Warped Passages - Lisa Randall [75]
In this chapter you will make a jump of your own, into the Standard Model of particle physics, the theory that describes the known elementary constituents of matter and the forces that act upon them.* The Standard Model, which represents the culmination of many surprising and exciting developments, is a stupendous achievement. You don’t need to remember all the details—I’ll repeat the names of all the particles or the nature of their interactions when I refer to them later on. But the Standard Model underlies many of the exotic, extra-dimensional theories that I will describe shortly, and as you learn about the recent exciting developments, a feeling for the Standard Model and its key ideas will contribute to a deeper understanding of matter’s fundamental structure and the way physicists think about the world today.
The Electron and Electromagnetism
When Vladimir I. Lenin used the electron as a metaphor in his philosophical book Materialism and Empirio-Criticism, he wrote that “the electron is inexhaustible,” referring to the layers of theoretical ideas and interpretation through which we interpret it. Indeed, today we understand the electron very differently than we did in the early twentieth century, before quantum mechanics revised our ideas.
But in a physical sense the opposite of Lenin’s quote is true: the electron is exhaustible. So far as has been determined, the electron is fundamental and indivisible. To a particle physicist, the electron, rather than having “inexhaustible” structure, is the simplest Standard Model particle to describe. The electron is stable and has no constituent parts, so we can characterize it completely by listing only a few properties, including mass and charge. (The Czech anti-Communist string theorist Luboš Motl quipped that this is not the only difference between his and Lenin’s perspectives.)
An electron will move towards the positively charged anode of a battery. A moving electron also responds to a magnetic force: as an electron moves through a magnetic field, its path will bend. Both these phenomena are the result of the electron’s negative charge, which makes the electron respond to electricity and magnetism.
Before the 1800s, everyone thought that electricity and magnetism were separate forces. But in 1819 the Danish physicist and philosopher Hans Oersted found that a current of moving charges generates a magnetic field. From this observation he deduced that there should be a single theory describing both electricity and magnetism: they must be two sides of the same coin. When a compass needle responds to a bolt of lightning, it confirms Oersted’s conclusion.
The classical theory of electromagnetism, still in use today, was developed in the nineteenth century and used the observation that electricity and magnetism are related. The notion of a field was also critical to this theory. “Field” is the name physicists give to any quantity that permeates space. For example, the value of the gravitational field at any point tells how strong the effect of gravity is there. The same goes for any type of field: the value of the field at any location tells us how intense the field is there.
In the latter half of the nineteenth century, the English chemist and physicist Michael Faraday introduced the concepts of electric and magnetic fields, and these concepts persist in physics today. Given that he had to temporarily abandon his formal education at the age of fourteen to help support his family, it is quite remarkable that he managed to do physics research that had such a revolutionary impact. Fortunately for him (and for the history of physics), he was apprenticed to a bookbinder who encouraged him to read the books on which he was working, and educate himself.
Faraday’s idea was that charges produce electric or magnetic fields everywhere in space, and these fields in turn act on other charged objects, no matter where those objects are. The magnitude of the effect of electric and magnetic fields on