Section 9.1 Introduction
One of the deepest and most persistent questions in physics is “What is the universe made of?” In this chapter we begin to address this question. The answer seems to be particles, and the harder we bang protons or electrons together, the more new types of particles appear. In this chapter, we discuss our current understanding of the fundamental building blocks of nature. Specifically, we discuss categories for particle types, some of their properties, and the way particles behave in various reactions. And most importantly, you'll see how conservation laws begin to make sense of the jumble of “elementary” particles. In the next two chapters, we will discuss how these particles interact with each.
A word about the term elementary particle: ideally, an elementary particle has a set of fixed properties, like mass, spin, charge, etc., and cannot be further subdivided into smaller constituents. Atoms were once considered elementary until they were found to be made of protons, neutrons, and electrons. Only a few of the particles discussed in this chapter are truly elementary. Thus the use of quotes.