Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!ut-sally!utastro!bill From: bill@utastro.UUCP (William H. Jefferys) Newsgroups: sci.physics Subject: Re: Speed of light Message-ID: <1392@utastro.UUCP> Date: Sun, 9-Nov-86 00:02:23 EST Article-I.D.: utastro.1392 Posted: Sun Nov 9 00:02:23 1986 Date-Received: Sun, 9-Nov-86 06:19:52 EST References: <241@sri-arpa.ARPA> Organization: U. Texas, Astronomy, Austin, TX Lines: 96 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)