Can Einstein's dream of unifying the known forces be realized? Phenomena governed by the strong, weak, and electromagnetic forces can be described by a unified mathematical theory, but for many years no one could see how to include gravity in the description. Today, however, one of the most exciting areas of theoretical physics is an approach that would unify all four forces. It is called string theory, and some believe it represents a scientific revolution on the scale of quantum mechanics. Experiments at existing and planned accelerators will search for phenomena that are expected if string theory is correct, such as the existence of supersymmetric particles and the Higgs boson.
Why is there apparently more matter than antimatter? Every known type of particle has an antiparticle counterpart, with the same mass and opposite electric charge. (Neutral particles either are their own antiparticles [e.g., the photon and the neutral pion], or have distinct antiparticles [e.g., the neutron and the antineutron].) When a particle and its antiparticle come close together, they are annihilated. Generally, whenever matter is created, an equal amount of antimatter is also created, so one would expect matter and antimatter to have been present in equal amounts in the early universe. If that were true, however, the universe should now be an excruciatingly dull place, since almost all pairs of matter and antimatter particles would have had more than enough time to encounter and annihilate each other. Why is there so much matter around—in the form of galaxies, solar systems, planets, and people?
Of course, as in any branch of science, serendipity and unforeseen developments are bound to play a key role in shaping the course of this work. Just as the Hubble Space Telescope is used to study many different phenomena, not all of which were even known when it was being built, particle accelerators and detectors are used to investigate issues that are recognized or become amenable to experiment only after the instruments are running. Elementary-particle theorist Steven Weinberg observed recently that physicists frequently "do not know in advance what are the right questions to ask, and we often do not find out until we are close to an answer."
Whatever future research in elementary particle physics reveals about the world around us, one thing is certain: It will inspire awe for the intrinsic beauty of the fundamental principles that shape our universe.
The following chapters report on the field of elementary-particle physics in a way that we think is accessible to readers without scientific backgrounds. Chapters 2, 3, and 4 present a comprehensive picture of the scientific status of the field today and how it reached this point. Chapters 5, 6, and 7 describe the research objectives and instruments for the next two decades. Chapters 8, 9, and 10 describe the structure of the field and how it relates to other branches of physics and technology and to society at large.
Finally, Chapter 11 presents the committee's conclusions concerning the health of elementary-particle physics and its recommendations for the future.