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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)
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