Way back on April 2, 2005, I introduced the four forces of nature. Then, from July to September, I wrote a series of essays describing quantum mechanics, the physics of extremely small scales (see archives at lower right-hand portion of the page). In fact, with varying degrees of success, the forces have been integrated with quantum mechanics to produce various quantum theories of the forces.
Today, I discuss Quantum Electrodynamics (QED), the quantum theory of the electromagnetic force. My main source on this topic is the book QED: The Strange Theory of Light and Matter, by the legendary physicist Richard P. Feynman. The book was based on a series of public lectures by Feynman; in my view, it starts off being reasonably understandable to a lay reader such as myself, but then increases in complexity.
In introducing the subject matter, Feynman goes back to the mid-1800's work of James Clerk Maxwell, one of the pioneering scientists in studying the connection between electricity and magnetism:
Because the theory of quantum mechanics could explain all of chemistry and the various properties of substances, it was a tremendous success. But still there was the problem of the interaction of light and matter. That is, Maxwell's theory of electricity and magnetism had to be changed to be in accord with the new principles of quantum mechanics that had been developed. So a new theory, the quantum theory of the interaction of light and matter, which is called by the horrible name "quantum electrodynamics," was finally developed by a number of physicists in 1929 (pp. 5-6).
Feynman, born in 1918, was thus just a youngster at the time of QED's founding (he died in 1988). It was around 1948, when Feynman was only about 30, that he made a major contribution to QED. As he notes in the book, he, Julian Schwinger, and Sin-Itiro Tomonaga came up with a breakthrough in how to calculate things in QED. For that, Feynman, Schwinger, and Tomonaga won the 1965 Nobel Prize. Writes Feynman in QED:
At last, we had a quantum theory of electricity and magnetism with which we could calculate!... The theory of quantum electrodynamics has now lasted for more than fifty years, and has been tested more and more accurately over a wider and wider range of conditions. At the present time I can proudly say that there is no significant difference between experiment and theory! (p. 7).
Feynman's discussion deals with photons (particles of light) and electrons. In keeping with the probabilistic nature of quantum mechanics (which I covered on September 10), Feynman begins with the example of "partial reflection." If one shines a light down on a piece of glass (about which Feynman notes, "... a piece of glass is a terrible monster of complexity -- huge numbers of electrons are jiggling about," p. 16), an average of 4% of the photons will bounce back up off the glass and 96% of photons will go through the glass.
Feynman then introduces a method for using arrows to conduct these probabilistic calculations, and then moves on to more complex examples. The use of arrows to depict processes in particle physics has been formalized in Feynman diagrams.
Another of the forces, the weak nuclear force, has also been integrated with QED, as part of "electroweak" theory. In the coming weeks, I will write about the quantum theory of the strong nuclear force (Quantum Chromodynamics) and about attempts to unify quantum mechanics with the remaining force, gravity.