# 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]

The “average” is more precisely stated as the harmonic mean round-trip speed. This principle states what is the most that can be empirically verified. We may then adopt either of the following conventions, as convenient:

(A) The reception leg of the path of light in vacuo takes no time, while the other legs take the time needed to make the harmonic mean round-trip speed equal to c.

or

(B) To an observer at rest relative to the source of light the one-way speed of light in vacuo moves with nominal speed c.

(A) A leg of light travel could take no time if the speeds of the other legs are such that the harmonic mean speed equals c. In this way, the Galilean transformation of length is preserved for the reception leg, which is the one observed. Interchanging length and duration leads to the Galilean transformation of duration.

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.