# Mean speed of light postulate

Einstein stated his second postulate as (see here):

light is propagated in vacant space, with a velocity c which is independent of the nature of motion of the emitting body.

Since the one-way speed of light cannot be measured, but only the round-trip (or two-way) speed, let us adopt Winnie’s Round-Trip Light Principle as a postulate:

(RTLP) The average round-trip speed of any light-signal propagated (in vacuo) in a closed path is equal to a constant c in all inertial frames of reference. [Special Relativity Without One-way Velocity Assumptions, Part II, John A. Winnie, p.221]

This is the most that can be empirically verified. We may then adopt one of the following conventions, as convenient:

(A) The final observed leg of the path of light in a vacuum takes no time.

and

(B) To an observer at rest relative to the source of light the speed of light in a vacuum has the nominal speed c.

(A) Since the harmonic mean speed of light is c, the speeds of the other legs of light travel are at least c/2 such that the mean speed equals c. In this way, the Galilean transformation is preserved for the final leg. And interchanging length and duration leads to an alternate version of the Galilean transformation.

This accords with common ways of speaking. Even astronomers speak of where a star is now, rather than pedantically keep saying where it was so many years ago. Physical theory should be in accord with observation of the physical world as much as possible. This is an example of how amateur scientists can help re-integrate science and common life.

(B) Although the nominal speed of light is c, the relative one-way speed may be greater or lesser than this. See post here.