Another great 20th century contribution of Albert Einstein is the general theory of relativity. It provides deep insights into the nature of gravity. In this theory, heavy massive bodies such as the Earth and Sun cause spacetime to curve, much in the same way as a bowling ball — when placed on a bed — depresses the bed’s surface. An object moving in such a curved spacetime no longer moves at a constant speed in a constant direction — it accelerates, just like a marble, when thrown onto the bed with the bowling ball, moves toward the bowling ball. See animation. Since, by definition, forces are things that create accelerations, the curvature of spacetime is seen to be the source of the gravitational force. Two interesting consequences of general relativity are the black hole and the expansion of the universe (Click here to read a review of book The Inflationary Universe).
General Relativity
Another great 20th century contribution of Albert Einstein is the general theory of relativity. It provides deep insights into the nature of gravity. In this theory, heavy massive bodies such as the Earth and Sun cause spacetime to curve, much in the same way as a bowling ball — when placed on a bed — depresses the bed’s surface. An object moving in such a curved spacetime no longer moves at a constant speed in a constant direction — it accelerates, just like a marble, when thrown onto the bed with the bowling ball, moves toward the bowling ball. See animation. Since, by definition, forces are things that create accelerations, the curvature of spacetime is seen to be the source of the gravitational force. Two interesting consequences of general relativity are the black hole and the expansion of the universe (Click here to read a review of book The Inflationary Universe).