On Wed, 13 Feb 2019, (Steve Willner) wrote:
(Eric Flesch) writes:

Are you suggesting that a local measurement of c at z=1 would return
a value twice as large as measured at z=0? I'm pretty sure that's
ruled out by observations.

We have no observations (i.e., local measurements) at the place of
z=1, we can only look at there from here. Photons from there are
fossils from that place, but when reaching here, their path & speed
are governed by the spatial manifold of this place here.

SW Isn't [changing c] ruled out by observations?

No because we've observed only here in this local place.

??? There are plenty of redshifts for high-z objects, and optical and
radio redshifts agree with each other. Moreover, CO redshifts agree
with H I redshifts, both measured with radio techniques. That
wouldn't be the case if c were varying because the 21 cm line is a
hyperfine splitting that has a different dependence on c than
ordinary lines.

No, the frequency is invariant but the wavelengths compress with the
slowing light -- so it looks the same as if both here & there were
flat. My model may be wrong but not for that reason.

Optical (grating) spectrographs measure wavelength. If there were a
change, why wouldn't that be measured?

Let's see, you'd need a standard-candle-type source of light known to
be precisely identical across the years (otherwise we just say 4000A
is the same now as then), confidence in the accuracy of the earlier
measurements, and readiness to accept a shift in the values. That's
a hurdle enough without our having disabled our own tools by tying the
length of the meter to c. What a terrible idea that was to do that.