Speed of light has also been shown in experiment. It should even be possible for the group velocity to become infinite or negative, with pulses travelling instantaneously or backwards in time. None of these options, however, allow information to be transmitted faster than "c". It is impossible to transmit information with a light pulse any faster than the speed of the earliest part of the pulse (the front velocity). It can be shown that this is (under certain assumptions) always equal to "c". It is possible for a particle to travel through a medium faster than the phase velocity of light in that
Variable speed of light 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. While Einstein first mentioned a variable speed of light