On the indifference of photons to the velocity of their own source
Crucial in this regard was the experiment conducted by Michelson and Morley in 1887 in Cleveland, Ohio. There were observed fringes formed by the interference of light passing a certain distance along and across the direction of flight of the Earth in outer space. At that time, it was believed that light has a fixed speed with respect to the hypothetical, practically immobile aether filling space. If so, in the laboratory coordinate system the velocity of the Earth in space must be deducted from the velocity of light in the aether. Therefore, it has been expected that the light would go longer in the longitudinal direction than in the transverse one. That has to be marked by the interference pattern but has not happened. Similar but more accurate experiments have been conducted more consistently for about half a century, though with zero results. Today, the experimentally permissible uncertainty of the speed of light in vacuum is ten to the 17th power times less than the value of the speed itself.