Warped Passages - Lisa Randall [39]
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?