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Executive Summary Gravity is one of the four fundamental forces of nature. It is an immediate fact of everyday experience, yet it presents us with some of the deepest theoreti- cal and experimental challenges in contemporary physics. Gravity is the weakest of the four fundamental forces, but, because it is a universal attraction between all forms of energy, it governs the structure of matter on the largest scales of space and time, including the structure of the universe itself. As one of the fundamental interactions, gravity is central to the quest for a unified theory of all forces, whose simplicity would emerge at very high energies or, equivalently, at very small distances. Gravitational physics is thus a two-frontier science. On the large scales of astrophysics and cosmology it is central to the understanding of some of the most exotic phenomena in the universe black holes, pulsars, quasars, the final des- tiny of stars, and the propagating ripples in the geometry of spacetime called gravitational waves. On the smallest scales it is concerned with the quantized geometry of spacetime, the unification of all forces, and the quantum initial state of the universe. Its two-frontier nature means that gravitational physics is a cross-disciplinary science overlapping astrophysics and cosmology on large scales and elementary-particle and quantum physics on small scales. The theory that bridges this enormous range of scales is Einstein's 1915 general theory of relativity. The key ideas of general relativity are that gravity is the geometry of four-dimensional spacetime, that mass produces spacetime cur- vature while curvature determines the motion of mass, and that all freely falling bodies follow paths independent of their mass (an idea that is called the principle of equivalence). 1

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2 GRAVITATIONAL PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME When gravitational fields are weak and vary only slowly with time, the effects of general relativity are well approximated by Newton's 300-year-old theory of gravity. However, general relativity predicts qualitatively new phe- nomena when gravitational fields are strong, are rapidly varying, or can accumu- late over vast spans of space or time. Black holes, gravitational waves, closed universes, and the big bang are some examples. Further, when the principles of classical general relativity are united with quantum theory, quantum uncertainties can be expected in the geometry of spacetime itself. The focus of modern gravi- tational physics has naturally been on exploring such relativistic and quantum phenomena. Gravitational physics is one of the oldest subjects in physics. Yet the expan- sion of opportunities in both experiment and theory has made it one of the most rapidly changing areas of science today. A short list of some of the important achievements of the past decade illustrates this point: The confirmation of the existence of gravitational waves by the observed shortening of the orbital period of a binary pulsar. The detection of the fluctuations in the cosmic background radiation (the light from the big bang) that are the origin of today's galaxies, stars, and planets. The development of a new generation of high-precision tests (to parts in a thousand billion) of the equivalence principle that underlies general relativity, and the verification of general relativity's weak-field predictions to better than parts in a thousand. . The identification of candidate black holes in x-ray binary stars and in the centers of galaxies. Black holes are no longer a theorist's dream; they are central to the explanation of many of astronomy's most dramatic phenomena. The use of gravitational tensing as a practical astronomical tool to inves- tigate the structure of galaxies and to search for the dark matter in the universe. The increasing use of large-scale numerical simulations to solve Einstein's difficult nonlinear equations. These simulations can predict the effects of strong gravity that will be seen in the next generation of observations of gravitational phenomena. The discovery of "critical phenomena" in gravitational collapse analo- gous to those that occur in transitions between different states of matter. The development of string theory and the quantum theory of geometry as promising candidates for the union of quantum mechanics and general relativity. The first descriptions of the quantum states of black holes. The development of powerful mathematical tools to study the physical regimes where Einstein's theory can break down. .

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EXECUTIVE SUMMARY 3 The Committee on Gravitational Physics (COP) foresees that the transforma- tion of the science of gravitational physics will accelerate in the next decade, driven by new experimental, observational, and theoretical opportunities. A single theme runs through the most important of these opportunities: the explora- tion of strong gravitational fields. Among the specific opportunities the COP believes could be realized in the next decade if appropriate resources are made available are the following: . The first direct detection of gravitational waves by the worldwide net- work of gravitational wave detectors now under construction. The first direct observation of black holes by the characteristic gravita- tional radiation they emit in the last stages of their formation. The use of gravitational waves to probe the universe of complex astro- nomical phenomena by the decoding of the details of the gravitational wave signals from particular sources. The continuing transformation of cosmology into a data-driven science by the wealth of measurements expected from new cosmic background radiation satellites, new telescopes in space and on the ground, and new systematic surveys of the large-scale arrangements of the galaxies. The first unambiguous determination of the basic parameters that charac- terize our universe, its age and fate, the matter of which it is made, how much of that matter there is, and the curvature of space on large scales. The unambiguous measurement of the value of the cosmological con- stant, with profound implications for our understanding of the fate of the uni- verse, and also for particle physics and quantum gravity. The use of gamma-ray, x-ray, optical, infrared, and radio telescopes on Earth and in space to detect new black holes in orbit about companion stars and to explore the extraordinary properties of the geometry of space in the vicinity of black holes that are predicted by general relativity. The measurement of the dragging of inertial frames due to the rotation of Earth at the 1 percent level by the Gravity Probe B mission scheduled for launch in 2000. Dramatically improved tests of the equivalence principle that underlies general relativity. The understanding of the predictions of Einstein's theory in dynamical, strong-field, realistic situations through the implementation of powerful numeri- cal simulations and sophisticated mathematical techniques untrammeled by weak- field assumptions, special symmetries, or other approximations. The development of current ideas in string theory and the quantum theory of geometry to achieve a finite, workable union of quantum mechanics, gravity, and the other forces of nature, potentially resulting in a fundamentally new view .

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4 GRAVITATIONA:L PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME of space and time. The application of this new theory to predict the outcome of black hole evaporation and the nature of the big bang singularity. The continued development within quantum gravity of a theory of the quantum initial condition of the universe capable of making testable predictions of cosmological observations today. If these opportunities are realized, the CGP expects the next decade of re- search in gravitational physics to be characterized by (1) a much closer integra- tion of gravitational physics with astrophysics, cosmology, and elementary-par- ticle physics, (2) much larger experiments yielding much more data and requiring international collaboration, (3) a much closer relationship between theory and experiment, and (4) a much wider, more important role for computation in gravi- tational physics. In light of such opportunities, the CGP identified the following unordered list of highest-priority goals for gravitational physics: Receive gravitational waves and use them to study regions of strong gravity. Explore the extreme conditions near the surface of black holes. Measure the geometry of the universe and test relativistic gravity on cosmological scales; explore the beginning of the universe. Test the limits of Einstein's general relativity and explore for new physics. Unify gravity and quantum theory. . . In making this list, the CGP assumed that the scientific objectives of a number of projects now under way will be achieved, e.g., Gravity Probe B. construction of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Chandra X-ray satellite, and the MAP cosmic background satellite. Although fully en- dorsed by the CGP, these projects do not appear in its recommendations.

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EXECUTIVE SUMMARY s The COP makes several recommendations for reaching these goals. The four areas of recommended actions are listed in priority order, with the highest- pnonty area given first. The recommendations within each of the four categories have equal weight. 1. Gravitational Waves The search for gravitational waves divides naturally into the high-frequency gravitational wave window (above a few hertz) accessible by experiments on Earth, and the low-frequency gravitational wave window (below a few hertz) accessible only from space. Both windows are important, and the CGP has not prioritized one over the other. The highest priority is to pursue both of these sources of information. The High-Frequency Gravitational Wave Window Carry out the first phase of LIGO scientific operations. Enhance the capability of LIGO beyond thefirst phase of operations, with the goal of detecting the coalescence of neutron star binaries. Support technology development that will provide the foundation for fu- ture improvements in LIGO's sensitivity. The Low-Frequency Gravitational Wave Window Develop a space-based laser interferometer facility able to detect the gravitational waves produced by merging supermassive black holes. 2. Classical and Quantum Theory of Strong Gravitational Fields Support the continued development of analytic and numerical tools to obtain and interpret strong-field solutions of Einstein's equations. Support research in quantum gravity, to build on the exciting recent progress in this area. . 3. Precision Measurements Dramatically improve tests of the equivalence principle and of the gravi- tational inverse square law. . Continue to improve experimental testing of general relativity, making use of available technology, astronomical capabilities, and space opportunities.

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6 GRAVITATIONA:L PHYSICS: EXPLORING THE STRUCTURE OF SPACE AND TIME 4. Astronomical Observations The astronomical observations recommended below have strong arguments for support from astronomy and astrophysics. The ones listed are those that the COP expects will have the greatest impact on gravitational physics in the next decade. . Use gamma-ray, x-ray, optical, infrared, and radio telescopes on Earth and in space to study the environment near black holes. Measure the temperature and polarization;fluctuations of the cosmic back- ground radiation from arcminute scales to scales of tens of degrees. Search for additional relativistic binary systems. Launch all-sky gamma-ray and x-ray burst detectors capable of detecting the electromagnetic counterparts to LIGO events. . Use astronomical observations of supernovae and gravitational lenses to infer the distribution of dark matter and to measure the cosmological constant. If these recommendations are implemented, the COP believes that the next decade in gravitational physics could see as significant a transformation of the field as occurred in the late 1960s and early 1970s. This transformation will take the subject further into the arena of strong gravitational fields, with stronger coupling from experiment than ever before, leading to a deeper understanding of the central place of gravitational physics in resolving the fundamental questions of contemporary physics.