Introduction

Concept of a black hole

Black holes have attracted people's imaginations perhaps more than any other kind of object in the cosmos. Remarkably the concept of a black hole dates back more than 200 years. In 1783, the Cambridge cleric John Michell speculated in a lecture to the Royal Society about the effects of the Sun's gravity on the light it was radiating. Michell was aware of the finite speed of light (determined by Roemer in the seventeenth century from observations of eclipse timings of Jupiter's moons) and believed that photons from the Sun (he called them ‘corpuscles’) would be slowed down as they left the Sun due to its gravity. His speculation was to point out that if the Sun's diameter were 500 times larger and of the same density, then its mass would be 108M⊗, and gravity would prevent light from escaping the Sun at all. A similar conjecture was put forward by Laplace in 1795.

However, our modern concept of a black hole stems from Einstein's theory of general relativity (GR) and the first exact solutions, derived by Karl Schwarzschild in 1916, of Einstein's equations. Under GR, the effect of a massive body's gravity is to curve the space-time around it, forcing light to follow a curved path (called a ‘geodesic’). If the body is sufficiently massive and compact, then this curvature closes in on itself, and any light emitted by the body will never escape – hence the term black hole.