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Gravitational waves may be directly detected, and the predictions of Einstein's theory for the structure of black holes may be checked against data for the first time. Questions such as the origin of the chemical elements and the nature of extremely energetic cosmic accelerators will be understood more deeply. All of this has given birth to a rich new interplay of physics and astronomy.

Unifying the Forces of Nature

Experiment and theory together will provide a new understanding of the basic constituents of matter. The mystery of the nature of elementary particles deepened in the 1990s with the discovery of the extraordinarily heavy top quark and the observation of oscillations in neutrinos from the Sun and the upper atmosphere, suggesting that neutrinos have extremely tiny masses. During the decade ahead the unknown physics responsible for elementary-particle masses and other properties will begin to reveal itself in experiments at a new generation of high-energy colliders. Possibilities range from the discovery of new and unique elementary particles to more exotic scenarios involving fundamental changes in our description of space and time.

Determining this new physics is an important step toward an historic goal: the discovery of a unified theoretical description of all the fundamental forces of nature—the strong nuclear force, the electroweak forces, and gravity. The most promising and exciting framework for unifying gravity with the other forces is string theory, which proposes that all elementary particles behave like strings at very tiny distances. String theory has also given birth to new and vibrant intersections between physics and pure mathematics. This decade will see much progress toward the goal of discovering a unified theory of the forces of nature.

The scale and complexity of the physics necessary to advance these six priority areas will require increased levels of strategically directed investment and international cooperation. In the United States, the investment must be broad based: from the federal government, from industry, from colleges and universities, and from other supporters of physics research and education. The committee believes that as a result of this focused investment, the decade ahead will see dramatic progress in the above areas and in the many other frontiers described in this overview and in the earlier volumes of the survey. There is little doubt that the new ideas and technologies developed in these quests will enable progress in all the sciences, contribute to the needs of the nation, and benefit the lives of people everywhere.

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