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.

And didn't Einstein originally just assume the speed of light as a constant?

Here's an explanation, take it or leave it as you will.

Two scientists named Michaelson and Morley conducted an experiment to measure the speed of the earth through the 'ether' - a material characteristic to space assumed to exist. After all, 'when light waves, what waves?' In other words, what propogates light through what appears to be a vacuum? The theory said there was something called ether that existed even in a vacuum.

In their experiment, they set up a right-angled apparatus, one leg of which was aligned with the direction of the earth's motion around the sun, and the other perpendicular to it. By measuring the difference in the time it took a light pulse to travel both legs, you could get a measure of the speed of the earth through the ether which is what 'waved' when a light wave went by (since the ether would drag the light along with it).

They didn't find any difference. A host of other similar experiments showed that, regardless of the circumstances, the speed of light (in a vacuum) was always the same to the limits of accuracy of the measurement.

So, Einstein didn't 'assume' the speed of light was constant. That's what the data showed.

Two other guys names Lorentz and Fitsgerald developed a relationship that quantified how much things at very high speed behaved differently than those at normal speeds. This 'Lorentz-Fitgerald contraction' was SQRT(1 - V**2/C**2).

Einstein came along and in his 'Special' theory of relativity showed that the speed of light as measured by an observer is constant regardless of his own velocity if the rate of time passes differently for the observer based on his velocity, using the Lorentz-Fitsgeral contraction to quantify the amount of change in perceived rate of time passage.

Then Einstein extended from the 'Special Theory Relativity' to the 'General Theory of Relativity' by devloping a mathematical expression for the curvature of the universe which related linear dimensions to time, with the units worked out by combining t (time) with c (speed of light). The mathematical expression is called a 'tensor', and it's as good an example of how you can't 'speak' real science without mathematics as I've ever bumped my head up against.

One good thing about the General Theory of Relativity is that it provided an explanation for gravity. It was always a challenge to conventional physics to explain action at at distance without an interaction phenomenon. How does the earth know the sun is over there pulling on us? And how does the sun manage to grab the earth and yank it around without a string between the two? The curvature of space described by solving the Einstein tensor for local conditions offers the prediction that the earth is following an equal-value (in the tensor) line around the sun even as it changes direction. In other words, the earth goes 'straight', but 'straight' is not straight in the Euclidean sense. Instead, the 'straight' travel of a body in motion is actually to follow an equipotential line in the Einstein tensor value for space.

The other good thing about the General Theory (okay, there are lots of them, but this is already a long note) is that it predicts that light itself obeys gravity, despite having no rest mass for the conventional Newtonian model to act upon. This is provable by lots of experimental data, so the General Theory gained a lot of credibility.

Now, to wrap it up, if the speed of light is not a constant, then the Einstein tensor doesn't provide a solution to the motion of bodies in space-time. There's another variable that makes it impossible to solve. There is an awful lot of observable data that would need another explanation. (Obviously, if the speed of light is almost a constant, then the Einstein tensor is almost right, and still very useful for lots of situations.)

I don't know if this data on the natural nuclear reactor proves the speed of light is variable or not, but there are lots of challenges with a totally constant, for all time, speed of light, too (as mentioned in the article), so it'll be interesting to see what happens.

That's only one of the cdk theories out there.

CDK refers to the decay of "c." It cannot do what it set out to do with a speed of light that used to be slower and is speeding up. You see, what it set out to do is explain why we "think" we see objects millions or billions of years old. (They are apparently millions, even billions of light-years away. There would not have been time in a young universe for light from them to reach us in any manner unless light used to be much, much faster and not long ago.) If light had slowed down dramatically very recently, that should help cram quasars into a 6000 year old universe. If light is speeding up, the problem only gets worse.

Again, if light-speed is really changing, according to this study it has speeded up a tiny fraction in 2 billion years.

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