Editing Variable speed of light (section)

== Relation to other constants and their variation ==
{{further|Time-variation of fundamental constants}}

=== Gravitational constant ''G'' ===
{{See also | Dirac large numbers hypothesis}}
In 1937, [[Paul Dirac]] and others began investigating the consequences of natural constants changing with time.<ref>{{cite journal |author=Dirac |first=Paul A. M. |year=1938 |title=A New Basis for Cosmology |journal=[[Proceedings of the Royal Society A]] |volume=165 |issue=921 |pages=199–208 |bibcode=1938RSPSA.165..199D |doi=10.1098/rspa.1938.0053|s2cid=121069801 }}</ref> For example, Dirac proposed a change of only 5 parts in 10<sup>11</sup> per year of the [[Gravitational constant|Newtonian constant of gravitation]] ''G'' to explain the relative weakness of the [[gravitational force]] compared to other [[fundamental forces]]. This has become known as the [[Dirac large numbers hypothesis]].

However, [[Richard Feynman]] showed<ref>{{cite book |last1=Feynman |first1=Richard P. |title=The Feynman Lectures on Physics |title-link=The Feynman Lectures on Physics |last2=Leighton |first2=R. |last3=Sands |first3=M. |publisher=Addison Wesley Longman |year=2006 |isbn=0-8053-9045-6 |edition=definitive |volume=1 |language=en-us |chapter=7: The Theory of Gravitation |orig-year=1964}}</ref> that the gravitational constant most likely could not have changed this much in the past 4 billion years based on geological and solar system observations, although this may depend on assumptions about ''G'' varying in isolation. (See also [[strong equivalence principle]].)

=== Fine-structure constant ''α'' ===
{{See also|Fine-structure constant#Potential variation over time}}
One group, studying distant quasars, has claimed to detect a variation of the fine-structure constant<ref>{{cite journal |author=Webb |first1=J. K. |last2=Murphy |first2=M. T. |last3=Flambaum |first3=V. V. |last4=Dzuba |first4=V. A. |last5=Barrow |first5=J. D. |last6=Churchill |first6=C. W. |last7=Prochaska |first7=J. X. |last8=Wolfe |first8=A. M. |year=2001 |title=Further Evidence for Cosmological Evolution of the Fine Structure Constant |journal=Physical Review Letters |volume=87 |issue=9 |pages=091301 |arxiv=astro-ph/0012539 |bibcode=2001PhRvL..87i1301W |doi=10.1103/PhysRevLett.87.091301 |pmid=11531558 |s2cid=40461557}}</ref> at the level in one part in 10<sup>5</sup>. Other authors dispute these results. Other groups studying quasars claim no detectable variation at much higher sensitivities.<ref>{{cite journal |author=Chand |first1=H. |last2=Srianand |first2=R. |last3=Petitjean |first3=P. |last4=Aracil |first4=B. |year=2004 |title=Probing the cosmological variation of the fine-structure constant: results based on VLT-UVES sample |journal=Astron. Astrophys. |volume=417 |issue=3 |pages=853–871 |arxiv=astro-ph/0401094 |bibcode=2004A&A...417..853C |doi=10.1051/0004-6361:20035701 |s2cid=17863903}}</ref><ref>{{cite journal |author=Srianand |first1=R. |last2=Chand |first2=H. |last3=Petitjean |first3=P. |last4=Aracil |first4=B. |year=2004 |title=Limits on the time variation of the electromagnetic ne-structure constant in the low energy limit from absorption lines in the spectra of distant quasars |journal=Physical Review Letters |volume=92 |issue=12 |pages=121302 |arxiv=astro-ph/0402177 |bibcode=2004PhRvL..92l1302S |doi=10.1103/PhysRevLett.92.121302 |pmid=15089663 |s2cid=29581666}}</ref><ref>{{cite journal |author=Levshakov |first1=S. A. |last2=Centurion |first2=M. |last3=Molaro |first3=P. |last4=D'Odorico |first4=S. |year=2005 |title=VLT/UVES constraints on the cosmological variability of the fine-structure constant |journal=Astron. Astrophys. |volume=434 |issue=3 |pages=827–838 |arxiv=astro-ph/0408188 |bibcode=2005A&A...434..827L |doi=10.1051/0004-6361:20041827 |s2cid=119351573}}</ref>

The [[natural nuclear reactor]] of [[Oklo]] has been used to check whether the atomic [[fine-structure constant]] ''α'' might have changed over the past 2 billion years. That is because ''α'' influences the rate of various nuclear reactions. For example, {{SimpleNuclide|link=yes|samarium|149}} captures a neutron to become {{SimpleNuclide|samarium|150}}, and since the rate of neutron capture depends on the value of ''α'', the ratio of the two [[samarium]] isotopes in samples from Oklo can be used to calculate the value of ''α'' from 2 billion years ago. Several studies have analysed the relative concentrations of radioactive isotopes left behind at Oklo, and most have concluded that nuclear reactions then were much the same as they are today, which implies ''α'' was the same too.<ref>{{cite journal |last=Petrov |first=Yu. V. |author2=Nazarov, A. I. |author3=Onegin, M. S. |author4=Sakhnovsky, E. G.  |year=2006 |title=Natural nuclear reactor at Oklo and variation of fundamental constants: Computation of neutronics of a fresh core |journal=Physical Review C |volume=74 |issue=6 |pages=064610 |doi=10.1103/PHYSREVC.74.064610 |arxiv = hep-ph/0506186 |bibcode = 2006PhRvC..74f4610P |s2cid=118272311 }}</ref><ref>{{Cite journal|last1=Davis|first1=Edward D.|last2=Hamdan|first2=Leila|date=2015|title=Reappraisal of the limit on the variation in ''α'' implied by the Oklo natural fission reactors|journal=Physical Review C|volume=92|issue=1|pages=014319|bibcode=2015PhRvC..92a4319D|doi=10.1103/physrevc.92.014319|arxiv=1503.06011|s2cid=119227720}}</ref>

[[Paul Davies]] and collaborators have suggested that it is in principle possible to disentangle which of the dimensionful constants (the [[elementary charge]], the [[Planck constant]], and the speed of light) of which the fine-structure constant is composed is responsible for the variation.<ref>{{cite journal |last1=Davies |first1=P. C. W. |last2=Davis |first2=Tamara M. |last3=Lineweaver |first3=Charles H. |year=2002 |title=Cosmology: Black holes constrain varying constants |journal=Nature |volume=418 |issue=6898 |pages=602&ndash;603 |bibcode=2002Natur.418..602D |doi=10.1038/418602a |pmid=12167848 |s2cid=1400235}}</ref> However, this has been disputed by others and is not generally accepted.<ref>{{cite arXiv |eprint=hep-th/0208093|last1= Duff|first1= M. J.|title= Comment on time-variation of fundamental constants|year= 2002}}</ref><ref>{{cite journal |last1=Carlip |first1=S. |last2=Vaidya |first2=S. |name-list-style=amp |year=2003 |title=Black holes may not constrain varying constants |journal=Nature |volume=421 |issue=6922 |pages=498 |arxiv=hep-th/0209249 |bibcode=2003Natur.421..498C |doi=10.1038/421498a |pmid=12556883 |s2cid=209814835}}</ref>