Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [115]
For Born, Schrödinger's equation described a probability wave. There were no real electron waves, only abstract waves of probability. 'From the point of view of our quantum mechanics there exists no quantity which in an individual case causally determines the effect of a collision', wrote Born.58 And he confessed, 'I myself tend to give up determinism in the atomic world.'59 Yet while the 'motion of particles follows probability rules', he pointed out, 'probability itself propagates according to the law of causality'.60
It took Born the time between his two papers to fully grasp that he had introduced a new kind of probability into physics. 'Quantum probability', for want of a better term, was not the classical probability of ignorance that could in theory be eliminated. It was an inherent feature of atomic reality. For example, the fact that it was impossible to predict when an individual atom would decay in a radioactive sample, amid the certainty that one would do so, was not due to a lack of knowledge but was the result of the probabilistic nature of the quantum rules that dictate radioactive decay.
Schrödinger dismissed Born's probability interpretation. He did not accept that a collision of an electron or an alpha particle with an atom is 'absolutely accidental', i.e. 'completely undetermined'.61 Otherwise, if Born was right, then there was no way to avoid quantum jumps and causality was once again threatened. In November 1926, he wrote to Born: 'I have, however, the impression that you and others, who essentially share your opinion, are too deeply under the spell of those concepts (like stationary states, quantum jumps, etc.), which have obtained civic rights in our thinking in the last dozen years; hence, you cannot do full justice to an attempt to break away from this scheme of thought.'62 Schrödinger never relinquished his interpretation of wave mechanics and the attempt at a visualisability of atomic phenomena. 'I can't imagine that an electron hops about like a flea', he once memorably said.63
Zurich lay well outside the golden quantum triangle of Copenhagen, Göttingen and Munich. As the new physics of wave mechanics spread like wildfire through Europe's physics community in the spring and summer of 1926, many were eager to hear Schrödinger discuss his theory in person. When the invitation arrived from Arnold Sommerfeld and Wilhelm Wien to give two lectures in Munich, Schrödinger readily accepted. The first, on 21 July, to Sommerfeld's 'Wednesday Colloquium', was routine and well-received. The second, on 23 July, to the Bavarian section of the German Physical Society, was not. Heisenberg, who at the time was based in Copenhagen as Bohr's assistant, had returned to Munich in time to hear both of Schrödinger's lectures before going on a hiking tour.
As he sat in the packed lecture theatre for a second time, Heisenberg listened quietly until the end of Schrödinger's talk, entitled 'New results of wave mechanics'. During the question-and-answer session that followed, he became increasingly agitated until he could no longer remain silent. As he rose to speak, all eyes were on him. Schrödinger's theory, he pointed out, could not explain Planck's radiation law, the Frank-Hertz experiment, the Compton effect, or the photoelectric effect. None could be explained without discontinuity and quantum jumps – the very concepts that Schrödinger wanted to eliminate.
Before Schrödinger could reply, with some in the audience already expressing their disapproval at the remarks of the 24-year-old, an annoyed Wien stood up and intervened. The old physicist, Heisenberg told Pauli later, 'almost threw me out of the room'.64 The pair had a history going back to Heisenberg's days as a student in Munich and his poor showing during the oral examination