Editing Acceleration (section)

== Relation to relativity ==

===Special relativity===
{{main|Special relativity|Acceleration (special relativity)}}
The special theory of relativity describes the behaviour of objects travelling relative to other objects at speeds approaching that of light in vacuum. [[Newtonian mechanics]] is exactly revealed to be an approximation to reality, valid to great accuracy at lower speeds. As the relevant speeds increase toward the speed of light, acceleration no longer follows classical equations.

As speeds approach that of light, the acceleration produced by a given force decreases, becoming [[infinitesimally]] small as light speed is approached; an object with mass can approach this speed [[asymptotically]], but never reach it.

===General relativity===
{{main|General relativity}}
Unless the state of motion of an object is known, it is impossible to distinguish whether an observed force is due to [[gravity]] or to acceleration—gravity and inertial acceleration have identical effects. [[Albert Einstein]] called this the [[equivalence principle]], and said that only observers who feel no force at all—including the force of gravity—are justified in concluding that they are not accelerating.<ref name="Greene">{{cite book |title=The Fabric of the Cosmos: Space, Time, and the Texture of Reality |title-link=The Fabric of the Cosmos |last=Greene |first=Brian |date=8 February 2005 |author-link=Brian Greene |isbn=0-375-72720-5 |publisher=Vintage |page=67}}</ref>