Just about everyone has heard of black holes. But many nonscientists are not exactly sure what these bizarre objects are and what they are capable of doing.
Twentieth-century predictions that black holes might exist naturally raised the question of how such superdense objects could form. Over time, scientists came to realize that there might be more than one answer to this question, depending on the size of the black hole.
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.
Long before scientists began dreaming about possibly capturing, controlling, and exploiting black holes for humanity's benefit, they dreamed of detecting and directly observing these cosmic oddities in the first place. Einstein, Schwarzschild, Kerr, and many others became certain that black holes could exist, but for a long time they could find no convincing evidence that they do exist.
As long as stellar black holes were the only kind of black holes for which science could find even indirect evidence, the universe seemed a far less scary place than it does today. After all, stellar black holes did not appear to pose any major short- or long-term danger to the universe as a whole or to the existence of life within it.
So far, stellar and galactic black holes have been considered in light of how their major properties—extreme gravity, accretion disks, quasars, and so forth—affect matter, space, and time in the universe surrounding them. Very little has been said about what happens inside a black hole other than that matter is either crushed or falls down the hole's gravity well forever.