Why Does E=mc2_ - Brian Cox [83]
Before we leave the subject of testing the Standard Model, we cannot resist one other example from a quite different type of experiment. Electrons (and many other elementary particles) behave like tiny magnets, and some very beautiful experiments have been designed to measure these magnetic effects. These aren’t collider experiments. There is no brutal smashing together of matter and antimatter here. Instead, very clever experiments allow the scientists to measure the magnetism to an astonishing one part per trillion. It is a staggering precision, akin to measuring the distance from London to New York to an accuracy much less than the thickness of a human hair. As if that weren’t amazing enough, the theoretical physicists have been hard at work too. They have calculated the same thing. Calculations like this used to be done using nothing more than a pen and some paper, but these days even the theorists need good computers.
Nevertheless, starting with the Standard Model and a cool head, theorists have calculated the predictions of the Standard Model, and their result agrees exactly with the experimental number. To this day the theory and experiment are in agreement to ten parts per billion. It is one of the most precise tests of any theory that has ever been made in all of science. By now, and thanks in no small part to LEP and the electron magnetism experiments, we have a great deal of confidence that the Standard Model of particle physics is on the right lines. Our theory of nearly everything is in fine shape—except for one last detail, which is actually a fairly big detail. What are those last two lines of the master equation?
We are guilty in fact of hiding one crucial piece of information that is absolutely central to our quest in this book. Now is the time to let the cat out of the bag. The requirement of gauge symmetry seems to demand that all of the particles in the Standard Model have no mass. That is plain wrong. Things do have mass and you do not need a complicated scientific experiment to prove it. We’ve spent the entire book so far thinking about it, and we derived the most famous equation in physics, E = mc2, and that very definitely has an “m” in it. The final two lines of the master equation are there to fix this problem. In understanding those final two lines we will complete our journey, for we will have an explanation for the very origin of mass.
The problem of mass is very easy to state. If we try to add mass directly into the master equation, then were are doomed to spoil gauge symmetry. But as we have seen, gauge symmetry lies at the very heart of the theory. Using it, we were able to conjure into being all of the forces in nature. Worse still, theorists proved in the 1970s that abandoning gauge symmetry is not an option, because then the theory falls apart and stops making sense. This apparent impasse was solved by three groups of people working independently of each other in 1964. François Englert and Robert Brout working in Belgium, Gerald Guralnik, Carl Hagen, and Tom Kibble in London, and Peter Higgs in Edinburgh all wrote landmark papers that led to what later became known as the Higgs mechanism.
What would constitute an explanation of mass? Well, suppose you started out with a theory of nature in which mass never reared its head. In such a theory, mass simply