An Old Man's Toy: Gravity at Work and Play in Einstein's Universe by A. Zee (Macmillan: $21.95; 272 pages)

Physics began in 1687 with the publication of Isaac Newton's "Principia Mathematica," which showed that the law of gravity governs falling apples, the moon and the planets. One law of physics rules everything in the heavens and on Earth. It was one of the greatest intellectual achievements of all time.

But in this century, Einstein showed that Newton didn't have it quite right. Gravity is not merely a force that acts between two bodies. Rather, gravity is a symptom of the very essence of space and time. Gravity is deeper than Newton thought.

Nowadays, however, theoretical physicists don't spend much time thinking about gravity. They are interested in quantum mechanics, subatomic particles, dark matter, superstrings and the like. This is a big mistake, says physicist A. Zee in "An Old Man's Toy." Gravity is still the name of the game, he says. It explains all we know about physics and cosmology, and it points to essential paradoxes that remain unresolved.

"Typically," Zee writes, "graduate students are required to take two or even three years of quantum physics, while Einstein's theory of gravity is brushed over in a half-year optional course. . . . But gravity is not to be denied. If it doesn't get more stage time, it's gonna call its agent! The audience hears ominous rumblings from the wings. In its petulance, gravity is threatening to disrupt the entire play."

Zee's book is a tour of modern physics from the point of view of gravity. There is no shortage of popular and semi-popular books that explain current theories of the Big Bang, the origin of matter, the formation of stars and galaxies, subatomic physics and superstring theory.

"An Old Man's Toy" covers this ground from a different perspective, and it succeeds both in spinning out a new idea and in making it accessible. Zee's writing is clear, straightforward and profound. He is a very good explainer.

"We spoke figuratively of gravity grumbling in the wings, waiting for its chance on center stage," Zee writes. "In fact, gravity is the stage!"

Of course, reading about physics is never going to be like reading a romance novel. It requires attention. But that attention is handsomely repaid. As in so many areas of thought, in physics, the closer one examines the premises, the more elusive they become.