Editing Acceleration (section)

===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>