Neils Bohr – Physicist Nonpareil

While the finish given to our picture of the world by the theory of relativity has already been absorbed into the general scientific consciousness, this has scarcely occurred to the same extent with those aspects of the general problem of knowledge which have been elucidated by the quantum – NEILS BOHR

After Albert Einstein, Neils Bohr was probably the most respected theoretical physicist of the 20th century.

He was the scion of a very distinguished scientific family in Copenhagen, Denmark. His father Christian was a Professor of physiology and his brother Harold a mathematician of great distinction. His own son Agee, was a brilliant physicist who also won the 1975 Nobel Prize for physics.

Bohr obtained his doctorate in 1911 from the university of Copenhagen, where he was educated. He spent four extremely productive years with Rutherford in Manchester; Bohr then returned to Denmark where he became, in 1918, Director of the newly created institute of theoretical physics.

The institute became one of the most exciting research centres in the world under Bohr. A galaxy of physicists from around the world were to pass through it and eventually it was to bestow on the orthodox account of Quantum theory, the apt description of the ‘ Copenhagen interpretation’.

Bohr published in 1913 a classic paper on the constitution of atoms and molecules, in which he used the quantum of energy ” h” introduced into physics by Max Planck in 1900, to rescue Rutherford’s account of atomic structure from a vital objection and also to account for the line spectrum of hydrogen. The first problem Bohr faced was to explain the stability of the atom. Rutherford’s 1911 model of the atom with the electrons orbiting a central nucleus (the so- called planetary model) was theoretically unstable. This was because, unlike Planets orbiting the sun, electrons of charged particles, which according to classical physics radiate energy and consequently spiral in toward the nucleus.

Bohr theorised that there were ‘stationary’ orbits for the electrons in which the electron did not radiate energy. He further assumed that such orbits occurred when the electrons had definite values of angular momentum, specifically values h/2 pi, 2h/2pi, 3h/2 pi etc. where his Planck’s “h” constant. Using this idea he was able to calculate energies, E1, E2, E3 etc. for possible orbits of the electrons. He further postulated that emission of light occurred when an electron moved from one orbit to a lower – energy orbit; absorption was accompanied by a change to a higher- energy orbit. In each case the energy difference produced radiation of energy hv, where v is the frequency. In 1913 he realised that, using this idea, he could obtain a theoretical formula similar to the empirical formula of Johannes Balmer for a series of lines in the hydrogen spectrum.

Bohr received the Nobel Prize for Physics for this work in 1922. The Bohr theory was developed further by Arnold Sommerfeld.

Bohr also made other major contributions to this early development of quantum theory. The correspondence principle (1916) is his principle that the quantum-theory description of the atom corresponds to classical physics at large magnitudes.

Bohr publicly formulated the’ complementarity’ principle 1927. He argued against continuing attempts to eliminate such supposed difficulties at the wave-particle duality of light and many other atomic phenomena. His starting point was the impossibility to distinguish satisfactorily between the actual behaviour of atomic objects, and their interactions with the measuring instruments that serve to define by conditions under which the phenomena appear. If you examine light with one instrument the argument went, it undulates like a wave; select another and it scatters like a particle. His conclusion was that evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complimentary in the sense that only the totality of the phenomenon exhausts the possible information about the objects. It was a principle Bohr remained faithful to even representing it on his coat of arms in 1947 with the motto ‘contraria sunt complementa’ above the Yin/ Yang symbols. Together with indeterminacy principle of Heisenberg and the probability waves of Max Born, the principle emerged from the 1930 Solvay conference (the last one Einstein attended) as the most authoritative and widely accepted theory to describe atomic phenomena.

Bohr also made major contributions tothe work on radioactivity that led to the discovery and exploitation of nuclear fission. Bohr’s liquid- drop model of the nucleus, which was published in 1936, provided the basics of the first theoretical account of fission worked out in collaboration with John Wheeler in 1939. It was also Bohr who in 1939, made the crucial suggestion that fission was more likely to occur with the rarer isotope uranium 235 than the more common variety uranium 238.

In 1943 Bohr, who had a Jewish mother, felt it necessary to escape from occupied Denmark. He made his way to Los Alamos in America where he served as a consultant on the atomic bomb project. He was quick to appreciate the consequences of using such weapons and in 1944, made an early approach to Roosevelt and Churchill proposing that such obvious danger could perhaps be used to bring about a rapprochement between Russia and the West. Scientists were in unique position, he argued, in having their soviet contacts and the knowledge to make the first approach. Much of Bohr’s time after the war was spent working, among Scientists, for adequate controls of nuclear weapons and in 1955, he organised the first atoms for peace conference in Geneva. Humanity will be confronted with dangers of unprecedented character unless, in due time, measures can be taken to forestall a disastrous competition in such formidable armaments and to establish an international control of the manufacture and use of the powerful materials.

He predicted the existence of a Zirconium-like element, which was named Hafnium after the Latin name for Copenhagen, where it was discovered. Later, the element Bohrium was named after him. Robert Oppenheimer credited Bohr with acting “as a scientific father figure to the younger men”, most notably Richard Feynman.

 Bohr received numerous honours and accolades. In addition to the Nobel Prize, he received the Hughes Medal in 1921, the Matteucci Medal in 1923, the Franklin Medal in 1926, the Copley Medal in 1938, the Order of the Elephant in 1947, the Atoms for Peace Award in 1957 and the Sonning in 1961. He became foreign member of the Royal Netherlands Academy of Arts and Sciences in 1923, and of the Royal Society in 1926. The Bohr model’s semicentennial was commemorated in Denmark on 21 November 1963 with a postage stamp depicting Bohr, the hydrogen atom and the formula for the difference of any two hydrogen energy levels:

There are a couple of interesting anecdotes concerning Bohr.

An American scientist once visited the offices of the great Nobel prize winning physicist, in Copenhagen. He was amazed to find that over Bohr’s desk was a horseshoe, securely nailed to the wall, with the open end up in the approved manner (so it would catch the good luck and not let it spill out). The American said with a nervous laugh, “Surely you don’t believe the horseshoe will bring you good luck, do you Professor Bohr?” After all, as a scientist Bohr chuckled mischievously. “I believe no such thing, my good friend. Not at all. I am scarcely likely to believe in such foolish nonsense. However, I am told that a horseshoe will bring you good luck whether you believe in it or not.”

Einstein liked inventing phrases such as “God does not play dice with the universe,” and “The Lord is subtle but not malicious.” On one occasion Bohr answered, “Einstein, stop telling God what to do.”

There is another version. Einstein described his “private opinion” of quantum physics in one of the 1945 letters by referencing a phrase that he had already made famous: “God does not play dice with the universe.” In the letter, he wrote: “God tirelessly plays dice under laws which he has himself prescribed.” Apparently Bohr replied. ” God does play dice with the universe, but he throws them where they can’t be seen”. This last sentence has been attributed to Stephen Hawking also.