Prologue: 'Not even wrong'The physicist Wolfgang Pauli was, with Heisenberg, Schrödinger and Dirac, one of the early leaders in the development of quantum mechanics.He was renowned for being a tough audience, exclaiming 'wrong' (falsch), or 'completely wrong' (ganz falsch) when he disagreed with a speaker.Near the end of his life, when asked his opinion of a recent article by a younger physicist, he sadly said 'it is not even wrong' (Das ist nicht einmal falsch) . . .A scientific idea is 'not even wrong' if it is so incomplete that it cannot be used to make predictions that could be compared to observations to see if the idea is wrong.
Prologue: 'Not even wrong'The physicist Wolfgang Pauli was, with Heisenberg, Schrödinger and Dirac, one of the early leaders in the development of quantum mechanics.He was renowned for being a tough audience, exclaiming 'wrong' (falsch), or 'completely wrong' (ganz falsch) when he disagreed with a speaker.Near the end of his life, when asked his opinion of a recent article by a younger physicist, he sadly said 'it is not even wrong' (Das ist nicht einmal falsch) . . .A scientific idea is 'not even wrong' if it is so incomplete that it cannot be used to make predictions that could be compared to observations to see if the idea is wrong.
The founding fathers of quantum mechanics had already perceived the essence of the difficulties of quantum mechanics; today, after almost a century, the discussions are still lively and, if some very interesting new aspects have emerged, at a deeper level the questions have not changed so much. What is more recent, nevertheless, is a general change of attitude among physicists: until about 1970 or 1980, most physicists thought that the essential questions had been settled, and that “Bohr was right and proved his opponents to be wrong”. This was probably a consequence of the famous discussions between Bohr, Einstein, Schrödinger, Heisenberg, Pauli, de Broglie, and others (in particular at the Solvay meetings [1–3], where Bohr's point of view had successfully resisted Einstein's extremely clever attacks). The majority of physicists did not know the details of the arguments. They nevertheless thought that the standard “Copenhagen interpretation” had clearly emerged from the infancy of quantum mechanics as the only sensible attitude for good scientists. This interpretation includes the idea that modern physics must contain indeterminacy as an essential ingredient: it is fundamentally impossible to predict the outcome of single microscopical events; it is impossible to go beyond the formalism of the wave function (or its generalization, the state vector ∣ψ) and complete it.
Abstract There is a certain similarity in the early lives of Planck and of Heisenberg [1,2]. Both had fathers who were professors at the University of Munich, both visited the Maximilians-Gymnasium in Munich, and both went to see a professor in Munich to seek advice for their studies. We have mentioned at the beginning of the Prologue how Jolly in vain tried to dissuade Planck from studying physics. Heisenberg, in his autobiographic book [3], which is devised as a series of conversations, tells us of a similar experience. In the autumn of 1920, when he was about to enter the university, his first choice was mathematics. He wanted not only to do course work but also learn about problems of current interest. This, he thought, he could do best by participating in a professor’s seminar usually reserved to advanced students and Ph.D. candidates. He asked for an interview with Lindemann , the most distinguished mathematician in Munich, who was famous for having proved that the number π is transcendental. But the old professor and the young student found no understanding, partly because Lindemann’s dog noisily interfered. The result was that Heisenberg opted for theoretical physics and went to ask Sommerfeld to be admitted to his seminar. Sommerfeld agreed. At that time Pauli, only a year older than Heisenberg, was already actively doing research under Sommerfeld. The two young men became life-long friends.
Quantum theory was not the work of one person. During a period from ca. 1900 to 1927, a collection of highly talented minds worked toward a communal understanding of the new theory and experimental results. ere were many wrong turns and not everyone who made decisive contributions fully agreed with the end result. Figure 14.1 attempts to summarize the chain of in uences in terms of ideas leading to the end products, at least as far as 1927 is concerned. Not everyone is represented here (Wolfgang Pauli, made several important contributions). I have focussed on people whose ideas will be helpful to us in understanding the nature of quantum theory. Inevitably, not everyone will agree with my choice. Figure 14.1 is only an outline at best, and the number of arrows connecting a person to the rest of the group is not an entirely fair representation of how in uential they were, as it takes no account of the importance of the ideas involved. However, as we work our way through this outline history, you might nd it useful to refer back to the gure from time to time to check your bearings.
Quantum mechanics is our most successful physical theory. It underlies our very detailed understanding of atomic physics, chemistry, and nuclear physics, and the many technologies to which physical systems in these regimes give rise. Additionally, relativistic quantum mechanics is the basis for the standard model of elementary particles, which very successfully gives a partial unification of the forces operating at the atomic, nuclear, and subnuclear levels. However, from its inception the probabilistic nature of quantum mechanics, and the fact that “quantum measurements” in the orthodox formulation appear to require the intervention of non-quantum mechanical “classical systems,” have led to speculations by many physicists, mathematicians, and philosophers of science that quantum mechanics may be incomplete. Among the Founding Fathers of quantum theory, Einstein and Schrödinger were both of the opinion that quantum mechanics is in some way unsatisfactory, and this view has been amplified in more recent profound work of John Bell, among others. In an opposing camp, many others in the physics, mathematics, and philosophy communities have attempted to provide an interpretational foundation in which quantum mechanics remains a complete and self-contained system. Among the Founding Fathers, Bohr, Born, and Heisenberg maintained that quantum mechanics is a complete system, and a number of recent proposals have been made to improve upon or to provide alternatives to their “Copenhagen Interpretation.” The debate continues, and has spawned an enormous literature.
At the beginning of the twentieth century, it became clear that microscopic particles do not obey Newton's laws of classical mechanics. The idea of light quanta was at odds with the established wisdom that light is a wave phenomenon, of course, but no other explanation of the photoelectric effect seemed possible. The next step in the development of quantum mechanics was taken by Niels Bohr, who introduced postulates about the discreteness of the energy of the bound electrons in an atom similar to the discreteness of the energy of light quanta. Modern quantum mechanics was founded by Werner Heisenberg, Max Born, Erwin Schroedinger, Paul Adrian Maurice Dirac, Wolfgang Pauli, Enrico Fermi, and others. In quantum mechanics, it is in general fundamentally impossible to determine the exact outcome of an experiment; one can only determine the probability of any given outcome.