MacTutor
Niels Bohr
Times obituary
STRUCTURE OF THE ATOM
Professor Niels Bohr, the Danish physicist, whose death yesterday at the age of 77 is reported by Reuters from Copenhagen, had been widely regarded, ever since Einstein died, as the greatest physicist alive. He was the leading spirit in the development of modern quantum theory, on which our understanding of atoms is based, and he has had considerable influence on the philosophy of our time.
He was born on October 7, 1885, in Copenhagen as the eldest son of Christian Bohr. Professor of Physiology at the university of that city. His younger brother, Harold, became a famous mathematician: both acquired early fame in Danish football. Niels Bohr studied at Copenhagen University and took his doctorate in 1911. Then he came to England to work for a short time under Professor J. J. Thomson at the Cavendish Laboratory, Cambridge; in the spring of 1912, he went to the University of Manchester, where Professor Ernest (later Lord) Rutherford had just discovered the atomic nucleus.
As a mathematical physicist, he clearly saw the great difficulties in Rutherford's assumption that each atom is a small solar system, with several negatively charged electrons circulating, like miniature planets, around their "sun," the positively charged nucleus; such a system, under the laws of classical physics, would inevitably collapse very quickly, sending out its energy in the form of radiation. But he also saw that Planck's quantum law, which stated that radiation was not emitted continuously but always in finite parcels, offered a possibility of accounting for the stability of atoms and also for the numerical—and so far quite mysterious—relations between their spectral lines.
THE QUANTUM LAW
By applying quantum law to the simplest atom of all, he formulated his model of the hydrogen atom, which shocked the world of physics in 1913. Its features—electrons allowed to run only on specified circles, radiation emitted only when an electron jumped from one circle to another—were frankly unbelievable: yet it accounted so accurately for so many observed facts that it had to be taken seriously. Newton's mechanics, the rock on which all physics had been built, had to be changed to fit the atomic domain, and that took about 15 years; indeed, the ground of physics still quakes from the great upheaval which Bohr initiated and which he had guided ever since with indefatigable patience and penetrating insight.
Bohr saw quite early that in the atomic world we do not deal with objects that behave in a certain way whether we observe them or not. The laws of quantum theory imply that we cannot help disturbing what we want to observe. With large objects, the disturbance is negligible; switching the light on in a dark room does not make the furniture jump. But an atom or electron will jump even if only a single radiation quantum acts on it; every time we "see" an atom its motion changes significantly. There is no way of telling what it does when we are not looking; all we can do is calculate the probability that the next observation will give a certain result. Bohr stressed that it depends on the kind of observation whether the object behaves like a particle or a wave; those two aspects he called "complementary", implying that no possible observation could show both at once. This idea of complementarity between different aspects of things is an important extension of philosophical thought, and Bohr has suggested interesting applications in biology, psychology, and sociology.
The atomic nucleus Bohr realized did not possess the planetary structure of an atom but was more nearly like a drop of a drop of liquid in which the molecules collide all the time, and in 1936 he gave the theory of nuclei a new direction by showing how the quantum theory could be applied to such a jostling crowd. The discovery of uranium fission in 1938 showed that a large nucleus could indeed divide just as a large raindrop might be torn in two by the wind, and the drop-like behavior of nuclei was later demonstrated by many experiments based on the brilliant theoretical work of one of Bohr's sons, Aage Bohr.
RETURN TO MANCHESTER
After a year in Copenhagen, Bohr returned to Manchester in 1914 for two years as a reader in mathematical physics; but in 1916 he was appointed professor of theoretical physics at the University of Copenhagen. He was largely responsible for the creation in 1920 of an Institute for Theoretical Physics, which came to house an increasing number of experimental physicists and their instruments, including (since 1937) the first cyclotron to operate in Europe.
That institute soon became a Mecca for theoretical physicists from all over the world, and after 1933 a refuge for many scientists who had fled from Hitler's Germany. Their social center was the mansion "Gamle Carlsberg," given to the nation by the founder of the well-known brewery and placed at Niels Bohr's disposal in 1932. Here, under the motherly care of Bohr's beautiful wife, Margrethe (the daughter of the mathematician, Professor Nörlund, whom he married in 1912 and by whom he had four sons), students and scholars of all nations gathered to eat and talk and listen to music, and often to sit quite literally at Bohr's feet, trying to catch his challenging remarks, subtle comments, and gentle jokes, spoken in his soft, Danish voice.
But Bohr did not hesitate to make his authority heard in wider contexts. When it became known, in 1942, that Hitler planned to terminate his so far lenient treatment of Danish Jews, Bohr (whose mother had come from Jewish stock), like many others, fled to Sweden in a small boat on a moonless night; There he persuaded the King of Sweden to offer unconditional sanctuary to all Jewish refugees from Denmark. Taken to England soon afterward, and then to the United States, he urged that thought be given to the entirely new political problems which the atom bomb—then under construction—would create. And again in 1950, when the warning of Hiroshima and Nagasaki seemed in danger of being forgotten, he addressed an eloquent appeal to the United Nations, pleading for an "open world" where no war could be prepared secretly and where mutual confidence might grow and spread.
With his heavy build and large, hairy hands, Niels Bohr looked more like a peasant than a scholar; but his huge head with deep-set eyes under bushy eyebrows is not easily forgotten, nor the sudden sunny smile that seemed to deprecate what he had said, lest he be taken too seriously. The tentative character of all fundamental scientific advance was always in his mind, from the day when he first proposed his hydrogen atom, stressing that it was merely a model of something as yet 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, whether in some ways he went too far—as some physicists believe the future will tell.
Bohr received many honors, among them them the Nobel Prize in 1922; election as Foreign Member of the Royal Society in 1926; the highest Danish decoration, the Order of the Elephant; the Atoms for Peace Award in 1957; and the Sonning Prize last year.
___________________________________
Mr. M. W. Perrin writes:
The death of Professor Niels Bohr has broken one more link with the days, during and immediately after the war, when the atomic bomb was produced as a practical consequence of the advances in nuclear physics to which he had made a great contribution. He appreciated the full significance of this with, perhaps, earlier and clearer foresight than anyone else.
The British wartime "Directorate of Tube Alloys" maintained communication with Professor Bohr in Copenhagen, and arrangements were made, in October 1943, to bring him and one of his sons, Aage, to England in a special Mosquito flight. It was my privilege to be intimately concerned with this, and with his subsequent journeys between this country and the United States, for which he used a British passport made out in the name of Mr. Nicholas Baker.
His first concern was reaching London was to establish contact with Lord Waverley and, through his friend, Judge Frankfurter, with Mr. Roosevelt in the United States. The leading scientists in both countries were, of course, eager to get the advice and help of Professor Bohr, but he was always more anxious to see that the statesmen clearly appreciated the implications of the large-scale release of nuclear energy and understood that the nuclear bomb would not be just another military weapon. It was in this connection that he was anxious to try to get agreement with Sir Winston Churchill and Mr. Roosevelt that they, with Russia, should announce a decision to refrain from using the weapon and thus attempt to eliminate war and make scientific advances available for the good of all humanity.
The actual course of events was a continuing disappointment to Bohr, but those who knew him well and heard him speak learned to love and respect his good will and great wisdom and know that he never missed any opportunity of furthering his hopes and, in this, he was greatly helped his wife and family.
_________________________
SHARING OF NUCLEAR KNOWLEDGE
Sir Henry Dale, O.M., writes:—
The appreciation by Mr. Michael Perrin in your issue of November 21, has already done most of what was immediately needed to amplify the reference in your obituary notice to Niels Bohr's personal and naturally outstanding contributions to the British and American wartime researches, and the eventually cooperative enterprise, aimed at the large-scale release of nuclear energy, and resulting in the production of the first atomic bombs much sooner, in fact, than it had earlier seemed reasonable to expect.
As an official member then of Sir John Anderson's small and confidential communication, to represent the more general interests and opinions of science and scientists, I was also brought into specially intimate touch with Bohr behind the scenes, during both the periods of his work on these problems in this country; in the first instance from the time of his arrival in London in October 1943, and again, and more frequently, after he returned from the United States in April 1944.
By then, the completion of an atomic bomb could be foreseen in a matter of months, or a year; and it could further be predicted with an approach to confidence that the object with which the project had first been undertaken with such urgency, namely, to forestall any possibility of a prior success in this direction by Nazi Germany, could by then be discounted. The question of what was then to be done with and about the bombs thus needed an early decision by those responsible for national and allied policies. For Niels Bohr, it had become, by then, a matter of urgent anxiety.
As Mr. Perrin has said, while in the United States, Bohr had made contact with Roosevelt through his friendship with Mr. Justice Frankfurter; and he had thus been charged, through the same channel, with a message to Sir Winston Churchill, seeming to offer an opportunity for a discussion on matters of such a moment. This is not the occasion for discussion of the reasons for Bohr's failure in this approach. I can fully confirm, however, Perrin's suggestion with regard to Bohr's essential aim and to his dread of the danger which has now been all too fully realized. He was passionately convinced, and I, with him and others too, I believe, who held positions of great authority here, that the only real chance of safety for mankind, under the revolutionary conditions created by these tremendous discoveries, would have been found in an open offer to share the new knowledge with all mankind, in return for a binding agreement to surrender all further claims to national secrecy about science
Niels Bohr will certainly be remembered as one of the world's great men of science in the lineage of Newton, Rutherford, and Einstein; but I believe that the memorial which he would most have wished for himself would have been a renewed and truly determined effort, even now, to follow where attempts were made to give a lead, to get back to a policy which might have been tried before the bombs were dropped, and before the decision was made to keep so much of science still shackled with a wartime secrecy, and thus to maintain a standing and disastrous threat to all mankind, in place of the unexampled prosperity and happiness which the new science might yet be used to offer in such abundance.
STRUCTURE OF THE ATOM
Professor Niels Bohr, the Danish physicist, whose death yesterday at the age of 77 is reported by Reuters from Copenhagen, had been widely regarded, ever since Einstein died, as the greatest physicist alive. He was the leading spirit in the development of modern quantum theory, on which our understanding of atoms is based, and he has had considerable influence on the philosophy of our time.
He was born on October 7, 1885, in Copenhagen as the eldest son of Christian Bohr. Professor of Physiology at the university of that city. His younger brother, Harold, became a famous mathematician: both acquired early fame in Danish football. Niels Bohr studied at Copenhagen University and took his doctorate in 1911. Then he came to England to work for a short time under Professor J. J. Thomson at the Cavendish Laboratory, Cambridge; in the spring of 1912, he went to the University of Manchester, where Professor Ernest (later Lord) Rutherford had just discovered the atomic nucleus.
As a mathematical physicist, he clearly saw the great difficulties in Rutherford's assumption that each atom is a small solar system, with several negatively charged electrons circulating, like miniature planets, around their "sun," the positively charged nucleus; such a system, under the laws of classical physics, would inevitably collapse very quickly, sending out its energy in the form of radiation. But he also saw that Planck's quantum law, which stated that radiation was not emitted continuously but always in finite parcels, offered a possibility of accounting for the stability of atoms and also for the numerical—and so far quite mysterious—relations between their spectral lines.
THE QUANTUM LAW
By applying quantum law to the simplest atom of all, he formulated his model of the hydrogen atom, which shocked the world of physics in 1913. Its features—electrons allowed to run only on specified circles, radiation emitted only when an electron jumped from one circle to another—were frankly unbelievable: yet it accounted so accurately for so many observed facts that it had to be taken seriously. Newton's mechanics, the rock on which all physics had been built, had to be changed to fit the atomic domain, and that took about 15 years; indeed, the ground of physics still quakes from the great upheaval which Bohr initiated and which he had guided ever since with indefatigable patience and penetrating insight.
Bohr saw quite early that in the atomic world we do not deal with objects that behave in a certain way whether we observe them or not. The laws of quantum theory imply that we cannot help disturbing what we want to observe. With large objects, the disturbance is negligible; switching the light on in a dark room does not make the furniture jump. But an atom or electron will jump even if only a single radiation quantum acts on it; every time we "see" an atom its motion changes significantly. There is no way of telling what it does when we are not looking; all we can do is calculate the probability that the next observation will give a certain result. Bohr stressed that it depends on the kind of observation whether the object behaves like a particle or a wave; those two aspects he called "complementary", implying that no possible observation could show both at once. This idea of complementarity between different aspects of things is an important extension of philosophical thought, and Bohr has suggested interesting applications in biology, psychology, and sociology.
The atomic nucleus Bohr realized did not possess the planetary structure of an atom but was more nearly like a drop of a drop of liquid in which the molecules collide all the time, and in 1936 he gave the theory of nuclei a new direction by showing how the quantum theory could be applied to such a jostling crowd. The discovery of uranium fission in 1938 showed that a large nucleus could indeed divide just as a large raindrop might be torn in two by the wind, and the drop-like behavior of nuclei was later demonstrated by many experiments based on the brilliant theoretical work of one of Bohr's sons, Aage Bohr.
RETURN TO MANCHESTER
After a year in Copenhagen, Bohr returned to Manchester in 1914 for two years as a reader in mathematical physics; but in 1916 he was appointed professor of theoretical physics at the University of Copenhagen. He was largely responsible for the creation in 1920 of an Institute for Theoretical Physics, which came to house an increasing number of experimental physicists and their instruments, including (since 1937) the first cyclotron to operate in Europe.
That institute soon became a Mecca for theoretical physicists from all over the world, and after 1933 a refuge for many scientists who had fled from Hitler's Germany. Their social center was the mansion "Gamle Carlsberg," given to the nation by the founder of the well-known brewery and placed at Niels Bohr's disposal in 1932. Here, under the motherly care of Bohr's beautiful wife, Margrethe (the daughter of the mathematician, Professor Nörlund, whom he married in 1912 and by whom he had four sons), students and scholars of all nations gathered to eat and talk and listen to music, and often to sit quite literally at Bohr's feet, trying to catch his challenging remarks, subtle comments, and gentle jokes, spoken in his soft, Danish voice.
But Bohr did not hesitate to make his authority heard in wider contexts. When it became known, in 1942, that Hitler planned to terminate his so far lenient treatment of Danish Jews, Bohr (whose mother had come from Jewish stock), like many others, fled to Sweden in a small boat on a moonless night; There he persuaded the King of Sweden to offer unconditional sanctuary to all Jewish refugees from Denmark. Taken to England soon afterward, and then to the United States, he urged that thought be given to the entirely new political problems which the atom bomb—then under construction—would create. And again in 1950, when the warning of Hiroshima and Nagasaki seemed in danger of being forgotten, he addressed an eloquent appeal to the United Nations, pleading for an "open world" where no war could be prepared secretly and where mutual confidence might grow and spread.
With his heavy build and large, hairy hands, Niels Bohr looked more like a peasant than a scholar; but his huge head with deep-set eyes under bushy eyebrows is not easily forgotten, nor the sudden sunny smile that seemed to deprecate what he had said, lest he be taken too seriously. The tentative character of all fundamental scientific advance was always in his mind, from the day when he first proposed his hydrogen atom, stressing that it was merely a model of something as yet 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, whether in some ways he went too far—as some physicists believe the future will tell.
Bohr received many honors, among them them the Nobel Prize in 1922; election as Foreign Member of the Royal Society in 1926; the highest Danish decoration, the Order of the Elephant; the Atoms for Peace Award in 1957; and the Sonning Prize last year.
___________________________________
Mr. M. W. Perrin writes:
The death of Professor Niels Bohr has broken one more link with the days, during and immediately after the war, when the atomic bomb was produced as a practical consequence of the advances in nuclear physics to which he had made a great contribution. He appreciated the full significance of this with, perhaps, earlier and clearer foresight than anyone else.
The British wartime "Directorate of Tube Alloys" maintained communication with Professor Bohr in Copenhagen, and arrangements were made, in October 1943, to bring him and one of his sons, Aage, to England in a special Mosquito flight. It was my privilege to be intimately concerned with this, and with his subsequent journeys between this country and the United States, for which he used a British passport made out in the name of Mr. Nicholas Baker.
His first concern was reaching London was to establish contact with Lord Waverley and, through his friend, Judge Frankfurter, with Mr. Roosevelt in the United States. The leading scientists in both countries were, of course, eager to get the advice and help of Professor Bohr, but he was always more anxious to see that the statesmen clearly appreciated the implications of the large-scale release of nuclear energy and understood that the nuclear bomb would not be just another military weapon. It was in this connection that he was anxious to try to get agreement with Sir Winston Churchill and Mr. Roosevelt that they, with Russia, should announce a decision to refrain from using the weapon and thus attempt to eliminate war and make scientific advances available for the good of all humanity.
The actual course of events was a continuing disappointment to Bohr, but those who knew him well and heard him speak learned to love and respect his good will and great wisdom and know that he never missed any opportunity of furthering his hopes and, in this, he was greatly helped his wife and family.
_________________________
SHARING OF NUCLEAR KNOWLEDGE
Sir Henry Dale, O.M., writes:—
The appreciation by Mr. Michael Perrin in your issue of November 21, has already done most of what was immediately needed to amplify the reference in your obituary notice to Niels Bohr's personal and naturally outstanding contributions to the British and American wartime researches, and the eventually cooperative enterprise, aimed at the large-scale release of nuclear energy, and resulting in the production of the first atomic bombs much sooner, in fact, than it had earlier seemed reasonable to expect.
As an official member then of Sir John Anderson's small and confidential communication, to represent the more general interests and opinions of science and scientists, I was also brought into specially intimate touch with Bohr behind the scenes, during both the periods of his work on these problems in this country; in the first instance from the time of his arrival in London in October 1943, and again, and more frequently, after he returned from the United States in April 1944.
By then, the completion of an atomic bomb could be foreseen in a matter of months, or a year; and it could further be predicted with an approach to confidence that the object with which the project had first been undertaken with such urgency, namely, to forestall any possibility of a prior success in this direction by Nazi Germany, could by then be discounted. The question of what was then to be done with and about the bombs thus needed an early decision by those responsible for national and allied policies. For Niels Bohr, it had become, by then, a matter of urgent anxiety.
As Mr. Perrin has said, while in the United States, Bohr had made contact with Roosevelt through his friendship with Mr. Justice Frankfurter; and he had thus been charged, through the same channel, with a message to Sir Winston Churchill, seeming to offer an opportunity for a discussion on matters of such a moment. This is not the occasion for discussion of the reasons for Bohr's failure in this approach. I can fully confirm, however, Perrin's suggestion with regard to Bohr's essential aim and to his dread of the danger which has now been all too fully realized. He was passionately convinced, and I, with him and others too, I believe, who held positions of great authority here, that the only real chance of safety for mankind, under the revolutionary conditions created by these tremendous discoveries, would have been found in an open offer to share the new knowledge with all mankind, in return for a binding agreement to surrender all further claims to national secrecy about science
Niels Bohr will certainly be remembered as one of the world's great men of science in the lineage of Newton, Rutherford, and Einstein; but I believe that the memorial which he would most have wished for himself would have been a renewed and truly determined effort, even now, to follow where attempts were made to give a lead, to get back to a policy which might have been tried before the bombs were dropped, and before the decision was made to keep so much of science still shackled with a wartime secrecy, and thus to maintain a standing and disastrous threat to all mankind, in place of the unexampled prosperity and happiness which the new science might yet be used to offer in such abundance.
