“Heisenberg’s starting point was the philosophical judgment, that a physical theory should not concern itself with things like electron orbits in atoms that can never be observed. This is a risky assumption, but in this case it served Heisenberg well.”

— Steven Weinberg

Source: Lectures on Quantum Mechanics (2012, 2nd ed. 2015), Ch. 1: Historical Introduction

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Steven Weinberg 46

American theoretical physicist 1933

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“In the interior of the atom, Bohr had tried the plan of retaining the particle-electron and modifying the classical mechanics. Heisenberg took the opposite course, his procedure amounting in effect to retaining the classical mechanics, at least in form, and modifying the electron. Actually, the electron dropped out all together, because it exists only as a matter of inference and not of direct observation. For the same reason, the new theory contains no mention of atoms, nuclei, protons, or of electricity in any shape or form. The existences of all these are matters of inference, and Heisenberg's purely mathematical theory could no more make contact with them than with the efficiency of a turbine or with the price of wheat.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“In 1925 Heisenberg made a new attempt, on entirely novel lines, to obtain an explanation of atomic spectra. Working in collaboration with Bohr, he had come to the conclusion that the imperfections of Bohr's theory had been a consequence of assuming too simple a model for the atom. For Bohr had not only assumed that the atom consisted of particles moving through space and time, but also that the particles inside atoms were of the same kind as the electrons outside atoms.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“Quantum mechanics explains the physical stability of the atom by predicting its allowed discrete energy levels and defining the wave functions (also called atomic orbitals) associated with each energy level. The orbitals determine the probability density for finding the electrons at particular locations in the atom when the electrons are in a specific quantum state.”

David W. Oxtoby (1951) President of Pomona college

Principles of Modern Chemistry (7th ed., 2012), Ch. 5 : Quantum Mechanics and Atomic Structure

“Heisenberg now approached the problem from a new philosophical angle. He discarded all models, pictures and parables, and made a clear distinction between sure knowledge we gain from observation of nature and the conjectural knowledge we introduce when we use models, pictures and parables. Sure knowledge… can only be numerical, so that Heisenberg's results were inevitably mathematical in form, and could not disclose anything about the true nature of physical properties or entities.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“With Heisenberg's uncertainty principle, the fundamental role of experimental errors became a basic feature of physics.”

Léon Brillouin (1889–1969) French physicist

[Léon Brillouin, Science and Information Theory, second edition, Academic Press, New York, 1962, 0-48643-918-6, 304]

“…a detailed mathematical discussion shows that whatever kind of wave-packet we select to represent the electron, the product of the two uncertainties of position and momentum can never be less than h, which is precisely what Heisenberg found…”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“Heisenberg finds that facts of observation lead uniquely and inevitably to the theoretical structure known as matrix mechanics. This shows that the total radiation in any region of empty space can change only by a single complete quantum at a time. Thus not only in the photo-electric phenomenon, but in all other transfers of energy through space, energy is always transferred by complete quanta; fractions of a quantum can never occur. This brings atomicity into our picture of radiation just as definitely as the discovery of the electron and its standard charge brought atomicity into our picture of matter and of electricity.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“No fairer destiny could be allotted to any physical theory, than that it should of itself point out the way to the introduction of a more comprehensive theory, in which it lives on as a limiting case.”

Albert Einstein book Relativity: The Special and the General Theory

Es ist das schönste Los einer physikalischen Theorie, wenn sie selbst zur Aufstellung einer umfassenden Theorie den Weg weist, in welcher sie als Grenzfall weiterlebt.
Über die spezielle und die allgemeine Relativitätstheorie (1920) Tr. Robert W. Lawson, Relativity: The Special and General Theory (1920) pp. 90-91.
1920s

“To remove these one had to account for an “observer” (that is at least for one subject):
(i) Observations are not absolute but relative to an observer’s point of view (i. e., his coordinate system: Einstein);
(ii) Observations affect the observed so as to obliterate the observer’s hope for prediction (i. e., his uncertainty is absolute: Heisenberg).
After this, we are now in the possession of the truism that a description (of the universe) implies one who describes it”

Heinz von Foerster (1911–2002) Austrian American scientist and cybernetician

observes it
Source: 1980s, Notes on an epistemology for living things, 1981, p.258

“Heisenberg’s starting point was the philosophical judgment, that a physical theory should not concern itself with things like electron orbits in atoms that can never be observed. This is a risky assumption, but in this case it served Heisenberg well.”

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Steven Weinberg 46

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