Theory of Relativity Breakthrough

An Albert Einstein Theory

Einstein's Theory of Relativity

Einstein's special relativity was "special" because it dealt only with the specific case of intertial reference frames. An inertial reference frame is a body that is either at rest or that moves with a constant velocity. In contrast, his general theory of relativity accounts not only for these, but also for bodies that accelerate (i.e., change their velocity). Einstein began his theory with a thought experiment--that is, an experiment carried out only in the mind of the experimenter. This experiment imagines a physicist in a room on Earth dropping a ball to the ground. The ball falls to the floor at an accelerating rate because of the force of gravity. (link here) However, the physicist would observe the same phenomenon in an accelerating spaceship in a region of outer space without gravity: upon the ball's release, it would hang suspended in mid-air as the floor of the spaceship rushed up to hit it. To the physicist inside the ship, however, the ball would appear to "fall" toward the floor exactly as it did in the room on earth. Thus, it would be impossible for the physicist inside the spaceship to distinguish between gravitation and any other acceleration. Indeed, this was the essence of Einstein's "equivalence principle," which posits the equivalence of physical effects within reference frames at rest within a gravitational field (like the room) and within reference frames accelerating in the absence of any gravitational field (as in the rocket). The equivalence principle also posits the equivalence of gravitational mass (the measure of the force a body exerts on another) and inertial mass (the measure of a body's resistance to being accelerated)

(www.Sparknotes.com)