Warped Passages (14 page)

Figure 30.
The atom consists of electrons circulating around a tiny nucleus. The nucleus is composed of positively charged protons and charge-neutral neutrons.

Figure 31.
The proton and neutron are composed of more elementary quarks bound together through the strong force.

The Nobel Prize-winning physicist Stephen Weinberg coined the term “Standard Model” to label the well-established particle physics theory that describes the interactions of these fundamental building blocks of matter—the electron, the up quark, and the down quark—as well as other fundamental particles that we will get to momentarily. The Standard Model also describes three of the four forces through which the elementary particles interact: electromagnetism, the weak force, and the strong force. (It usually omits gravity.)

Although gravity and electromagnetism were known for hundreds of years, no one understood the last two less familiar forces until the second half of the twentieth century. Those weak and strong forces act on fundamental particles and are important for nuclear processes. They permit quarks to bind together and nuclei to decay, for example.

If we wanted, we could also include gravity in the Standard Model. We usually don’t though, because gravity is far too weak a force to be of any consequence at the distance scales that are relevant to particle physics at experimentally accessible energies. At very high energies and very small distances, our usual notions about gravity break down; this is relevant to string theory, but it does not happen on measurable distance scales. When studying elementary particles, gravity is important only in certain extensions of the Standard Model, such as the extra-dimensional models we will consider later on. For all other predictions about elementary particles, we can forget about gravity.

Now that we’ve entered the world of fundamental particles, let’s look around a little and take stock of our neighbors. The up quark, the down quark, and the electron lie at the core of matter. However, we now know that there also exist additional, heavier quarks and other heavier electron-like particles that are nowhere to be found in ordinary material.

For example, whereas the electron has a mass of about one-half of one-thousandth that of a proton, a particle called the
muon
, with
precisely the same charge as the electron, has a mass that is two hundred times greater than the electron’s. A particle called the
tau
, which also has the same charge, has a mass that is ten times greater still. And experiments at high-energy colliders have discovered even heavier particles in the past thirty years. To produce them, physicists needed the large amount of highly concentrated energy that today’s high-energy particle colliders can create.

I realize that this section was billed as a tour inside matter, but the particles I am talking about now are not inside the stable objects of the material world. Although all known matter consists of elementary particles, heavier elementary particles are not constituents of matter. You won’t find them in your shoelaces, on your table top, or on Mars, or in any other physical object that we know about. But these particles are currently created today at high-energy collider experiments, and they were a part of the early universe immediately after the Big Bang.

Nonetheless, these heavy particles are essential components of the Standard Model. They interact through the same forces as the more familiar particles do, and will very likely play a role in a deeper understanding of matter’s most basic physical laws. I’ve listed the Standard Model particles in Figures 32 and 33. I’ve included neutrinos and
force-carrying gauge bosons, which I’ll tell you more about in Chapter 7 when I discuss all the elements of the Standard Model in detail.

Figure 32.
The matter particles of the Standard Model and their masses. Particles in the same column have identical charges but different masses.

Figure 33.
The force-carrying gauge bosons of the Standard Model, their masses, and the forces they communicate.

No one knows why the heavy Standard Model particles exist. The questions of their purpose, what role they play in the ultimate underlying theory, and why their masses are so different from those of the constituents of more familiar matter are some of the major mysteries facing the Standard Model. And these are only a few of the puzzles that the Standard Model leaves unresolved. Why, for example, are there four forces and no others? Could there be others we haven’t yet detected? And why is gravity so much weaker than the other known forces?

The Standard Model also leaves open a more theoretical question, the one that string theory hopes to address: how do we reconcile quantum mechanics and gravity consistently at all distance scales? This question differs from the others in that it doesn’t concern currently visible phenomena, but is instead a question about the intrinsic limitations of particle physics.

Both types of unanswered question—those that concern visible and purely theoretical phenomena—give us reasons to look beyond the Standard Model. Despite the Standard Model’s power and success, we’re confident that more fundamental structure awaits discovery and that the search for more fundamental principles will be rewarded. As the composer Steve Reich elegantly put it in the
New York Times
(when making an analogy to a piece he wrote), “First there were just atoms, then there were protons and neutrons, then there were quarks, and now we’re talking about string theory. It seems like every 20, 30, 40, 50 years a trapdoor opens and another level of reality opens up.”
^*

Experiments at current and future particle colliders are no longer looking for the ingredients of the Standard Model—those have all been found. The Standard Model nicely organizes these particles according to their interactions, and the full complement of Standard Model particles is now known. Instead, experimenters are looking for particles that should be even more interesting. Current theoretical models include the Standard Model ingredients, but add new elements
to address some of the questions that the Standard Model leaves unresolved. We hope that current and future experiments will provide clues that will allow us to distinguish among them and find the true underlying nature of matter.

Although we have experimental and theoretical hints about the nature of a more fundamental theory, we are unlikely to know what is the correct description of nature until higher-energy experiments (that probe shorter distances) provide the answer. As we will see later on, theoretical clues tell us that experiments in the next decade will almost certainly discover something new. It probably won’t be definitive evidence of string theory, which will be very difficult to discover, but we’ll see that it could be something as exotic as new relations in spacetime, or new and as yet unseen extra dimensions—new phenomena that feature in string theory as well as other particle physics theories. And despite the broad scope of our collective imagination, these experiments also have the potential to reveal something that no one has yet thought of. My colleagues and I are very curious about what that will be.

Preview

We know about the structure of matter we just visited as a result of the critical physics developments of the last century. These stupendous advances are essential to any more comprehensive theory of the world we might come up with and were also major achievements in themselves.

Starting in the next chapter, we’ll review those developments. Theories grow out of the observations and deficiencies of progenitor theories, and you can better appreciate the role of more recent advances by becoming acquainted with these remarkable earlier developments. Figure 34 indicates some of the ways in which the theories we will discuss interconnect. We’ll see how each of these theories was built using the lessons from older ones and how newer theories filled in gaps that were detected only after the older theories were complete.

We’ll begin with the two revolutionary ideas of the early twentieth century: relativity and quantum mechanics, through which we learned
about the shape of the universe and the objects it contains, and the composition and structure of the atom. We’ll then introduce the Standard Model of particle physics, which was developed in the 1960s and 1970s to predict the interactions of the elementary particles we just encountered. We’ll also consider the most important principles and concepts in particle physics: symmetry, symmetry breaking, and scale dependence of physical quantities, through which we’ve learned a great deal about how matter’s most elementary components create the structures we see.

Figure 34.
The fields of physics we will encounter and how they are connected.

However, despite its many successes, the Standard Model of particle physics leaves some fundamental questions unanswered—questions so basic that their resolution promises new insight into the building blocks of our world. Chapter 10 presents one of the most interesting and mysterious aspects of the Standard Model: the origin of the elementary particles’ masses. We’ll see that we almost certainly need a more profound physical theory than the Standard Model if we are to explain the masses of known particles and the weakness of gravity.