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7 Gravitation Theory: I Few areas of physics apply to such a broad range of phenomena as does Einstein's General Theory of Relativity. Gravity, which it de- scribes, governs our universe on its largest scales and its smallest. Current attempts to construct a quantum theory of gravity confront physics at its most fundamental level. At the same time, relativity is an important element in the variety of physics being used to construct models of pulsars, x-ray sources, quasars, and the universe itself. The theory of relativity is deep and central and has broad application. As a consequence it is actively worked on and has many ties to other disciplines. The General Theory of Relativity was proposed in final form by Einstein in 1916. It is at once a theory of gravity and a theory of the structure of space-time; it attributes the gravitational interaction to the geometric curvature of our space-time continuum. The late 1960s saw the start of a period of exciting development in the area of relativity. In pelt this was brought on by new astronomical discoveriesthe discov- eries of pulsars, galactic x-ray sources, possibly black holes, and the 3-K cosmic radiation. Further stimulation came from the fruitful interaction of particle physics and quantum gravity in the development of field-theory techniques that could be applied to both areas and the progress in particle physics toward energy scales where quantum gravity must be important. Fundamental discoveries within the field (the singularity theorems and the Hawking radiation to give just two 59

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60 GRA VITATION examples) deepened our understanding, provided new confidence, and enabled the theory to be extended into new domains. This exciting period of development is still under way. Gravity is a very weak force by the standards of elementary-particle physics. The only reason that it is the dominant force on astrophysical length scales is that it is always attractive; unlike electromagnetism, it cannot be canceled, neutralized, or shielded against. Even in our solar system, gravity is a weak force by relativistic standards matter velocities are everywhere less than the speed of light by a factor of 10-3. The crucial effects that mark the difference between Newtonian theory and general relativistic gravity and between general relativity and alternative relativity theories are therefore very small in our solar system. Nevertheless, there are times and places in the universe where gravity is strong, such as in neutron stars, black holes, and the big bang. Even the universe itself is a highly relativistic system in that recession velocities approach the speed of light for the most distant known objects. The General Theory of Relativity, as a fundamental theory of physical interactions, contains three major kinds of purely gravitational elementary objects: gravitational waves, black holes, and isolated universes. The simplest universe is the flat, empty universe of Min- kowski space-time. Other simple universes are the homogeneous, isotropic cosmological models, which are good models for our uni- verse. These elementary objects can be combined in various ways: Gravitational waves can propagate, either on a flat background or in a universe. Black holes can also inhabit either. Black holes can be formed by the implosion of sufficiently intense pulses of gravitational waves, as well as by the gravitational collapse of matter. When black holes collide, they coalesce to form a larger black hole, and some gravitational waves are radiated. When the effects of quantum mechanics are included, new processes appear. Black holes decay by quantum emission of particles and eventually disappear (Hawking radiation). Universes can in principle tunnel into one another by quantum-mechanical barrier penetration. Quantum effects are important for the structure of a big-bang singular- ity.