Quantum physics

The endangered Schrödinger's Cheshire elephant.

Anyone who is not shocked by quantum physics has not understood it.
— Niels Bohr (1885 – 1962), quoted by John Gribbin, In Search of Schrödinger’s Cat, Corgi, 1985.

Finally, in 1926, an ‘uncommon-sensy’ theory was developed to explain the ‘new type of behavior’ of electrons in matter. It looked cockeyed, but in reality it was not: it was called the theory of quantum mechanics. The word ‘quantum’ refers to this peculiar aspect of nature that goes against common sense.
— Richard Feynman (1918 – 1988), QED, Penguin Books, 1990, 5.

Only connect
— Edward Morgan (E. M.) Forster (1879 – 1970), Howard’s End, motto on title page.

Of course, if electrons were waves there would be no difficulty. We think we understand the regular reflection of light and X-rays — and we should understand the reflection of electrons as well if electrons were only waves instead of particles.

This observation, though true, does not seem a particularly valuable one. It is rather as if one were to see a rabbit climbing a tree, and were to say, ‘Well, that is a rather strange thing for a rabbit to be doing, but after all, there is nothing to get excited about. Cats climb trees — so that, if the rabbit were only a cat, we would understand its behaviour perfectly.’

Of course, the explanation might be that what we took to be a rabbit was not a rabbit at all, but was actually a cat. Is it possible that we have been wrong all this time in supposing that they are particles, and that they are actually waves?
— Clinton Davisson, Franklin Institute Journal, 205, 597. (my source: New Scientist, 14 April 1977)

It seems to me one thing can be asserted: despite the importance and the extent of the progress accomplished by physics in the last centuries, as long as physicists were unaware of the existence of quanta, they were unable to comprehend anything of the profound nature of physical phenomena, for, without quanta, there would be neither light nor matter.
— Louis de Broglie (1892 – 1987), The Revolution in Physics, 1953, 14.

Had I known that we were not going to get rid of this damned quantum jumping, I never would have involved myself in this business.
— Erwin Schrödinger (1887 – 1961), quoted by John Gribbin, In Search of Schrödinger’s Cat, Corgi, 1985.

The quantum theory is primarily a practical branch of physics, and as such it is brilliantly successful. It has given us the laser, the electron microscope, the transistor, the superconductor and nuclear power. At a stroke it explained chemical bonding, the structure of the atom and nucleus, the conduction of electricity, the mechanical and thermal properties of solids, the stiffness of collapsed stars, and a host of other important physical phenomena.
— Paul Davies, God and the New Physics, Penguin Books, 1990, 101.

We are all familiar with crime waves; not waves of any substance, but waves of probability. Where the crime wave is most intense, there is the greatest probability of a felony. The quantum wave is also a wave of probability. It tells you where you can expect the particle to be, and what chance it may have of such-and-such a property, such as rotation or energy. The wave thus encapsulates the inherent uncertainty and unpredictability of the quantum factor.
— Paul Davies, God and the New Physics, Penguin Books, 1990, 108.

The modern physicist is a quantum theorist on Monday, Wednesday, and Friday and a student of gravitational relativity theory on Tuesday, Thursday, and Saturday. On Sunday he is neither, but is praying to his God that someone, preferably himself, will find the reconciliation between the two views.
— Norbert Wiener (1894 – 1964), I am a mathematician, the later life of a prodigy, 1953.

Physicists use the wave theory on Mondays, Wednesdays and Fridays and the particle theory on Tuesdays, Thursdays and Saturdays.
— William Henry Bragg (1862 – 1942) (attrib.)

God runs electromagnetics by wave theory on Monday, Wednesday, and Friday, and the Devil runs them by quantum theory on Tuesday, Thursday, and Saturday.
— Anonymous.

… the general public has, I believe, few ideas even vague ones about the quantum theory. It must be said, this is excusable, for quanta are a very mysterious thing. In my case, I was about twenty when I began to work with them and I have been pondering over them a quarter of a century; very well, I must humbly confess that if in the course of these meditations I have come to understand some of their aspects a little better, I do not yet know exactly what is hidden behind the mask which covers their true face. Nevertheless, it seems to me that one thing can be asserted: despite the importance and the extent of the progress accomplished by physics in the last centuries, as long as the physicists were unaware of the existence of quanta, they were unable to comprehend anything of the profound nature of physical phenomena for, without quanta, there would be neither light nor matter and, if one may paraphrase the Gospels [John 1:3], it can be said that without them ‘nothing was made that has been made’.
— Louis Victor Pierre Raymond de Broglie (1892 – 1987), The Revolution in Physics: a non-mathematical survey of quanta (1953).

There is one particular question the answer to which will, in my opinion, lead to an extensive elucidation of the whole [quantum] problem. What happens to the light energy of a light-quantum after its emission? Does it pass outwards in all directions, according to Huygens’ wave theory, continually increasing in volume and tending towards infinite dilution? Or does it, as in Newton’s emanation theory, fly like a projectile in one direction only? In the former case, the quantum would never again be in a position to concentrate its energy at a spot strongly enough to detach an electron from its atom; while in the latter case it would be necessary to sacrifice the chief triumph of Maxwell’s theory the continuity between the static and the dynamic fields and with it the classical theory of the interference phenomena which accounted for all their details, both alternatives leading to consequences very disagreeable to the modern theoretical physicist.
— Max Planck (1858 – 1947), conclusion to 1919 Nobel Lecture.

These results, and others I have obtained in the course of the work, are very difficult to explain on the assumption that the radiation from beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions. The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons. The capture of the alpha-particle by the Be9 [⁹Be] nucleus may be supposed to result in the formation of a C12 [¹²C] nucleus and the emission of a neutron. From the energy relations of this process the velocity of the neutron emitted in the forward direction may well be about 3 x 109 cm per sec. The collisions of this neutron with the atoms through which it passes will give rise to the recoil atoms, and the observed energies of the recoil atoms are in fair agreement with this view.
— Sir James Chadwick (1891 – 1974), Nature, February 27, 1932, 129(3252), 312.

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