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Dirac’s Mechanics of q Numbers (1925)

S. Brandt

Published Apr 22, 1992• 0 citations
Publisher

Abstract

Abstract The celebrated three-men paper, which gave the first extensive account of the new quantum mechanics, started by Heisenberg in the summer of 1925, appeared in print in February 1926 (see Episode 37). On its first page it contains a note added in proof : ‘In a paper by P. Dirac [1] which has appeared in the meantime independently some of the results [in the Born–Jordan paper and in the three-men paper] and new conclusions drawn from the theory have been given.’ How did it happen that in the short time available an unknown outsider could have done independent and new work?

DOI
10.1093/oso/9780199544691.003.0039

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Abstract The celebrated three-men paper, which gave the first extensive account of the new quantum mechanics, started by Heisenberg in the summer of 1925, appeared in print in February 1926 (see Episode 37). On its first page it contains a note added in proof : ‘In a paper by P. Dirac [1] which has appeared in the meantime independently some of the results [in the Born–Jordan paper and in the three-men paper] and new conclusions drawn from the theory have been given.’ How did it happen that in the short time available an unknown outsider could have done independent and new work?
Abstract The breakthrough to quantum mechanics was, as we have seen in Episode 37, achieved by Heisenberg in the summer of 1925 and the theory, in the form now known as matrix mechanics, was completed a little later by Born, Heisenberg, and Jordan. From his first days as a student Heisenberg had belonged to groups which worked on quantum theory. He was, so to speak, totally immersed in the field and reached his solution at the age of twenty-three. At the turn of the year 1925/26 quantum mechanics was created again, now clad as wave mechanics [1], by Schrödinger, who did not belong to an active research group in the field and who was then thirty-eight. As soon as it appeared wave mechanics was popular with physicists, because it was based on familiar differential equations and not on matrix formalism which was practically unknown.
The 1925 paper “On quantum mechanics” by M. Born and P. Jordan, and the sequel “On quantum mechanics II” by M. Born, W. Heisenberg, and P. Jordan, developed Heisenberg’s pioneering theory into the first complete formulation of quantum mechanics. The Born and Jordan paper is the subject of the present article. This paper introduced matrices to physicists. We discuss the original postulates of quantum mechanics, present the two-part discovery of the law of commutation, and clarify the origin of Heisenberg’s equation. We show how the 1925 proof of energy conservation and Bohr’s frequency condition served as the gold standard with which to measure the validity of the new quantum mechanics.
Heisenberg's breakthrough in his July 1925 paper that set in motion the development of Quantum Mechanics through subsequent papers by Born, Jordan, Heisenberg and also Dirac (from 1925 to 1927) is reexamined through a modern lens. In this paper, we shall discuss some new perspectives on (i) what could be the guiding intuitions for his discoveries and (ii) the origin of the Born-Jordan-Heisenberg canonical quantization rule. From this vantage point we may get an insight into Einstein's Quantum Riddle (Lande1974,Sommerfeld1918,Born1926) and a possible glimpse of what might come next after the last 100 years of Heisenberg's quantum mechanics.
Abstract This is the second of two volumes on the genesis of quantum mechanics in the first quarter of the twentieth century. It covers the period 1923–1927. It traces how the old quantum theory of Bohr and Sommerfeld gave way to the matrix mechanics of Heisenberg and others, and to the wave mechanics of Schrödinger. It then turns to the emergence of statistical transformation theory and the Hilbert space formalism unifying these two forms of the new quantum theory. It uses the metaphor of an arch built on a scaffold to capture these developments. The volume provides detailed analyses of the classic papers of Heisenberg, Born, Jordan, Pauli, Dirac, De Broglie, Einstein, Schrödinger, von Neumann, and others, which helped to lay the foundations of modern quantum mechanics. Drawing on correspondence and later reminiscences of these authors and synthesizing much of the extensive secondary literature on the breakthrough to modern quantum mechanics in the mid-1920s, this volume presents an authoritative new account of the genesis of quantum mechanics.
The report by Born and Heisenberg on ‘quantum mechanics’ may seem surprisingly difficult to the modern reader. This is partly because Born and Heisenberg are describing various stages of development of the theory that are quite different from today's quantum mechanics. Among these, it should be noted in particular that the theory developed by Heisenberg, Born and Jordan in the years 1925–6 and known today as matrix mechanics (Heisenberg 1925b [1], Born and Jordan 1925 [2], Born, Heisenberg and Jordan 1926 [4]) a differs from standard quantum mechanics in several important respects. At the same time, the interpretation of the theory (the topic of Section II of the report) also appears to have undergone important modifications, in particular regarding the notion of the state of a system. Initially, Born and Heisenberg insist on the notion that a system is always in a stationary state (performing quantum jumps between different stationary states). Then the notion of the wave function is introduced and related to probabilities for the stationary states. At a later stage, probabilistic notions (in particular, what one now calls transition probabilities) are extended to arbitrary observables, but it remains somewhat unclear whether the wave function itself should be regarded as a fundamental entity or merely as an effective one. This may reflect the different routes followed by Born and by Heisenberg in the development of their ideas. The common position presented by Born and Heisenberg emphasises the probabilistic aspect of the theory as fundamental, and the conclusion of the report expresses strong confidence in the resulting picture.