The Experimental Origins of Quantum Mechanics
Quantum mechanics, with its controversial probabilistic nature and curious blending of waves and particles, is a very strange theory. It was not invented because anyone thought this is the way the world should behave, but because various experiments showe
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Quantum mechanics, with its controversial probabilistic nature and curious blending of waves and particles, is a very strange theory. It was not invented because anyone thought this is the way the world should behave, but because various experiments showed that this is the way the world does behave, like it or not. Craig Hogan, director of the Fermilab Particle Astrophysics Center, put it this way: No theorist in his right mind would have invented quantum mechanics unless forced to by data.1 Although the first hint of quantum mechanics came in 1900 with Planck’s solution to the problem of blackbody radiation, the full theory did not emerge until 1925–1926, with Heisenberg’s matrix model, Schr¨odinger’s wave model, and Born’s statistical interpretation of the wave model.
1.1 Is Light a Wave or a Particle? 1.1.1 Newton Versus Huygens Beginning in the late seventeenth century and continuing into the early eighteenth century, there was a vigorous debate in the scientific community 1 Quoted in “Is Space Digital?” by Michael Moyer, Scientific American, February 2012, pp. 30–36.
B.C. Hall, Quantum Theory for Mathematicians, Graduate Texts in Mathematics 267, DOI 10.1007/978-1-4614-7116-5 1, © Springer Science+Business Media New York 2013
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1. The Experimental Origins of Quantum Mechanics
over the nature of light. One camp, following the views of Isaac Newton, claimed that light consisted of a group of particles or “corpuscles.” The other camp, led by the Dutch physicist Christiaan Huygens, claimed that light was a wave. Newton argued that only a corpuscular theory could account for the observed tendency of light to travel in straight lines. Huygens and others, on the other hand, argued that a wave theory could explain numerous observed aspects of light, including the bending or “refraction” of light as it passes from one medium to another, as from air into water. Newton’s reputation was such that his “corpuscular” theory remained the dominant one until the early nineteenth century.
1.1.2 The Ascendance of the Wave Theory of Light In 1804, Thomas Young published two papers describing and explaining his double-slit experiment. In this experiment, sunlight passes through a small hole in a piece of cardboard and strikes another piece of cardboard containing two small holes. The light then strikes a third piece of cardboard, where the pattern of light may be observed. Young observed “fringes” or alternating regions of high and low intensity for the light. Young believed that light was a wave and he postulated that these fringes were the result of interference between the waves emanating from the two holes. Young drew an analogy between light and water, where in the case of water, interference is readily observed. If two circular waves of water cross each other, there will be some points where a peak of one wave matches up with a trough of another wave, resulting in destructive interference, that is, a partial cancellation between the two waves, resulting in a small amplitude of the combined wave at that point. At other
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