$ Mathematically speaking, a vacuum is ZERO particle COUNT.
Experimental MEASUREMENTs in AiR vary with AMBiENT particle COUNT.!!
(EinsteiN SPECiFiED ZERO particle-COUNT, for the VELOCiTY of light.)
(And, STP&g AiR is 26 Orders-of-Magnitude LESS EMPTY than "vacuum".)

SPECiFiCALLY ..EinsteiN's ((focus)) is on a *ZERO particle-COUNT*.!!
(Light VELOCiTY v VARiEs ..inversely, WiTH the PATH-particle-COUNT.)
Light-VELOCiTY c is, NOW, a MATHEMATiCAL constant ..just as is pi.!!

Light-VELOCiTY v "constancy", is ONLY per *CONSTANT particle-COUNT*.
Light VELOCiTY v *DECREASES*, inversely, with PATH-particle-COUNT.!!

$ CONCLUSiON:
Light VELOCiTY v *iNCREASES*, inversely, with PATH-particle-COUNT.!!
(Note, VARiOUS vacu ONLY exist in squeezed-SWiSS-cheese-LiKE minds.)

Brian A M Stuckless
^

jem wrote: Harry wrote: "jem" wrote in message
news:B2hlf.3958$fz5.1074@dukeread04... [John Kennaugh wrote earlier:
3/ It cannot be the speed relative to the observer because -=-

insert ..see top of PAGE, dooOPs.!!

-=- there is no causality whereby an observer's speed can effect
the speed at which light leaves the source besides which light
leaves the source when there is no observer so it can only be a
function of the source.] SNIP

Your claim was that the speed at which light is measured to leave
its source can't depend on the speed of the measurer, but the
*fact* that it *does* in a logically consistent theory (e.g. SR),
means that your claim is wrong. It's as simple as that.

What a big mistake!

but *whose* mistake?

Instead, in SRT it does not depend on the speed of either the
source, the receiver, or the measurer. It is measured to be
constant (in vacuum and at constant gravitational potential)
relative to any calibrated inertial coordinate system. This
system may be virtual, and all three may be moving
arbitrarily relative to it.

So at what speed does an SR observer measure light to leave its
source, when the observer is approaching that source at speed v?
(Read carefully this time). Is that speed independent of v?
Zero particle-count-error-bars.!! (i.e. in-vacu) by Uncle Tom.