Is the speed of light changing?
(August 2002)


A 'Brief Communication' in the August issue of Nature caused a certain amount of excitement in the Australian media this month, when journalists thought it meant that three Australian scientists, Paul Davies, Tamara Davis and Charles Lineweaver, were saying that the speed of light was changing. In fact, they were not quite saying that, but they were saying that it was more likely than that the charge on the electron was changing, which was the alternative answer to an interesting piece of research reported last year.

This news led to an Australian press feeding frenzy of claims that Einstein had been proved wrong, that we were seeing a paradigm shift and so on and so forth. Here at websterworld, we just said ''oh yes, that's the story we ran last year, revisited''. To see what we were recalling, though, you will need to read Is the fine structure constant changing? from August 2001.

Far from being the start of a paradigm shift (if it is one, that actually began a year earlier), the new paper looks at the fine structure constant, which is a combination of three variables, and suggests that one of the three variables, the charge on the electron, could not have changed. By implication, they suggest that this means one of the other variables, the speed of light, must have changed.

At first sight, this seems to make little sense, given that there is a third variable, Planck's constant, which might also have changed, but physicists say that it is easier to fit current models if Planck's constant is ignored and assumed to be constant, while the charge on the electron and the speed of light are examined as the prime suspects. At that point, we called Charles Lineweaver (one of the present authors) and ]ohn Webb (one of the 2001 authors) and quizzed them more closely, and talked to a couple of other physicists as well.

For starters, every known cosmological model argues that as you get closer to the Big Bang, coupling constants change, because if they did not, symmetry would not have been maintained. Now we know very well that c, the speed of light, is only constant for a vacuum, and that light passing through air, water, glass or a diamond, travels more slowly. What happens, say the physicists, if the properties of the vacuum changeover time as well? If we change the dielectric constant of a vacuum, then the vacuum becomes an 'ether' that controls the speed of light.

The key issue is that nobody is too surprised to hear about changes close to the Big Bang, but the work described in 2001 is looking at times long after that, in the past billion to 10 billion years, when everything should have settled down. Perhaps the vacuum is changing slowly, and if it is, that makes last year's work very exciting, because it is the first evidence of that, but nobody would be entirely bemused by it. On the other hand, a change in Planck's constant is just, well, intuitively wrong, they seem to be saying.

Now the first thing to note here is that e, the charge on the electron, is a totally different thing from the c we meet in the one scientific equation that everybody knows e=mc2. That e represents the energy obtained when a mass is converted to energy. When a heavy nucleus splits, the products are always found to have a little less mass than what you began with, and the loss of mass is accompanied by energy - and the amount can be calculated by applying e=mc2.

Every electron, and every proton, has the same unit charge, and in our time, but what would be the consequence of that value changing? Basically, the second law of thermodynamics would be broken, violated, and necessarily thrown out.

Scientific laws are there to be found wrong, but every single bit of science we have comes down to that single law, one way or another. It is more than a law in the ordinary sense, it is a fundamental rule, the very foundation of science, and the suggestion that it might be violated is just not acceptable. If the second law of thermodynamics were truly violated on a large scale (or worse, on a cosmic scale), none of science would make any sense. The whole universe runs in accordance with the Second law, and could not exist without it.

In that environment, it is understandable that the authors favour the simpler model, one where the speed of light may have been changing. That makes sense in terms of what we know, it is plausible - and there seems to be just a hint there that the physicists find the idea exciting, because it could lead in interesting directions, and even scientists are human.

What we have now isa set of observations, several competing explanations, and an awareness that one of the explanations, a change in the value of e, seems to be unlikely, given what it would lead to. So it is possible that the speed of light is changing, it is plausible that the speed of light is changing, it is not yet entirely probable that the speed of light is changing, ever so slowly, across time. One thing is certain: the non-scientists who believe they can prove evolution did not happen by postulating a change in the speed of light can forget this one - the change and the rate of change is far too small to save their nebulous notions. There is no chance of a 6000-year-old Earth here, but we can confidently predict that this will not stop them trying.

©WebsterWorld Pty Ltd/contributors 2002


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