Chapter 15 - Space-Time and Gravitation

Abstract

This chapter discusses the theory of gravitation in space-time. According to the general theory of relativity, gravity is the result of a curvature of space-time. A weak curvature is responsible for Newtonian gravitational fields, such as that on the Earth. However, in objects such as black holes, the curvature is so strong that nonlinear effects produce a space-time singularity, which can force the curvature to evolve chaotically. The resulting rapid variations in curvature generate disturbances—gravitational waves—that propagate through the universe. With a network of detectors, it should be possible to determine the angular position of an event from the times of arrival of the signals at different detectors. If the distance to an event can be inferred from some parameters of the signals, and the red shift of the host cluster is known, it should be possible to obtain Hubble's constant in an unambiguous way, which provides a much more reliable cosmological scale. The Laser Interferometer Gravitational Observatory (LIGO) project is now building facilities for detecting and studying gravitational waves at two sites in the United States. The LIGO project involves the construction of three laser interferometers: a single interferometer with 4-km-long arms at one site, and two interferometers, with 2-km- and 4-km-long arms sharing the same vacuum system, at the other site. Correlating the outputs of the three interferometers at two widely separated sites would make it possible to distinguish signals because of the gravitational waves from bursts of instrumental and environmental noise.