Saving Bohr

Simon Burton

Michael D. Gordin

Françoise Balibar

SUMMARY. — The idea of complementarity may be viewed as the core of Bohr's strategy, in 1927, to save both observation and definition. To this end, Bohr sought to incorporate, within the new quantum theory, the usual concepts of space and time, as well as the invariance principles and their associated conservation laws. At the time, Bohr saw himself as following in the footsteps of Einstein, who, in 1905, used a similar strategy to resolve the apparent contradiction between the constancy of the velocity of light and galilean invariance. By clarifying Bohr's deliberately vague notions, Dirac made the search for invariance transformations the aim of quantum physics. Dirac's work thus illustrates the importance of Bohr's of 1927 for the future development of theoretical physics.

Hrvoje Nikolić

In 1930, Einstein argued against the consistency of the time–energy uncertainty relation by discussing a thought experiment involving a measurement of the mass of the box which emitted a photon. Bohr seemingly prevailed over Einstein by arguing that Einstein's own general theory of relativity saves the consistency of quantum mechanics. We revisit this thought experiment from a modern point of view at a level suitable for an undergraduate readership and find that neither Einstein nor Bohr was correct. Instead, this thought experiment should be thought of as an early example of a system demonstrating nonlocal 'EPR' quantum correlations, five years before the famous Einstein–Podolsky–Rosen paper.

Paul A. Flowers, Edward J. Neth, William R. Robinson +2 more

Michela Massimi

The exclusion principle was the final outcome of Pauli's struggle to understand some spectroscopic anomalies in the early 1920s: doublets were observed in the spectra of alkali metals, singlets and triplets in the spectra of the alkaline earths, and even more anomalous patterns were observed when chemical elements were placed in an external magnetic field (anomalous Zeeman effect and Paschen–Back effect). These anomalous spectra challenged the old quantum theory, and prompted a radical theoretical change (Section 2.1). From 1920 to 1924 Alfred Landé, Werner Heisenberg, and Niels Bohr were all engaged in trying to save the traditional spectroscopic model (the so-called atomic core model) and to reconcile it with the observed anomalies. The impasse was solved only with Pauli's introduction of a fourth degree of freedom for the electron, and the consequent demise of the atomic core model (Section 2.2). What Pauli called the ‘twofoldness’ [ Zweideutigkeit ] of the electron's angular momentum was soon reinterpreted as the electron's spin (Section 2.3). Pauli's exclusion rule was announced in this semi-classical spectroscopic context that characterized the revolutionary transition from the old quantum theory to the new quantum theory around 1925. The prehistory of Pauli's exclusion principle Atomic spectra and the Bohr–Sommerfeld theory of atomic structure The existence of spectral lines had been known to scientists since the beginning of the nineteenth century when Wollaston and Fraunhofer first observed the dark absorption lines in the spectrum of the Sun.