Gravity and Early Predictions of Black Holes

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. This is partly because their name can be somewhat misleading. A black hole is not simply an empty hole, or void, in space, but instead a cosmic entity having very substantial mass. (Mass is the measurable quantity of matter possessed by physical objects.) A black hole does have a hole, or tunnel, as part of its structure; but unlike a void, it possesses a number of physical properties as well, some of which can be measured by human instruments.

For example, a black hole’s large mass generates an equally large gravitational field, which scientists can detect. Gravity is a force or property of all matter that attracts one object to another. In small objects such as molecules, pebbles, people, houses, and so forth, gravitational attractions are very slight and neither noticeable to the human senses nor measurable by instruments; only on planetary and larger scales do these attractions become obvious and easily measurable. Thus, Earth’s gravity holds the Moon in orbit around the planet, and the gravity of the Sun, the star at the center of our solar system, keeps Earth and the other planets in orbit around it. Likewise, a black hole exerts a gravitational attraction on any objects that happen to stray too close to it. “In fact,” writes astronomer Thomas T. Arny,

the gravitational field generated by a black hole is no different from that generated by any other body of the same mass. For example, if the sun were suddenly to become a black hole with the same mass it has now [something that could not actually happen], the Earth would continue to orbit it just as it does now.