Editing Variable speed of light (section)

=== 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]].)