In the 17th century, Sir Isaac Newton developed the law of universal gravitation. This law recognized that every mass in the universe exerts an attraction on every other mass. Newton postulated that gravity was a universal force that affected all matter. Newton's theory served to explain the forces of attraction between the Earth and nearby objects, as well as the forces that affect the motion of planets and other astronomical bodies. In the early 20th century, Albert Einstein's special theory of relativity led to a major reinterpretation and clarification of Newton's theory of gravity. Scientists now believe that gravity affects energy as well as matter, and that gravity is actually a fundamental warping of the fabric of space-time. Some theorize that the force of this warping is transmitted by as-yet-unobserved particles called gravitons.