Warped Passages - Lisa Randall [84]
The nonzero mass of the weak gauge boson is critical to the success of the weak force theory. The mass is the reason that the weak force acts only over very short distances and is so weak as to be almost nonexistent at longer distances. The weak gauge bosons are different in this respect from the photon and graviton, both of which are massless. Because the photon and the graviton, the particle that communicates the gravitational force, carry energy and momentum but have no mass, they can communicate forces across great distances.
The concept of massless particles might sound strange, but from the particle physics perspective it is nothing very remarkable. The masslessness of the particles tells us that they travel at the speed of light (after all, light is composed of massless photons), and also that energy and momentum always obey a particular relation: energy is proportional to momentum.
The carriers of the weak force, on the other hand, do have mass. And from the perspective of particle physics, a massive gauge boson—not a massless one—is the oddity. The key development that paved the way for the theory of the weak force was understanding the origin of the weak gauge boson masses, which make the distance dependence of the weak force so different from that of electromagnetism. The mechanism that gives rise to the weak gauge boson masses, known as the Higgs mechanism, is the subject of Chapter 10. As we will see in Chapter 12, the underlying theory—that is, the precise model that gives particles their mass—is one of the biggest puzzles facing particle physicists today. One of the attractions of extra dimensions is that they might help solve this mystery.
Quarks and the Strong Force
A physicist friend once explained to one of my sisters that he worked on “the strong force which is called the strong force because it is so strong.” Although she did not find this particularly edifying, the strong force is in fact aptly named. It is an extremely powerful force. It binds together the constituents of the proton so powerfully that ordinarily they never separate. The strong force is only tangentially relevant to later parts of this book, but here I’ll give some basic facts about it for completeness.
The strong force, described by the theory called quantum chromo-dynamics (QCD), is the last of the Standard Model forces that we can explain with gauge boson exchange. It too was discovered only in the last century. The strong gauge bosons are known as gluons because they communicate the force, the “glue,” that binds strongly interacting particles together.
In the 1950s and 1960s, physicists discovered many particles in rapid succession. They gave the individual particles various Greek-letter names such as the ∏ pronounced “pion”), the (pronounced “eta”), and the ? (pronounced “Delta”—written with a capital “D” to reflect the case of the Greek letter). Collectively, these particles are called hadrons, after the Greek word hadros, which means “fat, heavy.”
Indeed, hadrons were all much more massive than the electron. They were mostly comparable in mass to the proton, which has 2,000 times the electron’s mass. The enormous multiplicity of hadrons was a mystery until the physicist Murray Gell-Mann* suggested in the 1960s that the many hadrons were not fundamental particles but were instead themselves composed of particles that he named quarks.
Gell-Mann got the word “quark” from a poem in James Joyce’s Finnegans Wake: “Three quarks for Muster Mark! Sure he hasn’t got much of a bark. And sure any he has it’s all beside the mark.” This, so far as I can deduce, is pretty much unrelated to the physics of quarks except for two things: there were three of them, and they were difficult to understand.*
Gell-Mann proposed that there are three varieties of quark†—they’re now called up, down, and strange—and that the numerous hadrons corresponded to the many possible combinations of quarks that could be bound together. If his proposal was correct, hadrons would have to fall neatly into predictable patterns. As was