GR regards gravity as the response of a mass to the local curvature of space and time caused by some other mass or field of energy. In other words, concentrations of mass cause distortions—dimples, really—in the fabric of space and time. These distortions guide the moving masses along straight-line geodesics,††† though they look to us like the curved trajectories we call orbits. The twentieth-century American theoretical physicist John Archibald Wheeler said it best, summing up Einstein’s concept as, “Matter tells space how to curve; space tells matter how to move.”††††
