Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [69]
Niels Bohr did as much as anyone to ensure that personal relations between scientists on opposing sides were restored as quickly as possible after the war. As a citizen of a neutral country, Bohr felt no resentment towards his German colleagues. He was among the first to extend an invitation to a German scientist when he asked Arnold Sommerfeld to lecture in Copenhagen. 'We had long discussions on the general principle of the quantum theory and the application of all kinds of detailed atomic problems', Bohr said after Sommerfeld's visit.55 Excluded for the foreseeable future from international meetings, German scientists and their hosts knew the value of these personal invitations. So when he received one from Max Planck to give a lecture on the quantum atom and the theory of atomic spectra in Berlin, Bohr gladly accepted. When the date was fixed as Tuesday, 27 April 1920, he was excited at the prospect of meeting Planck and Einstein for the first time.
'His must be a first-rate mind, extremely critical and far-seeing, which never loses track of the grand design', was Einstein's assessment of the young Dane, six years his junior.56 It was October 1919 and such an appraisal was a spur for Planck to get Bohr to Berlin. Einstein had long been an admirer. In the summer of 1905 as the creative storm that had broken loose in his mind began to subside, Einstein found nothing that was 'really exciting' to tackle next.57 'There would of course be the topic of spectral lines,' he told his friend Conrad Habicht, 'but I believe that a simple relationship between these phenomena and those already investigated does not exist at all, so that for the moment, the thing looks rather unpromising to me.'58
Einstein's nose for a physics problem ripe for attack was second to none. Having passed on the mystery of spectral lines, he came up with E=mc2, which said that mass and energy were interconvertible. But for all he knew, God Almighty was having a laugh at his expense by leading him 'around by the nose'.59 So when in 1913 Bohr showed how his quantised atom solved the mystery of atomic spectra, it appeared to Einstein 'like a miracle'.60
The uneasy mixture of excitement and apprehension that had taken hold of his stomach as Bohr made his way from the station to the university vanished as soon as he met Planck and Einstein. They put him at his ease by moving quickly from pleasantries to talk of physics. The two men could not have been more dissimilar. Planck was the epitome of Prussian formality and rectitude, while Einstein with his big eyes, unruly hair and trousers that were just a little too short gave the impression of a man at ease with himself, if not the troubled world in which he lived. Bohr accepted Planck's invitation to stay at his home during the visit.
His days in Berlin, Bohr said later, were spent 'discussing theoretical physics from morning to night'.61 It was the perfect break for the man who just loved to talk physics. He particularly enjoyed the lunch that the younger university physicists had thrown for him, from which they excluded all the 'bigwigs'. It was a chance for them to quiz Bohr after his lecture had left them 'somewhat depressed because we had the feeling that we had understood very little'.62 Einstein, however, understood perfectly well what Bohr was arguing and he did not like it.
Like virtually everyone else, Bohr did not believe in the existence of Einstein's light-quanta. He accepted, like Planck, that radiation was emitted and absorbed in quanta, but not that radiation itself was quantised. For him there was just too much evidence in favour of the wave theory of light, but with Einstein in the audience, Bohr told the assembled physicists: 'I shall not consider the problem of the nature of radiation.'63 However,