Einstein’s Theory at the Extremes: Gravitational Waves and Black Holes

Abstract

Einstein’s theory of gravity, General Relativity, is one of humankind’s greatest intellectual achievements. Over the last century, its interpretation of the gravitational interaction in terms of spacetime curvature has been brilliantly confirmed by many different experiments. Two astonishing predictions of General Relativity are gravitational waves—spacetime perturbations that travel at the speed of light—and black holes—mysterious objects made solely from pure spacetime fabric which challenge the laws of physics. Once considered only as bizarre solutions to Einstein’s equations, black holes have now acquired a central role in astrophysics, cosmology, and even in foundational physics and philosophy. The recent radio images of supermassive black holes are providing us with invaluable information about the dynamics of matter in the vicinity of an event horizon. Gravitational waves carry information about the most dramatic astrophysical processes in the Universe, such as black-hole collisions. Their detection has inaugurated the era of gravitational-wave astronomy, which is reshaping astrophysics, cosmology, and fundamental physics. Thanks to future experiments, black holes, and gravitational waves will shed new light on the unknown Universe. In this chapter, we will explore the theory of these fascinating objects and the observations that have confirmed their existence.