Editing Equivalence principle (section)

{{Short description|Hypothesis that inertial and gravitational masses are equivalent}}
{{About|the principle in gravitation|the principle in electromagnetism|surface equivalence principle}}
{{Use dmy dates|date=September 2020}}
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[[File:Elevator gravity.svg|thumb|upright=1.5|A falling object behaves exactly the same on a planet or in an equivalent accelerating frame of reference.]]
{{General relativity sidebar |fundamentals}}

The '''equivalence principle''' is the hypothesis that the observed equivalence of [[Gravitational mass|gravitational]] and [[Mass#Inertial mass|inertial mass]] is a consequence of nature. The weak form, known for centuries, relates to masses of any composition in free fall taking the same trajectories and landing at identical times. The extended form by [[Albert Einstein]] requires [[special relativity]] to also hold in free fall and requires the weak equivalence to be valid everywhere. This form was a critical input for the development of the theory of [[general relativity]]. The strong form requires Einstein's form to work for stellar objects. Highly precise experimental tests of the principle limit possible deviations from equivalence to be very small.