As a writer on physics, I’m always seeking new metaphors for understanding Einstein’s general theory of relativity, and while working on my last book, Spooky Action at a Distance, I thought I’d compare the warping of space and time to the motion of Earth’s tectonic plates. Einstein explained gravity as the bending of spacetime. A well-hit baseball arcs through the air to an outfielder’s glove because it is following the contours of spacetime, which the planet’s mass has resculpted. The mutabilit...