🚀 go-pugleaf

The constancy of the speed of light does not have a physically
invariant meaning. Whether you claim that the speed of light is 
constant, or that it is changing, the statement has meaning only 
with reference to the _physical method_ that was used to make 
_measurements_ of the speed of light. Different measurement 
methods can be expected to give different results.

For example, you could choose the Krypton-86 standard as your 
standard of length, and the atomic second as your standard of time.
This was done until very recently.

Or you could choose the length of the platinum-iridium 
bar stored at the International Bureau of Weights and Measures 
in Paris as your standard of length, and you could use the old 
definition of the second in terms of the Earth's orbital 
period as your standard of time.

The speed of light might be constant if you used one of these 
methods and variable if you used the other; or both methods could 
give variable results (varying at the same or different rates), or 
both could give constant results. What would happen in each case 
is a subject of experimental determination.

Once a method of measurement has been specified, one finds that 
the measured speed of light will vary as some monomial in certain 
_dimensionless_ physical parameters. For example, if we used the 
first method to measure the speed of light we would find that

	c ~ 1/(alpha^2 * beta)

where alpha is the fine structure constant and beta is the ratio
of electron to proton mass. The speed of light, measured in this
way, would be found to vary if and only if either alpha or beta
(or both) varied.

On the other hand, the speed of light determined using the second 
method varies as

	c ~ 1/(alpha * alphaG * beta)

where alphaG is the dimensionless gravitational coupling
constant G*M^2/ch, and where M in turn is the proton mass.
If only alphaG varied, then the first method would give a constant
speed of light and the second a variable one; similarly, the second
method could give a constant result and the first variable; and so 
on, depending on how the _dimensionless_ parameters happen to vary.

Of course, as was pointed out by several others, under the current
definition of physical standards, the speed of light is an absolute
constant, by definition.

The point is that only changes in _dimensionless_ parameters
have an invariant physical meaning. So it is ultimately meaningless 
to talk about "variations in the speed of light". Any physical
measurement that measures changes in the speed of light can in
reality only reflect changes in the underlying dimensionless 
parameters.

As far as I have been able to determine, all _practical_ schemes for
measuring "changes" in the speed of light would have to reflect
underlying changes in alpha, alphaG, or beta.

To my knowledge, no changes in any of these fundamental parameters
have been firmly established experimentally. van Flandern claimed 
to have detected a change in the ratio of the gravitational and 
atomic seconds, but his results have not been confirmed and have 
been strongly questioned. The limit on the cosmological variation 
of alpha set by the Okolo natural nuclear reactor is very stringent, 
as is the limit on beta (actually beta*gp, where gp is the form 
factor of the proton) from observations of quasars.

This is discussed at great length in Barrow and Tipler's recent
book, "The Anthropic Cosmological Principle", especially section
4.5.

As far as Barry Setterfield is concerned (he is the Creationist 
whose theory about "variations" in the speed of light brought 
this whole subject up), the variations from current values of 
the parameters that his theory would require are many, many
orders of magnitude, so large that life could not have existed
on Earth at the "creation" 6000 years ago.  For example, making 
reasonable assumptions I calculate that Adam and Eve would have been 
squashed into a "primordial ooze" only a micron thick, because 
the molecular forces holding their bodies together would have 
been unable to withstand the (relatively much stronger) gravity 
of Earth implied by his theory. So much for "Creation Science".

-- 
Glend.	I can call spirits from the vasty deep.
Hot.	Why, so can I, or so can any man; But will they come when you
	do call for them?    --  Henry IV Pt. I, III, i, 53

	Bill Jefferys  8-%
	Astronomy Dept, University of Texas, Austin TX 78712   (USnail)
	{allegra,ihnp4}!{ut-sally,noao}!utastro!bill	(UUCP)
	bill@astro.UTEXAS.EDU.				(Internet)