100 Years of Relativity Space-Time Structure: Einstein and Beyond ...

150 R. H. Priceform a hole. In terms of the proper time that describes the physics local tothe collapsing surface, passage through the event horizon is continuous andsmooth.II. Observing the unobservable black holeNo information can propagate outward through the event horizon, butthe compactness of a black hole means that the gravitational influence ofthe hole on its near environment gives rise to exotic phenomena (bendingof light rays, generation of X-rays, gravitational waves. . . ).III. Newtonian-like points, or spacetime regionsIt is a matter of distance from the hole. In a study of the properties ofspacetime very near a black hole, the focus is on the defining properties, thenonsingular one-way nature of the event horizon. At distances several timesr g , however, the main influence of the hole is its 1/r 2 pull. It therefore actson its environment like a Newtonian gravitational source and simple argumentsabout relative frames and momentum conservation at large distancesshow that the hole must move like a Newtonian object.IV. Black holes: simple or exceedingly unsimpleThe stationary Schwarzschild and Kerr spacetimes are very clean andsimple. Dynamical black holes are sufficiently complicated that an understandingof them can come only through numerical relativity, and thatunderstanding is still some time in the future.The history of the black hole has very much paralelled the place ofEinstein’s theory in science. The Schwarzschild geometry dates to the verystart of general relativity. There was limited interest in general relativity,and limited understanding of black holes among most astronomers until theearly 60s. New astronomical discoveries, starting with quasars, opened thepossibility that Einstein’s theory had an astrophysical role beyond cosmology.Now, almost a half century later, the transition is complete. It is nowall but taken for granted that supermassive black holes lie at the centers ofmany, perhaps all, galaxies, and that certain point-like X-ray sources areevidence of stellar mass holes.AcknowledgmentsThis work was supported by National Science Foundation grantPHY0244605, and NASA grant ATP03-0001-0027. I thank Frederick Jenetfor useful comments on the manuscript.