The structure of Stern-Gerlach experiments and Ludwig's approach to quantum theory

Сергей Рашковский

K. Heyde, John L. Wood

Jared R. Stenson, Jean-François S. Van Huele

M. Arsenijević, J. Jeknić-Dugić, M. Dugić

We employ the Stern-Gerlach experiment to highlight the basics of a minimalist, non-interpretational top-down approach to quantum foundations. Certain benefits of the here highlighted "quantum structural studies" are detected and discussed. While the top-down approach can be described without making any reference to the fundamental structure of a closed system, the hidden variables theory ?a la Bohm proves to be more subtle than it is typically regarded.

Břetislav Friedrich

ABSTRACT The bulk of experiments and observations that lined the path to quantum mechanics amounted to a backlog of data in need of explanation. A notable exception was the Stern–Gerlach experiment (SGE) designed to test the predictions of the old quantum theory as embodied by the 1913–1916 Bohr–Sommerfeld–Debye atom model. After its initial triumphs, such as the explanation of the astrophysically observed spectrum of , this model was frowned upon in some quarters, mainly because of its ad hoc nature. Otto Stern (1888–1969)—and his close colleague and friend, Max von Laue (1879–1960)—even took a vow: “If this nonsense of Bohr should, in the end, prove to be right, we will quit physics.” Stern would go further yet and, in 1920, devise an experiment—the SGE—to disprove Bohr's model. However, contrary to Stern's expectations, the SGE ruled unequivocally in favor of the model and its amendments. Wolfgang Pauli (1900–1958) quipped: “That should convert even the non‐believer Stern.” And it did: Confronted with the outcome of the SGE, Stern noted in 1961: “I still have objections to the idea of beauty of quantum mechanics. But she is correct.” Ironically, the SGE would epitomize quantum mechanics as we know it: whether quantum measurement, state preparation, coherence, or entanglement—in addition to the quantization of angular momentum and its spatial projections—the SGE has it all. It now seems that the SGE may even impact future studies of the relationship of quantum theory to other pillars of physics, most notably, general relativity. The descendants of the SGE include transformative techniques for the study of the quantum properties of nuclear, atomic, molecular, and condensed‐matter systems, with magnetic resonance imaging being among the most widely used. Key Points Edward Pickering; Williamina Fleming; Pickering series; Bohr‐Sommerfeld‐Debye model of the atom; Otto Stern; Stern‐Gerlach experiment; Bohr magneton; Old Quantum Theory; Spin; nuclear magneton; magnetic dipole moment of the proton; matter‐wave duality; Quantum Mechanics.

Daniel E. Platt

While the Stern–Gerlach experiment is an old and familiar problem, no analysis of this experiment is presented in the pedagogical literature using modern quantum mechanical techniques. Expositions of the Stern–Gerlach experiment are usually based on ‘‘old quantum theory,’’ i.e., semiclassical Bohr–Sommerfeld quantum mechanics. The experiment is also a popular one in discussions of the postulate of quantum measurement, which asserts that the process of measuring an observable forces the state vector of the system into an eigenvector of that observable, and the value measured will be the eigenvalue of that eigenvector. The most enduring of the philosophical arguments associated with quantum mechanics ultimately revolves around the mechanism of quantum measurement. Since the postulate of quantum measurement is expressed in terms well outside the realm of the old quantum mechanics, the traditional analysis of the Stern–Gerlach experiment cannot reveal any information about the relationship between dynamics and measurement. Therefore, a modern analysis of the Stern–Gerlach experiment must be made if this experiment is to reveal any information about the relationship between dynamics and the mechanism of quantum measurement. This paper presents such an analysis, and develops some of the implications for the theory of quantum measurement.