Gravity is the universal attractive interaction between masses, described at low energy by Newton's law of universal gravitation and at all energies by Einstein's general theory of relativity. In general relativity, gravity is the curvature of spacetime produced by energy and momentum. Gravitational acceleration at Earth's surface is approximately 9.81 m/s². Gravitational waves, predicted by general relativity and first observed directly in 2015 by LIGO, are ripples in spacetime caused by accelerating masses.