1885-1962

Niels Henrik David Bohr was one of the most respected theoretical physicists of the twentieth century. By introducing conceptions borrowed from the quantum theory Bohr succeeded in working out a picture of atomic structure that with later improvements still fitly serves as an elucidation of the physical and chemical properties of the elements. In 1922 Bohr received the Nobel Prize for physics for his work on the structure of atom.

During World War II Bohr did his best to rehabilitate Jewish scientists who where forced to leave Germany and the countries occupied by Hitler. He himself made a dramatic escape from Denmark and fled to the USA. In 1943 Bohr worked on the atom bomb project. However, after realising the great danger from such bomb he spent the rest of his life working on peaceful ways of using atomic energy.

Bohr was a great leader and mentor. He had an insatiable curiosity. There were few who came in contact with Bohr and were not inspired to put forth their best. He drew the best students of physics from all over the world. Above all he was a great humanist.

Bohr really epitomised the heroic image of a scientist. He had no hesitation to admit when he was in error. Einstein once commented : “He (Bohr) utters his opinions like one perpetually groping and never like one who believes to be in possession of a definite truth.” He continued to work till his death. In fact a diagram drawn on his study blackboard the night before his death to overcome the arguments of Albert Einstein has been kept unchanged.

Bohr was born on October 07, 1885. His mother Ellen Adler was the daughter of a prosperous Jewish banker and politician. His father Christian Bohr was a son of a school teacher. Christian Bohr was a Professor of Physiology at the University of Copen- hagen and had a deep interest in science, art and philosophy. Christian had published his first scientific paper at the age of twenty-two. Christian was politically and socially progressive. He was a religious skeptic and an early advocate of women’s rights. Christian was also a sports enthusiast and was instrumental in popularising soccer in Denmark. Christian encouraged his children by providing opportunities for them to explore their interests. Bohr grew up in an environment that encouraged independent development, human compassion and culture. His parents instilled in him a great love of knowledge and its pursuit.

Bohr had an elder sister Jenny and younger brother, Harald who distinguished himself as a mathematician. The two brothers remained best friends thoughout their life. His sister pursued a career teaching history and Danish. Bohr and also his brother Harald completed their elementary and high school studies at Gamelholm Grammar School. Bohr was not always first in his class. However, he was regularly third or fourth from the top. He displayed great abilities in mathematics and science. Bohr was an avid and excellent athelete.

After matriculation at the Gamelholm Grammar School in 1903 Bohr joined the University of Copenhagen. His brother followed him there. Bohr developed a fascination for poetry and memorised many stanzas in German and Danish. He also read philosophy including the works of the Danish philosopher Soren Aabye Kierkegaard (1813-55). At the university Bohr and his brother studied philosophy under their father’s friend Harold Hoffding. They had also formed a discussion group with some of their classmates. Bohr also enjoyed fiction. He particularly valued a little book titled Tale of a Danish Student by the Danish writer Paul Martin Moller. In this book the student is to sort out the many dualities inherent in life. For example Moller wrote : “Thus on many occasions man divides himself into two persons, one of whom tries to fool the other, while a third one, who in fact is the same as the other two, is filled with wonder at this confusion. In short, thinking becomes dramatic and quietly acts the most complicated plots with itself, and the spectator again and again becomes actor”. Bohr was always fascinated with duality — two things at once.

In 1905, the Royal Danish Academy of Sciences and Letters had proposed an award for the best paper on the surface tension of liquids. Bohr decided to participate in the competition. At that time he was 19 years old. His father made him available the facilities of his physiology laboratory. Lord Rayleigh (1842-1919) had proposed that it was possible to determine the surface tension of a liquid if a few factors were known, for example, the length of waves that formed on a jet of the liquid, the speed of the jet and its cross section. Bohr devised a method for producing a jet of water that would always have the same speed and cross section. He worked in the nights for months. Finally he submitted a paper. Though the paper was inconclusive, Bohr while extending Raleigh’s basic theory about surface tension of liquids, raised some important questions. He was declared one of the winners of the competition. A paper based on this work was published in 1909 in Philosophical Transactions of the British Royal Society. The paper was titled “Determination of the Surface Tension of Water by the Method of Jet Vibration.”

Bohr received his bachelor’s degree from the University of Copenhagen in 1907. He continued to work as a graduate student. He took his Master’s degree in Physics in 1909 and his Doctor’s degree in 1911. His thesis work was on the subject of the electron theory of metals. Bohr defended his thesis, which he dedicated to the memory of his father `with deepest gratitude’ on May 13, 1911. Bohr’s father had died few months before this at the age of 56 and he was buried in one of Copenhagen’s oldest cemeteries near the graves of the physicist Hans Christian Oersted (1777-1851) and the Danish writer Hans Christian Andersen (1805-75). Commenting on Bohr’s defence of his PhD work, a local newspaper reported : “Dr. Bohr, a pale and modest young man, did not take much part in the proceedings, the short duration of which a record… The words Bohr had written and the questions he had raised were literally so new and unusual that no one was equipped to question them.” Bohr’s doctoral work remains to this day a classic on the subject.

Bohr could never master the language he spoke or wrote. In fact in his school, Bohr’s worst subject had been Danish composition. It is said that even for writing a postcard Bohr would first prepare a draft. Bohr was not at all comfortable in writing. He dictated entire doctoral thesis to his mother. While Bohr’s father thought that a PhD student should write his won thesis but his mother firmly believed the task was hopeless. Most of Bohr’s later work and correspondence was dictated to his wife and his secretaries or co-workers. He took long time to write a paper. Seven or eight drafts were very common. Bohr shaped his ideas while orally communicating with other fellow physicists.

Before his death, Bohr’s father had helped arrange a grant for his son’s post- graduate work in England for a year. The grant was given by Carlsberg Brewery, producer of excellent quality beer. Bohr arrived in Cambridge, England, in late September 1911 to work under the guidance of Joseph John Thomson (1850-1940), the discoverer of the electron and head of the famous Cavendish Laboratory. The first Director of the Cavendish Laboratory was James Clerk Maxwell (1831-1879) who was succeeded by Lord Rayleigh. Thomson had taken the place of Rayleigh at the age of 28. At the time when Bohr reached Cambridge Thomson was 50 years old. Ernest Rutherford (1871-1937) had also come to Cambridge to work under Thomson but 16 years earlier than Bohr. In the very first meeting with Thomson, Bohr did not hesitate to point out his reservations on Thomson’s theory of atom. Bohr had thought a great deal about Thomson’s `plum pudding’ atom, and he was almost convinced that it could not be correct. It may be noted that based on his discovery of the electron, Thomson suggested in 1898 that atoms were spheres of positively charged matter with negatively charged electrons embedded in them in a uniform manner–something like a `plum pudding’. He presented Thomson his PhD thesis on the application of electron theory to metals – with the hope that Thomson might read it and discuss it with him. For Bohr to meet Thomson was a great event. He wrote to his brother: “Things are going so well for me. I have just been talking to J.J. Thomson and have explained to him, as well as I could, my ideas about radiation, magnetism, etc. If you could only knew what it meant to me to talk to such a man. He was extremely nice to me, and we talked about so much; and I do believe that he thought there was some sense in what I said. He is now going to read (my thesis) and he invited me to have dinner with him Sunday at Trinity College; then he will talk with me about it.”

Bohr had started working on cathode ray production at the suggestion of Thomson. But apparently he did not enjoy his work. Otherwise he enjoyed his stay at Cambridge. He joined a soccer club and did ice skating. After a few months Bohr went to meet Thomson again. The meeting was very cordial but Bohr realised that Thomson had not read his thesis. Bohr was greatly discouraged. Later in his life, Bohr while commenting on his stay at Cambridge, would say : “The whole thing was very interesting in Cambridge but it was absolutely useless.” He wanted to change his work place and accordingly he contacted Rutherford, who was then working at Manchester. Rutherford welcomed Bohr’s idea but he also advised him to first complete the work at Thomson’s laboratory. Bohr completed the work and told Thomson that “he would like to work with Rutherford as he would like to know something about radioactivity”.

Bohr arrived at Manchester in March 1912. Here he found the atmosphere quite stimulating. Under the leadership of Rutherford the physics laboratory of the Manchester University was fast emerging as one of the most productive in the world. Rutherford created around himself an atmosphere of intellectual excitement and openness. E. Andrade, one of Rutherford’s collaborators while commenting on Rutherford’s style of working, wrote : “Although there was no doubt as to who was the boss, everybody said what he liked without constraint… He was always full of fire and infectious enthusiasm when describing work into which he had put his heart and always generous in his acknowledgement of the work of others.”

Each afternoon all people working in Rutherford’s laboratory used to meet on tea. Rutherford also participated at these daily get-togethers. Besides discussing their research work they would discuss politics and sports. Ideas were freely exchanged. It was a time when so many important things were happening in physics and so nobody lacked an interesting topic to discuss.

In Manchester, Bohr was placed under George Charles von Hevesy (1885-1966), who was also of the Bohr’s age. At the time Hevesy was trying to separate radioactive decay products from their parent substance, a problem undertaken at the instance of Rutherford. It was Hevesy who developed the science of using radioactive traces in medical and biological research. Hevesy was awarded the 1943 Nobel Prize for chemistry. Bohr greatly profited from Hevesy’s extensive knowledge of radiochemistry. Bohr also undertook an eight-week laboratory course in the experimental methods of radioactive research. One of his instructors was Hans Wilhelm Geiger (1882-1945). It may be noted that Geiger, a pioneer in nuclear physics, developed a variety of instruments and techniques used for detecting and counting individual charged particles. Geiger alongwith E Marsden investigated the scattering of alpha particles by gold leaf (1909), a work which led Rutherford to propose his nuclear theory of atom. After completing this course Bohr started studying the absorption of alpha particles in aluminum at the instance of Rutherford. Commenting on his impression of Rutherford’s working style Bohr wrote to his brother : “…Rutherford is a man you can rely on ; he comes regularly and enquired how things are going on–talks about the smallest details… Rutherford is such an outstanding man and really interested in the work of all the people around him….”

After completing his one year post-doctoral study Bohr left Manchester for his homeland on July 24, 1912. At that time Bohr’s country Denmark was not a proper place for doing research work in physics. Then Denmark had only one university – the University of Copenhagen. The University had only one professorship in physics and which was then occupied by Christian Christiansen, Bohr’s teacher. When Christiansen resigned from the post on August 31, 1912 it went to Martin Knudsen though Bohr had also applied for the post. Even Bohr could not get the docentship (a much lower paid position) in physics as Knudsen recommended his own assistant for this post. Bohr had to content himself with a teaching assistant offered by Knudsen.

Irrespective of his position in the University, Bohr started working in real earnest. He wanted to see how the quantum theory could be applied to explain the structure of atom. He had started working in this direction while he was in Manchester. Bohr was quite convinced that to demonstrate that Rutherford’s model is a physical reality would require altogether a new approach. That is how he turned to quantum theory. Bohr had commented later : “It was clear and, that was the point about the Rutherford atom, that we had something from which we could not proceed at all in any other way than by radical change.” The quantum theory originated from a paper of Max Karl Ernst Ludwig Planck (1858-1947) published in 1900. In this paper titled `On the Theory of the Law of Energy Distribution in the Continuous Spectrum’ Planck proposed that certain experimental results could best be understood if it were assumed that substances emit light only of certain energies and not other. In other words Planck assumed energy changes to take place in small discrete installments or quanta. The quanta is a Latin word and it means `How much’. In mathematical term Planck’s idea can be expressed as E=nh ? where E is the energy of the light source, n is a positive integer (i.e. 0,1,2,3, and so on), ? (nu) is frequency and h is a constant now called Planck’s constant. Thus each energy has a fixed value. Einstein went one step forward. In 1905 he proposed that light not only comes in quanta but it is a bundle of quanta or of discrete particles. Thus light or electromagnetic radiation is a flow of these discrete particles. And the intensity of radiation or light is the flux of these quanta.

The first problem Bohr faced was to explain the stability of the atom. As Rutherford had proposed that the atom’s mass would be contained in the nucleus, and an equal number of negatively charged electrons would be found in motion somewhere outside the nucleus. This model, the so-called planetary model, was proposed by Rutherford in 1911. However, this model had a fundamental problem. This model was theoretically unstable. Its stability could not be explained by the laws of classical or Newtonian physics. Unlike planets orbiting the Sun, electrons are charged particles. In the 19th century, Michael Faraday (1791-1867) and Maxwell had shown that an electrically charged particle gives off radiation if it is diverted from straight path. So as an electron moves in a circular path it would emit radiation and consequently it would lose energy and the electron would describe smaller and smaller tracks with a declining period of revolution and finally rush in towards the positive nucleus. Thus the track would be a spiral.

Bohr proposed that the electrons could revolve around the nucleus in only “certain orbits” or certain energy levels, each orbit having a different radius. And as long as electrons revolve around the nucleus in such “allowed orbits” they do not radiate or lose electromagnetic radiation or energy, even though they have accelerated motion around the nucleus. But electrons could jump spontaneously from one allowed orbit to another and then they would absorb or release energy in packets or quanta. If electrons move inward, toward the nucleus, into an orbit having a smaller radius, they would release energy. Conversely when they move away from the nucleus into orbit of a larger radius they would absorb energy.

By the end of 1913 Bohr published three papers which have come to be known as `Bohr’s 1913 trilogy’. These papers were titled:

1. On the Constitution of Atoms and Molecules (Part – I)
2. Systems Containing only a Single Nucleus (Part –II)
3. Systems Containing Several Nuclei (Part – III)

Bohr sent the draft of the first paper to Rutherford for his comments on March 16, 1913. Rutherford in his letter dated March 20, 1913 wrote :”… your ideas as to the mode of origin of the spectrum of hydrogen are very ingenious and seems to work out well…but the mixture of Planck’s ideas with the old mechanics make it very difficult to form a physical idea of what is the basis of it. … There appears to me one grave difficulty in your hypothesis, which I have no doubt you fully realize, namely, how does an electron decide what frequency it is going to vibrate at when it passes from one stationary state to the other ! It seems to me that you would have to assume that the electron knows beforehand where it is going to stop.”

Despite his apprehension Rutherford decided to communicate the paper to Philosphical Magazine, after correcting Bohr’s English and making necessary changes. So he concluded the abovementioned letter by saying : “I suppose you have no objection to my using my judgment to cut any matter I may consider necessary in your paper ! Please reply !”

Instead of writing a letter Bohr personally came to Manchester to convince Rutherford communicating the paper intact. They together analysed the paper section by section and Rutherford after listening Bohr’s argument sent the paper after making few corrections to Bohr’s English. All the three papers were published by the end of 1913 in the same journal.

Today the far reaching implications of these papers in the growth of physics are well-known. In these papers Bohr not only gave a highly useful model of the atom but he also showed that quantum mechanics was a fundamental part of how nature worked. It may be noted that Newtonian physics or the classical physics which explained the working of nature on the larger scale failed to explain the behaviours of the subatomic particles. Bohr’s ideas were radical. Most of the scientists were not ready to accept them. As pointed out by Otto Robert Frisch (1904-79), “That picture was so unorthodox at the time that a number of physicists…had sworn to give up physics if that nonsense (Bohr’s atomic model) was true”. Even scientists like Thomson, Lord Rayleigh and Einstein were not much enthused. In 1914 Rutherford said : “while it is too early to say whether the theories of Bohr are valid, his contributions…are of great importance and interest.” And again in the same year Rutherford said : “N. Bohr has faced the difficulties by bringing in the idea of the quantum. At all events there is something going on which is inexplicable by the older mathematics.” Towards the end of his life Einstein commented : “That this insecure and contradictory foundation (of physics in the early part of the 20th century) was sufficient to enable a man of Bohr’s unique instinct and tact to discover the major laws of the spectral lines and of the electron shells of the atoms together with their significance for chemistry appeared to me like a miracle and appears to me as a miracle even today. This is the highest form of musicality in the sphere of thought.”

It should be noted that Bohr’s model of the atom was by no means the last word. Ideas of the atom have undergone substantial changes since his announcement in 1933. Bohr himself knew that his model was nothing but a sketchy approximation of reality. As Frisch would later recall : “Bohr himself was very much aware of the crudeness of that model ; it resembled the atom no more than a quick pencil sketch resembles a living human face. But he also knew how profoundly difficult it would be to get a better picture”.

In 1914 Bohr was appointed as Professor of Theoretical Physics. It was Bohr, who for the first time, started teaching of theoretical physics as a separate subject at Copenhagen University. But before Bohr took up this assignment he was offered by Rutherford a two-year readership at Manchester. Bohr decided to take up the opportunity to work directly with Rutherford. After taking permission from the University that he can join the post after two years Bohr left for Manchester. In 1916 Bohr returned to Denmark. It may be noted that the first World War was in full swing at that time.

In 1917 Bohr submitted a proposal to the University of Copenhagen for establishing an institute of theoretical physics as part of the University. The proposal was accepted by the University after the end of the war and Bohr collected about $20,000 for constructing a building to house the institute. When Bohr was in the process of laying the foundation of his institute he got an offer from Rutherford of a permanent Professorship in Mathematical Physics at Manchester University where a new centre was created for conducting research in modern physics. Rutherford wrote : “You know how delighted we would be to see you working with us again. I think the two of us could try and make physics boom, well think it over and let me know your mind as soon as you can. Possibly you might think of visiting us as soon as the seas are clear.” Further he continued “I wish I had you here to discuss the meaning of some of my results on collision of nuclei. I think I have got some rather startling results.”

For any young scientist it was a great honour, an offer coming from a great scientist like Rutherford. Moreover, financially it was more lucrative to work in an English university than in Denmark. But Bohr, being a great patriot, did not accept the offer. Thus instead of opting for working in an established laboratory and as a colleague of Rutherford he decided to stay in Denmark to establish his proposed institute. Building the institute in war-ravaged economy was not an easy task. But Bohr’s resourcefulness somehow made it possible. The Institute of Theoretical Physics was formally inaugurated in September 1921. Bohr became its first director, a post he held till his death. Bohr started living on the upper floor of the Institute. Bohr made the Institute the ultimate place for theoretical physics in the world. To quote Spangenburg and Moser : “During the 1920s and 1930s, the Institute for Theoretical Physics in Copenhagen, headed by Bohr commanded an influence over the world of scientific thought equaled only by Aristotle’s Lyceum in Athens. Theoretical physicists went there from all over the world, during a time often called the heroic age of atomic physics.”

Bohr’s charismatic personality and his revolutionary contribution to physics drew the best young minds from all over the world. In this context it is interesting to quote what Otto Frisch had to say about Bohr : “He had a soft voice with a Danish accent, and we were not always sure whether he was speaking English or German; he spoke both with equal easy and kept switching. Here, I felt, was Socrates come to life, tossing us challenges on higher plane, drawing wisdom out of us which we didn’t know we had, and which of course we hadn’t.”

In 1916 Bohr introduced the concept of correspondence principle – the principle that quantum mechanics has a limit in which it is equivalent to classical mechanics. Thus this principle to some extent brings the new theory nearer to the classical physics.

It an attempt to reconcile quantum and classical physics — two equally plausible but mutually exclusive ideas – Bohr proposed his `Complementarity Principle’ in 1927 Bohr observed that a phenomenon can be visualised in two mutually exclusive ways, but at the same time both visualisaitons can remain valid in their own terms. For example, light may undulate like a wave in one instrument but it may scatter in another instrument. This means, Bohr argued, evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomenon exhaust the possible information about the object. As Frisch pointed out “it is a bit as if reality was painted on both sides of a canvas so that you could only see one aspect of it clearly at any time”. Together with the indeterminacy principle of Werner Heisenberg (1901-76) and the probability waves of Max Born (1882-1970) Bohr’s complementarily principle has emerged as the most authoritative and widely accepted theory to describe atomic phenomena.

Bohr’s contribution in the field of radioactivity is quite significant. Bohr formulated the law of radioactive displacement. According to this law when radioactive element emits alpha particle it moves to places to the left on the Periodic Table (down in atomic number) but if it emits a beta particle, it moves to the right one place (up in atomic number). Bohr’s liquid drop model of the nucleus proposed in 1936 provided the basis for the first theoretical account of fission worked out in collaboration with John Wheeler in 1939. It was Bohr who had first suggested that the fission was more likely to occur with the rarer isotope uranium 235 than the more common variety uranium 238.

Bohr played an important role in advancing the study of physics in Europe. The idea of establishing CERN (Counsel European pour la Recherche Nucleaire or European Council for Nuclear Research) took shape in 1951 in a conference in Copenhagen in 1951. CERN is an international centre for theoretical and experimental physics. Contrary to Bohr’s expectation CERN was located in Geneva and not in Copenhagen, though initially for some time the theoretical branch of CERN was located in Copenhagen. After CERN Bohr helped establish a theoretical physics consortium, called Nordita (Nordisk Institute Theoretisk Atomfysik). The idea was not to compete with CERN. Denmark, Norway and Sweden participated in the establishment of Nordita, Subsequently Finland also joined. Bohr was associated with the Denmark’s Atomic Energy Commission since its inception.

Denmark was occupied by the Germans. Bohr, who had a Jewish mother, felt it necessary to escape from the occupied Denmark and eventually made his way to Los Alamos in the USA where he served as a consultant on the atomic bomb project. Bohr’s son Aage Niels Bohr also worked here as a Junior Scientific Officer. By mere presence of Bohr, the project, ‘which looked so macabre’ seemed to be hopeful . To quote Weiskopf : “In Los Alamos, we were working on something which is perhaps the most questionable, the most problematic thing a scientist can be faced with. At that time physics, our believed science, was pushed into the most cruel part of reality and we had to live it through. We were, most of us at least, young and somewhat inexperienced in human affairs, I would say. But suddenly in the midst of it, Bohr appeared in his Alamos.

It was the first time we became aware of the sense in all these terrible things, because, Bohr right a way participated not only in the work but in our discussion. Every great and deep difficulty bears in itself its own solution… This we learned from him”.

Most of Bohr’s time after the war was spent working among scientists for adequate control of nuclear weapons. In 1955 Bohr orgainsed the first Atoms for Peace Conference in Geneva.

Bohr visited India in 1960 at the invitation of Indian Science Congress Association. He attended its session in Mumbai, where he delivered two lectures on human knowledge and atoms and on the principles of quantum physics. He also visited Kolkata, Chennai, Agra and Delhi. It is to be noted that the then Prime Minister of India Jawaharlal Nehru accompanied Bohr during these visits.

Bohr died on November 18, 1962. On his death the New York Times wrote : “With the passing of Niels Bohr the world has lost not only one of the great scientists of this century but also one of the intellectual giants of all time.”

Books written by Niels Bohr

1. The Theory of Spectra and Atomic Constitution, Cambridge: Cambridge University Press; 1922.
2. Atomic Theory and the Description of Nature, Cambridge: Cambridge University Press 1934.
3. The Unity of Knowledge, New York : Doubleday & Co., 1955.
4. Atomic Physics and Human Knowledge, New York : John Wileys. 1958.

Books on Niels Bohr

1. Niels Bohr : A Century Volume. Edited by A.P. French and P.J. Kennedy Cambridge, MA Harward University Press 1985

2. Niels Bohr : The Man, His Science, and the World They Changed by Ruth Moore. Cambridge, (Massachusetts) : MIT Press 1985.

3. Niels Bohr’s Times : In Physics, Philosophy, and Policy by Abraham Pais. New York : Oxford University Press, 1991.

4. Niels Bohr : His life and Work as Seen by His Friends. Edited by S. Rozental, New York : John Wiley, 1967.

5. Niels Bohr : Gentle Genius of Denmark by Ray Spangenburg and Diane K Moser. Hyderabad : Universities Press (India) Limited 1999.

6. Niels Bohr : A Profile Edited by A.N. Mitra, L.S. Kothari, V.Singh, S.K. Trehan : New Delhi, Indian National Science Academy, 1985.

Niels Bohr
Hans Christian Andersen
Albert Einstein
Richard Feynman
Hitler
Soren Abye Kierkegaard
Lord Rayleigh
Joseph Jahn Thomson
James Clerk Maxwell
Ernst Rutherford
George Charles von Hevesy
Hans Wilhelm Geiger
Max Karl Ludwig Planck
Werner Heisenberg
Max Born
Aage Niels Bohr