Editing Special relativity (section)

===Basis===
Unusual among modern topics in physics, the theory of special relativity needs only mathematics at high school level and yet it fundamentally alters our understanding, especially our understanding of the concept of [[time]].<ref name=Mermin-2009/>{{rp|ix}} Built on just two postulates or assumptions, many interesting consequences follow. 

The two postulates both concern observers moving at a constant speed relative to each other. The first postulate, the {{slink|#principle of relativity}}, says the laws of physics do not depend on objects being at absolute rest: an observer on a moving train sees natural phenomena on that train that look the same whether the train is moving or not.<ref name=Mermin-2009/>{{rp|5}} The second postulate, constant speed of light, says observers on a moving train or on in the train station see light travel at the same speed. A light signal from the station to the train has the same speed, no matter how fast a train goes.<ref name=Mermin-2009/>{{rp|25}}

In the theory of special relativity, the two postulates combine to change the definition of "relative speed". Rather than the simple concept of distance traveled divided by time spent, the new theory incorporates the speed of light as the maximum possible speed. In special relativity, covering ten times more distance on the ground in the same amount of time according to a moving watch does not result in a speed up as seen from the ground by a factor of ten.<ref name=Mermin-2009/>{{rp|28}}