“Classical mechanics has been developed continuously from the time of Newton and applied to an ever-widening range of dynamical systems, including the electromagnetic field in interaction with matter. The underlying ideas and the laws governing their application form a simple and elegant scheme, which one would be inclined to think could not be seriously modified without having all its attractive features spout. Nevertheless it has been found possible to set up a new scheme, called quantum mechanics, which is more suitable for the description of phenomena on the atomic scale and which is in some respects more elegant and satisfying than the classical scheme. This possibility is due to the changes which the new scheme involves being of a very profound character and not clashing with the features of the classical theory that make it so attractive, as a result of which all these features can be incorporated in the new scheme.”

— Paul Dirac, book Principles of Quantum Mechanics

I. The Principle of Superposition - 1. The Need for a Quantum Theory
The Principles of Quantum Mechanics (4th ed. 1958)

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theoretical physicist 1902–1984

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“The development of quantum mechanics in the 1920s was the greatest advance in physical science since the work of Isaac Newton. It was not easy; the ideas of quantum mechanics present a profound departure from ordinary human intuition. Quantum mechanics has won acceptance through its success. It is essential to modern atomic, molecular, nuclear, and elementary particle physics, and to a great deal of chemistry and condensed matter physics as well.”

Steven Weinberg (1933) American theoretical physicist

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“The classical concept of 'physical entity', be it particle, wave, field or system, has become a problematic concept since the advent of relativity theory and quantum mechanics. The recent developments in modern quantum mechanics, with the performance of delicate and precise experiments involving single quantum entities, manifesting explicit non-local behavior for these entities, brings essential new information about the nature of the concept of entity.”

Diederik Aerts (1953) Belgian theoretical physicist

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“I would not call [entanglement] one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.”

Erwin Schrödinger (1887–1961) Austrian physicist

italics in the original

DISCUSSION OF PROBABILITY RELATIONS BETWEEN SEPARATED SYSTEMS (1935)

“The actual heart of quantum mechanics is that the objects in its equations are connected to the observations very differently than the classical counterparts have been.”

Luboš Motl (1973) Czech physicist and translator

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James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“This fallacious result is not… a peculiarity of classical mechanics; it is given also by a very wide class of possible systems of mechanics. This being so, no minor modification of the classical mechanics can possibly put things right. Something far more drastic is needed; we are called upon to surrender either the [1] continuity or the [2] causality of classical mechanics, or else the possibility of [3] representing changes by motions in time and space.”

James Jeans (1877–1946) British mathematician and astronomer

Physics and Philosophy (1942)

“It has been generally believed that only the complex numbers could legitimately be used as the ground field in discussing quantum-mechanical operators. Over the complex field, Frobenius' theorem is of course not valid; the only division algebra over the complex field is formed by the complex numbers themselves. However, Frobenius' theorem is relevant precisely because the appropriate ground field for much of quantum mechanics is real rather than complex.”

Freeman Dyson (1923) theoretical physicist and mathematician

[The threefold way: algebraic structure of symmetry groups and ensembles in quantum mechanics, Jour. Math. Phys., 3, 1962, 1199–1215, https://books.google.com/books?id=nnyNUidX1OMC&pg=PA410] (p. 1200)

“It has always seemed to me that the hard thing to understand is not quantum mechanics, but where classical mechanics comes from (in the sense of how it emerges from a “measurement”).”

Peter Woit (1957) American physicist

Not Even Wrong (blog)

“Newton's laws of motion and gravitation achieved their original success when applied to the solar system. The first definite evidence that they were applicable on a larger scale came from the study of binary stars towards the eighteenth century. In recent times the limitations of Newton's theory have become apparent. Even on the scale of the solar system, it has been challenged by Einstein's.”

Gerald James Whitrow (1912–2000) British mathematician

"Newton and the Stars" (10 Oct 1957)

“I think that at that time and practically ever since, the underlying sense of form in my work has been the system of the Universe, or part thereof. For that is a rather large model to work from.”

Alexander Calder (1898–1976) American artist

1950s - 1960s, Excerpt, What Abstract Art Means to Me (1951)

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