in a relative change of c. Dicke assumed a refractive index (eqn. 5) and proved it to be consistent with the observed value for light deflection. In a comment related to Mach's principle, Dicke suggested that, while the right part of the term in eq. 5 is small, the left part, 1, could have "its origin in the remainder of the matter in the universe". Given that in a universe with an increasing horizon more and more masses contribute to the above refractive index, Dicke considered a cosmology where c decreased in time, providing an alternative explanation to the cosmological redshift. Subsequent proposals Variable speed of light models, including Dicke's, have been developed which agree with all known tests of general relativity. Other models claim to shed light on the equivalence principle or make a link to Dirac's large numbers hypothesis. Several hypotheses for varying speed of light, seemingly in contradiction to general relativity theory, have been published, including those of Giere and Tan (1986) and Sanejouand (2009). In 2003, Magueijo gave a review of such hypotheses. Cosmological models with varying speeds of light have been proposed independently by Jean-Pierre Petit in 1988, John Moffat in 1992, and the team of Andreas Albrecht and João Magueijo in 1998 to explain the horizon problem of cosmology and propose an alternative to cosmic inflation. Relation to other constants and their variation Gravitational constant G In 1937, Paul Dirac and others began investigating the consequences of natural constants changing with time. For example, Dirac proposed a change of only 5 parts in 1011 per year of the 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 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 One group, studying distant quasars, has claimed to detect a variation of the fine-structure constant at the level in one part in 105. Other authors dispute these results. Other groups studying quasars claim no detectable variation at much higher sensitivities. The natural nuclear reactor of Oklo has been used to check whether the atomic fine-structure constant α might have