Editing Equivalence principle (section)

== Concept ==

In classical mechanics, [[Newton's law of universal gravitation|Newton's equation of motion in a gravitational field]], written out in full, is:
: [[inertial mass]] × [[acceleration]] = [[gravitational mass]] × [[gravitational field|gravitational acceleration]] 
Careful experiments have shown that the inertial mass on the left side and gravitational mass on the right side are numerically equal and independent of the material composing the masses. The equivalence principle is the hypothesis that this numerical equality of inertial and gravitational mass is a consequence of their fundamental identity.<ref name="EinsteinMeaning">{{cite book|last1=Einstein|first1=Albert|title=The Meaning of Relativity|url=https://archive.org/details/meaningrelativit00aein|url-access=limited|date=2003|publisher=Routledge|isbn=9781134449798|page=[https://archive.org/details/meaningrelativit00aein/page/n67 59]}}</ref>{{rp|32}}

The equivalence principle can be considered an extension of the principle of relativity, the principle that the laws of physics are [[Invariant (physics)|invariant]] under uniform motion. An observer in a windowless room cannot distinguish between being on the surface of the Earth and being in a spaceship in deep space accelerating at 1''g'' and the laws of physics are unable to distinguish these cases.<ref name="EinsteinMeaning" />{{rp|33}}