One of Einstein’s 1905 papers described the special theory of relativity, which explained that moving objects become more massive as they approach the speed of light, clocks slow down, and objects flatten into pancakes. In 1916, Einstein published his general theory of relativity, showing that mass warps the structure of space and time, accelerating objects emit gravitational waves, and clocks slow down in a gravitational field. In the 1920s and 1930s, physicists developed the set of ideas known as quantum mechanics to explain the puzzling behavior of the subatomic world; these fundamental insights contributed to some of the most important technologies of the 20th century, including the semiconductors that have made possible the proliferation of modern electronic devices. Also in the 1920s and 1930s, astronomers produced evidence indicating that the universe is expanding, which suggests that all matter was created in an event known as the big bang, which took place more than 13 billion years ago. Studies of materials revealed new phenomena such as superconductivity, nuclear fission, and the coherent emission of light (leading to the development of the laser). These astonishing insights into the nature of the physical world produced new fields of physics (such as nuclear physics, condensed matter physics, and particle physics), generated knowledge that found applications throughout the sciences and in technology, and created a base of understanding that has helped remake our world.

The field of elementary particle physics (or, simply, “particle physics,” which is the term used most often in this report) took shape in the first half of the 20th century as physicists began to study the fundamental constituents of matter and their interactions (Box 1-1). Both experimentation and theory have been critical to the advance of particle physics. For example, early in the 20th century, certain puzzling experimental results caused physicists to seek new and more fundamental explanations of the laws of nature. This search led to Einstein’s startling new theories of space and time and of gravity, as well as to the equally revolutionary development of quantum mechanics by physicists such as Max Planck, Niels Bohr, Werner Heisenberg, Max Born, and Erwin Schrödinger. The second half of the century witnessed a blossoming of particle physics as experiments tested existing hypotheses and inspired new ones. Many of those experiments involved particle accelerators, which convert matter to energy and back to matter again, as described by Einstein’s equation, E = mc2. In recent decades, accelerator experiments have become enormous undertakings involving thousands of scientists and engineers and intellectual and financial contributions from countries around the world. In addition, a spectrum of much smaller, less expensive, but also highly valuable experiments has measured the special properties of particles and particular interactions among particles. Most recently, astronomical data from satellites and ground-based facilities have produced extremely useful information for particle physics. The nascent field of particle astrophysics has brought a deeper apprecia-



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