As Sir Isaac Newton himself replied in response to similar questions, "hypotheses non fingo." Which roughly translates as "I don't have a clue." That such a simple question, about so common a phenomenon, has defied a direct answer for centuries might explain why the physics world has been atwitter lately over a novel attempt to resolve the riddle. A flurry of recent papers have examined this new idea, which mixes principles from string theory and black hole physics with basic old-fashioned thermodynamics. If this notion is right, gravity turns out to be a special sort of entropy, a result of...