Quantum Electrodynamics (QED) is the theory of how light and matter interacts. It is one of the most successful theories ever devised by a human being. Because it represents an attempt by humans to understand the universe, QED is worth looking at in a creative thinking class. Today’s lecture will look at two things, first, the genesis of the idea, and the story behind it’s main creator and chief exponent–Richard Feynman, and second, an account of one application of the theory itself. By the end of the lecture, I’d like you to be able to do a little physics.
This is a guest lecture for CTYI's gifted kid's class, and I'm looking forward to it. The fultext of the lecture is after the jump as well.
CTYI_QED
Here are the lecture notes in .pdf.
CTYI Lecture
Quantum Electrodynamics: The Story of the Theory of Light
Stephen Kinsella
Quantum Electrodynamics (QED) is the theory of how light and matter interacts. It is one of the most successful theories ever devised by a human being. Because it represents an attempt by humans to understand the universe, QED is worth looking at in a creative thinking class. Today’s lecture will look at two things, first, the genesis of the idea, and the story behind it’s main creator and chief exponent–Richard Feynman, and second, an account of one application of the theory itself. By the end of the lecture, I’d like you to be able to do a little physics.
Physics has a history of synthesizing many things observed in nature (called phenomena) into one or two general laws or theories. Physicists search constantly for one small rule or equation, which will give them the means to predict what will happen in any given natural situation.
In the beginning of the science proper, people thought heat, motion, sound, gravity, and light were separate and didn’t have much to do with one another. Sir Isaac Newton in 1696 showed us the laws of motion; at a sweep humans saw that apparently different things were aspects of the same phenomenon. Sound, for example, is just the motion of atoms in the air, and today the phenomenon of sound is more or less completely understood. Heat is also the motion of atoms in a substance. The science of the rules of motion is called mechanics.
In 1896, James Maxwell showed us electricity and magnetism were related, through the theory of electromagnetism. To make the synthesis work, Maxwell had to assume light was a wave. After Maxwell, we had laws of motion, laws of gravitation, and laws of electromagnetism.
In the early 1900’s, a new theory set about explaining what matter was. This electron theory of matter set about understanding the behaviour of atoms as if they were the stars and planets whose motions Newton’s laws explained. The electron theory of matter was a failure, and it wasn’t until 1926 that a new theory emerged which started to explain how matter and light interacted.
This theory, called quantum mechanics, was very strange because, down at the atomic level, it turns out, things are very strange. The reason this new theory was called quantum was because to use it requires thinking in a very strange and unusual way.
But in spite of its complication and strangeness, the theory was a great success. It explained all of chemistry (all of it) as the interaction of atoms in a specific way. The only thing the new quantum theory didn’t explain was the interaction of light and matter. Nobel laureate Paul Dirac described the situation well, noting "It seems that some essentially new physical ideas are here needed."
The theory of quantum electrodynamics, developed by Julian Schwinger, Sin-Itro Tomonaga, and Richard Feynman, resolves the issue of trying to calculate how light and matter (as electrons) interacts. The theory has stood firm for over 70 years, and, in the light of many experimental tests of its validity, seems to be correct. The accuracy of the theory is staggering: If you wanted to measure the distance between New York and Los Angeles, if your measurement was as accurate as the QED theory, you would get the measurement correct to within the width of a human hair!
What does the theory do? QED describes all phenomena of the physical world, except gravitation and certain radioactive phenomena. Gasoline burning, the motion of atoms, why copper is a certain weight, why iron is so stiff. QED is that good.
How did Feynman come up with the idea?
In his 1965 Nobel lecture, Feynman describes the process by which he came up with the theory of QED.
Feynman’s Nobel lecture begins with a statement of the problem as it stood when he was a young undergraduate. The problem was not simple: when electrons interact, and one calculates the force of the interaction, one gets a number very close to one. When you attack the problem theoretically, the formula cranked out nonsense. So the formula needed to get fixed. Feynman was the man to do it.
Let’s get the problem very clear in our heads: the problem is to describe how charged particles interact with one another at a distance, by exchanging photons.
In phase one, Feynman began by considering what would happen if the particle were just to act on itself, not anything else. This turned out to be a false way of looking at the problem, but in the process, Feynman has gathered a large amount of knowledge about the different ways you can write down all there is to know about physics. This first phase took about 3 years.
In the second phase, as a graduate student, Feynman considered 2 electrons interacting with one another. This viewpoint turned out to be wrong as well, but he was on the right track.
In the third phase, Feynman discarded what had come before him, creating diagrams to calculate the forces acting upon one another. These diagrams are called Feynman diagrams, and I want to show you how to use them in this lecture.
References
Feynman, R. P. QED: The Strange Theory of Light and Matter, Penguin, 1985.
Feynman, R.P. “The Development of the Space-Time View of Quantum Electrodynamics”, Nobel Prize Lecture,http://nobelprize.org/nobel_prizes/physics/laureates/1965/feynman-lecture.html, 1965.
Feynman, R.P. Six Not So Easy Pieces, Penguin Books, 1997.