Niels Bohr
Niels Bohr; drawing by David Levine

One achievement that stands apart, in the complex history of ideas of the twentieth century, is the development of our concept of the structure of matter. It was a steady development, which penetrated deeper and deeper into the inner structure of the atom, ever broadening with each step our understanding of material things. Modern scientific progress is usually described as a series of revolutions and upheavals, in which old ideas are destroyed by a new theory. This description, however, overlooks the fundamental fact that scientific development is intrinsically evolutionary. Indeed, each of the new “revolutionary” ideas in modern science was a refinement of the old system of thought, a generalization or an extension. Relativity did not replace Newton’s mechanics—orbits of satellites are still calculated with Newton’s theory—but extended its application to extreme velocities and established the general validity of a common conceptual basis for both systems of mechanics and electricity. Quantum theory came perhaps nearest to being revolutionary, but even its ideas, such as the uncertainty principle, must be considered as a refinement, an application of mechanics to very small systems; quantum theory did not change the validity of classical mechanics to the motion of bodies of larger size.

Although the steady and incessant growth of our understanding of material structure may have helped to steady the minds of the scientists who live in this century of upheaval, it clearly did not have that effect on society itself. Growth of this kind necessarily brings with it more and more ways of dealing with new materials—new forms of energy and new ways of using them. This again, necessarily, changes the quality of life at an ever-increasing rate. We are left at odds with our accepted system of values when we face the new human problems created by this change.

Nothing could be better suited to illustrate this problem than a study of the life of Niels Bohr. Bohr was a great physicist. He ranks with the greatest figures in the history of science, with Galileo, Newton, Maxwell, and Einstein. It was he who began the development of our concepts of the structure of matter and kept this development going for half a century. To a greater degree than any other scientist, he was involved in the human consequences of his science, in its impact on politics and society.

Bohr was born in 1885; his life as a scientist began about 1905 and lasted for fifty-seven years. In 1905 Einstein published his first paper on special relativity, only a few years after Planck’s great discovery of the quantum of action. Niels Bohr had the luck to be alive at this important moment, or perhaps it was mankind’s luck that he was present at this critical point in the history of science. What a time to be a physicist! Bohr began his work when the structure of the atom was still unknown, and ended it when atomic physics had reached maturity, when the atomic nucleus was put to industrial use for the production of electric power, to medical use in cancer treatment, and also, regrettably, to military and political use.

BOHR’S WORK CAN BE DIVIDED into four periods, all of which are marked by his influence on the development of atomic science. The first phase began with his meeting with Rutherford in 1912 and ended in 1922, when he founded the Institute of Theoretical Physics in Copenhagen. During this decade Bohr introduced the concept of the “quantum state” and created an intuitive method of dealing with atomic phenomena. During the second phase, from 1922 to 1930, he gathered around him in his new Institute several of the most productive physicists of the world who, under his leadership, developed the ideas of quantum mechanics. This is the conceptual structure, replacing Bohr’s original intuitive method, which gives an adequate description of the inner workings of the atom. During the third period, 1930-40, he applied the new quantum concepts to electromagnetic and later to nuclear fields, and the exploration of the structure of the atomic nucleus. Then came the Second World War and the last period of his life, in which he became deeply involved in the social, political, and human consequences of the new discoveries.

Let us return to the first phase which began with the publication, in 1913, of his work on the quantum orbits of the hydrogen atom. This remarkable paper proposed to explain the as yet unexplained properties of the atom by introducing into physics the completely new concept of the “quantum state.” Bohr’s ideas were based upon previous work by Planck and Einstein, which suggested that energy and light are emitted and absorbed in discrete units or bundles—quanta. He applied the idea of the quantum to the structure of the atom. Hardly any other paper in the literature of physics has generated so many new theories and discoveries. In the ten years following its publication, much that had not previously been understood fell into place: the structure of the spectra of elements, the process of absorption and emission of light, the reasons for the periodic system of elements, the puzzling sequence of properties of the ninety-two different atomic species. Quality, the specificity of chemical substances, was reduced to quantity, to the number of electrons per atom. All of this rested on Bohr’s assumption that the atomic orbits were quantized—i.e., that only certain specified patterns of orbits were admitted within the atom, at that time still a provisional hypothesis. Bohr’s contemporaries, however, took this assumption quite literally, although Bohr warned them both in his papers and at meetings that his could not be the final explanation, that something fundamental remained to be discovered before what was going on in the quantization of the atom could be properly understood.


From 1922-30 the quantum came to be fully understood. It was a period without parallel in the history of science, the most fruitful and most interesting eight years in modern physics. Bohr had found a new way of working; he no longer worked alone, but in collaboration. It was his great strength to have assembled around him at the Institute for Theoretical Physics the most active and gifted physicists of the world, including Klein, Kramers, Pauli, Heisenberg, Ehrenfest, Gamow, Bloch, Casimir, and Landau. This extraordinary collaboration produced the foundations of the quantum concept and first conceived and discussed the uncertainty relation. During this period, the particle-wave antinomy was for the first time understood. In lively informal discussions the deepest problems of the structure of matter were brought to light.

IT IS DIFFICULT to convey the excitement of intellectual life in Copenhagen at the time. Bohr’s career was then at its height. It was here that he created the “Kopenhagener Geist,” the special style which he imposed upon phyics. A giant among his colleagues, he was an equal in this group of optimistic, talented young people, approaching the deepest questions of nature in a spirit of freedom and joy. When I was a young man, I had the privilege of going to Copenhagen and I remember that I was somewhat startled by some of the jokes that crept into the discussions. They seemed to me disrespectful. When I mentioned my feelings to Bohr he replied, “There are things that are so serious that you can only joke about them.”

During this great period of physics Bohr and his small group of scientists touched the nerve of the universe. Man’s eyes were opened to the inner workings of Nature which before had been obscure. Once the fundamental tenets of atomic mechanics were established, it was possible to understand and calculate almost every phenomenon in the world of atoms, including atomic radiation, the chemical bond, the structure of crystals, the metallic state. Before that time, the environment was composed of unknown forces: electric, adhesive, chemical, and elastic. All of these were reduced to one—the electromagnetic force. In the course of only a few years, the basis was formed for a science of atomic phenomena, which grew into the vast body of knowledge available to us today. Never before was so much explained by so few people in so short a time.

A few words are appropriate here to stress the impact of these new ideas. Previously chemistry and physics were thought to be separate. Chemistry was the science of matter and its specific properties. The atom was a concept of chemistry—the atom of gold, of oxygen, of silver—different specific entities whose existence was noted, but not understood. Physics on the other hand, was a science of general properties, of motion, of strain and stress, of electric and magnetic fields. No one could yet answer the question: “Where do the specific properties of matter come from?”

The specificity of the atoms seemed a miracle. What prevented Nature from producing a gold atom which is slightly different from other gold atoms? Shouldn’t there be intermediate atoms which are not quite gold but halfway to silver? Why couldn’t there be a continuous change from gold to silver? What keeps all atoms of one species exactly alike: why are they not altered by the rough treatment they suffer when the material is heated or subject to other outside influences? This question was even more acute and disconcerting when Rutherford found out that the atoms are little solar systems with the atomic nucleus as the center sun and electrons circling around it as its planets. Such systems should be extremely sensitive to collisions and other disturbing influences.

Bohr saw that there was a connection between these atomic properties and quantum theory. He tried to formulate this connection by postulating the existence of “quantum states” characteristic of each species of the atomic system. Electrons can assemble around the nucleus only in certain well-defined modes—the quantum states. Under normal conditions the electrons of a specific atomic system would invariably assemble in that mode which has the lowest energy. It is a stable configuration, since any change would be possible only if enough energy could be supplied to reach the next quantum state, which is a definite step higher in the energy scale. It is that configuration which is responsible for the typical properties of the different kinds of atoms.


SO FAR THIS IS A FORMULATION only of the strange fact that atoms have specific qualities. But Bohr also showed how to calculate correctly the energy of these quantum states in certain simple cases. The real significance of this work emerged when it became clear that Bohr’s new concept was intimately connected with the dual nature of electrons, whose motions are sometimes observed as a particle motion, sometimes as a wave motion. This duality, one of the most far-reaching discoveries in modern physics, was predicted by Louis de Broglie in 1924, before it was confirmed by experiments. The quantum states turned out to be vibrations of electron waves confined to the space surrounding the nucleus, as was shown for the first time by the Austrian physicist Schroedinger in 1925. A most exciting situation: The specific atom states could be understood as harmonic vibrations of electron waves under the confining influence of the electric attraction of the nucleus. The specific properties of the elements were based upon a natural interplay of vibrations. The old dream of “the harmony of the spheres” seemed to be revived.

But this situation was also deeply disturbing. How can it be that electrons exhibit properties of both waves and particles at the same time? There seemed to be an irreconcilable contradiction between electronic particles revolving around the nucleus, and vibrating electronic waves. Obviously, the mere existence of atoms with well-defined specific properties points to something strange happening in the atom; otherwise little solar systems of electrons would not exhibit such behavior. The discovery of the wave-particle nature of the electron only reinforced this. Wouldn’t it be natural under these conditions to use the finest means of observation to find out what the detailed atomic structure is like, and finally settle the question as to whether the electron is a wave or a particle?

Bohr, however, approached this problem by refuting the view that the solution could be found by closer observation of the atom. For Nature is arranged in a way which makes this direct approach impossible, because no observation of so tiny an object as the atom can be made without influencing it. The quantum state has a peculiar way of escaping ordinary observation, because the very act of such observation would obliterate the conditions of its existence. The quantum state is a form of motion which cannot be divided into parts and followed up point by point, as we do when we describe the motion of a planet around the sun. It must be considered in its characteristic entirety and indivisibility. The quantum properties can unfold only when the atom is left undisturbed—when the perturbations to which it is exposed contain less energy than would be necessary to cross the threshold into the next quantum state. Here we find the atom with its characteristic properties, and it behaves like an indivisible entity. When we try to look into the details of the quantum state by some sharp instrument of observation, we necessarily pour much energy into it and destroy the quantum state. In fact, when an atom is given a large amount of energy, it behaves like an ordinary solar system. The characteristic quantum properties are lost. The necessary coarseness of our means of observation—light comes in quanta and so do all other forms of energy—makes “exact” observation in the old sense impossible. Hence the famous “uncertainty principle” formulated by Heisenberg in 1926, when he was working with Bohr in Copenhagen. The quantum state represents a novel state of matter which cannot be described in the old-fashioned way. This state exhibits features that do not occur with objects of our ordinary experience. This is why we must use more abstract terms when we describe atomic reality. It may seem incredible to the uninitiated that an electron behaves in certain situations like a wave, and in others like a particle. But this is just part of the reality we face in the world of atoms.

Bohr introduced the term “complementarity” for this complex state of affairs. He was so fascinated with this new mode of argument that he tried to apply it to other aspects of human thought. For example, the problem of free will can be looked at in a similar way. The awareness of personal freedom in making decisions seems a straightforward factual experience. But when we analyze the process, and follow each step in its causal connection the experience of free decision tends to disappear. A related “complementarity” is found in the well-known paradox of thinking about the thinking process, and also in the juxta-position of reasoning and acting. The legal approach to human problems often shows features contradictory to the human approach, which could be resolved by a “complementary” approach. Bohr, an enthusiastic skier, sometimes used the following simile, which can be understood perhaps only by fellow skiers. When you try to analyze a christiania turn in all its detailed movements, it will evanesce and become an ordinary stem turn, just as the quantum state turns into classical motion when analyzed by sharp observation.”

One of the most debated applications of complementarity was Bohr’s attempt to formulate, according to this principle the problem of life. In a celebrated talk given in 1933, he expressed the idea that the apparent contradictions between the phenomena of life and the laws of physics and chemistry could be seen in the light of complementarity in the following sense: any attempt to verify in all details the validity of physics in a living cell would necessarily kill the cell and destroy the very object of investigation. Thus living matter might be a new and different state of matter, which would not be at variance with the laws of physics, but would be outside their valid application. Recent developments in biology have made Bohr’s idea somewhat less attractive. It turned out that the phenomena of life may not be in such irreconcilable contradiction to the laws of physics as the quantum state was to classical physics. Bohr gave a second talk shortly before he died, in which he revised to some extent his original ideas.

WE HAVE SPENT much time in commenting upon the first and the second periods of Bohr’s life. Indeed, he reverted to the idea of complementarity for the rest of his life, trying to formulate it in new and better ways. After 1930 he turned to nuclear physics. Atomic properties had been explained by the peculiar vibrations which the electron waves make when they are confined close to the atomic nucleus by the electric attraction between the nucleus and the electrons. The same phenomenon was found to reappear again within the nucleus, albeit on a much smaller scale and on a much higher scale of energy. The nucleus is a system of protons and neutrons held together by a strong nuclear force. Again one finds quantum states of nucleus with their characteristic properties based upon vibrations of waves: here it is a matter of neutron and proton waves confined by the nuclear force. Nuclear structure presents impressive evidence for the general validity of quantum mechanics. Here the physicists discovered a repeat performance of the same principles on a new level, with a few characteristic differences: in the atom the nucleus dominates the electron motions or vibrations because of its big charge and its heavy weight. in the nucleus we face a republican regime where all constituents have equal mass and exert an equal force on each other. Bohr was the first to analyze the typical properties of nuclear quantum states stemming from this difference. We can only touch on this topic here; nevertheless, the following point should be mentioned for its importance in the history of science. The weight of Bohr’s personality was so great that for a decade the typical differences between nuclear and atomic states were at the center of interest, whereas the similarities between these states were pushed aside; it was as late as 1950 when Meyer and Jensen discovered the shell structure of nuclei analogous to the shell structure of atoms.

The fission of uranium was discovered when Bohr was deeply involved in his studies of nuclear structure. Obviously this phenomenon captured Bohr’s interest, and he wrote a fundamental paper on this process with the American physicist John Wheeler, which had a decisive influence on the development of nuclear energy.

When Bohr pursued a new problem he always used to find a “victim” among the younger physicists who happened to be in Copenhagen. This lucky man had the privilege of working with him day and night while Bohr tried to explain his ideas to the “victim” until they became clear to both of them. The most important part of this collaboration was the writing of a joint paper on the subject. The “victim” was supposed to write down the sentences while Bohr dictated. As the days went by, they worked away at each sentence until it expressed the desired idea. Indeed the ideas took their real shape only during those attempts of formulation. The interaction between thought and language always fascinated Bohr. He often said that any attempt to express a thought involves some change, some irrevocable interference with the essential idea, and this interference is all the stronger, the clearer one tries to express oneself. Here again there is a “complementarity,” as he frequently pointed out, between clarity and truth, between Klarheit und Wahrheit, as he liked to say.

This is why Bohr was not a very clear lecturer. He was intensely interested in what he had to say, but he was too much aware of the intricate web of ideas, of all possible cross-connections. This awareness made his talks fascinating but hard to follow. Miss Moore’s biography quotes a delightful story which Bohr liked to tell about a young man who was sent by his own village to another town to hear a great rabbi. When he returned he reported: “The rabbi spoke three times; 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 talk was a great and unforgettable experience. I understood nothing and the rabbi himself didn’t understand much either.”

The work on uranium fission brought Bohr inevitably into a realm where physics and human affairs are irrevocably intertwined. Even before these discoveries, he had been deeply concerned with human problems. He was aware, earlier than many of his colleagues, that atomic physics played, and would play, a decisive part in civilization and in the fate of mankind—that science cannot be separated from the rest of the world. The events of world history brought home this point earlier than he had expected. By the 1930s, the ivory tower of pure science had already broken down. A stream of refugees from the Nazi regime came to Copenhagen and found help and support from Bohr: James Franck, Hevesy, Placzek, Frisch, and many others found a haven in Copenhagen where they could pursue their scientific work. In addition, Bohr’s Institute became the center for other scientists who needed help: Bohr himself found refuge for many of them in England or in the United States. Then came the years of war. When Denmark was occupied by the Nazis in April, 1940, Bohr was in close touch with the Danish Resistance. He refused to collaborate with the Nazi authorities, and was forced to leave Denmark. He escaped to Sweden, later came to England, and then to the United States, where he spent the rest of the war years.

IN AMERICA the fourth period of his life began. In Los Alamos he joined a large group of scientists who, at that time, were working on the exploitation of nuclear energy for war purposes. He did not shy away from this, the most problematic of all scientific activities. He faced it squarely, as a necessity, but at the same time it was his idealism, his foresight, and his hope for peace that inspired so many people at that place of war to think about the future and to prepare their minds for the tasks ahead. He believed that, in spite of death and destruction, man had a positive future, one that could be transformed by scientific knowledge. During the war Bohr engaged in a one-man campaign to inform the leading statesmen of the West of the dangers and the hope of the atomic bomb. He wished to convince men in power to make use of this new momentous achievement for the creation of a more open world, in which science would bring East and West together. He met with many statesmen, including Roosevelt and Churchill, and quickly learned the difficulties and pitfalls of diplomatic life. Although he was able to convince Roosevelt and several other important statesmen, his meeting with Churchill turned out to be a complete failure. Churchill was opposed to any sharing of secrets with Russia, and even went so far as to accuse Bohr of being too friendly with the Russians.

Bohr’s political ideas did not come to anything. Nor did any other attempt during these times to make nuclear technology international, as a means of avoiding a nuclear armament race between powerful nations. Shortly after the war, an International Atomic Energy Commission at the United Nations was created under the chairmanship of Bohr’s old friend and collaborator, H. Kramers, but the grim realities of the East-West conflict prevented the Commission from coping with the much more serious realities of nuclear war. Bohr, and others who shared his thinking—there were many all over the world—were deeply disappointed. Bohr ended his efforts for an international agreement on nuclear weapons with his famous Letter to the United Nations, written in 1950, in which he set down his thoughts about the necessity of an open world.

Bohr returned to his native Denmark after the war. Deeply revered by his fellow citizens he became something of a national hero, a rather unusual role for a scientist, and he became deeply involved in Danish affairs. In the last decade of his life, Bohr spent much of his time on creating international scientific enterprises. He help to found the Scandinavian Institute of Atomic Physics, NORDITA and a new European Laboratory for modern fundamental research in physics in which all European countries were to participate. He helped to build the European Center of Nuclear Research—CERN—at Geneva. CERN, the home of one of the world’s largest particle accelerators, became a symbol of Europe’s renaissance in fundamental science, a field in which the US had become the acknowledged leader. Physical research required a great many people and large machines. The high energy accelerators made it possible to go beyond the structure of the nucleus and to explore the structure of its constituents, the world within the proton and neutron. Bohr recognized this as a logical continuation of what he and his friends had started. He saw the necessity of a large scale, of an international scale, in physics. In no other human endeavor are the narrow limits of nationality or politics more obsolete than in the search for more knowledge about the universe.

A NUMBER OF BOOKS have appeared recently about Niels Bohr and the development of quantum physics. Ruth Moore’s sympathetic biography is an impressive study of Bohr’s life and background, his scientific career, the circle of his friends and collaborators; it contains a fascinating account of Bohr’s political efforts. Miss Moore is not a physicist, nor had she ever met Bohr. Her book is at its best when it deals with his personal relations, and particularly in its account of Bohr’s odyssey from one statesman to another after the war. Much of the factual information on these matters is taken from the remarkable book Britain and Atomic Energy 1939-45 by Margaret Gowing (St. Martin’s Press), a book that deserves more attention in this country; its emphasis on some of the details in the history of atomic energy are often neglected in books written in the US, such as the initial contributions of the English and French physicists to the development of nuclear energy, as well as the efforts of Bohr and his friends to bring about early internationalization of atomic energy.

Miss Moore’s rendering of the life and spirit of the Bohr Institute is excellent, even though all of her information is second hand. Even the scientific problems are well reported, particularly the famous discussions between Bohr and Einstein, the latter of whom was never at ease with Bohr’s views on Quantum Mechanics. The book contains a few grave errors in its treatment of Bohr’s physics and one wonders why Miss Moore did not have the manuscript examined more carefully by an expert.

For the development of Bohr’s ideas, rather than Bohr’s personality, the reader should turn to The Questioners by Barbara L. Cline. This excellent study, written for the non-specialist, manages to make clear complicated modern developments in physics, from Planck to Einstein, Bohr, and Heisenberg. It is readable, fascinating, and almost always correct. Perhaps only a non-specialist and a woman can give an account of science which can be understood.

Gamow’s book, Thirty Years that Shook Physics, deals with the same subject as Miss Cline’s book, but not so successfully. Gamow was a close colleague of Bohr and has first-hand knowledge of what was going on at Copenhagen. This is an advantage when he describes daily life at Bohr’s Institute, and he has some amusing anecdotes to tell. His expert knowledge, however, puts him at a disadvantage when he tries to explain the intricacies of quantum mechanics. For he knows it too well, and is not aware of what the general reader may not be able to follow. The most delightful part of his book is an English translation of a play, a recast extract of Goethe’s Faust, lampooning the situation in modern physics, which was presented at one of the famous jocular sessions at physics events in Copenhagen.

There will soon be available an English translation from the Danish of a memorial volume, Niels Bohr: His Life and Work, Related by his Friends and Collaborators. It contains many articles by distinguished scientists, public figures in Danish life who worked with Bohr or were acquainted with him, and friends and relatives of Bohr. The book includes interesting contributions by such famous scientists as Heisenberg, Dirac, Frisch, and Casimir, and an important firsthand account by his son Aage of Bohr’s political efforts in connection with the atomic bomb. Aage Bohr was his father’s constant companion and secretary during these difficult years and is therefore a reliable witness. Other articles are perhaps of less interest, being somewhat hero-worshipping in tone. There are a great many anecdotes, some of which are repeated in several contributions, and some of which should not have been included at all, since not every joke of Bohr’s was worth preserving. But the most charming and revealing article is a short piece by Bohr’s son, Hans, who writes about his father with love and genuine admiration, in that simple style which is so typically Danish. One is profoundly moved by this reflection on a great man.

This Issue

April 20, 1967