While Einstein's first mentioned a variable speed of light in 1907, he reconsidered the idea more thoroughly in 1911.

Another consequence of the squashing is that the propagation of particles is deformed, even of light, leading to a variable speed of light.

One consequence of this theory is a variable speed of light, where photon speed would vary with energy, and some zero-mass particles might possibly travel faster than 'c'.

Variable speed of light (VSL) is a hypothesis that states that the speed of light, usually denoted by c, may be a function of space and time.

Some theorists (such as Dirac and Milne) have proposed cosmologies that conjecture that physical "constants" might actually change over time (e.g. Variable speed of light or Dirac varying-G theory).

Variable speed of light occurs in some situations of classical physics, as equivalent formulation of accepted theories, but also in various alternative theories of gravitation and cosmology, many of them non-mainstream.

French astrophysicist Jean-Pierre Petit, a senior researcher at the National Center for Scientific Research, published an earlier 1988 theory involving variable speed of light in the journal Modern Physics Letters A.

The de Sitter effect was described by de Sitter in 1913 and used to support the special theory of relativity against a competing 1908 emission theory by Walter Ritz that postulated a variable speed of light.