Editing Theory of relativity (section)

== 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]].