Editing Inertial frame of reference (section)

===General relativity===
{{Main|General relativity|Introduction to general relativity}}
{{See also|Equivalence principle|Eötvös experiment}}
General relativity is based upon the principle of equivalence:<ref name=Morin>{{Cite book|title=Introduction to Classical Mechanics |author=David Morin |page=[https://archive.org/details/introductiontocl00mori/page/649 649] |url=https://archive.org/details/introductiontocl00mori |url-access=registration |quote=acceleration azimuthal Morin. |isbn=978-0-521-87622-3 |publisher=Cambridge University Press |date=2008}}</ref><ref name=Giancoli>{{Cite book|title=Physics for Scientists and Engineers with Modern Physics |author=Douglas C. Giancoli |url=https://books.google.com/books?id=xz-UEdtRmzkC&q=%22principle+of+equivalence%22&pg=PA155
|page=155 |date=2007 |publisher=Pearson Prentice Hall |isbn=978-0-13-149508-1 }}</ref>
{{blockquote|<i>There is no experiment observers can perform to distinguish whether an acceleration arises because of a gravitational force or because their reference frame is accelerating.</i>|Douglas C. Giancoli, ''Physics for Scientists and Engineers with Modern Physics'', p. 155.}}

This idea was introduced in Einstein's 1907 article "Principle of Relativity and Gravitation" and later developed in 1911.<ref name=General_theory>A. Einstein, "[http://www.relativitycalculator.com/pdfs/On_the_influence_of_Gravitation_on_the_Propagation_of_Light_English2.pdf On the influence of gravitation on the propagation of light] {{Webarchive|url=https://web.archive.org/web/20201224033225/http://www.relativitycalculator.com/pdfs/On_the_influence_of_Gravitation_on_the_Propagation_of_Light_English2.pdf |date=24 December 2020 }}", ''Annalen der Physik'', vol. 35, (1911) : 898–908</ref> Support for this principle is found in the [[Eötvös experiment]], which determines whether the ratio of inertial to gravitational mass is the same for all bodies, regardless of size or composition. To date no difference has been found to a few parts in 10<sup>11</sup>.<ref name=NRC>{{Cite book|title=Physics Through the Nineteen Nineties: Overview |page=15 |url=https://books.google.com/books?id=Hk1wj61PlocC&q=equivalence+gravitation&pg=PA15
|isbn=0-309-03579-1 |date=1986 |author=National Research Council (US) |publisher=National Academies Press }}</ref> For some discussion of the subtleties of the Eötvös experiment, such as the local mass distribution around the experimental site (including a quip about the mass of Eötvös himself), see Franklin.<ref name=Franklin>{{Cite book|title=No Easy Answers: Science and the Pursuit of Knowledge |author=Allan Franklin |page=66 |url=https://books.google.com/books?id=_RN-v31rXuIC&q=%22Eotvos+experiment%22&pg=PA66
|isbn=978-0-8229-5968-7 |date=2007 |publisher=University of Pittsburgh Press }}</ref>

Einstein's [[general relativity|general theory]] modifies the distinction between nominally "inertial" and "non-inertial" effects by replacing special relativity's "flat" [[Minkowski Space]] with a metric that produces non-zero curvature. In general relativity, the principle of inertia is replaced with the principle of [[geodesic (general relativity)|geodesic motion]], whereby objects move in a way dictated by the curvature of spacetime. As a consequence of this curvature, it is not a given in general relativity that inertial objects moving at a particular rate with respect to each other will continue to do so. This phenomenon of [[geodesic deviation]] means that inertial frames of reference do not exist globally as they do in Newtonian mechanics and special relativity.

However, the general theory reduces to the special theory over sufficiently small regions of [[spacetime]], where curvature effects become less important and the earlier inertial frame arguments can come back into play.<ref>{{cite book |title=Information Theory and Quantum Physics: Physical Foundations for Understanding the Conscious Process |first1=Herbert S. |last1=Green |publisher=Springer |date=2000 |isbn=354066517X |page=154 |url=https://books.google.com/books?id=CUJiQjSVCu8C}} [https://books.google.com/books?id=CUJiQjSVCu8C&pg=PA154 Extract of page 154]</ref><ref>{{cite book |title=Theory of Special Relativity |first1=Nikhilendu |last1=Bandyopadhyay |publisher=Academic Publishers |date=2000 |isbn=8186358528 |page=116 |url=https://books.google.com/books?id=qMOyfi_i0j8C}} [https://books.google.com/books?id=qMOyfi_i0j8C&pg=PA116 Extract of page 116]</ref> Consequently, modern special relativity is now sometimes described as only a "local theory".<ref>{{cite book |title=Cosmological Inflation and Large-Scale Structure |first1=Andrew R. |last1=Liddle |first2=David H. |last2=Lyth |publisher=Cambridge University Press |date=2000 |isbn=0-521-57598-2 |page=329 |url=https://books.google.com/books?id=XmWauPZSovMC}} [https://books.google.com/books?id=XmWauPZSovMC&pg=PA329 Extract of page 329]</ref> "Local" can encompass, for example, the entire [[Milky Way galaxy]]: The astronomer [[Karl Schwarzschild]] observed the motion of pairs of stars orbiting each other. He found that the two orbits of the stars of such a system lie in a plane, and the perihelion of the orbits of the two stars remains pointing in the same direction with respect to the [[Solar System]]. Schwarzschild pointed out that that was invariably seen: the direction of the [[angular momentum]] of all observed double star systems remains fixed with respect to the direction of the angular momentum of the Solar System. These observations allowed him to conclude that inertial frames inside the galaxy do not rotate with respect to one another, and that the space of the Milky Way is approximately Galilean or Minkowskian.<ref>[http://www.mpiwg-berlin.mpg.de/Preprints/P271.PDF In the Shadow of the Relativity Revolution] {{Webarchive|url=https://web.archive.org/web/20170520084821/http://www.mpiwg-berlin.mpg.de/Preprints/P271.PDF |date=20 May 2017 }} Section 3: The Work of Karl Schwarzschild (2.2 MB PDF-file)</ref>