**In the
considerable amount of (welcome!) e-mail we recieve from this site, we have been
continuously getting requests for a simple explanation of relativity. We
have therefore decided that it would be more appropriate to create a new
section of the time travel web site dedicated to answering this very important
question for our visitors. As always, your comments are welcome.
The theory of relativity is in fact two theories. The special theory of
relativity (1905) and the general theory of relativity (1915). The special
theory gives a unified account of the laws of mechanics and of
electromagnetism. Einstein rejected the concepts of absolute space and time and
made two postulates (a) the laws of nature are the same for all observers in
uniform relative motion and (b) the speed of light is the same for all such
observers.
The transformation of time implies that two events that are simultaneous
according to one observer will not necessarily be so according to another in
uniform relative motion. It will appear to two observers in uniform relative
motion that each other's clock runs slowly. This is the phenomenon of time
dilation.
A mathematical formulation of the special theory of relativity is based on the
idea that an event is specified by four co-ordinates: three spatial
co-ordinates and one time co-ordinate. These co-ordinates define a
four-dimensional space and the motion of a particle can be described by a curve
in this space**

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