According to this fine popular primer, nobody knows what gravity is, but few readers will feel that their time was wasted.
No one thought about gravity before Aristotle, writes science writer Panek (The 4 Percent Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality, 2011), but all ancient cultures knew that some things were “up” (the heavens, the gods), and earthly matter was “down.” Everything on Earth fell down, but the heavens stayed up, and few thinkers wondered why. “Reasoning,” writes the author, “…was what Aristotle would introduce into the conversation: methodology, not mythology.” However, he came to the wrong conclusion, maintaining that objects fell because they are drawn toward the center of the universe, which sat at the center of the Earth. Heavenly objects, being perfect, were exempt. Newton’s concept of universal attractive force and the inverse square law were not original, but his outstanding mathematics, which predicted movements of bodies anywhere in the universe, made him a superstar in Britain. Natural philosophers of other nations pointed out that a force that acted magically across empty space was clearly nonsense. Because Newton’s math worked so well, they came around, but plenty of thoughtful scientists remained unhappy. Panek paraphrases physicist and philosopher Ernst Mach: “the theory of gravitation had disguised its philosophical shortcomings by proving its reliability and usefulness. But the philosophical shortcomings remained. They’d just become respectable.” Einstein solved the problem in 1915 by more dazzling mathematics demonstrating that matter warps nearby space-time. Bodies moving through this distorted space seem to change direction, giving the appearance of a force acting on them. Many bizarre consequences—black holes, gravitational lenses, the slowing of time—follow naturally. Philosophically inclined readers may complain that scientists still don’t know what gravity is, but the remainder will enjoy Panek’s expert description of the spectacular things that gravity does.
A useful primer on a force that still inspires mystery.