A. Special Theory, General Theory
Albert Einstein's most noted accomplishment is his theory of relativity. This theory was developed in two major stages.
The first stage is known as the special theory of relativity. Its essential idea is that neither space nor time are absolute things, but relative things that depend on one's frame of reference, while the combination of space and time is a single, nonrelative entity, which remains the same regardless of one's frame of reference.
In this single entity (called "spacetime"), time turns out to be a relative dimension, like the three dimensions of space. Which points in space you find to have the same altitude depends on where you stand on earth-change your location and you'll find those same points now have different altitudes. Similarly, which events in spacetime you see as happening at the same time depends on how you're moving-move with a different velocity and you'll find the same events occur at different times. Accounting for time as a relative direction in spacetime, much as the vertical is a relative direction in space, turns out to provide a lot of information about physical phenomena and the laws that govern them. For example, Einstein took account of how space and time are related to demonstrate the equivalence of mass and energy ("E=mc2").
The second stage, called the general theory of relativity, picks up where the special theory left off. In its treatment of time as one of the dimensions of geometry, the special theory depicts spacetime as a four-dimensional analog of a two-dimensional plane. Just as a place lacks curvature, so does the spacetime of the special theory. But according to the general theory, spacetime is analogous to a more general type of surface, which can be curved instead of flat. And just as time's being a direction has physical consequences, spacetime's having curvature also has physical consequences. It will be easier to understand these consequences once we have a clear idea of how a thing like space, or spacetime, can be curved; however, one consequence is worth mentioning now: gravity.
More about that later. (.....continued)