Albert Einstein's theory of relativity is actually two separate theories: his *special theory
of relativity* , postulated in the 1905 paper, *The Electrodynamics of Moving Bodies* and
his *theory of general relativity* , an expansion of the earlier theory, published as *The
Foundation of the General Theory of Relativity* in 1916. Einstein sought to explain situations
in which Newtonian physics might fail to deal successfully with phenomena, and in so doing proposed
revolutionary changes in human concepts of time, space and gravity.

The special theory of relativity was based on two main postulates: first, that the speed of light is constant for
all observers; and second, that observers moving at constant speeds should be subject to the same
physical laws. Following this logic, Einstein theorized that time must change according to the
speed of a moving object *relative* to the frame of reference of an observer. Scientists have
tested this theory through experimentation - proving, for example, that an atomic clock ticks more
slowly when traveling at a high speed than it does when it is not moving. The essence of Einstein's
paper was that both space and time are *relative* (rather than absolute), which was said to
hold true in a *special* case, the absence of a gravitational field. Relativity was a stunning
concept at the time; scientists all over the world debated the veracity of Einstein's famous
equation, E=mc2, which implied that matter and energy were equivalent and, more specifically, that
a single particle of matter could be converted into a huge quantity of energy. However, since the
special theory of relativity only held true in the absence of a gravitational field, Einstein
strove for 11 more years to work gravity into his equations and discover how relativity might work
generally as well.

According to the theory of general relativity, matter causes space to curve. It is posited that
gravitation is not a force, as understood by Newtonian physics, but a curved *field* (an area
of space under the influence of a force) in the space-time continuum that is actually created by
the presence of mass. According to Einstein, that theory could be tested by measuring the
deflection of starlight traveling near the sun; he correctly asserted that light deflection would
be twice that expected by Newton's laws. This theory also explained why the light from stars in a
strong gravitational field was closer to the red end of the spectrum than
those in a weaker one.

For the final thirty years of his life, Einstein attempted to find a unified
field theory, in which the properties of all matter and energy could be expressed in a single
equation. His search was confounded by quantum theory's *uncertainty
principle* , which stated that the movement of a single particle could never be accurately
measured, because speed and position could not be simultaneously assessed with any degree of
assurance. Although he was unable to find the comprehensive theory that he sought, Einstein's
pioneering work has allowed countless other scientists to carry on the quest for what some have
called "the holy grail of physicists."

See a simple introduction to the theory of relativity:

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