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{{main|Equivalence principle}}
When constructing his [[general theory of relativity]], [[Albert Einstein|Einstein]] made the following observation: a freely falling object in a gravitational field will not be able to detect the existence of the field by making local measurements ("a falling man feels no gravity").<ref name=Zee>{{cite book|url=https://books.google.com/books?id=B7OdsjBg5WcC&pg=PA17&dq=a+falling+man+feels+no+gravity&hl=en&newbks=1&newbks_redir=|pages=5-7, 10-17, 21-27, 230-242|title=Einstein's Universe: Gravity at Work and Play|first=A. |last=Zee|publisher=Oxford University Press|year=2001|access-date=June 18, 2025}}</ref> Einstein was then able to complete his general theory by arguing that the physics of curved spacetime must reduce over small regions to the physics of simple inertial mechanics (in this case [[special relativity]]) for small free-falling regions.<ref name=Zee /> Einstein referred to this as "the happiest idea of my life" (Zee (2001), p. 17.<ref name=Zee /><ref>{{cite book|url=https://books.google.es/books?id=mbUGZYzs878C&pg=PR13&dq=%22the+happiest+idea+of+my+life%22+Einstein&hl=es&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwjQgquThIqGAxVk_7sIHVxaB3kQ6AF6BAgIEAI#v=onepage&q=%22the%20happiest%20idea%20of%20my%20life%22%20Einstein&f=false|title=Relativity: The Special and General Theory|first=Albert |last=Einstein|publisher=Barnes & Noble Publishing|year=2004|page=XIII|access-date=June 18, 2025}}</ref>
==Laboratory frame{{anchor|Laboratory|Laboratory frame}}==
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