Niels Henrik David Bohr, Hon.F.R.S.E.

by N Kemmer

Niels Bohr's death on November 18, 1962, brought to physicists throughout the world quite special feelings of loss and grief. Bohr's standing as, a leader in theoretical physics was rivalled by none. He had been associate, teacher, host and friend to so many during his full scientific life that there can be few centres of physics in the world in which his absence will not be felt personally.

To those who did not know him and to future generations his personality will no doubt seem to have a paradoxical side: here was the theorist whose thinking will for ever be associated with the idea of precise analysis of limits to experimentation, whose every pronouncement emphasizes the need for theory to be based on that which is observable - yet this was the Niels Bohr whose friends affectionately remember his childlike helplessness in relation to objects in the inanimate world around him.

Here was the physicist whose published work abounds in landmarks in the history of physics - yet this was the Niels Bohr whose failure to communicate with his fellow men in speech or lecture and whose reluctance to put pen to paper were legendary.

Here was the man whose native land honoured him above all fellow citizens, who occupied a mansion of honour and was host to royalty - yet this was the Niels Bohr renowned among his friends for his simplicity and modesty.

In the spirit of Bohr's own Principle of Complementarity it must simply be recorded that such indeed was the whole man.

Niels Bohr was born on October 7, 1885, the son of Christian Bohr, Professor of Physiology at Copenhagen University. His brilliant record at school and at Copenhagen University is remembered, but so is his distinction as a football player, though in this latter field he was surpassed by his brother, Harald, the mathematician.

Bohr emerges as a great figure in the international scientific scene around 1913. After completing his doctoral dissertation, on the electron theory of metals in 1911, and gaining the degree of Dr. Phil. he visited England and in Manchester made his first contact with Ernest Rutherford. This was the beginning of a remarkable association which will always be remembered in the history of atomic physics. It is so well known that brief mention must suffice here. Rutherford, by brilliant experiment and straightforward reasoning had just demonstrated the existence of the atomic nucleus; in so doing he provided the first real insight into the structure of matter. But at the same time new difficulties in the way of consistent description of atomic structure were revealed: a stable nuclear atom is an impossibility according to classical physical theory. In 1913 Bohr published his three great papers On the Constitution of Atoms and Molecules, which contained the idea of stable non-radiating atomic states and the complete theory of the hydrogen atom. They point the way to the construction of a detailed theory of atomic phenomena, based partly on classical physics and partly on an entirely new principle stemming from the concept of a smallest "quantum of action" first introduced by Planck in 1900. In the hands of Bohr and others, among whom Sommerfeld deserves particular mention, this new Quantum Theory of atomic structure had many successes during the next decade, but it was not an entirely consistent description of phenomena. The quantum concept demanded a complete break with classical thinking and many physicists were unable to accept such a radical step. Bohr himself, however, was entirely convinced that experimental evidence did not leave room for anything less drastic. This need for a departure from classical ideas was central to Bohr's work in this period; at the same time, in stating his Correspondence Principle and analyzing our knowledge in the light of it, Bohr made clear how the vast body of evidence supporting classical physics could still be valid in the limiting case of large quantum numbers. Experimentation in the atomic domain had for the first time taken physicists beyond the region in which the classical approximation is acceptable.

In 1914 Bohr became Schuster Reader in Mathematical Physics at Manchester and although he did not hold that post for long, it consolidated the great friendship with Rutherford which lasted till Rutherford's death.

Since 1916 Bohr headed the Theoretical Physics Department of Copenhagen University, and from I 92o he directed his great "Universitetets Institut for Teoretisk Fysik" there, soon to become famous throughout the world as the great centre of research at which nearly all the leading theorists of the period could be found at one time or another. Today there are many busy institutes devoted to theoretical physics and it is difficult to appreciate the unique position enjoyed by the Copenhagen Institute in the 'twenties and 'thirties.

In 1922 Bohr was awarded the Nobel Prize for Physics.

The transitional stage in atomic theory came to an end in 1925 when Heisenberg, Born and Jordan formulated their Quantum Mechanics. This theory was followed shortly afterwards by Schrödinger's Wave Mechanics, but the two were soon recognized to be equivalent and henceforth they developed as one. Quantum Mechanics provides the permanent foundation of the entire edifice of modern theoretical physics. It represents an even clearer break with classical physics than Bohr's original description of the behaviour of atoms, and much deep analysis was needed to demonstrate that it is a consistent and acceptable description of reality. It is characteristic of Bohr that he was in the lead among those concerned with the foundations of the theory. It was not difficult, just after 1925, for any competent theorist to push forward significantly our understanding of a particular atomic phenomenon, by treating it according to the rules of the new mechanics, but the problem that occupied Bohr was the meaning of these rules themselves, which from a classical standpoint would appear to be self-contradictory. In terms of the notion of Complementarity, Bohr analysed the situation and showed to the satisfaction of most physicists that the apparent contradictions never relate to observations or their interpretation, and arise only because the mental picture one must use to correlate experience is of a new kind, unforeseen in classical physics. It must nevertheless be recorded that a minority of physicists have found themselves unable to accept this view of Quantum Mechanics, so strongly advocated by Bohr. In particular, Einstein disagreed with Bohr's position throughout his life, in spite of long and forceful, though always friendly, debates.

In the history of physics the 1920'S mark the unravelling of atomic structure; in the 1930's attention turned to the structure of the nucleus and Niels Bohr again took a lead in the new field. In 1936 he put forward the "compound nucleus" picture which, like so much of his earlier work, became the starting point of a new trend of theoretical exploration. Characteristically, this picture of the nucleus led away again from the pattern of thinking, by now firmly established in the theorists' minds, which had proved appropriate for the description of the outer atom. Bohr stresses that in the conditions of strong many-particle interactions existing in the nucleus, quite different types of dynamic behaviour must be expected - and indeed are observed. In some respects a more classical type. of description then becomes possible again, and in this spirit Bohr himself, together with Wheeler, gave a full theoretical account of nuclear fission shortly after its discovery in 1939.

At this point the history of physics and the story of Bohr's life merge with world history. There is no need to retell here the consequences of the fact that knowledge and understanding of nuclear fission were acquired on the eve of world conflict. Bohr spent the early years of German occupation in Denmark, where his many acts of aid to colleagues and friends in danger and distress are remembered with deep gratitude. Then, with arrest imminent, he escaped to Sweden, thence to England and the United States. It was inevitable that his cooperation was sought in the enormous project of building military weapons based on nuclear fission. At Los Alamos, under the code name of Nicholas Baker, Bohr became one of the leaders of the illustrious team of physicists who had put pure research to one side, convinced of the vital need to beat the enemy in the race for this weapon. But even while on that work, Bohr was conscious of a truth not then clearly discerned, that the power now accessible to man could well mean the destruction of the human race and that conditions necessary for survival must be worked out rationally. He was a leading advocate of the need to embrace new ideals of mutual trust and openness in international relations; later, in 195o, he gave the fullest expression to these views in his "Open Letter" to the United Nations.

Back in Copenhagen after the war, Bohr gradually relinquished his direct responsibilities in his great Institute, but his influence on scientific life in Copenhagen, and internationally, remained great. It was his presence above all that made Copenhagen the natural place, first for the theoretical division of CERN, the European Nuclear Research organization (now entirely at Geneva) and later for the Nordic Institute for Theoretical Atomic Physics, NORDITA. The Copenhagen Institute is now directed by Niels Bohr's son, Aage, who has established himself as a leader among the present generation of nuclear theorists. Mrs Margarethe Bohr survives her husband; the esteem and affection in which she and her husband were held throughout a long and happy partnership will continue to go out to her from all their many friends throughout the world.

Bohr was the Society's Senior Honorary Fellow at the time of his death, and his name has occupied an honoured place on the Roll since 1927.

Niels Bohr's RSE obituary by N Kemmer appeared in Royal Society of Edinburgh Year Book 1964, 10-14.

[See also Biographical Memoirs of Fellows of the Royal Society 9 1963, 37-53.]