Editing Principle of relativity (section)

===In special relativity===
{{main|Special relativity}}

[[Joseph Larmor]] and [[Hendrik Lorentz]] discovered that [[Maxwell's equations]], used in the theory of [[electromagnetism]], were invariant only by a certain change of time and length units. This left some confusion among physicists, many of whom thought that a [[luminiferous aether]] was incompatible with the relativity principle, in the way it was defined by [[Henri Poincaré]]:

{{Quotation|The principle of relativity, according to which the laws of physical phenomena should be the same, whether for an observer fixed, or for an observer carried along in a uniform movement of translation; so that we have not and could not have any means of discerning whether or not we are carried along in such a motion.|Henri Poincaré, 1904<ref>{{Cite book|author=Poincaré, Henri|year=1904–1906|chapter=[[s:The Principles of Mathematical Physics|The Principles of Mathematical Physics]]|title=Congress of arts and science, universal exposition, St. Louis, 1904|volume=1|pages=604–622|publisher=Houghton, Mifflin and Company|place=Boston and New York}}</ref>}}

In their 1905 papers on [[Annus Mirabilis Papers#Papers|electrodynamics]], Henri Poincaré and [[Albert Einstein]] explained that with the [[Lorentz transformations]] the relativity principle holds perfectly. Einstein elevated the (special) principle of relativity to a [[postulate]] of the theory and derived the Lorentz transformations from this principle combined with the principle of the independence of the speed of light (in vacuum) from the motion of the source. These two principles were reconciled with each other by a re-examination of the fundamental meanings of space and time intervals.

The strength of special relativity lies in its use of simple, basic principles, including the [[covariance and contravariance of vectors|invariance]] of the laws of physics under a shift of [[inertial reference frame]]s and the invariance of the speed of light in vacuum. (See also: [[Lorentz covariance]].)

It is possible to derive the form of the Lorentz transformations from the principle of relativity alone. Using only the isotropy of space and the symmetry implied by the principle of special relativity, one can show that the space-time transformations between inertial frames are either Galilean or Lorentzian. Whether the transformation is actually Galilean or Lorentzian must be determined with physical experiments. It is not possible to conclude that the speed of light ''c'' is invariant by mathematical logic alone. In the Lorentzian case, one can then obtain relativistic interval conservation and the constancy of the speed of light.<ref name=Friedman>Yaakov Friedman, ''Physical Applications of Homogeneous Balls'', Progress in Mathematical Physics '''40''' Birkhäuser, Boston, 2004, pages 1-21.</ref>