https://en.wikipedia.org/wiki/Variable_speed_of_light
Notable attempts to incorporate a variable speed of light into physics have been made by Einstein in 1911, by Robert Dicke in 1957, and by several researchers starting from the late 1980s.
The speed of light in vacuum instead is considered a constant, and defined by the SI as 299792458 m/s. Variability of the speed of light is therefore equivalent with a variability of the SI meter and/or the SI second.
VSL should not be confused with faster than light theories; nor should it be confused with the fact that the speed of light in a medium is slower than the speed of light in vacuum depending on the medium's refractive index.
(“The principle of the constancy of the speed of light can be kept only when one restricts oneself to space-time regions of constant gravitational potential.”)
https://en.wikipedia.org/wiki/Variable_speed_of_light
Notable attempts to incorporate a variable speed of light into physics have been made by Einstein in 1911, by Robert Dicke in 1957, and by several researchers starting from the late 1980s.
The speed of light in vacuum instead is considered a constant, and defined by the SI as 299792458 m/s. Variability of the speed of light is therefore equivalent with a variability of the SI meter and/or the SI second.
VSL should not be confused with faster than light theories; nor should it be confused with the fact that the speed of light in a medium is slower than the speed of light in vacuum depending on the medium's refractive index.
(“The principle of the constancy of the speed of light can be kept only when one restricts oneself to space-time regions of constant gravitational potential.”)
https://en.wikipedia.org/wiki/Variable_speed_of_light