Editing Equivalence principle (section)

== History ==
{{See also|History of gravitational theory}}

By [[experiment]]ing with the [[acceleration]] of different materials, [[Galileo Galilei]] determined that [[gravitation]] is independent of the amount of [[mass]] being accelerated.<ref name=Everitt>{{Cite journal |last1=Everitt |first1=C.W.F. |last2=Damour |first2=T. |last3=Nordtvedt |first3=K. |last4=Reinhard |first4=R. |date=October 2003 |title=Historical perspective on testing the Equivalence Principle |url=https://linkinghub.elsevier.com/retrieve/pii/S0273117703903358 |journal=Advances in Space Research |language=en |volume=32 |issue=7 |pages=1297–1300 |doi=10.1016/S0273-1177(03)90335-8|bibcode=2003AdSpR..32.1297E }}</ref>

[[Isaac Newton]], just 50 years after Galileo, investigated whether gravitational and inertial mass might be different concepts. He compared the periods of pendulums composed of different materials and found them to be identical. From this, he inferred that gravitational and inertial mass are the same thing. The form of this assertion, where the equivalence principle is taken to follow from empirical consistency, later became known as "weak equivalence".<ref name=Everitt/>

A version of the equivalence principle consistent with special relativity was introduced by [[Albert Einstein]] in 1907, when he observed that identical physical laws are observed in two systems, one subject to a constant gravitational field causing acceleration and the other subject to constant acceleration, like a rocket far from any gravitational field.<ref name="WhittakerII"/>{{rp|152}}  Since the physical laws are the same, Einstein assumed the gravitational field and the acceleration were "physically equivalent". Einstein stated this hypothesis by saying he would:

{{blockquote|...assume the complete physical equivalence of a gravitational field and a corresponding [[accelerated reference frame|acceleration of the reference system]].|Einstein, 1907<ref>Einstein, Albert. "On the relativity principle and the conclusions drawn from it." Jahrb Radioaktivitat Elektronik 4 (1907): 411–462.</ref>}}

In 1911 Einstein demonstrated the power of the equivalence principle by using it to predict that clocks run at different rates in a [[gravitational potential]], and light rays [[bending of starlight|bend]] in a gravitational field.<ref name="WhittakerII">{{cite book | last=Whittaker | first=Sir Edmund | title=A History of the Theories of Aether and Electricity | publisher=Courier Dover Publications | date=1989-01-01 | isbn=0-486-26126-3 | volume=2}}</ref>{{rp|153}} He connected the equivalence principle to his earlier principle of special relativity:

{{blockquote| This assumption of exact physical equivalence makes it impossible for us to speak of the absolute acceleration of the system of reference, just as the usual theory of relativity forbids us to talk of the absolute velocity of a system; and it makes the equal falling of all bodies in a gravitational field seem a matter of course.|Einstein, 1911<ref>Einstein, Albert. "On the Influence of Gravitation on the Propagation of Light." Annalen der Physik 35.898–908 (1911): 906.</ref>}}

Soon after completing work on his theory of gravity (known as [[general relativity]])<ref>Lorentz, Hendrik Antoon, et al. [https://books.google.com/books?id=oy_Z3o7p6zwC The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity]. United Kingdom, Dover Publications, 1923.</ref>{{rp|111}} and then also in later years, Einstein recalled the importance of the equivalence principle to his work:
{{blockquote| The breakthrough came suddenly one day. I was sitting on a chair in my patent office in Bern. Suddenly a thought struck me: If a man falls
freely, he would not feel his weight. I was taken aback. This simple thought experiment made a deep impression on me. This led me to the theory of gravity.| Einstein, 1922<ref>Einstein, Albert, [https://web.archive.org/web/20151222085312/http://inpac.ucsd.edu/students/courses/winter2012/physics2d/einsteinonrelativity.pdf ''How I Constructed the Theory of Relativity''], translated by Masahiro Morikawa from the text recorded in Japanese by Jun Ishiwara, Association of Asia Pacific Physical Societies (AAPPS) Bulletin, Vol. 15, No. 2, pp. 17–19, April 2005. Einstein recalls events of 1907 in a talk in Japan on 14 December 1922.</ref>}}

Einstein's development of general relativity necessitated some means of empirically discriminating the theory from other theories of gravity compatible with [[special relativity]]. Accordingly, [[Robert H. Dicke|Robert Dicke]] developed a test program incorporating two new principles – the {{section link||Einstein equivalence principle}}, and the {{section link||Strong equivalence principle}} – each of which assumes the weak equivalence principle as a starting point.