The hole in the heart of quantum electrodynamics

Pair production is the creation of a particle and its antiparticle. Some say it was first observed in 1929, but it's usually accredited to Carl Anderson in 1932. He used a cloud chamber and an electromagnet to investigate cosmic rays. He effectively split a gamma photon over an atomic nucleus to create an electron and an antielectron. He called the latter the positive electron, which was soon shortened to positron: Image from schoolphysics However whilst he realised that he’d discovered the positron, he didn’t realise that he’d performed pair…

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The photon

The photon has a wave nature, which is why we can refract and diffract light. But what sort of a wave nature? When you try to find a picture, a lot of illustrations depict the photon as some kind of wave train. Even Feynman diagrams do this. Image by bitwise, see Wikipedia commons The photon is shown as a squiggly line, sometimes with an arrowhead, something like this: ⇝. That suggests you could split a photon lengthwise and end up with two photons, each with the…

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What is a photon?

OK, so quantum electrodynamics is said to date from 1929 when it was the same thing as quantum field theory. However it immediately suffered from the “problem of infinities”. So much so that some say most workers in the field doubted its correctness, whilst others say physicists believed a conceptual change was needed. As to what, see the conceptual foundations and the philosophical aspects of renormalization theory by Tian Yu Cao and Silvan Schweber dating from 1993. They say QFT embodies a reductionist view, but “serious doubt has often been cast on the…

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Quantum electrodynamics

Quantum electrodynamics arguably goes back to Werner Heisenberg and Wolfgang Pauli in 1929, when it was the same thing as quantum field theory. But as pointed out by Robert Oppenheimer in 1930, it suffered from the "divergence issue", also known as the “problem of infinities”. So much so that some historians say most workers in the field doubted its correctness, and some say the accepted wisdom was that it was no good. Others say physicists were overwhelmed by the problems and believed that a conceptual change…

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Quantum electrodynamics in the 1930s

Quantum electrodynamics or QED is said to be the quantum field theory or QFT which gives “a complete account of matter and light interaction”. Some say it was developed by Sin-Itiro Tomonaga, Julian Schwinger, and Richard Feynman in the 1940s: Image from Rod Nave’s hyperphysics But some say it started with Pascual Jordan in 1925, some say it started with Dirac in 1927, and some say it started with Heisenberg and Pauli’s “canonical” papers of 1929 and 1930. In the history of QFT Meinard Kuhlmann says…

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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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The principle of equivalence and other myths

Once you know that an optical clock goes slower when it’s lower because light goes slower when it’s lower, you soon understand why light curves. Not because it follows the curvature of spacetime. Because the speed of light is spatially variable, like Einstein said. Then once you know about the wave nature of matter and electron spin, you soon understand why matter falls down, and why the Newtonian deflection of matter is only half the deflection of light. Then once you know how gravity works, you…

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How gravity works

Once you understand that there is no time flowing inside an optical clock, then you understand that an optical clock goes slower when it’s lower because light goes slower when it’s lower. Because the speed of light varies in the room you’re in. After that you understand that light waves curve downwards in a gravitational field for the same reason that sonar waves curve downwards in an ocean. Search the ES310 sonar propagation webpage and you find where the US Navy said it: “Recall how differences…

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The speed of light is not constant

The speed of sound in air is sometimes said to be 343.2 metres per second or 768 miles per hour. But actually, it varies. It usually decreases with altitude up to about 11 kilometres above sea level. That’s about 36,000 feet, which is typical for a passenger jet. At that altitude the speed of sound is circa 295 m/s or 660 mph, which is one reason why passenger jets don’t fly as fast as you might like. Interestingly enough, the speed of sound typically decreases with…

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The nature of time

I think it’s important to understand the nature of time. That’s because I think it leads to other things. I also think you can gain an understanding of the nature of time by being empirical. For example we use phrases like time flows and time passes, but when you look for the empirical evidence of time flowing or passing, you can’t find any. That’s because there isn’t any. I can hold my hands up a foot apart and show you the gap, the space between them.…

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A detective story

So why isn't the future what it used to be? I think it’s something of a detective story, one where you have to look back at the history. In 1831 Michael Faraday was doing his ground-breaking experiments, showing how electricity and magnetism were interrelated. Then in 1865 James Clerk Maxwell developed the theory, and in 1880 we had light bulbs courtesy of Joseph Swan and Thomas Edison. In 1905 Einstein gave us E=mc², saying there was an awful lot of energy in matter. Then in 1934…

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The future isn’t what it used to be

The future isn’t what it used to be. When I was a boy it was really something. I was born in 1956 when buildings were black and children played out. Other things were different too. I remember visiting my grandparents when I was very young. They lived in a small terraced house in Gorton, Manchester, near Belle Vue Zoo and the red rec and a massive factory called Peacocks. I followed the cat up the steep narrow stairs and saw the chamber pot under the high-legged bed. I was…

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