History

Quantum electrodynamics in the 1920s

Quantum electrodynamics is often shortened to QED. As for what it is exactly, I find it difficult to say. Wikipedia says it’s the relativistic quantum field theory of electrodynamics, and gives “a complete account of matter and light interaction”. But that’s not enough. The Encyclopaedia Britannica says it’s a quantum field theory which “describes mathematically not only all interactions of light with matter but also those of charged particles with one another”. That’s not enough either. Particularly since it’s defining QED in terms of other things…

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A potted history of quantum mechanics

There’s some ambiguity when it comes to quantum mechanics. Some people apply the term widely, others apply it to the theory that was developed in the 1920s to replace the old quantum theory. There’s some ambiguity with that too, in that the old quantum theory was primarily an atomic model proposed by Niels Bohr in 1913 and extended by Arnold Sommerfeld in 1916. It didn’t include the quantum nature of light, which arguably began with Max Planck’s black-body paper in 1900. Or with Albert Einstein’s photoelectric…

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The old quantum theory and Bohr

In 1911 Niels Bohr was in Cambridge. He had a Carlsberg fellowship grant and a postdoc position at the Cavendish lab under JJ Thomson. However there were issues, and he ended up transferring to Manchester and Ernie Rutherford. You can read about it in JJ Thomson and the Bohr atom by John Heilbron dating from 1977. Heilbron says “far from being merely ‘scientific curiosities’ JJ Thomson's seemingly naive models actually contained some of the fundamental ideas of Niels Bohr's revolutionary quantum theory of the atom”. He…

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The old quantum theory begins

Quantum physics or quantum theory can arguably be traced back to Gustav Kirchhoff's researches on the solar spectrum. See his 1860 paper on the relation between the radiating and absorbing powers of different bodies for light and heat. Kirchhoff spoke about black body radiation and what came to be known as the black body problem. When a body is heated such that it emits light, the colour of its glow depends on its temperature alone. As you heat a poker in a fire it glows cherry…

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