Breakfast with Einstein by Chad Orzel

Author:Chad Orzel
Language: eng
Format: epub, pdf
Tags: Quantum physics;Physics;Quantum mechanics;Astronomy;Everyday;Fun;Science;Exotic science;Phenomena;Big bang;Black holes;TED talk;Phones;Photographs;Electronics;Popular science;Pop science
Publisher: Oneworld Publications
Published: 2018-09-27T09:53:44+00:00

From Orbits to Pilot Waves to Probability

Along with its oversimplicity—its status as a spherical cow—the other outstanding problem with the Bohr model was the seeming arbitrariness of the quantum condition for allowed orbits. That is, why should only integer multiples of the angular momentum be allowed in the first place? Sommerfeld’s extension of the theory provided a richer variety of orbits, but they still lacked a convincing basis in some physical property of the electrons inside an atom.

The first step toward an answer to this problem came from a French graduate student from an aristocratic family, Louis-Victor-Pierre-Raymond de Broglie (generally just shortened to “Louis de Broglie”), who picked up on a connection to the other branch of emerging quantum theory, regarding the nature of light. In his PhD thesis, de Broglie suggested a parallel between light and matter—if light waves have particle nature, then maybe a particle like an electron should have an associated wave, with the same inverse relationship between the wavelength and momentum seen for light: doubling the electron’s momentum should cut its wavelength in half. In this wave picture, the Bohr-Sommerfeld quantum model takes on an obvious physical meaning: If you trace out the electron wave around one of the “stationary state” orbits with the principal quantum number n, when you get back to the starting point, the wave has oscillated n times. The allowed orbits are those for which the electron wave wrapped around the orbit forms a standing-wave pattern just like the standing-wave modes of light used to set up the black-body problem back in Chapter 2.

The idea of electrons as waves was an extremely bold suggestion—one popular story says that de Broglie’s PhD committee had no idea what to make of his thesis, until Einstein was invited to weigh in and declared it “a first feeble ray of light on this worst of our physics enigmas.” Luckily, it is also an eminently testable idea, and within a few years direct experimental evidence emerged. In the US, Clinton Davisson and Lester Germer saw wave diffraction in a beam of electrons bouncing off a crystal of nickel, a discovery made entirely by accident. While they were running their experiment, a break in their vacuum system let in air that oxidized their nickel sample. To clean the surface, they heated it to a high temperature, which caused it to partially melt, and as it cooled it formed much larger crystals, leading to more dramatic (and thus more easily observed) diffraction peaks. Visiting the UK some years later, Davisson was surprised to hear Max Born citing his odd experiment as evidence of the wave nature of electrons. Subsequent experiments confirmed this explanation, though, and Davisson shared the 1937 Nobel Prize for discovering the wave nature of the electron with George Thomson, at the University of Aberdeen, who’d observed diffraction of electrons sent through thin films of grease. Thomson’s father was J. J. Thomson, who won the 1906 Nobel Prize for showing that the electron is a particle (see Chapter 3)—dinner-table conversation in the Thomson family must have been interesting.

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