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Gravity, or gravitation is one of the fundamental forces of the universe. It is an attraction, or pull, between any two objects with mass. We discuss it in three parts:

Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.

1. Everyday sense: the force which causes objects to fall to the ground
2. Newton's laws: how gravity keeps the Solar System and most major astronomical objects together
3. Einstein's theory of general relativity: the role of gravity in the universe

Some physicists think gravity is caused by gravitons, but they are still unsure.