# Relativity in Simple Terms

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.

The special theory of relativity is concerned with relative motion between non-accelerated frames of reference. The general theory deals with general relative motion between accelerated frames of reference.  In accelerated systems of reference, certain fictitious forces are observed, such as the centrifugal and Coriolis forces found in rotating systems.  These are known as fictitious forces because they disappear when the observer transforms to an non-accelerated system.

The predictions of general relativity only differ from Newton's theory by small amounts and most tests of the theory have been carried out through observations in astronomy.  For example, it explains the shift in the perihelion of Mercury, the bending of light or other electromagnetic radiation in the presence of large bodies, and the Einstein shift.