Fuld Hall. At the same time, Einstein himself used the adjoining small assistant's office: he had a dislike of the big one, which he did not use anyway. After we had entered, Bohr asked me to sit down (`I always need an origin for the coordinate system') and soon started to pace furiously around the oblong table in the center of the room. He then asked me if I could note down a few sentences as they emerged during his pacing. At such sessions, Bohr never had a full sentence ready. He would often dwell on one word, coax it, implore it, to find the continuation. This could go on for several minutes. At the moment the word was `Einstein.' There was Bohr, almost running around the table and repeating `Einstein ... Einstein ...' It would have been curious sight for someone not familiar with him. After a little while he walked to the window, gazed out, repeating every now and then: `Einstein ... Einstein ...'

    At that moment the door opened very softly and Einstein tiptoed in. He indicated to me with a finger on his lips to be very quiet, an urchin smile on his face. He was to explain a few minutes later the reason for his behavior. Einstein was not allowed by his doctor to buy tobacco. However, the doctor had not forbidden him to steal tobacco, and this was precisely what he set out to do now. Always on tiptoe, he made a beeline for Bohr's tobacco pot, which stood on the table at which I was sitting. Meanwhile Bohr, unaware, was standing at the window, muttering `Einstein ... Einstein ...' I was at a loss what to do, especially because I had at that moment not the faintest idea what Einstein was up to.

    Then Bohr, with a firm `Einstein', turned around. There they were, face to face, as if Bohr had summoned him forth. It is an understatement to say that for a moment Bohr was speechless. I myself, who had seen it coming, had felt distinctly uncanny for a moment, so I could well understand Bohr's own reaction. A moment later the spell was broken when Einstein explained his mission and soon we were all bursting with laughter.

    The periods of closest contact which I had with Bohr are those mentioned so far. In subsequent years I saw him often, either in Denmark or in the US, but no longer for protracted periods of time.

    In the fall of 1961 we were both present at the Solvay Congress in Brussels. It was the 50th anniversary of the first one, and Bohr gave an account, both charming and fascinating, of the developments during that period. He was present at the report I gave at that meeting, after which we talked in the corridor and spoke of the future of particle physics. It was the last time I spoke to him.

Descent and early years

Niels Bohr descended from an upper class background. His father was professor of physiology at the University of Copenhagen, its rector during 1905-6, and was twice proposed for the Nobel Prize in physiology or medicine. His mother hailed from a Jewish banking family. Her father was co-founder of two major Danish banks and a member of parliament. Niels was born on October 7, 1885, at Ved Stranden 14, one of Copenhagen's handsomest mansions, then the home of his maternal grandmother. He had an older sister, Jenny, and a younger brother, Harald, who became a renowned mathematician. All who knew Niels in his early years have recalled a close-knit, harmonious, and stimulating family. When I saw the brothers together, it struck me that one could see in Harald's face—but not in Niels'—some traits of his Jewish ancestry. Soon after Niels' birth the family moved to the professorial apartment at Bredgade 62, where he would live until he received his doctorate.

    In his school years Niels was tall and strong like a bear. During his adolescence he would occasionally beat-up classmates. He did well in school but was not ambitious. From early on he showed special gifts for mathematics and physics, was not good at foreign languages, but very good at Danish. In physical exercises he was one of the best, especially at soccer. His brother was even better at that, becoming a member of the Danish soccer team that won the silver medal at the 1908 Olympics.

    There is a charming story about Bohr and soccer. After he had been appointed professor in Copenhagen, he presented himself at a public audience to the King, as is the Danish custom. Dress: morning coat and white gloves, the latter not to be removed when shaking hands with the monarch. Accordingly Bohr called on Christian X, a rather stiff military type. I have it on good authority that this event went about as follows. After the introduction the King said he was pleased to meet the famous soccer player Bohr, whereupon Niels replied something like, I am sorry but Your Majesty must be thinking of my brother. The King was taken aback since according to the rules of the game one does not contradict the monarch during a public audience. So Christian started all over again, saying how pleased he was, etc. Bohr now became very uncomfortable and replied that indeed he was a soccer player, but that he had a brother who was the famous soccer player. Whereupon the King said, `Audiensen er forbi' (the audience is over), and Bohr took his leave, walking backwards, as custom demands.

* * *

In 1903 Bohr entered Copenhagen University. He had chosen physics as his major, astronomy, chemistry, and mathematics as minor subjects. His chemistry teacher has recalled that Bohr was second to none in breaking glassware. Oh, that must be Bohr, he is said to have remarked when one day the laboratory was rocked by explosions.

    However that may be, Bohr's first scientific publication describes some lovely physics experiments he had performed. In order to execute these he had to cope with a complication. The University had no physics laboratory. So he did this work in his father's physiology laboratory. He dictated the resulting paper to Harald, the first example of his life-long routine to do the work himself but let others do the writing.

    In 1910 Niels obtained his master's degree, and in May 1911 he publicly defended his doctor's thesis wearing the customary white tie and tails. A newspaper noted that the majority of those attending were soccer players.

    The thesis is entitled: `Studies on the electron theory of metals.' It is an extension of the classical theory initiated by Lorentz. Of particular interest is his inevitable failure to explain classically some paradoxes related to the Hall effect. About these he stated:

It does not seem possible at the present stage of the development of the electron theory to explain the magnetic properties of bodies from this theory.

It could just be that these experiences encouraged him to push into areas beyond, into the mysteries of quantum physics, as he was soon to do.

    The year 1911 can be said to mark the end of the first phase of Bohr's life. Meanwhile the second phase had begun. In 1909 he first met his future wife, Margrethe Norlund. They were married on August 1, 1912. The finest comment on the meeting of Niels and Margrethe was made shortly after Niels' death by Richard Courant, friend of the Bohrs for many decades:

Some people have speculated about the lucky circumstances which combined to make Niels so successful. I think the ingredients of his life were by no means matters of chance but deeply ingrained in the structure of his personality ... It was not luck, rather deep insight, which led him to find in young years his wife, who, as we all know, had such a decisive role in making his whole scientific and personal activity possible and harmonious.

I have been privileged to know Margrethe well. She was a most charming yet formidable lady.

Bohr, father of the atom

In 1911, Bohr sent the following letter to the Carlsberg Foundation:

    In English: `The undersigned takes the liberty of requesting a travel stipend of 2500 kroner for one year's study at Universities abroad.' The amount requested—and received—may seem puny, but remember that since that time the Danish kroner has suffered deflation by a factor of about forty. More striking is the fact that neither a vita nor a research proposal was appended, showing that Bohr was already sufficiently known by the powers that be.

    The following September Bohr arrived at the Cavendish Laboratory in Cambridge, hoping to work under the guidance of J. J. Thomson. I have been told that their first meeting went roughly as follows. He entered the office, opened J. J.'s book Conduction of Electricity Through Gases, pointed at a certain formula and politely said: `This is wrong.' About that encounter Bohr said, later in life:

It was a disappointment that Thomson was not interested to learn that his calculations were not correct. That was also my fault. I had no great knowledge of English and therefore I did not know how to express myself. And I could say only that this is incorrect. And he was not interested in the accusation that it was not correct ... Thomson was a genius who actually showed the way to everybody. Then some young man could make things a little better ... The whole thing was very interesting in Cambridge but it was absolutely useless.

    Then, while still attached to Cambridge, Bohr met Rutherford and his whole life changed.

* * *

Times were not yet ripe for tackling the theory of atomic structure when the twentieth century began. As one physicist has written: `It is perhaps not unfair to say that, for the average physicist of that time, speculations about atomic structure were something like speculations about life on Mars—very interesting for those who like that kind of thing, but without much hope of support from convincing scientific evidence and without much bearing on scientific thought and development.' The first major step toward clarifying this issue was Rutherford's discovery that the bulk of the atomic mass resides in a tiny central body, the nucleus. That work was published in May 1911—half a year before Bohr met Rutherford for the first time. Which goes to show once again that it is not enough to be smart, one also has to be at the right place, Rutherford's laboratory in Manchester, and at the right time, March 1912, when Bohr moved there.

    Rutherford was the most important scientific figure in Bohr's life, not just because his discovery of the nucleus led to Bohr's most important work, his discovery of the structure of the atom as a whole, but also because Rutherford's personal and scientific style profoundly influenced Bohr. Later he said of him: `To me he had almost been like a second father.'

    Rutherford had stated that an atom consists of a nucleus plus electrons orbiting around it, either unaware or unconcerned of the paradoxes this picture posed. Bohr's early thoughts on that problem are contained in a memorandum sent to Rutherford on July 6, 1912. The most important line in this document concerns his realization that its understanding demands a new `hypothesis for which there will be given no attempt at a mechanical foundation (as it seems hopeless),' (my italics). He had realized that atomic stability cannot be understood on the basis of classical physics but that one somehow needed the young quantum theory.

    Later in July Bohr returned to Denmark to get married, and to take up a junior position at the University. Friends have characterized him at that time as: `Somewhat introvert, saintly, extremely friendly, yet shy ... an incessant worker, always in a hurry, serenity and pipe smoking came later.'

* * *

It was only in February 1913 that Bohr realized the need for spectroscopic data to reach his goal. This resulted in a paper, published in July, that marks the birth of quantum dynamics. His main piece of information was the divine guess made by Johann Balmer, a Swiss schoolmaster, who in 1885 had proposed his formula

vmn = R]1/n2 -1/m2],

(n = 1, 2, 3, ... m > n and integer) for the hydrogen spectrum. Already in 1885 the `Rydberg constant' R was known to one part in ten thousand, correct to one part in a thousand:

R = 3,2916 x [10.sup.15] inverse secs.

    Now to Bohr's paper on hydrogen. He begins by noting that, according to the classical theory, `the electron will no longer describe stationary orbits,' but will fall inward to the nucleus due to energy loss by radiation. Then he plunges into the quantum theory. His first postulate: an atom has a state of lowest energy, the ground state, which by assumption does not radiate, one of the most audacious hypotheses ever introduced in physics. His second postulate: higher `stationary states' of an atom will turn into lower ones such that the energy difference E is emitted in the form of a light quantum with frequency v given by E = hy (h is Planck's constant).

    I shall not discuss in detail Bohr's quantum constraint on the hydrogen orbits. Suffice it to say that it is equivalent to the one we learned in school: the orbital angular momentum L is restricted to the values

L = n(h/2[Pi]), n = 0, 1, 2, ...

Not only does the Balmer formula follow at once, but R now appears in terms of fundamental constants

    R = (2[Pi]2e4m/h.sup.3]) inverse secs.

This prediction of R, `inside the experimental errors in the constants entering in the expression for the theoretical value,' is the first triumph of quantum dynamics and the most important equation that Bohr derived in his life. It represented a triumph over logic. Never mind that discrete orbits violated the laws of physics then known. Nature had told Bohr that he was right anyway and had advised him that a new logic was called for which should also explain the serious conflict of the Bohr theory with classical causality: how does an electron choose beforehand to which stationary state it shall pass when emitting a light quantum? All these issues were clarified after 1925 when quantum mechanics arrived. Bohr was of course well aware of all these problems. As was written of him at the time of his death: `The tentative character of all scientific advance was always on his mind, from the day he first proposed his hydrogen atom, stressing that it was merely a model beyond his grasp. He was sure that every advance must be bought by sacrificing some previous certainty and he was forever prepared for the next sacrifice.'

    Bohr's work of 1913 caused a veritable explosion of activity in quantum physics, in Europe as well as in the United States. As he wrote to Rutherford, `The whole field has from a very lonely state suddenly got into a desperately crowded one.' I must restrict myself to Bohr's own contributions during the next few years. These are: his formulation of the correspondence principle which states, roughly speaking, that for large wave lengths the theory should be in formal accord with classical mechanics and electrodynamics; his prediction of the ratio of Rydberg constants for singly ionized helium and hydrogen, agreeing with experiment to five significant figures; his formulation of selection rules for electric dipole transitions; and his proof that the chemical properties of the elements are largely determined by the configuration of the outermost shell of electrons, which may be said to make him the founder of quantum chemistry. He also struggled with one of the several problems which his theory could not cope with: the spectrum of helium, not understood until 1926.

    The best characterization of Bohr's activities during those years was given in 1949 by the 70-year-old Einstein: `That this insecure and contradictory foundation was sufficient to enable a man of Bohr's unique instinct and tact to discover the major laws of the spectral lines appeared to me as a miracle—and appears to me as a miracle even today. This is the highest sphere of musicality in the sphere of thought.'

    Those years of struggle in the clair-obscur left an indelible mark on Bohr's style, once again best expressed by Einstein: `He utters his opinions like one perpetually groping and never like one who believes to be in the possession of definite truth.' As Bohr himself often used to say, never express yourself more clearly than you think.

Bohr as administrator and fundraiser

In April 1916 Bohr was appointed to the newly created chair of theoretical physics in Copenhagen. On March 3, 1921 his own Institute for Theoretical Physics (the later Niels Bohr Institute) was formally opened. Soon physicists from far and wide came to work there, the world's leading center in theoretical physics during the 1920s and 1930s. The international character of the enterprise was manifest from the start. By 1930 some 60 physicists, hailing from Austria, Belgium, Canada, China, Germany, Holland, Hungary, India, Japan, Norway, Poland, Romania, Switzerland, the United Kingdom, the United States, and the USSR had spent time in Copenhagen. By the time of Bohr's death the number of visitors who spent at least a month in Copenhagen had risen to over 400. At the opening ceremonies of the Institute Bohr had stated its main theme to be: `To introduce a constantly renewed number of young people into the results and methods of science,' covering not only theory but also a lively experimental program, all directed and supervised by Bohr himself, who also personally oversaw several extensions of his Institute's buildings. Among the mighty contributions produced in Copenhagen by those young people I note: Heisenberg's uncertainty relations; Dirac's transformation theory, the Dirac statistics, and his first paper on quantum electrodynamics; Frisch and Meitner's theory of nuclear fission; and the experimental discovery of a new element, hafnium, named after Copenhagen. Moreover, beginning in 1929, Bohr organized a series of international conferences at his Institute, remembered as the most outstanding gatherings of their kind during these years. No wonder that in those days he was called `director of atomic theory.'

    So did Bohr fulfill his tasks as teacher and administrator. But that was still not all. Bohr also became his own raiser of funds, from Denmark and abroad. When in 1923 he came to the United States to negotiate funding by the Rockefeller Foundation, The New York Times called him `A modern Viking who comes on a great errand ... Working with Dr. Bohr is regarded by scientists as working with the foremost of the exponents of the new atomic physics, which is revolutionizing science.' Bohr is unique, I believe, in being able to combine all these activities with an intense and most important research program of his own at the frontiers of physics. He worked under strains which stretched his formidable physical strength to the limit—and beyond. A few times, overexertion would force him to take some weeks' rest.

* * *

In 1922 Bohr received the Nobel Prize. These days, such awards make front page news in the world press. It was not always like that. To find the first word of Bohr's prize in The New York Times, turn to page 4, the middle of column 2, of its November 10 edition to find, in its entirety, the following item:

Nobel prize for Einstein

The Nobel Committee has awarded the physics prize for 1921 to Albert Einstein, identified with the theory of relativity, and that for 1922 to Professor Neils [sic] Bohr of Copenhagen.

    At the traditional Nobel banquet, Bohr proposed `A toast to the vigorous growth of the international work on the advancement of science which is one of the high points of existence in these, in many respects, sorrowful times", words spoken shortly after the end of the First World War.


In 1925 quantum mechanics arrived. In March 1927 Heisenberg stated his uncertainty principle. On September 16, 1927, at the Volta Meeting in Como, Bohr enunciated for the first time the principle of complementarity, which embodies the physical interpretation of the uncertainty relations.

    From then on and for the rest of his life Bohr focussed on the language of science, the way in which we communicate. In 1927 he stated straight away his main theme:

Our interpretation of the experimental material rests essentially upon the classical concepts.

It sounds simple enough but is also most profound. Let me enlarge. In the classical era one verified the validity of theories by comparing them with experimental observations made with balances, thermometers, voltmeters, etc. The theories have been modified in the quantum era but—and this was Bohr's point—their validity continues to be verified by the same readings of a balance's equilibrium position, a thermometer's mercury column, a voltmeter's needle, etc. The phenomena may be novel, their modes of detection may have been modernized, but detectors should be treated as classical objects; their readings continue to be described in classical terms.

    `The situation thus created is of a peculiar nature,' Bohr remarked. Consider for example the question: can I not ask for the quantum mechanical properties of a detector, say a voltmeter? The answer is yes, I can. Next question: but should I then not abandon the limited description of the voltmeter as a classical object, and rather treat it quantum mechanically? The answer is yes, I must. But in order to register the voltmeter's quantum properties I need another piece of apparatus with which I again make classical readings. In Bohr's own rather cryptic words: `The concept of observation is in so far arbitrary as it depends upon which objects are included in the system to be observed.'

    These considerations led Bohr to introduce his refinement of language demanded by quantum mechanics. Thus he said (I paraphrase): the question of whether an electron is a particle or a wave is a sensible question in the classical context, where the relation between the object of study and detector either needs no specification or else is a controllable relation. In quantum mechanics that question is meaningless, however. There one should rather ask: does the electron (or any other object) behave like a particle or like a wave? That question is answerable, but only if one specifies the experimental arrangement by means of which `one looks' at the electron. That is what Bohr meant in Como, where he introduced the concept of complementarity:

An independent reality in the ordinary [that is, classical] physical sense can ... neither be ascribed to the phenomena nor to the agencies of observation ... The very nature of the quantum theory ... forces us to regard the space-time coordination and the claim of causality, the union of which characterizes the classical theories, as complementary but exclusive features of the description, symbolizing the idealization of observation and definition, respectively.

    Let me say what Bohr meant in my own words. Wave and particle behavior mutually exclude each other. The classical physicist would say: if two descriptions are mutually exclusive, then at least one of them must be wrong. The quantum physicist will say: whether an object behaves as a particle or as a wave depends on your choice of experimental arrangement for looking at it. He will not deny that particle and wave behavior are mutually exclusive but will assert that both are necessary for the full understanding of the object's properties.

    It can be said that with Bohr's paper of 1927 the logic of quantum mechanics as we know it today reached it closure. It was Heisenberg who coined the term der Kopenhagener Geist, the spirit of Copenhagen, for this fundamentally new interpretation of the foundation of quantum physics.

    Bohr's Como lecture did not bring down the house, however. He himself would later frown on expressions he used there, such as `disturbing the phenomena by observation.' Such language may have contributed to the considerable confusion that for so long has reigned around this subject. It has been said of his philosophical writings: `Against the sometimes maddening frustration brought about by these essays is the fact that nobody has succeeded in saying anything better in the 60 years since Bohr started talking about the problem.' To me these writings are not at all frustrating, however, since I have had the privilege of many discussions with Bohr about these problems.

    A month after Como the great masters gathered at the fifth Solvay Congress in Brussels. It was there that Einstein expressed for the first time his critical attitude toward quantum mechanics, which he maintained until his death. This forced Bohr to improve his language. His finest expose is found in the volume for Einstein's 70th birthday. I know that article so well because in Princeton I helped him prepare it. In this paper he recalled his position regarding the so-called EPR paradox, a misnomer since that paper is logically flawless. They simply concluded that Bohr's view is incompatible with the assumption that quantum mechanics is complete. Einstein's statement that quantum mechanics is `not reasonable' is his problem.

    Also in that Einstein book Bohr repeated the correct language which it had taken him 20 years to formulate that clearly: to define the term `phenomenon' to include both the object of study and the mode of observation:

Phrases often found in the physical literature, as `disturbance of phenomena by observation' or `creation of physical attributes of objects by measurements', represent a use of words like `phenomena' and `observation' as well as `attribute' and `measurement' which is hardly compatible with common usage and practical definition and, therefore, is apt to cause confusion. As a more appropriate way of expression, one may strongly advocate limitation of the use of the word phenomenon to refer exclusively to observations obtained under specified circumstances, including an account of the whole experiment.

* * *

I am now ready to state why I consider Bohr to be not only a major figure in physics but also one of the most important twentieth-century philosophers. As such he must be considered the successor to Kant, who had considered causality as a `synthetic judgement a priori,' not derivable from experience. Causality is, in Kant's own words, `a rule according to which phenomena are sequentially determined. Only by assuming that rule is it possible to speak of experience of something that happens.' This view must now be considered passé. Since Bohr the very definition of what constitutes a phenomenon has undergone changes that, unfortunately, have not yet sunk in sufficiently among professional philosophers.

    Again according to Kant, constructive concepts are intrinsic attributes of the `Ding an sich,' a viewpoint desperately maintained by Einstein, but abandoned by quantum physicists. In Bohr's words: `Our task is not to penetrate in the essence of things, the meaning of which we don't know anyway, but rather to develop concepts which allow us to talk in a productive way about phenomena in nature.' After Bohr's death Heisenberg wrote that Bohr was `primarily a philosopher, not a physicist,' a judgement that is arguable yet particularly significant if one recalls how greatly Heisenberg admired Bohr's physics.

* * *

From many discussions with Bohr I know that complementarity was his contribution most precious to him. There lay his inexhaustible source of identity in later life. He would not consider himself a philosopher, however, as witness his favorite definition of an expert and a philosopher. An expert is someone who starts out knowing something about some things, goes on to know more and more about less and less, and ends up knowing everything about nothing. Whereas a philosopher is someone who starts out knowing something about some things, goes on to know less and less about more and more, and ends up knowing nothing about everything. I like to think that Pascal's words, `To ridicule philosophy is truly philosophical,' might have appealed to him.

    Much has been written on how Bohr was influenced by his reading of various philosophers. I consider such speculations as far-fetched, to say the least. I do know, however, that he admired William James, and spoke with respect of Buddha and Lao Tse.

    Bohr gave much thought to extending complementarity to fields outside physics, being well aware that those ideas were mostly preliminary. The fields he probed were biology, where his ideas are now obsolete; human cultures where on the issue of nature versus nurture he emphatically sided with the latter; and psychology, where I consider his ideas of lasting value. Samples: `Words like thoughts and sentiments ... have since the origins of language been used in a typically complementary manner.' And: `It must be recognized that in any situation which calls for the strict application of justice there is no room for display of love and that, conversely, the ultimate exigencies of a feeling of love may conflict with all ideas of justice.' Such complementary modes of thinking have had a lasting and liberating influence on my own life.

    The principal and all-pervasive theme in these thoughts was the use of language. He was fond of telling the following story which he liked to apply to himself. Once upon a time a young rabbinical student went to hear three lectures by a famous rabbi. Afterwards he told his friends: `The first talk was brilliant, clear and simple. I understood every word. The second was even better, deep and subtle. I didn't understand much, but the rabbi understood all of it. The third was by far the finest, a great and unforgettable experience. I understood nothing and the rabbi didn't understand much either.' He also would often use some of his self-created aphorisms such as: It is not enough to be wrong, one must also be polite; or: Some subjects are so serious that one can only joke about them.

    Finally I mention Bohr's own succinct summary of his philosophy, where the emphasis is once again on the use of language.

There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature ... What is it that we humans depend on? We depend on our words. Our task is to communicate experience and ideas to others. We are suspended in language.

Bohr's role in nuclear physics and biology

On December 11, 1931, the Danish Academy of Sciences and Letters chose Bohr to be the next occupant of the Aeresbolig, the residence of Honor in the Carlsberg breweries. At about that time Bohr commenced the new task of redirecting his Institute toward the young field of nuclear physics, moving the experimental thrust from atomic spectroscopy to nuclear processes. That demanded a lot of money for construction, equipment, and operating expenses. This time Bohr the fundraiser pulled out all the stops. On April 25, 1938, the 25th anniversary of Bohr's completion of the hydrogen atom paper, the new laboratory space was inaugurated in the presence of King Christian X. Whereupon, a newspaper wrote: `The institute locks its doors again for publicity. One prefers to work unnoticed until results are available.' Those began coming in late 1938, when the Copenhagen cyclotron, which ranks among the first of its kind in Europe, began producing an intense source of neutrons generated by a 4 MeV deuteron beam. In 1939 another accelerator, this one of Cockcroft-Walton type, began producing 1 MeV neutrons. Also in that year plans took shape for building a 2 MeV van de Graaff, not used for scientific work until 1946.

    Meanwhile the miserable Nazi time in Germany had begun, leading Niels and his brother Harald to join the board of the Danish committee for support to refugee intellectuals. Niels found financial support for offering temporary hospitality to a number of physicists who, with few exceptions, were to rise to prominence elsewhere.

    As if all that was not enough, Bohr also managed, in the 1930s, to make an important contribution to the theory of nuclear reactions. These, he proposed, should be treated as a two-step process. In the first, the incoming projectile merges with the bombarded nucleus into a single unit, the compound nucleus, which is well separated in time from the second stage, the break-up of this compound. Hans Bethe has written: `The compound nucleus dominated the theory of nuclear reactions at least from 1936 to 1954 ... At Los Alamos when we tried to get [probabilities] the compound nucleus model could explain many phenomena.'

    Bohr's final contribution to nuclear physics (and his last major one to physics), made in 1939 at age 53, was his discovery that only the rare isotope U235 undergoes fission when uranium is bombarded with slow neutrons. This led to his well-known papers with Wheeler on fission theory.

    There is still more in the 1930s, to wit, Bohr's own role in promoting applications of nuclear physics to biology. In the 1920s he had attracted Georg von Hevesy to Copenhagen, where he (H.) made the first applications of isotopic tracers to the life sciences. Hevesy came back again in 1935-43 to continue his tracer studies, which led to applications to nuclear medicine, a discipline unquestionably founded by Hevesy, with Bohr acting as godfather.

    Finally, as if he did not have enough to do, Bohr was president of the Kongelige Danske Videnskabernes Selskab (Royal Danish Academy of Sciences and Letters) from 1939 until his death in 1962.

* * *

The Second World War changed Bohr's life profoundly. In 1943, warned of his impending arrest by the Germans, he escaped to England. To the end of the War he served as consultant for atomic bomb projects. For the rest of his life his main concern was the political implications of the new weapons, as you will hear from my friend Ove Nathan.

The final years

I turn to Bohr's final years, beginning with him at age 60, still in full vigor, still running up steps two at a time.

    Bohr has now reached the zenith of his influence. He has become a public figure of the first rank, he and Margrethe often being called Denmark's second royal family. High dignitaries visit his home. On his 70th birthday (1955) the King and Queen come to congratulate him, and the Prime Minister addresses the Danish people by radio to honor him. Foreign notables visit Carlsberg, among them Queen Elizabeth II and Prince Philip of England, the Queen of Siam, the Crown Prince (now the Emperor) of Japan, Jawaharlal Nehru (Prime Minister of India), David Ben Gurion (Prime Minister of Israel), Adlai Stevenson. When the newspaper Politiken asks its readers to vote on which men and women have put the greatest stamp on developments in Denmark during that era, Bohr tops the list.

    Along with these consuming activities, the Bohrs still found time for, and took pleasure in, receiving junior and senior physicists in their home.

    For some of the post-war years Bohr remained active in research but more and more he assumed the roles of elder statesman and senior philosopher. He continued writing on complementarity, gave many occasional addresses and traveled widely, to the United States, where in 1950 I helped him put finishing touches to his open letter to the UN, to Iceland, Israel, Yugoslavia, Greenland, India, the USSR. He was active in the founding of CERN, of NORDITA, and of laboratories at Risø, vibrant parts of the Bohr legacy.

* * *

On Sunday, November 18, 1962, Bohr died of heart failure in his Carlsberg home. Physicists and other friends, young and old, sent expressions of sorrow and sympathy to Margrethe Bohr and her family. So did dignitaries from various parts of the world.

    President Kennedy wrote to Mrs Bohr: `I am deeply saddened by Professor Bohr's death. American scientists, indeed all American citizens who knew doctor Bohr's name and his great contributions, have respected and venerated him for more than two generations ... We are forever indebted to him for the scientific inspiration he brought along on his many visits to the United States, and especially for his great contribution to the atomic center at Los Alamos. Please accept my condolences and deep sympathy.' Other messages included those from the Prime Minister of Israel, the King of Sweden, and the Chancellor of West Germany. At CERN the flags of all member nations flew at half mast. Bohr was eulogized at the United Nations in New York. At a UNESCO meeting in Paris a minute of silence was observed.

    Bohr's ashes were interred in the family grave in Assistens Kirkegaard in Copenhagen, where they now rest next to those he loved most: his wife (who died 22 years later), his parents, his brother Harald, and his son Christian.

* * *

After the death of Rutherford in 1937, Bohr's speech in his memory included these words.

His untiring enthusiasm and unerring zeal led him on from discovery to discovery and among these the great landmarks of his work, which will forever bear his name, appear as naturally connected as the links in a chain.
Those of us who had the good fortune to come in contact with him will always treasure the memory of his noble and generous character. In his life all honours imaginable for a man of science came to him, but yet he remained quite simple in all his ways. When I first had the privilege of working under his personal inspiration he was already a physicist of the greatest renown, but nonetheless he was then, and always remained, open to listen to what a young man had on his mind. This, together with the kind interest he took in the welfare of his pupils, was indeed the reason for the spirit of affection he created around him wherever he worked ... The thought of him will always be to us an invaluable source of encouragement and fortitude.

    I know of no better way of concluding than by applying these words to Niels Bohr himself.

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Table of Contents

Introduction 1
1 Niels Bohr 6
2 Max Born 30
3 Paul Dirac 48
4 Albert Einstein 78
5 Mitchell Feigenbaum 84
6 Res Jost 106
7 Oskar Klein 122
8 Hendrik Kramers 148
9 Tsung Dao Lee and Chen Ning Yang 172
10 John Von Neumann 184
11 Wolfgang Pauli 210
12 Isidor I. Rabi 264
13 Robert Serber 280
14 George Uhlenbeck 288
15 Viktor Weisskopf 326
16 Eugene Wigner 330
Onomasticon 353
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  • Anonymous

    Posted March 8, 2003

    People in science.

    What makes Pais' book especially compelling and captivating is that he knew the main players in Science over the period of a lifetime. And then the unique quality of his writing! The result is a page turner. We are given a glimps into the personal lives of Bohr, of Dirac, of Einstein, of von Neumann, of Pauli, and of others of the major profiles in science in the twentieth century;-- and Pais offers his own thoughts on their science as well. Based on having worked with them...Pais also wrote landmark biographies of Bohr and Einstein. Highly recommended.

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