Editing Theory of relativity (section)

== Special relativity ==
{{Main|Special relativity}}
[[File:Einstein_Portrait.png|thumb|180px|Albert Einstein, physicist, 1879-1955, Graphic: Heikenwaelder Hugo,1999]]
Special relativity is a theory of the structure of [[spacetime]]. It was introduced in Einstein's 1905 paper "[[On the Electrodynamics of Moving Bodies]]" (for the contributions of many other physicists and mathematicians, see [[History of special relativity]]). Special relativity is based on two postulates which are contradictory in [[classical mechanics]]:
# The [[laws of physics]] are the same for all observers in any [[inertial frame of reference]] relative to one another ([[principle of relativity]]).
# The [[speed of light]] in [[vacuum]] is the same for all observers, regardless of their relative motion or of the motion of the [[light]] source.

The resultant theory copes with experiment better than classical mechanics. For instance, postulate 2 explains the results of the [[Michelson–Morley experiment]]. Moreover, the theory has many surprising and counterintuitive consequences. Some of these are:
* [[Relativity of simultaneity]]: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
* [[Time dilation]]: Moving [[clock]]s are measured to tick more slowly than an observer's "stationary" clock.
* [[Length contraction]]: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
* [[Speed of light#Upper limit on speeds|Maximum speed is finite]]: No physical object, message or field line can travel faster than the speed of light in vacuum.
** The effect of gravity can only travel through space at the speed of light, not faster or instantaneously.
* [[Mass–energy equivalence]]: {{nowrap|1=''E'' = ''mc''<sup>2</sup>}}, energy and mass are equivalent and transmutable.
* [[Mass in special relativity|Relativistic mass]], idea used by some researchers.<ref name=":0">{{Cite web|title = The Theory of Relativity, Then and Now|url = http://www.smithsonianmag.com/innovation/theory-of-relativity-then-and-now-180956622/?no-ist|access-date = 2015-09-26|first = Brian|last = Greene}}</ref>

The defining feature of special relativity is the replacement of the [[Galilean transformation]]s of classical mechanics by the [[Lorentz transformation]]s. (See [[Maxwell's equations]] of [[electromagnetism]].)