Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [97]
'Hopefully you will then take atomic theory forward in good measure and solve several of the problems with which I have tormented myself in vain and which are too difficult for me', Pauli wrote to Bohr.23 'I hope also that Heisenberg will then bring back home a philosophical attitude in his thinking.' By the time the young German arrived, Bohr had been well briefed. Throughout the two-week visit, the principles of physics rather than any particular problem was the focus of their discussions as Bohr and Heisenberg strolled through Faelledpark next to the institute or chatted over a bottle of wine in the evenings. Many years later, Heisenberg described his time in Copenhagen in March 1924 as a 'gift from heaven'.24
'I shall, of course, miss him (he is a charming, worthy, very bright man, who has become very dear to my heart), but his interest precedes mine, and your wish is decisive for me', Born wrote to Bohr after Heisenberg received an invitation for an extended stay in Copenhagen.25 Due to spend the forthcoming winter semester teaching in America, Born would not need the services of his assistant until May the following year. At the end of July 1924, having successfully completed his habilitation thesis and gained the right to teach at German universities, Heisenberg left for a three-week hiking tour around Bavaria.
When he returned to Bohr's institute on 17 September 1924, Heisenberg was still only 22 years old, but had already written or co-written an impressive dozen papers on quantum physics. He still had much to learn and knew that Bohr was the man to teach him. 'From Sommerfeld I learned optimism, in Göttingen mathematics, from Bohr physics', he said later.26 For the next seven months, Heisenberg was exposed to Bohr's approach to overcoming the problems that plagued quantum theory. While Sommerfeld and Born were also troubled by the same inconsistencies and difficulties, neither man was haunted like Bohr by them. He could hardly bring himself to talk of anything else.
From these intense discussions, Heisenberg 'realized how difficult it was to reconcile the results of one experiment with those of another'.27 Among these experiments was Compton's scattering of X-rays by electrons that supported Einstein's light-quanta. The difficulties just seemed to multiply with de Broglie's extension of wave-particle duality to encompass all matter. Bohr, having taught Heisenberg all that he could, had great hopes for his young protégé: 'Now everything is in Heisenberg's hands – to find a way out of the difficulties.'28
By the end of April 1925, Heisenberg was back in Göttingen, thanking Bohr for his hospitality and 'sad about the fact that I must carry on wretchedly alone by myself in the future'.29 Nevertheless, he had learned a valuable lesson from discussions with Bohr and in his ongoing dialogue with Pauli: something fundamental had to give. Heisenberg believed he knew what that might be as he tried to solve a long-standing problem: the intensitie of the spectral lines of hydrogen. The Bohr-Sommerfeld quantum atom could account for the frequency of hydrogen's spectral lines, but not how bright or dim they were. Heisenberg's idea was to separate what was observable and what was not. The orbit of an electron around the nucleus of a hydrogen atom was not observable. So Heisenberg decided to abandon the idea of electrons orbiting the nucleus of an atom. It was a bold step, but one he was now ready to take, having long detested attempts at pictorial representations of the unobservable.
As a teenager in Munich, Heisenberg 'was enthralled by the idea that the smallest particles of matter might reduce to some mathematical form'.30 At about the same time he came across an illustration in one of his textbooks that he found appalling. To explain how one