Editing Equivalence principle (section)

=== Tests of the Einstein equivalence principle ===
In addition to the tests of the weak equivalence principle, the Einstein equivalence principle requires testing the local Lorentz invariance and local positional invariance conditions.

Testing local Lorentz invariance amounts to testing special relativity, a theory with vast number of existing tests.<ref name=Will2014/>{{rp|12}} Nevertheless, attempts to look for quantum gravity require even more precise tests. The modern tests include looking for directional variations in the [[speed of light]] (called "clock anisotropy tests") and new forms of the [[Michelson–Morley experiment]]. The anisotropy measures less than one part in 10<sup>−20</sup>.<ref name=Will2014/>{{rp|14}}

Testing local positional invariance divides in to tests in space and in time.<ref name=Will2014/>{{rp|17}} Space-based tests use measurements of the [[gravitational redshift]], the classic is the [[Pound–Rebka experiment]] in the 1960s. The most precise measurement was done in 1976 by flying a hydrogen maser and comparing it to one on the ground. The [[Global Positioning System]] requires compensation for this redshift to give accurate position values.

Time-based tests search for variation of [[dimensionless]] [[fundamental physical constants|constants and mass ratios]].<ref name="Uzan">{{Cite journal |last=Uzan |first=Jean-Philippe |date=2003-04-07 |title=The fundamental constants and their variation: observational and theoretical status |url=https://link.aps.org/doi/10.1103/RevModPhys.75.403 |journal=Reviews of Modern Physics |language=en |volume=75 |issue=2 |pages=403–455 |doi=10.1103/RevModPhys.75.403 |arxiv=hep-ph/0205340 |bibcode=2003RvMP...75..403U |s2cid=118684485 |issn=0034-6861}}</ref> For example, Webb et al.<ref>{{cite journal |first1=John K. |last1=Webb |first2=Michael T. |last2=Murphy |first3=Victor V. |last3=Flambaum |first4=Vladimir A. |last4=Dzuba |first5=John D. |last5=Barrow |first6=Chris W. |last6=Churchill |first7=Jason X. |last7=Prochaska |first8=Arthur M. |last8=Wolfe |doi=10.1103/PhysRevLett.87.091301 |journal=Physical Review Letters |title=Further Evidence for Cosmological Evolution of the Fine Structure Constant |volume=87 |issue=9 |year=2001 |arxiv=astro-ph/0012539 |pmid=11531558 |bibcode=2001PhRvL..87i1301W |pages=091301|s2cid=40461557 }}</ref> reported detection of variation (at the 10<sup>−5</sup> level) of the fine-structure constant from measurements of distant [[quasar]]s. Other researchers dispute these findings.<ref>{{Cite journal |last1=Rocha |first1=G |last2=Trotta |first2=R |last3=Martins |first3=C.J.A.P |last4=Melchiorri |first4=A |last5=Avelino |first5=P.P |last6=Viana |first6=P.T.P |date=Nov 2003 |title=New constraints on varying α |url=https://linkinghub.elsevier.com/retrieve/pii/S1387647303001532 |journal=New Astronomy Reviews |language=en |volume=47 |issue=8–10 |pages=863–869 |doi=10.1016/j.newar.2003.07.018|arxiv=astro-ph/0309205 |bibcode=2003NewAR..47..863R |s2cid=9280269 }}</ref>

The present best limits on the variation of the fundamental constants have mainly been set by studying the naturally occurring [[Oklo]] [[natural nuclear fission reactor]], where nuclear reactions similar to ones we observe today have been shown to have occurred underground approximately two billion years ago. These reactions are extremely sensitive to the values of the fundamental constants.

{| class="wikitable"
|+ Tests of changes in fundamental constants<ref name=Will2014/>{{rp|19}}
|-
!Constant
!Year
!Method
!Limit on fractional change per year
|-
||[[weak interaction]] constant
||1976
||Oklo
||10<sup>−11</sup>
|-
||[[fine-structure constant]]
||1976
||Oklo
||10<sup>−16</sup>
|-
||[[electron]]–[[proton]] mass ratio
||2002
||quasars
||10<sup>−15</sup>
|}