Black holes. No term evokes the mystery of modern gravity more than this one. The mystery of black holes is more than an invention of popularizers of astronomy and relativity. Black holes were certainly a mystery to Einstein and his contemporaries. Yet today black holes are everywhere: in X-ray binaries, in the centers of galaxies, and of course in books, like this one, on relativity and gravity!
In this chapter: we study general relativity's most intriguing prediction: black holes. We look at the central place they have in Einstein's theory, their role in astronomy today, and the direction they are giving to efforts to unify gravity and quantum theory. We calculate orbits around black hole, examine the astronomical evidence for black holes, and learn about wormholes, the Hawking radiation, and black hole entropy.
Theorists attacked the problem of understanding black holes, not by using astronomical evidence, but by using lessons they had learned from quantum mechanics. Quantum thinking demanded that physicists ask only questions about things that could be measured, not about what is hidden from experiment. Thus, they can measure that light behaves sometimes as a particle (the photon) and sometimes as a wave, but they find it useless to ask what is a wave–particle.