(J. J. Lodder) writes:=20
> >> <tko...@netcologne.de
> >>> Nicolaas Vroom <nicolaa...@pandora.be
> >>>> Of course you could claim that the speed of light is constant.
> >>> The way that the SI units are defined now, the speed of light
> >>> in vacuum is indeed constant. If you measure anything else than
> >>> 299792458 m/s, recalibrate your measurement devices.
> >> The speed of light is now a defined quantity, that is true. However=
> >> that is merely a practical matter. If the speed of light really wer=
> >> variable, that could still be detected just as easily as before the
> >> redefinition.
> > It is incredible how much misunderstanding there is
> > on such a simple subject.
> I agree. :-)
> > To clear things up:
> > The speed of light cannot 'really' be variable.
> It is true that the metre is now defined as the distance travelled by
> light in a certain time and thus by definition the speed of light is=20
> constant. However, the metre (and the second) used to be defined=20
> differently than they are now.
Certainly, the definitions have changed several times even.
> Back then, it was certainly possible, in=20
> principle, to detect a change in the speed of light.=20
Yes, but what would this mean? 
The changes would have depended on the particular -human- choices
made for the definitions of those units.
And even so, my point stands.
The only physically sound way
of defining those independent length and time units
is to chose them in mutally compatible ways.
(so with a factor c between them,
for example both based on atomic hyperfine structure)
> One could perform the same experiment today.
That's another common misunderstanding.
Those experiments -are- done routinely in standards labs.
Only the name differs, they are nowadays called:
'the calibration of a secondary meter standard'.
> Nature doesn't know what the current SI definitions are.
A forteriori, nature doesn't know what units are at all.
All units are LGM or human inventions.
> The metre is now defined as it is a) as a practical matter and b)=20
> because we assume that the speed of light is constant.
We assume nothing about that.
We define it to be the case.
(and take the consequences, if any, somewhere else)
> If the speed of light did change, i.e. of one does observations like th=
> of R=C3=B8mer, Fizeau, etc., using, say, pendulum clocks as a reference=
> notice that it changes, then one has measured the change. The conseque=
> would not be to point to the SI definition and say that it cannot=20
> change, therefore we must modify other definitions (perhaps even=20
> periodically if the speed of light depends on time), but rather would b=
> to realize that our assumptions in the current definition of the metre
> are wrong and must be changed.
Certainly. If yesterday's pistons won't fit tomorrow's engines
we must overhaul all of our physics.
> Think about the definition of the metre and kilogram. Why are the=20
> original definitions not used? One reason is because on noticed that
> the mass of the reference kilogram has actually changed with time.
All that we can say is that we can't reproduce the relative mass
of certain chunks of metal as accurately as we thought we could.
(reasons mostly unknown)
That's why the kilo is now locked to Planck's constant,
with, one hopes, better reproducibility.
> By your logic, that should not have been possible, since, by definitio=
> the reference kilogram has a mass of exactly one kilogram.
> Nevertheless, the change was detected.
See above, no.
The other so-called kilograms may have changed instead.
Only if all metal kilograms drift in the same way wrt Planck
there will be a new problem.
 For your amusement:
there is creationist folklore about the speed of light.
Taking a handful of the very first measurements from the 19th century
(with large errors)
they conclude that the speed of light varies enormously.
And (you could not possibly have guessed it of course)
so enormously that the apparent age of the universe of billions of years
fits precisely with the creation of the earth 6 000 years ago.