Warped Passages - Lisa Randall [77]
The Photon
Maxwell’s classical electromagnetic theory made many successful predictions, but it predated quantum mechanics so it obviously didn’t include quantum effects. Today, physicists study the electromagnetic force with particle physics. The particle physics theory of electromagnetism includes the predictions of Maxwell’s well-studied and well-verified classical theory, but incorporates the predictions of quantum mechanics as well. It is therefore a more comprehensive and more accurate theory of electromagnetism than its classical predecessor. In fact, the quantum theory of electromagnetism has yielded incredibly precise predictions that have been tested with the unbelievable precision of one part in a billion.*
The quantum electromagnetic theory attributes the electromagnetic force to the exchange of the particle called the photon, the quantum of light that we considered in the previous chapter. The way it works is that an incoming electron emits a photon, which travels to another electron, communicates the electromagnetic force, and then disappears. Through their exchange, photons transmit, or mediate, a force. They act as confidential letters that convey information from one place to another, but are afterwards immediately destroyed.
We know that the electric force is sometimes attractive and sometimes repulsive: it’s attractive when oppositely charged objects interact, and repulsive when the charges have the same sign, either both positive or both negative. You might think of the repulsive force communicated by the photon as an interaction between two ice skaters throwing a bowling ball back and forth; each time one of them catches the ball, he slides away from the other across the ice. Attractive forces, on the other hand, are more like two novices tossing a frisbee to each other; unlike the ice skaters, who slide further apart, these beginning frisbee players would approach each other with each successive throw.
The photon is the first example we will encounter of a gauge boson, a fundamental, elementary particle that is responsible for communicating a particular force. (The word “gauge” sounds more daunting than it really is; physicists first used it in the late 1800s because of a tangential analogy to railroad gauges that tell you the distance between the rails—a term that was far more familiar a hundred years ago.) Weak bosons and gluons are other examples of gauge bosons. These particles communicate the weak and strong forces respectively.
Between the late 1920s and the 1940s, the English physicist Paul Dirac and the Americans Richard Feynman and Julian Schwinger—as well as Sin-Itiro Tomonaga working independently in post-war Japan—developed the quantum mechanical theory of the photon. They named the branch of quantum theory that they developed quantum electrodynamics (QED). Quantum electrodynamics includes all the predictions of the classical electromagnetic theory as well as particle (quantum) contributions to physical processes—that is, interactions that are generated by exchanging or producing quantum particles.
QED predicts how photon exchange produces the electromagnetic force. For example, in the process illustrated in Figure 47, two electrons enter the interaction region, exchange a photon, and then emerge with their resultant paths (speed and direction of motion, for example) influenced by the electromagnetic force that was communicated. Field theory associates numbers with each part of the diagram so that we can use it to make quantitative predictions. This picture is an example of a Feynman diagram, named after Richard Feynman, and is a pictorial way of describing interactions in quantum field theory. (Feynman was so proud of his invention that he had some diagrams painted on his van.)
Figure 47. The Feynman diagram, on the right, has several interpretations. One interpretation (reading bottom to top) is that two electrons enter an interaction region, exchange a photon, and two electrons