Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [119]
As he left the apartment disappointed at his failure to persuade Einstein, Heisenberg needed to make a decision. In three days' time, on 1 May, he was due in Copenhagen to begin his dual appointment as Bohr's assistant and as a lecturer at the university. However, he had just been offered an ordinary professorship at Leipzig University. Heisenberg knew it was a tremendous honour for one so young, but should he accept? Heisenberg told Einstein of the difficult choice he had to make. Go and work with Bohr, was his advice. The next day, Heisenberg wrote to his parents that he was turning down the Leipzig offer. 'If I continue to produce good papers,' he reassured himself and them, 'I will always receive another call; otherwise I don't deserve it.'15
'Heisenberg is now here and we are all very much occupied with discussions about the new development of the quantum theory and the great prospect it holds out', Bohr wrote to Rutherford in the middle of May 1926.16 Heisenberg lived at the institute in a 'cosy little attic flat with slanting walls' and a view of Faelled Park.17 Bohr and his family had moved into the plush and spacious director's villa next door. Heisenberg was such a regular visitor that he soon felt 'half at home with the Bohrs'.18 The enlargement and renovation of the institute had taken far longer than expected and Bohr was exhausted. Sapped of energy, he suffered a severe case of flu. As Bohr spent the next two months recovering, Heisenberg successfully used wave mechanics to account for the line spectrum of helium.
Once Bohr was back to his old self, living next door to him was something of a mixed blessing. 'After 8 or 9 o'clock in the evening Bohr, all of a sudden, would come up to my room and say, "Heisenberg, what do you think about this problem?" And then we would start talking and talking and quite frequently we went on till twelve or one o'clock at night.'19 Or he would invite Heisenberg over to the villa for a chat that lasted long into the evening, fuelled by glasses of wine.
As well as working with Bohr, Heisenberg gave two lectures a week on theoretical physics at the university in Danish. He was not much older than his students, and one of them could barely believe 'he was so clever since he looked like a bright carpenter's apprentice just returned from technical school'.20 Heisenberg quickly adapted to the rhythm of life at the institute and with his new colleagues enjoyed sailing, horse riding, and walking tours at the weekends. But there was less and less time for such activities after Schrödinger's visit at the beginning of October 1926.
Schrödinger and Bohr had failed to reach any sort of accord over the physical interpretation of either matrix or wave mechanics. Heisenberg saw how 'terribly anxious' Bohr was 'to get to the bottom of things'.21 In the months that followed, the interpretation of quantum mechanics was all that Bohr and his young apprentice talked about as they tried to reconcile theory and experiment. 'Bohr often came up to my room late at night to talk to me of the difficulties in quantum theory which tortured both of us', Heisenberg said later.22 Nothing caused them more pain than wave-particle duality. As Einstein told Ehrenfest: 'On the one hand waves, on the other quanta! The reality of both is firm as a rock. But the devil makes a verse out of this (which really rhymes).'23
In classical physics something can be either a particle or a wave; it cannot be both. Heisenberg had used particles and Schrödinger waves as they discovered their respective versions of quantum mechanics. Even the demonstration that both matrix and wave mechanics were mathematically equivalent had not yielded any deeper understanding of wave-particle duality. The crux of the whole problem, Heisenberg said, was that no one could answer the questions: 'Is an electron now a wave or is it a particle, and how does it behave if I do this and that and so on?'24 The harder Bohr and Heisenberg thought