Einstein announced his general theory of relativity to the world in 1915, and it has remained a backbone in the field of physics since that time. Though the equations, says Professor Strominger, are "quite complex and quite beautiful," the theory itself is rather simple. It postulates that gravity is an illusion resulting from the curvature of space and time. Einstein said that all matter - including the sun and the earth and humans - causes some curvature in its surrounding space/time dimensions. Under these circumstances, it's difficult to say what a straight line is: For example, the earth thinks it's moving straight ahead, but because of the curvature of space, it's actually circling around the sun. Everything from Newton's previous theory of gravity can be understood or ironed out using Einstein's theory of general relativity.