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{{Short description|Theoretical physics term}}

{{refimprove |date=June 2025}}

In [[theoretical physics]], a '''local reference frame''' ('''local frame''') refers to a [[coordinate system]] or [[frame of reference]] that is only expected to function over a small region or a restricted region of space or [[spacetime]].

The term is most often used in the context of the application of '''local inertial frames''' to small regions of a [[gravitational field]]. Although gravitational [[tidal forces]] will cause the background geometry to become noticeably [[non-Euclidean]] over larger regions, if we restrict ourselves to a sufficiently small region containing a cluster of objects falling together in an ''effectively'' uniform gravitational field, their physics can be described as the physics of that cluster in a space free from explicit background gravitational effects.

==Equivalence principle==

==Einstein and general relativity==

{{main|Equivalence principle}}

When constructing his [[general theory of relativity]], [[Albert Einstein|Einstein]] took the result that an accelerated body feels an apparent gravitational field ([[g-force|geeforce]]s), and inverted it: an object in a gravitational field, if it is ''not'' accelerated, will ''not'' be able to detect the existence of the field by making local measurements ("a falling man feels no gravity"). 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 freefalling regions.

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&dq=a+falling+man+feels+no+gravity&pg=PA17|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|isbn=978-0-19-514285-3 |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.com/books?id=mbUGZYzs878C&dq=%22the+happiest+idea+of+my+life%22+Einstein&pg=PR13|title=Relativity: The Special and General Theory|first=Albert |last=Einstein|publisher=Barnes & Noble Publishing|year=2004|page=XIII|isbn=978-0-7607-5921-9 |access-date=June 18, 2025}}</ref>

Einstein referred to this as "the happiest idea of my life".

==Laboratory frame{{anchor|Laboratory|Laboratory frame}}==

In physics, the '''laboratory frame of reference''', or '''lab frame''' for short, is a [[frame of reference]] centered on the [[laboratory]] in which the [[experiment]] (either real or thought experiment) is done. This is the reference frame in which the laboratory is at rest. Also, this is usually the frame of reference in which [[measurement]]s are made, since they are presumed (unless stated otherwise) to be made by laboratory instruments. An example of instruments in a lab frame, would be the [[particle detector]]s at the detection facility of a [[particle accelerator]].

==See also==

*[[EquivalenceBreit principleframe]]

*[[Center-of-mass frame]]

*[[Frame bundle]]

*[[Inertial frame of reference]]

*[[Local coordinates]]

*[[Local spacetime structure]]

*[[Local Lorentz covariance]]

*[[LocallyMinkowski flat spacetimespace]]

*[[Normal coordinates]]

==References==

{{reflist}}

{{DEFAULTSORT:Local Reference Frame}}

[[Category:Frames of reference]]

[[Category:General relativity]]

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