A luminous star with the same density as the Earth and a diameter 250 times that of the Sun would not, because of its attraction, allow any of its rays to reach us. It is therefore possible that the largest luminous bodies in the universe are, for that very reason, invisible. — Pierre-Simon Laplace, Exposition of the System of the World (1796) These sentences are in the first edition of the Exposition of the System of the World. Laplace wrote them at a time when people believed, like Newton, that light was made of particles. If light is a swarm of corpuscles, gravity must pull on them as it pulls on a stone. And then, if a star is compact enough, the light it emits will fall back, like a stone thrown upward.