Like ordinary stars, planets, and other celestial bodies, black holes, which astronomers have been able to detect in recent years, have certain physical properties that distinguish them from the others. Yet because of a black hole's extraordinary nature, especially the fact that it is black and invisible, very few of its properties can be directly measured from the outside. "From the outside," John Gribbin explains, you can [calculate] the mass of the hole from its gravitational attraction and the speed with which it rotates. If it has an electric charge, you could measure that as well. But those . . . properties are all you can ever measure. There is no way to tell what the matter that went into the hole was before it was swallowed up . . . whether it was a star, a great glob of water, or...