Gravity Lost, Gravity Regained could be said to characterize this latest exercise in science popularization by physicist Zee (Fearful Symmetry, 1987). Einstein's theory of gravity, discussed at length here, emphasizes the equivalence principle, inspired by Einstein's happy thought that when we fall freely we do not feel our own weight. This amounts to saying that in a small region of space the physical effects of a gravitational field—as perceived by the observer—are indistinguishable from the physical effects experienced by another observer accelerating at a constant rate in the absence of a gravitational field. Think of persons falling into a box furnished like a living room: an object dropped into the box is equivalent to the floor moving up to hit the object; think also of astronauts in orbit—they are not in a gravity-free environment; they are experiencing weightlessness because they are in free fall around the earth. The old man's toy was, in fact, a contraption given to Einstein on his 76th birthday to illustrate this brilliant insight of a half-century earlier. From equivalence to curved spacetime is an easy transition, for Einstein and for Zee. Blobs of mass make dents in space (like a trampoline), causing light to bend as it passes nearby. Gravity/four-dimensional spacetime/E=mc squared all come together in the Einsteinian framework. They don't come together with the quantum, however. And much of Zee's exegesis brilliantly constructs the universe according to electromagnetic, strong, and weak forces, starting at some point after the Big Bang. The problem, according to Zee, is accounting for that first zillionth of a second. Gravity cannot be fitted into current unified theories, no matter how bizarre. Therein lies the rub, and—for Zee—the hope that physicists who have for too long ignored gravity will discover, renew, and regain insights, † la Einstein, that will once again revolutionize physics.
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