Speed of light may have changed recently

The speed of light, one of the most sacrosanct of the universal physical constants, may have been lower as recently as two billion years ago - and not in some far corner of the universe, but right here on Earth.

The controversial finding is turning up the heat on an already simmering debate, especially since it is based on re-analysis of old data that has long been used to argue for exactly the opposite: the constancy of the speed of light and other constants.

A varying speed of light contradicts Einstein's theory of relativity, and would undermine much of traditional physics. But some physicists believe it would elegantly explain puzzling cosmological phenomena such as the nearly uniform temperature of the universe. It might also support string theories that predict extra spatial dimensions.

The threat to the idea of an invariable speed of light comes from measurements of another parameter called the fine structure constant, or alpha, which dictates the strength of the electromagnetic force. The speed of light is inversely proportional to alpha, and though alpha also depends on two other constants (see graphic), many physicists tend to interpret a change in alpha as a change in the speed of light. It is a valid simplification, says Victor Flambaum of the University of New South Wales in Sydney.

It was Flambaum, along with John Webb and colleagues, who first seriously challenged alpha's status as a constant in 1998. Then, after exhaustively analysing how the light from distant quasars was absorbed by intervening gas clouds, they claimed in 2001 that alpha had increased by a few parts in 105 in the past 12 billion years.

But then German researchers studying photons emitted by caesium and hydrogen atoms reported earlier in June that they had seen no change in alpha to within a few parts in 1015 over the period from 1999 to 2003 (New Scientist, 26 June) though the result does not rule out that alpha was changing billions of years ago.

Throughout the debate, physicists who argued against any change in alpha have had one set of data to fall back on. It comes from the world's only known natural nuclear reactor, found at Oklo in Gabon, West Africa.

The Oklo reactor started up nearly two billion years ago when groundwater filtered through crevices in the rocks and mixed with uranium ore to trigger a fission reaction that was sustained for hundreds of thousands of years. Several studies that have analysed the relative concentrations of radioactive isotopes left behind at Oklo have concluded that nuclear reactions then were much the same as they are today, which implies alpha was the same too.

That is because alpha directly influences the ratio of these isotopes. In a nuclear chain reaction like the one that occurred at Oklo, the fission of each uranium-235 nucleus produces neutrons, and nearby nuclei can capture these neutrons.

For example, samarium-149 captures a neutron to become samarium-150, and since the rate of neutron capture depends on the value of alpha, the ratio of the two samarium isotopes in samples collected from Oklo can be used to calculate alpha.

A number of studies done since Oklo was discovered have found no change in alpha over time. "People started quoting the reactor [data] as firm evidence that the constants hadn't changed," says Steve Lamoreaux of Los Alamos National Lab (LANL) in Albuquerque, New Mexico.

Now, Lamoreaux, along with LANL colleague Justin Torgerson, has re-analysed the Oklo data using what he says are more realistic figures for the energy spectrum of the neutrons present in the reactor. The results have surprised him. Alpha, it seems, has decreased by more than 4.5 parts in 108 since Oklo was live (Physical Review D, vol 69, p121701).

That translates into a very small increase in the speed of light (assuming no change in the other constants that alpha depends on), but Lamoreaux's new analysis is so precise that he can rule out the possibility of zero change in the speed of light. "It's pretty exciting," he says.

So far the re-examination of the Oklo data has not drawn any fire. "The analysis is fine," says Thibault Damour of the Institute of Advanced Scientific Studies (IHES) in Bures-sur-Yvette in France, who co-authored a 1996 Oklo study that found no change in alpha. Peter Moller of LANL, who, along with Japanese researchers, published a paper in 2000 about the Oklo reactor that also found no change in alpha, says that Lamoreaux's assumptions are reasonable.

The analysis might be sound, and the assumptions reasonable, but some physicists are reluctant to accept the conclusions. "I can't see a particular mistake," says Flambaum. "However, the claim is so revolutionary there should be many independent confirmations."

While Flambaum's own team found that alpha was different 12 billion years ago, the new Oklo result claims that alpha was changing as late as two billion years ago. If other methods confirm the Oklo finding, it will leave physicists scrambling for new theories. "It's like opening a gateway," says Dmitry Budker, a colleague of Lamoreaux's at the University of California at Berkeley.

Some physicists would happily accept a variable alpha. For example, if it had been lower in the past, meaning a higher speed of light, it would solve the "horizon problem".

Cosmologists have struggled to explain why far-flung regions of the universe are at roughly the same temperature. It implies that these regions were once close enough to exchange energy and even out the temperature, yet current models of the early universe prevent this from happening, unless they assume an ultra-fast expansion right after the big bang.

However, a higher speed of light early in the history of the universe would allow energy to pass between these areas in the form of light.

Variable "constants" would also open the door to theories that used to be off limits, such as those which break the laws of conservation of energy. And it would be a boost to versions of string theory in which extra dimensions change the constants of nature at some places in space-time.

But "there is no accepted varying-alpha theory", warns Flambaum. Instead, there are competing theories, from those that predict a linear rate of change in alpha, to those that predict rapid oscillations. John Barrow, who has pioneered varying-alpha theories at the University of Cambridge, says that the latest Oklo result does not favour any of the current theories. "You would expect alpha to stop [changing] five to six billion years ago," he says.

Before Lamoreaux's Oklo study can count in favour of any varying alpha theory, there are some issues to be addressed. For one, the exact conditions at Oklo are not known. Nuclear reactions run at different rates depending on the temperature of the reactor, which Lamoreaux assumed was between 227 and 527°C.

Damour says the temperature could vary far more than this. "You need to reconstruct the temperature two billion years ago deep down in the ground," he says.

Damour also argues that the relative concentrations of samarium isotopes may not be as well determined as Lamoreaux has assumed, which would make it impossible to rule out an unchanging alpha. But Lamoreaux points out that both assumptions about the temperature of the Oklo reactor and the ratio of samarium isotopes were accepted in previous Oklo studies.

Another unknown is whether other physical constants might have varied along with, or instead of, alpha. Samarium-149's ability to capture a neutron also depends on another constant, alpha(s), which governs the strength of the strong nuclear attraction between the nucleus and the neutron.

And in March, Flambaum claimed that the ratio of different elements left over from just after the big bang suggests that alpha(s) must have been different then compared with its value today (Physical Review D, vol 69, p 063506).

While Lamoreaux has not addressed any possible change in alpha(s) in his Oklo study, he argues that it is important to focus on possible changes in alpha because the Oklo data has become such a benchmark in the debate over whether alpha can vary. "I've spent my career going back and checking things that are 'known' and it always leads to new ideas," he says.

many dogmatic assumptions of cosmology.

Y'know, 'dogmatic' is one of those words that inherently amount to name-calling. Whoever is 'dogmatic' is automatically close-minded, stubborn, whatever. So I'm strongly spring-loaded to discount any opinions or discussion by someone who calls any other position 'dogmatic' in the same way I discount the opinions of someone who introduces any other ad hominem attack. You might consider that when trying to make a point, because - at least for me - it completely undermines anything else you might want to say. (The same applies if you quote or excerpt the position of someone who calls other positions 'dogmatic' if you expect that quoted position to be convincing.)

It was never an issue on whether the speed of light differs when it's not in a vacuum. I recognized that caveat in my original posting, which was already long enough that I didn't feel the need to go through the topic of non-vacuum conditions. The basic theory starts from that condition in the same sense that we talk about the speed of an aircraft in still air when discussing its performance. True, when flying into a headwind, the speed over the ground varies, but the performance in still air is a valid reference point from which additional variables can be introduced. In this case, the variable of interest is the speed of light as measured by observers with differing absolute velocities. Having one of those observers measure it in a vacuum while the other measures it in a transparent medium of higher index of refraction does not address the matter at hand. If, regardless of their own intrinsic velocities, differing observers measure the speed of light in the same conditions (i.e. vacuum) and get the same value to the limits of measurement, that's an important datum for theories to address. The point of bringing up some of the seminal experiments that demonstrated that datum was to address the specific issue of whether Einstein 'assumed' the speed of light was constant, or based his theory on the data that indicated it was constant. That's a key factor in evaluating the strength of his theory.

It may indeed be true that neutrinos preclude the very possibility of a vacuum - which does not absolutely undermine the theory. There is still a lot of discussion on whether neutrinos have any rest mass or charge, and while if they do they provide some convenient explanations for dark matter, convenience alone is not proof. Part of Einstein's theory is an explanation for why - at the speed of light - a travelling electromagnetic field can be self-sustaining without electrical charges to 'wave' around. It may be wrong, but the argument that there must be something to 'wave' begs the question. It does not prove Einstein is wrong - it assumes it as a given. Sorry, not convinced.

For Southack: I did read the article, and there are some subtleties that even the article mentions that make their success in part semantics. Here's another explanation for their results. The light pulse is absorbed by the sodium medium at one end, and then it is transmitted through the condensate to the other end not as 'light' (meaning an electromagnetic wave) but as a physical motion of the atoms on the condensate - which would be much slower than the speed of light - and then reradiated on the other end. This is analogous to capturing a sound wave in a microphone, transmitting it in a taut string, and then reradiating it at the other end. The original sound does indeed provide the energy that is transmitted in the string, but to say that the wave of alternate compression and expansion of air molecules is the same thing as the lateral vibrations of a string becomes as much semantics as fact. I'm not arguing with their results. In fact, their findings require an expansion of the very language we use and I honor their advance in science. But the key is how their results relate to the General Theory of Relativity as it relates to the speed of light through free space. That relationship is not clear from the article. In particular, I didn't find anything that declared the 'light' pulses in the condensate were not subject to gravity while in the condensate. The fact the light pulses were 'trapped' in a physical array of atoms which were themselves prevented from reacting to gravity by the levitation fields no more proves that the light pulses were unaffected by gravity than a car supported by a bridge proves the car is unaffected by gravity.

Once again, my attempt in replying to the original post by <1/1,000,000th% (among others) was to explain in a few paragraphs a very complex subject that can't be explained accurately in words anyway. (Take two courses in Tensor mathematics and call me in the morning.) Nothing in my post insisted that the speed of light (even in a vacuum) is constant, only that the available data say so - or at least, the data available to Einstein. Perhaps it's not. If so, it would explain some troublesome cosmological data, just as it makes inexplicable other data currently explained by the General Theory. Frankly, I don't have any personal stake one way or the other. If you don't find my explanation interesting, fine. Have a nice day.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.