General Relativity

To understand relativity one needs to first understand the concept of a reference frame. A reference frame is a conceptual system for locating objects and events in space and time. It consists of a "frame" or set of spatial coordinate axes (for example, north-south, east-west, and up-down) and of a "clock" (that is, any means of measuring time). Such a system is called a reference frame because any object or event's position and state of motion can, in principle, be described by referring to points on the axes of the frame and to readings given by clocks. A description of any object or event's location and speed depends on the reference frame on which the description is based. If, for example, you are riding in a car, you are at rest in a reference frame that is rigidly attached to the car but are moving in a reference frame that is rigidly attached to the road. The two frames are moving relative to each other. A key insight of Ein stein's was that there is no absolute reference frame. That is, the reference frame attached to the car is precisely as valid as the frame attached to the road. A reference frame that is moving at a constant velocity in a straight line is termed an intertial reference frame. A reference frame that is accelerating or rotating is termed a noninertial. As mentioned above, the theory of general relativity expands the theory of special relativity from inertial to noninertial reference frames.

One of the effects of special relativity was to combine our concepts of space and time into a unified concept: space-time. According to the space-time concept, space and time are not independent as they are in everyday experience and in Newtonian physics: that is, in relativistic space-time an observer's state of motion through space (velocity) has a real effect on how quickly time passes in their frame of reference (that is, a frame of reference moving with them) relative to observers in different frames of reference. General relativity allows that space-time to be "curved" by the matter contained in it, and explains gravity as a manifestation of curved space-time.