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Introduction Gravitational physics grew out of astronomy in the seven- teenth century. Newtonian gravitation, besides being an ade- quate and successful description of the gravitational interaction for everyday purposes, had a profound influence on the philosophy of natural science. It was the first example of the universality of physical laws; the force that caused Newton's apocryphal apple to fall was the same reason that the Moon falls toward the Earth, or the Earth toward the Sun. As has been known since the discovery of special relativity at the beginning of this century, Newtonian gravitation cannot be a complete theory of gravitation. It cannot be expressed in a relativistically covariant way and is not a field theory in the modern sense. The epoch between the formulation of special relativity in 1905 and the formulation of general relativity in 1916 witnessed the development of many theories of gravita- tion consistent with special relativity: scalar, vector, and tensor theories in four dimensions. A few of these theories survive, and several new theories have been developed in recent times. None carry the weight of general relativity, but they do serve to high- light the sensitive points in general relativity and help to generate a scheme for testing general relativity. General relativity imbeds gravitation in a theory that relates the structure of space en c! time to the distribution of matter. As 3

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4 such, the framework set by general relativity becomes the arena for the other interactions in nature. Indeed, Einstein and others devoted considerable effort after the discovery of general relativity to an unsuccessful attempt at trying to unify, through geometric considerations alone, all the known interactions in nature. It is ironic that recent attempts at this grand unification from quantum field theory appear to be possible for all fields but gravitation. The failure ~ in good measure due to the inability, at present, to formulate a viable quantum theory of gravity an outstanding theoretical problem. Most of the consequences of classical (nonquantized) general relativity have been theoretically studied over the past 70 years. In its pristine form, the theory has no free parameters. A modification to the theory, noted by Einstein shortly after its first formulation, is the addition of a new term in the field equations, which would only have an effect on space-time on the largest scales, in particular in cosmological solutions. In a purely mathematical sense the theory is complete and can be tested at any level. One could hold the extreme view that the agreement of general relativity with Newtonian gravitation in the low-velocity, weak-field limit is adequate empirical justification for the entire theoretical structure. A more critical viewpoint is that the failure of the theory in any test that can be devised ~ sufficient reason to render the entire theory invalid. In the past 20 years, substantial theoretical progress has been made toward understanding the inner structure of relativistic grav- itation. Extensive work has been done on formal solutions to the Einstein field equations and the meaning of the singularities in the theory. The properties of black hole solutions have been in- vestigated In sufficient depth to make firm predictions concerning the gravitational radiation emitted by them in encounters with other black holes or stars. In the past 10 years, numerical rel- ativity has become an active field} by which the solutions of the nonlinear differential equations of general relativity can be ~risual- ized. A particularly important aspect of the numerical modeling is the coupling of general relativity with fluid dynamics and mag- netohydrodynamics in the attempt to understand astrophysical phenomena such as stellar collapse, in which relativistic gravity may play a major role in the final stage. A technique h" been developed for weak gravitational fields that expands metric theo- ries of relativistic gravity, including general relativity, into a set of

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s terms multiplied by parameters associated with the degree that a particular physical concept or symmetry is incorporated into the theory. This parametric post-Newtonian (PPN) expansion serves as a useful framework with which to test relativistic gravitation. The profound consequences of general relativity are the phe- nomena that involve strong spac~time curvature and the large- scaTe global properties of space-time. In these regimes, gravitation dominates all other interactions and Newtonian gravitation is not even a fair approximation of reality. The unambiguous discovery of a black hole, and especially the observation of the gravitational ra- diation due to the interaction of ~ black hole with its surroundings, would constitute the strongest confirmation of general relativity. A complete understanding of cosrn~c evolution would provide as sharp a test; however, the prospects for this are not as bright considering the complexity of interpreting the astrophysical phe- nomena that are the source of cosmological data. General relativity makes specific predictions for new effects when the sources of the field are in motion. In analogy with elec- tricity and magnetism, the gravitational field can be divided into electric and magnetic terms. In a weak field, the electric terms correspond to Newtonian gravitation. The new magnetic terms will come into evidence when both source and test body are in relative motion. The magnetic interaction between the spinning Earth and a spinning gyro ~ measurable and ~ the basis of a new test of general relativity to be attempted In the space program within the next 10 years. Magnetic gravitation is expected to be a large effect in the field of a spinning black hole. It may play a role in astrophysical phenomena, in particular ~ the explanation of the jets in extragalactic double radio sources. Gravitational radiation is predicted in all relativistic theories of gravitation. The sources of the radiation are accelerating masses. In general relativity the lowest-order radiation is due to the tim~dependent quadrupole moment of the source. Binary stellar systems, aspheric stellar collapse, colliding stellar systems, and rotating neutron stars (pul- sars) are examples of sources of gravitational radiation that may be detectable on Earth or by space-based receiving systems. The direct detection of the radiation could serve to test general relativ- ~ty by confirming the propagation speed and polarization states of the waves. Furthermore, as already indicated, gravitational waves may be the best way to detect black holes and thereby test general relativity in the strong-field, high-velocity limit.

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6 Gravitational wave astronomy could well open a new view of the universe as it will expose phenomena that involve the coherent motions of large masses at relativistic speeds. There processes will generally occur in the innermost parts of active astrophysical re- gions and therefore be obscured from "view" in the electromagnetic astronomies. A particularly exciting although highly speculative prospect ~ the possible detection of gravitation radiation noise from the very earliest epoch of the universe that is completely obscured in the electromagnetic and even neutrino astronomers. The first observational evidence for relativistic gravitation was the small discrepancy in the orbital motion of the planet Mercury discovered in the nineteenth century. The orbit, taking account of the perturbations due to the other objects In the solar system, did not close on itself as predicted by Newtonian gravitation. Einstein was wed aware of this and considered the ability of general relativity to predict the motion of Mercury as strong evidence for the correctness of the then new theory. The perihelion advance of Mercury as wed as the bending of starlight by the Sun and the gravitational red shift of spectral lines emanating from the Sun relative to the same lines observed on Earth are the three classical tests suggested by Einstein. It is interesting to note that although the bending of light was observed in the welI-publicized Eddington eclipse expedition of 1918, a reliable measurement of this effect had to wait for the radio interferometer observations of quasi-stelIar sources passing near the Sun in the past decade. The gravitational red shift was not well measured until the middle 1960s by Mossbauer techniques on the ground and, more recently, by the comparison of two hydrogen maser clocks, one on the Earth's surface and the other in a suborbital trajectory. In the mid-196Os, a new test became possible with the emergence of radar astronomy that enabled the direct determination of the range to solar system bodies, thereby measuring directly the space-time curvature of the region around the Sun. One of the most fortuitous discoveries for the observational tests of relativistic gravitation is the binary pulsar system, which was uncovered in a general radio search for pulsars. This system appears to be composed of two neutron stars, one of which is a stable pulsar and therefore a ~good" clock. The kinematics of this system, observed through the Doppler shift of the pulsar signals as it orbits about its companion, has established several of the

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7 classical relativistic tests and furthermore is now demonstrating the first evidence for gravitational radiation damping. Besides the red shift experiment mentioned above, the space program has played an important role in experimental tests of general relativity. Laser ranging to the corner reflectors placed on the Moon by the Apollo program has allowed a test of the principle of equivalence for gravitational self-energy. The Viking program through the Mars Orbiter and Lander has provided range information without the perturbations due to planetary topogra- phy. The precision timing of radio signals across the solar system to Viking and back to Earth has established the best observation of the post-Newtonian time delay in the vicinity of the Sun. The precision ephemerides of Mars, in concert with those of the other planets in the solar system, have set the best limits on many of the post-Newtonian parameters, including limits on the change of G (Newtonian gravitational constant) with time. The common element in this historical perspective of progress in gaining empirical evidence for relativistic gravitation is the close coupling to technical advances In both direct experiments and observational techniques. There ~ no reason this should change in the future. The space program is expected to play a decisive role in this research.