Speed of light is only possible to verify experimentally that the two-way speed of light (for example, from a source to a mirror and back again) is frame-independent, because it is impossible to measure the one-way speed of light (for example, from a source to a distant detector) without some convention as to how clocks at the source and at the detector should be synchronized. However, by adopting Einstein synchronization for the clocks, the one-way speed of light becomes equal to the two-way speed of light by definition. The special theory of relativity explores the consequences of this invariance of "c" with the
One-way speed of light the time of flight of light signals through a fibre optic link between two hydrogen maser clocks. In 1992 the experimental results were analysed by Clifford Will who concluded that the experiment did actually measure the one-way speed of light. In 1997 the experiment was re-analysed by Zhang who showed that, in fact, only the two-way speed had been measured. The first experimental determination of the speed of light was made by Ole Christensen Rømer. It may seem that this experiment measures the time for light to traverse part of the Earth's orbit and thus determines its one-way speed, however,