Editing Principle of relativity (section)

==General principle of relativity==
The '''general principle of relativity''' states:<ref name=Moller>{{cite book |title=The Theory of Relativity |author=C. Møller |publisher=Oxford University Press |edition=2nd |location=Delhi |isbn=0-19-560539-X |url=https://books.google.com/books?id=xaFKAAAACAAJ&q=intitle:The+intitle:theory+intitle:of+intitle:relativity+inauthor:Moller |page=220 |year=1952}}</ref>{{Quotation|All systems of reference are equivalent with respect to the formulation of the fundamental laws of physics.|C. Møller ''The Theory of Relativity'', p. 220}}
That is, physical laws are the same in {{em|all}} reference frames—inertial or non-inertial. An accelerated charged particle might emit [[synchrotron radiation]], though a particle at rest does not. If we consider now the same accelerated charged particle in its non-inertial rest frame, it emits radiation at rest.

Physics in non-inertial reference frames was historically treated by a [[coordinate transformation]], first, to an inertial reference frame, performing the necessary calculations therein, and using another to return to the non-inertial reference frame. In most such situations, the same laws of physics can be used if certain predictable [[fictitious forces]] are added into consideration; an example is a uniformly [[rotating reference frame]], which can be treated as an inertial reference frame if one adds a fictitious [[Centrifugal force (fictitious)|centrifugal force]] and [[Coriolis force]] into consideration.

The problems involved are not always so trivial. Special relativity predicts that an observer in an inertial reference frame does not see objects he would describe as moving faster than the speed of light. However, in the non-inertial reference frame of [[Earth]], treating a spot on the Earth as a fixed point, the stars are observed to move in the sky, circling once about the Earth per day. Since the stars are light years away, this observation means that, in the non-inertial reference frame of the Earth, anybody who looks at the stars is seeing objects which appear, to them, to be moving faster than the speed of light.

Since non-inertial reference frames do not abide by the special principle of relativity, such situations are not [[contradiction|self-contradictory]].

===General relativity===
{{main|General relativity}}

General relativity was developed by Einstein in the years 1907–1915. General relativity postulates that the [[Symmetry (physics)#Local and global|global]] [[Lorentz covariance]] of special relativity becomes a [[Symmetry (physics)#Local and global|local]] Lorentz covariance in the presence of matter. The presence of [[matter]] "curves" [[spacetime]], and this [[curvature]] affects the path of free particles (and even the path of light). General relativity uses the mathematics of [[differential geometry]] and [[tensor]]s in order to describe [[gravitation]] as an effect of the [[geometry]] of [[spacetime]]. Einstein based this new theory on the general principle of relativity and named the theory after the underlying principle.