The fundamental force of attraction that all objects with mass have for each other. Like the electromagnetic force, gravity has effectively infinite range and obeys the inverse-square law. At the atomic level, where masses are very small, the force of gravity is negligible, but for objects that have very large masses such as planets, stars, and galaxies, gravity is a predominant force, and it plays an important role in theories of the structure of the universe. Gravity is believed to be mediated by the graviton, although the graviton has yet to be isolated by experiment. Gravity is weaker than the strong force, the electromagnetic force, and the weak force. Also called

*gravitation* See more at acceleration

*A Closer Look* With his law of universal gravitation, Sir Isaac Newton described *gravity* as the mutual attraction between any two bodies in the universe. He developed an equation describing an instantaneous gravitational effect that any two objects, no matter how far apart or how small, exert on each other. These effects diminish as the distance between the objects gets larger and as the masses of the objects get smaller. His theory explained both the trajectory of a falling apple and the motion of the planets—hitherto completely unconnected phenomena—using the same equations. Albert Einstein developed the first revision of these ideas. Einstein needed to extend his theory of *Special Relativity* to be able to understand cases in which bodies were subject to forces and acceleration, as in the case of gravity. According to Special Relativity, however, the instantaneous gravitational effects in Newton's theory would not be possible, for to act instantaneously, gravity would have to travel at infinite velocities, faster than the speed of light, the upper limit of velocity in Special Relativity. To overcome these inconsistencies, Einstein developed the theory of *General Relativity,* which connected gravity, mass, and acceleration in a new manner. Imagine, he said, an astronaut standing in a stationary rocket on the Earth. Because of the Earth's gravity, his feet are pressed against the rocket's floor with a force equal to his weight. Now imagine him in the same rocket, this time accelerating in outer space, far from any significant gravity. The accelerating rocket pushing against his feet creates a force indistinguishable from that of a gravitational field. Developing this *principle of equivalence*, Einstein showed that mass itself forms curves in space and time and that the effects of gravity are related to the trajectories taken by objects—even objects without mass, such as light. Whether gravity can be united with the other fundamental forces understood in quantum mechanics remains unclear.