Gravity is the force by which every object in the universe that has mass attracts every other object. Its effects only become obvious when large masses are involved. So, for instance, although we feel the strong downward pull of Earth's gravity, we feel no pull at all toward smaller masses such as coffee tables, vending machines, or other people. Yet a mutual attraction does exist between all things that have mass – you pull on Earth, just as Earth pulls on you. Until the beginning of the twentieth century, the only universal law of gravitation was that of Isaac Newton in which gravity was regarded as an invisible force which could act across empty space (see Newton's law of gravity). The force of gravity between two objects was proportional to the mass of each and inversely proportional to the square of their separation distance. Then, in 1913, Einstein published a revolutionary new theory of gravitation known as the general theory of relativity in which gravity emerges as a consequence of the geometry of spacetime. In the "rubber sheet" analogy of spacetime, masses such as stars and planets can be thought of as lying at the bottom of depressions of their own making. These gravitational wells are the spacetime craters into which any objects coming too close may fall (for example, matter plunging into a black hole) or out of which an object must climb if it is to escape (for example, a spacecraft leaving Earth for interplanetary space).
Quantum gravity is a theory, not yet achieved, that successfully merges quantum mechanics and general relativity, possibly involving modifications of one or both. String theory is an example of a theory of quantum gravity.