Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [111]
Heisenberg was even less diplomatic about the continuity that Schrödinger was trying to restore to the atomic realm where, as far as he was concerned, discontinuity ruled. 'The more I think about the physical portion of the Schrödinger theory, the more repulsive I find it', he told Pauli in June.38 'What Schrödinger writes about the visualizability of his theory "is probably not quite right", in other words it's crap.' Two months earlier, Heisenberg had appeared more conciliatory when he described wave mechanics as 'incredibly interesting'.39 But those who knew Bohr recognised that Heisenberg was employing exactly the sort of language favoured by the Dane, who always called an idea or an argument 'interesting' when in fact he disagreed with it. Increasingly frustrated as more of his colleagues abandoned matrix mechanics for the easier-to-use wave mechanics, Heisenberg finally snapped. He could hardly believe it when Born, of all people, started using Schrödinger's wave equation. In a fit of anger, Heisenberg called him a 'traitor'.
He may have been envious of the growing popularity of Schrödinger's alternative, but after its discovery it was Heisenberg who was responsible for the next great triumph of wave mechanics. He might have been annoyed at Born, but Heisenberg had also been seduced by the mathematical ease with which Schrödinger's approach could be applied to atomic problems. In July 1926 he used wave mechanics to account for the line spectra of helium.40 Just in case anyone read too much into his adoption of the rival formulation, Heisenberg pointed out that it was nothing more than expediency. The fact that the two theories were mathematically equivalent meant he could use wave mechanics while ignoring the 'intuitive pictures' Schrödinger painted with it. However, even before Heisenberg posted his paper, Born had used Schrödinger's palette to paint an entirely different picture on the same canvas when he discovered that probability lay at the heart of wave mechanics and quantum reality.
Schrödinger was not trying to paint a new picture, but attempting to restore an old one. For him there were no quantum jumps between different energy levels in an atom, but only smooth, continuous transitions from one standing wave into another, with the emission of radiation being the product of some exotic resonance phenomenon. He believed that wave mechanics allowed the restoration of a classical, 'intuitive' picture of physical reality, one of continuity, causality and determinism. Born disagreed. 'Schrödinger's achievement reduces itself to something purely mathematical,' he told Einstein, 'his physics is wretched.'41 Born used wave mechanics to paint a surreal picture of a reality with discontinuity, acausality and probability, instead of Schrödinger's attempt at a Newtonian-inspired old master. These two pictures of reality hang on different interpretations of the so-called wave function, symbolised by the Greek letter psi, , in Schrödinger's wave equation.
Schrödinger had known from the very beginning that there was a problem with his version of quantum mechanics. According to Newton's laws of motion, if the position of an electron is known at a certain time together with its velocity, then it is theoretically possible to determine exactly where it will be at some later time. However, waves are much more difficult to pin down than a particle. Dropping a stone into a pond sends ripples of waves across its surface. Exactly where is the wave? Unlike a particle, a wave is not localised at a single place, but is a disturbance that carries energy through a medium. Like people taking part in a 'Mexican wave', a water wave is just individual water molecules bobbing up and down.
All waves, whatever their size and shape, can be described by an equation that mathematically maps their motion, just as Newton's equations do for a particle. The wave function, , represents the wave itself and describes its shape at a