General Relativity

    This single sentence carries the seeds of general relativity. It began with Einstein thinking of the simple relative situation in which he imagined being in a closed box, like a lift. He would feel a force at his feet, interpreted as "weight". When he dropped a pen it would fall to the floor with constant acceleration. From experiments inside the lift, no one could tell if these effects were caused by a downward all-pervading force or if the box was being accelerated upwards by an unknown agent. He then applied his theory of special relativity to this situation and borrowed some advanced geometry to extend it to the Universe at large.

    First, Einstein abolished gravity as a 'force that acts through space'. He replaced it with geometry - a simple consequence of curvature of spacetime. Einstein showed that a lump of matter caused the spacetime around it to be curved. Two masses traveling near each other, such as the Earth and the moon, will move in 'straight lines through spacetime' - but we see them as moving in curved paths. The earth has more mass, so its spacetime curving effect is greater. Both bodies orbit around the same point, but the moon orbits in a larger, less curved path. The geometry works and the theory predicts the orbits of planets more accurately than Newton's theory of gravitation. The elliptical orbit of the planet Mercury swings around the sun in a way that only Einstein's theory could explain. The general theory of relativity is the basis of large scale cosmological theory-- including the black hole.