Vol. 7, Issue 4, Part F (2021)

Abstract
There is an alternative version of the light-speed constancy postulate which Einstein enunciated in his landmark 1905 paper on relativistic mechanics. The one that he proposed in his work was that the speed of light (c) in free space has the same value for all observers independent of their own state of motion. A review of the relevant experimental data indicates, however, that there is a fundamentally more restrictive version of the light-speed constancy assumption which fits in better with the facts, namely that only the speed of light in free space relative to its source always has the same constant value, whereas the value relative to any given observer depends critically on the latter’s state of motion. One doesn’t need to consider relevant experimental data to understand that Einstein’s version of the light-speed postulate is invalid. It is enough to consider the case of a light source emitting a light pulse at the same time that it leaves its original position. In particular, this example shows that that Einstein’s version of the postulate leaves open the distinct possibility that the speed of light relative to its source can be as high as 2c, in direct contradiction to his own proposal. It is also interesting to note that, although Einstein’s version stands in direct contradiction to the predictions of the classical (Galilean) velocity transformation, there is no such conflict when the alternative version is used instead which requires that the speed of flight in free space is only equal to c relative to the source from which it is emitted.