Quantum mechanics

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Those who are not shocked when they first come across quantum theory cannot possibly have understood it. ~ Niels Bohr

Quantum mechanics is a first quantized quantum theory that supersedes classical mechanics at the atomic and subatomic levels. It is a fundamental branch of physics that provides the underlying mathematical framework for many fields of physics and chemistry.

Quantum mechanics is sometimes used in a more general sense, to mean quantum physics.


I think I can safely say that nobody understands quantum mechanics. ~ Richard Feynman
  • Quantum mechanics was, and continues to be, revolutionary, primarily because it demands the introduction of radically new concepts to better describe the world. In addition we have argued that conceptual quantum revolutions in turn enable technological quantum revolutions.
    • Alain Aspect, "Introduction: John Bell and the second quantum revolution", in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics (2nd ed, 2004)
  • No other theory of the physical world has caused such consternation as quantum theory, for no other theory has so completely overthrown the previously cherished concepts of classical physics and our everyday apprehension of reality. For philosophers, it has been a romping ground of epistemological adventure of pessimism about science's ability to expose ultimate truth. For physicists, it has required a confrontation with the nature of physical reality and a heady inhalation of new attitudes. For all scientists and technologists, it has been the key to advances in all fields of endeavor, from genetics to superconductivity.
    The extraordinary feature of quantum theory is that although we do not understand it, we can apply the rules of calculation it inspires, and compute properties of matter to unparalleled accuracy, in some cases with a precision that exceeds that currently obtained from experiment.
  • … that what is proved, by impossibility proofs, is lack of imagination.
  • I am a Quantum Engineer, but on Sundays I Have Principles.
    • John Stewart Bell Opening sentence of his "underground colloquium" in March 1983, as quoted by Nicolas Gisin in an edition by J. S. Bell, Reinhold A. Bertlmann, Anton Zeilinger (2002). Quantum [un]speakables: from Bell to quantum information. Springer. p. 199. ISBN 3540427562. 
  • If the price of avoiding non-locality is to make an intuitive explanation impossible, one has to ask whether the cost is too great.
  • For those who are not shocked when they first come across quantum theory cannot possibly have understood it.
    • Niels Bohr, in 1952, quoted in Heisenberg, Werner (1971). Physics and Beyond. New York: Harper and Row. pp. 206. 
  • The power of the new quantum mechanics in giving us a better understanding of events on an atomic scale is becoming increasingly evident. The structure of the helium atom, the existence of half-quantum numbers in band spectra, the continuous spatial distribution of photo-electrons, and the phenomenon of radioactive disintegration, to mention only a few examples, are achievements of the new theory which had baffled the old.
    • Arthur Compton, Foreword to the English edition of The Physical Principles of the Quantum Theory by W. Heisenberg (1930)
  • For me, the important thing about quantum mechanics is the equations, the mathematics. If you want to understand quantum mechanics, just do the math. All the words that are spun around it don’t mean very much. It’s like playing the violin. If violinists were judged on how they spoke, it wouldn’t make much sense.
  • However unfamiliar this direct interparticle treatment compared to the electrodynamics of Maxwell and Lorentz, it deals with the same problems, talks about the same charges, considers the interactions of the same current elements, obtains the same capacitances, predicts the same inductances and yields the same physical conclusions. Consequently action-at-a-distance must have a close connection with field theory.
  • ...the "paradox" is only a conflict between reality and your feeling of what reality "ought to be."
    • Richard Feynman, in The Feynman Lectures on Physics, vol III, p. 18-9 (1965)
  • I think I can safely say that nobody understands quantum mechanics.
  • We have always had a great deal of difficulty understanding the world view that quantum mechanics represents. At least I do, because I'm an old enough man that I haven't got to the point that this stuff is obvious to me. Okay, I still get nervous with it.... You know how it always is, every new idea, it takes a generation or two until it becomes obvious that there's no real problem. I cannot define the real problem, therefore I suspect there's no real problem, but I'm not sure there's no real problem.
    • Richard Feynman, in Simulating Physics with Computers appearing in International Journal of Theoretical Physics (1982) p. 471.
  • Quantum theory was split up into dialects. Different people describe the same experiences in remarkably different languages. This is confusing even to physicists.
    • David Finkelstein, in Physical Process and Physical Law, in an edition by Timothy E. Eastman, Hank Keeton (2004). Physics and Whitehead: quantum, process, and experience. SUNY Press. p. 181. ISBN 0791459136. 
  • Physicists do not believe quantum mechanics because it explains the world, but because it predicts the outcome of experiments with almost miraculous accuracy. Theorists kept predicting new particles and other phenomena, and experiments kept bearing out those predictions.
  • Erwin with his psi can do
    Calculations quite a few.
    But one thing has not been seen:
    Just what does psi really mean?
    • Erich Hückel, translated by Felix Bloch and quoted in Traditions et tendances nouvelles des études romanes au Danemark (1988) by Ebbe Spang-Hanssen and Michael Herslund, p. 207; also in The Pioneers of NMR and Magnetic Resonance in Medicine : The Story of MRI‎ (1996) by James Mattson and Merrill Simon, p. 278
  • It is often stated that of all the theories proposed in this century, the silliest is quantum theory. In fact, some say that the only thing that quantum theory has going for it is that it is unquestionably correct.
  • In his standoff with Dr. Ramsay of Harvard last fall, Dr. Leggett suggested that his colleagues should consider the merits of the latter theory. "Why should we think of an electron as being in two states at once but not a cat, when the theory is ostensibly the same in both cases?" Dr. Leggett asked.
    Dr. Ramsay said that Dr. Leggett had missed the point. How the wave function mutates is not what you calculate. "What you calculate is the prediction of a measurement," he said.
    "If it's a cat, I can guarantee you will get that it's alive or dead," Dr. Ramsay said.
    David Gross, a recent Nobel winner and director of the Kavli Institute for Theoretical Physics in Santa Barbara, leapt into the free-for-all, saying that 80 years had not been enough time for the new concepts to sink in. "We're just too young. We should wait until 2200 when quantum mechanics is taught in kindergarten."
    • Dennis Overbye, "Quantum Trickery: Testing Einstein's Strangest Theory", The New York Times (Dec. 27, 2005)
  • Respectable scientists like de Broglie himself accept wave mechanics because it confers coherence and unity upon the experimental findings of contemporary science, and in spite of the astonishing changes it implies in connection with ideas of causality, time, and space, but it is because of these changes that it wins favor with the public. The great popular success of Einstein was the same thing. The public drinks in and swallows eagerly everything that tends to dispossess the intelligence in favor of some technique; it can hardly wait to abdicate from intelligence and reason and from everything that makes man responsible for his destiny.
    • Simone Weil, “Wave Mechanics,” On Science, Necessity, and the Love of God, R. Rees, trans. (1968), p. 75
  • Quantum theory does not trouble me at all. It is just the way the world works. What eats me, gets me, drives me, pushes me, is to understand how it got that way. What is the deeper foundation underneath it? Where does it come from? So that we won’t see it as something that is unwelcome by friends that we admire—John Bell and many others—it will be something that will make you say, ‘It couldn’t have been otherwise.’ We haven’t gotten to that stage yet, and until we do, we have not met the challenge that is right there. I continue to say that the quantum is the crack in the armor that covers the secret of existence. To me it’s a marvelous stimulus, hope, and driving force. And yet I am afraid that just the word—‘hope’—is what does not eat, or possess, or drive so many of our colleagues in the field. They’re content to take the theory for granted, rather than to find out where it comes from. But you would hardly feel the drive to find out where from if you don’t feel that the theory is utterly right. I have been brought up from ‘childhood’ to feel that it is utterly right. Here I was, reading that book of Weyl’s at the age of eighteen and just crazy about it.
  • The world is not as real as we think.… My personal opinion is that the world is even weirder than what quantum physics tells us.
    • Anton Zeilinger, quoted in Dennis Overbye, "Quantum Trickery: Testing Einstein's Strangest Theory", The New York Times (Dec. 27, 2005)

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