Figure 1:
(Top) In Einstein’s general theory of relativity, gravity is nothing more than the curvature of spacetime. A massive object, such as the sun, causes a deformation of the spacetime grid, while another object such as a planet or a light beam follows the shortest path (a “geodesic”) on this grid. To an observer, this looks like a deflection of the trajectory caused by gravity. (Bottom) A collapsing star can form a black hole so dense and massive that it creates a region of infinite curvature (a “singularity”) so that—inside the event horizon—light cannot escape. Current research in gravitation is attempting to modify general relativity to account for such objects consistent with quantum theory. The work of Hořava [1, 2] and Lu et al. [3] looks for ways to attack this problem.