Re: SR Postulate on Speed of Light


<john_doe_ph_d@xxxxxxxxx> wrote in message
news:1116554446.726529.218950@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I am confused by the typical wording that one finds for the second
> postulate of SR, namely that the speed of light is independent of the
> motion of the source. The speed of any wave depends only on the medium,
> so the motion of the source is not relevent. Consider a horn moving
> through the air. It emits a sound wave. Once the wave leaves the horn,
> its speed depends only on the properties of the air.
>
> Now it's true that for someone traveling with the horn, the speed of
> the sound would be greater in the forward direction than the backward
> direction, but this has more to do with the speed of the OBSERVER
> (relative to the medium).
>
> I am more comfortable with an alternate statement of the second
> postulate, namely that the speed of light is the same for all inertial
> observers.
>
> Are these two formulations of the second postulate equivalent?

Quite, the second formulation encompasses the first:
If the measured vacuum speed of light is the constant c in any inertial
frame, it is by definition independent of the motion of objects in that
frame and that includes light sources.

Harald


.

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