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{{Short description|Two interrelated physics theories by Albert Einstein}}
{{About|the scientific concept|philosophical or ontological theories about relativity|Relativism|the silent film|The Einstein Theory of Relativity}}
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[[File:BBH gravitational lensing of gw150914.webm|thumb|Video simulation of the merger [[GW150914]], showing [[spacetime]] distortion from gravity as the black holes orbit and merge]]

The '''theory of relativity''' usually encompasses two interrelated [[physics]] theories by [[Albert Einstein]]: [[special relativity]] and [[general relativity]], proposed and published in 1905 and 1915, respectively.<ref>{{Citation |author=Einstein A. |date=1916 |type=Translation 1920 |title=Relativity: The Special and General Theory|publisher=H. Holt and Company|location=New York|title-link=s:Relativity: The Special and General Theory }}</ref> Special relativity applies to all physical phenomena in the absence of [[gravity]]. General relativity explains the law of gravitation and its relation to the forces of nature.<ref name="londontimes" /> It applies to the [[cosmological]] and astrophysical realm, including astronomy.<ref name=relativity/>

The theory transformed [[theoretical physics]] and [[astronomy]] during the 20th century, superseding a 200-year-old [[Classical mechanics|theory of mechanics]] created primarily by [[Isaac Newton]].<ref name="relativity" /><ref name="spacetime" /><ref name="fitz-loren" /> It introduced concepts including 4-[[dimension]]al [[spacetime]] as a unified entity of [[space]] and [[time in physics|time]], [[relativity of simultaneity]], [[kinematics|kinematic]] and [[gravity|gravitational]] [[time dilation]], and [[length contraction]]. In the field of physics, relativity improved the science of [[elementary particle]]s and their fundamental interactions, along with ushering in the [[atomic age|nuclear age]]. With relativity, [[Physical cosmology|cosmology]] and [[astrophysics]] predicted extraordinary [[astronomy|astronomical phenomena]] such as [[neutron star]]s, [[black hole]]s, and [[gravitational wave]]s.<ref name="relativity">
{{cite encyclopedia
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{{cite encyclopedia
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== Development and acceptance ==
{{Main|History of special relativity|History of general relativity}}
{{General relativity sidebar}}

[[Albert Einstein]] published the theory of [[special relativity]] in 1905, building on many theoretical results and empirical findings obtained by [[Albert A. Michelson]], [[Hendrik Lorentz]], [[Henri Poincaré]] and others. [[Max Planck]], [[Hermann Minkowski]] and others did subsequent work.

Einstein developed [[general relativity]] between 1907 and 1915, with contributions by many others after 1915. The final form of general relativity was published in 1916.<ref name=relativity/>

The term "theory of relativity" was based on the expression "relative theory" ({{langx|de|Relativtheorie}}) used in 1906 by Planck, who emphasized how the theory uses the [[principle of relativity]]. In the discussion section of the same paper, [[Alfred Bucherer]] used for the first time the expression "theory of relativity" ({{langx|de|Relativitätstheorie}}).<ref>{{Citation|author=Planck, Max|date=1906 |title=Die Kaufmannschen Messungen der Ablenkbarkeit der β-Strahlen in ihrer Bedeutung für die Dynamik der Elektronen (The Measurements of Kaufmann on the Deflectability of β-Rays in their Importance for the Dynamics of the Electrons)|journal=Physikalische Zeitschrift|volume=7 |pages=753–761|title-link=s:Translation:The Measurements of Kaufmann }}</ref><ref>{{Citation|last=Miller |first=Arthur I.|date=1981|title=Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911)|location=Reading |publisher=Addison–Wesley|isbn=978-0-201-04679-3}}</ref>

By the 1920s, the physics community understood and accepted special relativity.<ref>{{cite book |title=The New Quantum Universe |edition=illustrated, revised |first1=Anthony J.G. |last1=Hey |first2=Patrick |last2=Walters |publisher=Cambridge University Press |date=2003 |isbn=978-0-521-56457-1 |page=227 |url=https://books.google.com/books?id=cTk-eVzT1oMC&pg=PA227|bibcode=2003nqu..book.....H }}</ref> It rapidly became a significant and necessary tool for theorists and experimentalists in the new fields of [[atomic physics]], [[nuclear physics]], and [[quantum mechanics]].

By comparison, general relativity did not appear to be as useful, beyond making minor corrections to predictions of Newtonian gravitation theory.<ref name="relativity" /> It seemed to offer little potential for experimental test, as most of its assertions were on an astronomical scale. Its [[Tensor analysis|mathematics]] seemed difficult and fully understandable only by a small number of people. Around 1960, general relativity became central to physics and astronomy. New mathematical techniques to apply to general relativity streamlined calculations and made its concepts more easily visualized. As astronomical [[phenomena]] were discovered, such as [[quasars]] (1963), the 3-kelvin [[microwave background radiation]] (1965), [[pulsar]]s (1967), and the first [[black hole]] candidates (1981),<ref name="relativity" /> the theory explained their attributes, and measurement of them further confirmed the theory.

== Special relativity ==
{{Main|Special relativity}}
[[File:Einstein_Portrait.png|thumb|180px|Albert Einstein, physicist, 1879-1955, Graphic: Heikenwaelder Hugo,1999]]
Special relativity is a theory of the structure of [[spacetime]]. It was introduced in Einstein's 1905 paper "[[On the Electrodynamics of Moving Bodies]]" (for the contributions of many other physicists and mathematicians, see [[History of special relativity]]). Special relativity is based on two postulates which are contradictory in [[classical mechanics]]:
# The [[laws of physics]] are the same for all observers in any [[inertial frame of reference]] relative to one another ([[principle of relativity]]).
# The [[speed of light]] in [[vacuum]] is the same for all observers, regardless of their relative motion or of the motion of the [[light]] source.

The resultant theory copes with experiment better than classical mechanics. For instance, postulate 2 explains the results of the [[Michelson–Morley experiment]]. Moreover, the theory has many surprising and counterintuitive consequences. Some of these are:
* [[Relativity of simultaneity]]: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
* [[Time dilation]]: Moving [[clock]]s are measured to tick more slowly than an observer's "stationary" clock.
* [[Length contraction]]: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
* [[Speed of light#Upper limit on speeds|Maximum speed is finite]]: No physical object, message or field line can travel faster than the speed of light in vacuum.
** The effect of gravity can only travel through space at the speed of light, not faster or instantaneously.
* [[Mass–energy equivalence]]: {{nowrap|1=''E'' = ''mc''<sup>2</sup>}}, energy and mass are equivalent and transmutable.
* [[Mass in special relativity|Relativistic mass]], idea used by some researchers.<ref name=":0">{{Cite web|title = The Theory of Relativity, Then and Now|url = http://www.smithsonianmag.com/innovation/theory-of-relativity-then-and-now-180956622/?no-ist|access-date = 2015-09-26|first = Brian|last = Greene}}</ref>

The defining feature of special relativity is the replacement of the [[Galilean transformation]]s of classical mechanics by the [[Lorentz transformation]]s. (See [[Maxwell's equations]] of [[electromagnetism]].)

== General relativity ==
{{Main|General relativity|Introduction to general relativity}}

General relativity is a theory of gravitation developed by Einstein in the years 1907–1915. The development of general relativity began with the [[equivalence principle]], under which the states of [[accelerated motion]] and being at rest in a [[gravity|gravitational field]] (for example, when standing on the surface of the Earth) are physically identical. The upshot of this is that [[free fall]] is [[inertia|inertial motion]]: an object in free fall is falling because that is how objects move when there is no [[force]] being exerted on them, instead of this being due to the force of [[gravity]] as is the case in [[classical mechanics]]. This is incompatible with classical mechanics and [[special relativity]] because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that [[Curved space|spacetime is curved]]. Einstein discussed his idea with mathematician [[Marcel Grossmann]] and they concluded that general relativity could be formulated in the context of [[Riemannian geometry]] which had been developed in the 1800s.<ref>{{cite journal | last1 = Einstein | first1 = A. | author-link2 = Marcel Grossmann | last2 = Grossmann | first2 = M. |date= 1913 | title = Entwurf einer verallgemeinerten Relativitätstheorie und einer Theorie der Gravitation |trans-title= Outline of a Generalized Theory of Relativity and of a Theory of Gravitation | journal = Zeitschrift für Mathematik und Physik | volume = 62 | pages = 225–261 }}</ref>
In 1915, he devised the [[Einstein field equations]] which relate the curvature of spacetime with the mass, energy, and any momentum within it.

Some of the consequences of general relativity are:
* [[Gravitational time dilation]]: Clocks run slower in deeper gravitational wells.<ref>
{{cite book
 |title=Feynman Lectures on Gravitation
 |first1=Richard Phillips |last1=Feynman
 |first2=Fernando B. |last2=Morínigo
 |first3=William |last3=Wagner
 |first4=David |last4=Pines
 |first5=Brian |last5=Hatfield
 |publisher=West view Press
 |date=2002
 |isbn=978-0-8133-4038-8
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 |url=https://books.google.com/books?id=jL9reHGIcMgC
}}{{Dead link|date=January 2023 |bot=InternetArchiveBot |fix-attempted=yes }}, Lecture 5</ref>
* [[precession#Relativistic (Einsteinian)|Precession]]: Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of [[Mercury (planet)|Mercury]] and in [[binary pulsar]]s).
* [[General relativity#Light deflection and gravitational time delay|Light deflection]]: Rays of [[light]] bend in the presence of a gravitational field.
* [[Frame-dragging]]: Rotating masses "drag along" the [[spacetime]] around them.
* [[Expansion of the universe]]: The universe is expanding, and certain components within the universe can [[accelerated expansion|accelerate the expansion]].

Technically, general relativity is a theory of [[gravitation]] whose defining feature is its use of the [[Einstein field equations]]. The solutions of the field equations are [[metric tensor (general relativity)|metric tensors]] which define the [[topology]] of the spacetime and how objects move inertially.

== Experimental evidence ==
Einstein stated that the theory of relativity belongs to a class of "principle-theories". As such, it employs an analytic method, which means that the elements of this theory are not based on hypothesis but on empirical discovery. By observing natural processes, we understand their general characteristics, devise mathematical models to describe what we observed, and by analytical means we deduce the necessary conditions that have to be satisfied. Measurement of separate events must satisfy these conditions and match the theory's conclusions.<ref name="londontimes">{{Cite news |last=Einstein |first=Albert |date=28 November 1919 |title=Time, Space, and Gravitation |newspaper=The Times |title-link=s:Time, Space, and Gravitation}}</ref>

=== Tests of special relativity ===
{{Main|Tests of special relativity}}
[[File:Michelson-Morley experiment (en).svg|thumb|A diagram of the [[Michelson–Morley experiment]]]]
Relativity is a [[Falsifiability|falsifiable]] theory: It makes predictions that can be tested by experiment. In the case of special relativity, these include the principle of relativity, the constancy of the speed of light, and time dilation.<ref name=faq>{{Cite web |editor1-last=Roberts |editor1-first=T |editor2-last=Schleif |editor2-first=S |editor3-last=Dlugosz |editor3-first=JM |date=2007 |title=What is the experimental basis of Special Relativity? |url=http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html |work=Usenet Physics FAQ |publisher=[[University of California, Riverside]] |access-date=2010-10-31}}</ref> The predictions of special relativity have been confirmed in numerous tests since Einstein published his paper in 1905, but three experiments conducted between 1881 and 1938 were critical to its validation. These are the [[Michelson–Morley experiment]], the [[Kennedy–Thorndike experiment]], and the [[Ives–Stilwell experiment]]. Einstein derived the [[Lorentz transformation]]s from first principles in 1905, but these three experiments allow the transformations to be induced from experimental evidence.

[[Maxwell's equations]]—the foundation of classical electromagnetism—describe light as a wave that moves with a characteristic velocity. The modern view is that light needs no medium of transmission, but Maxwell and his contemporaries were convinced that light waves were propagated in a medium, analogous to sound propagating in air, and ripples propagating on the surface of a pond. This hypothetical medium was called the [[luminiferous aether]], at rest relative to the "fixed stars" and through which the Earth moves. Fresnel's [[Aether drag hypothesis#Partial aether dragging|partial ether dragging hypothesis]] ruled out the measurement of first-order (v/c) effects, and although observations of second-order effects (v<sup>2</sup>/c<sup>2</sup>) were possible in principle, Maxwell thought they were too small to be detected with then-current technology.<ref name=maxb>{{Citation|last=Maxwell|first=James Clerk|date=1880|title=On a Possible Mode of Detecting a Motion of the Solar System through the Luminiferous Ether|journal=Nature|volume=21|issue=535|pages=314–315|doi=10.1038/021314c0 |bibcode = 1880Natur..21S.314. |title-link=s:Motion of the Solar System through the Luminiferous Ether|doi-access=free}}</ref><ref name="Pais 1982 111–113">{{cite book|last=Pais|first=Abraham|title="Subtle is the Lord&nbsp;...": The Science and the Life of Albert Einstein|url=https://archive.org/details/subtleislordscie00pais|url-access=registration|date=1982|publisher=Oxford Univ. Press|location=Oxford|isbn= 978-0-19-280672-7 |pages=[https://archive.org/details/subtleislordscie00pais/page/111 111–113]|edition=1st}}</ref>

The Michelson–Morley experiment was designed to detect second-order effects of the "aether wind"—the motion of the aether relative to the Earth. Michelson designed an instrument called the [[Michelson interferometer]] to accomplish this. The apparatus was sufficiently accurate to detect the expected effects, but he obtained a null result when the first experiment was conducted in 1881,<ref name=michel1>{{Cite journal |author = Michelson, Albert A. |title = The Relative Motion of the Earth and the Luminiferous Ether |journal = American Journal of Science |volume = 22 |issue = 128 |date = 1881 |pages = 120–129 |doi=10.2475/ajs.s3-22.128.120|title-link = s:The Relative Motion of the Earth and the Luminiferous Ether |bibcode = 1881AmJS...22..120M |s2cid = 130423116 }}</ref> and again in 1887.<ref name=michel2>{{Cite journal |author=[[Albert A. Michelson|Michelson, Albert A.]] & [[Edward W. Morley|Morley, Edward W.]] |title=On the Relative Motion of the Earth and the Luminiferous Ether |journal=American Journal of Science |volume=34 |issue=203 |date=1887 |pages=333–345 |doi=10.2475/ajs.s3-34.203.333|title-link=s:On the Relative Motion of the Earth and the Luminiferous Ether |bibcode=1887AmJS...34..333M |s2cid=124333204 }}</ref> Although the failure to detect an aether wind was a disappointment, the results were accepted by the scientific community.<ref name="Pais 1982 111–113"/> In an attempt to salvage the aether paradigm, FitzGerald and Lorentz independently created an [[ad hoc hypothesis|''ad hoc'' hypothesis]] in which the length of material bodies changes according to their motion through the aether.<ref>{{cite book|last=Pais|first=Abraham|title="Subtle is the Lord&nbsp;...": The Science and the Life of Albert Einstein|url=https://archive.org/details/subtleislordscie00pais|url-access=registration|date=1982|publisher=Oxford Univ. Press|location=Oxford|isbn= 978-0-19-280672-7|page=[https://archive.org/details/subtleislordscie00pais/page/122 122]|edition=1st}}</ref> This was the origin of [[FitzGerald–Lorentz contraction]], and their hypothesis had no theoretical basis. The interpretation of the null result of the Michelson–Morley experiment is that the round-trip travel time for light is [[isotropic]] (independent of direction), but the result alone is not enough to discount the theory of the aether or validate the predictions of special relativity.<ref name="robertson">{{cite journal|last=Robertson|first=H.P.|title=Postulate versus Observation in the Special Theory of Relativity|journal=Reviews of Modern Physics|date=July 1949|volume=21|issue=3|pages=378–382|bibcode = 1949RvMP...21..378R |doi = 10.1103/RevModPhys.21.378 |url=https://cds.cern.ch/record/1061896/files/RevModPhys.21.378.pdf|doi-access=free}}</ref><ref name="tw">{{cite book|last=Taylor|first=Edwin F.|title=Spacetime physics: Introduction to Special Relativity|date=1992|publisher=W.H. Freeman|location=New York|isbn=978-0-7167-2327-1|pages=[https://archive.org/details/spacetimephysics00edwi_0/page/84 84]–88|edition=2nd|author2=John Archibald Wheeler|url-access=registration|url=https://archive.org/details/spacetimephysics00edwi_0}}</ref>

[[File:Kennedy-Thorndike experiment DE.svg|left|thumb|The [[Kennedy–Thorndike experiment]] shown with interference fringes]]
While the Michelson–Morley experiment showed that the velocity of light is isotropic, it said nothing about how the magnitude of the velocity changed (if at all) in different [[inertial frame]]s. The Kennedy–Thorndike experiment was designed to do that, and was first performed in 1932 by Roy Kennedy and Edward Thorndike.<ref name=KT>{{cite journal |last=Kennedy |first=R.J. |author2=Thorndike, E.M. |date=1932 |title=Experimental Establishment of the Relativity of Time |journal=Physical Review |volume=42 |issue=3 |pages=400–418 |doi=10.1103/PhysRev.42.400 |url=http://pdfs.semanticscholar.org/ee2c/4c3e0a169f31c8983fdbd853d9e9e6d2f011.pdf |archive-url=https://web.archive.org/web/20200706022658/http://pdfs.semanticscholar.org/ee2c/4c3e0a169f31c8983fdbd853d9e9e6d2f011.pdf |url-status=dead |archive-date=2020-07-06 |bibcode = 1932PhRv...42..400K |s2cid=121519138 }}</ref> They obtained a null result, and concluded that "there is no effect ... unless the velocity of the solar system in space is no more than about half that of the earth in its orbit".<ref name="tw"/><ref>{{cite journal|last=Robertson|first=H.P.|title=Postulate versus Observation in the Special Theory of Relativity|journal=Reviews of Modern Physics|date=July 1949|volume=21|issue=3|page=381|doi=10.1103/revmodphys.21.378|bibcode = 1949RvMP...21..378R |url=https://cds.cern.ch/record/1061896/files/RevModPhys.21.378.pdf|doi-access=free}}</ref> That possibility was thought to be too coincidental to provide an acceptable explanation, so from the null result of their experiment it was concluded that the round-trip time for light is the same in all inertial reference frames.<ref name="robertson" /><ref name="tw" />

The Ives–Stilwell experiment was carried out by Herbert Ives and G.R. Stilwell first in 1938<ref>{{cite journal |last=Ives |first=H.E. |author2=Stilwell, G.R. |date=1938 |title=An experimental study of the rate of a moving atomic clock |journal=Journal of the Optical Society of America |volume=28 |issue=7 |pages=215 |bibcode=1938JOSA...28..215I |doi=10.1364/JOSA.28.000215 }}</ref> and with better accuracy in 1941.<ref name=Ives1941>{{cite journal |last=Ives |first=H.E. |author2=Stilwell, G.R. |date=1941 |title=An experimental study of the rate of a moving atomic clock. II |journal=Journal of the Optical Society of America |volume=31 |issue=5 |pages=369 |bibcode=1941JOSA...31..369I |doi=10.1364/JOSA.31.000369 }}</ref> It was designed to test the [[transverse Doppler effect]]{{Snd}} the [[redshift]] of light from a moving source in a direction perpendicular to its velocity—which had been predicted by Einstein in 1905. The strategy was to compare observed Doppler shifts with what was predicted by classical theory, and look for a [[Lorentz factor]] correction. Such a correction was observed, from which was concluded that the frequency of a moving atomic clock is altered according to special relativity.<ref name="robertson" /><ref name="tw" />

Those classic experiments have been repeated many times with increased precision. Other experiments include, for instance, [[Tests of relativistic energy and momentum|relativistic energy and momentum increase]] at high velocities, [[experimental testing of time dilation]], and [[modern searches for Lorentz violation]]s.{{citation needed|date=August 2024}}

=== Tests of general relativity ===
{{Main|Tests of general relativity}}
General relativity has also been confirmed many times, the classic experiments being the perihelion precession of [[Mercury (planet)|Mercury]]'s orbit, the [[gravitational lens|deflection of light]] by the [[Sun]], and the [[gravitational redshift]] of light. Other tests confirmed the [[equivalence principle]] and [[frame dragging]].

== Modern applications ==
Far from being simply of theoretical interest, relativistic effects are important practical engineering concerns. Satellite-based measurement needs to take into account relativistic effects, as each satellite is in motion relative to an Earth-bound user, and is thus in a different frame of reference under the theory of relativity. Global positioning systems such as [[GPS]], [[GLONASS]], and [[Galileo (satellite navigation)|Galileo]], must account for all of the relativistic effects in order to work with precision, such as the consequences of the Earth's gravitational field.<ref>Ashby, N. Relativity in the Global Positioning System. ''Living Rev. Relativ.'' '''6''', 1 (2003). {{doi|10.12942/lrr-2003-1}}{{cite web |url=http://relativity.livingreviews.org/Articles/lrr-2003-1/download/lrr-2003-1Color.pdf |title=Archived copy |access-date=2015-12-09 |url-status=dead |archive-url=https://web.archive.org/web/20151105155910/http://relativity.livingreviews.org/Articles/lrr-2003-1/download/lrr-2003-1Color.pdf |archive-date=2015-11-05 }}</ref> This is also the case in the high-precision measurement of time.<ref name=Francis2002>{{cite journal|last=Francis|first=S.|author2=B. Ramsey|author3=S. Stein|author4=Leitner, J.|author5=Moreau, J.M.|author6=Burns, R.|author7=Nelson, R.A.|author8=Bartholomew, T.R.|author9=Gifford, A.|title=Timekeeping and Time Dissemination in a Distributed Space-Based Clock Ensemble|journal=Proceedings 34th Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting|date=2002|pages=201–214|url=http://tycho.usno.navy.mil/ptti/ptti2002/paper20.pdf|access-date=14 April 2013|url-status=dead|archive-url=https://web.archive.org/web/20130217211012/http://tycho.usno.navy.mil/ptti/ptti2002/paper20.pdf|archive-date=17 February 2013}}</ref> Instruments ranging from electron microscopes to particle accelerators would not work if relativistic considerations were omitted.<ref>{{cite book |title=Einstein's Mirror |edition=illustrated |first1=Tony |last1=Hey |first2=Anthony J. G. |last2=Hey |first3=Patrick |last3=Walters |publisher=Cambridge University Press |date=1997 |isbn=978-0-521-43532-1 |page=x (preface) |url=https://archive.org/details/isbn_9780521435321|url-access=registration }}</ref>

== See also ==
* [[Doubly special relativity]]
* [[Galilean invariance]]
* [[List of textbooks on relativity]]

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{cite book |last=Einstein|first=Albert|title=Relativity: The Special and General Theory|date=2005|publisher=Pi Press|location=New York|isbn= 978-0-13-186261-6|edition=The masterpiece science|others=Translated by Robert W. Lawson}} 
* {{cite book |title = Relativity: The Special and General Theory|last = Einstein|first = Albert|publisher = Henry Holt and Company |date= 1920 |url = https://www.ibiblio.org/ebooks/Einstein/Einstein_Relativity.pdf}}
* {{cite book |last=Einstein|first=Albert|title=Albert Einstein, Autobiographical Notes|url=https://archive.org/details/autobiographical1979eins|url-access=registration|date=1979|publisher=Open Court Publishing Co.|location=La Salle, Illinois |isbn=978-0-87548-352-8|edition=A Centennial|author2=trans. Schilpp |author3=Paul Arthur }}
* {{cite book |last=Einstein|first=Albert|title=Einstein's Essays in Science|date=2009|publisher=Dover Publications|location=Mineola, New York |isbn=978-0-486-47011-5|edition=Dover|others=Translated by Alan Harris}}
* {{cite book |last=Einstein|first=Albert|title=[[The Meaning of Relativity]]|date=1956|orig-year=1922|publisher=Princeton University Press|edition=5}}
* [http://www.gutenberg.org/files/36276/36276-pdf.pdf?session_id=c6fd49e1b7f6dd81790ed141e999cf2bce859386 The Meaning of Relativity] Albert Einstein: Four lectures delivered at Princeton University, May 1921
* [https://web.archive.org/web/20151222085312/http://inpac.ucsd.edu/students/courses/winter2012/physics2d/einsteinonrelativity.pdf How I created the theory of relativity] Albert Einstein, 14 December 1922; [[Physics Today]] August 1982
* [https://www.britannica.com/science/relativity Relativity] [[Sidney Perkowitz]] [[Encyclopædia Britannica]]
{{refend}}

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