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The hunt for changes in the fine structure constant

A new generation of measurements are tightening the limits on how the fine structure constant may be changing

There’s no reason to think that the fundamental constants have always had the value we measure today. And there’s no reason to think they’re changing either. We simply don’t know one way or the other, although there is no shortage of theories predicting changes of some kind.

One constant that has attracted more interest than most is the fine structure constant, which can be thought of as the electromagnetic force between two electrons at a distance of one meter measured in units where the speed of light and Planck’s constant are set to unity. The fine structure constant determines, among other things, the distribution of certain spectral lines.

In 2000, John Webb at the University of New South Wales in Australia, and a few cobbers, announced that they’d found evidence that the fine structure constant must have been smaller some 10 billion years ago. Their evidence came from an analysis of the spectral lines from “quasi-stellar objects” that emitted their light some 10 billion years ago.

The result has always been controversial, however. And in 2004 another set of astrophysical data analysed in the same way seemed to show no variation. The accuracy of both these measurements was in the region of 10^-6.

Now two new generations of experiments are becoming possible that dramatically improve on this accuracy, say Ted Hansch and friends from the Max-Planck Institut fur Quantenoptik in Garching Germany. The first type use lab-based measurements to look for changes in the fine structure constant over a period of a few years. Using the the latest generation of amazingly precise lasers, various teams have shown that the fine structure constant cannot be changing by a factor of more than 10^-17 per year.

That doesn’t rule out bigger changes in the past that might show up in astronomical observations. But now these same highly accurate lasers are beginning to help calibrate the measurements of spectral lines from distant astronomical objects. This second new class of experiment threatens to place ever tighter limits on the way the fine structure constant may have changed in the past.

These measurements appear to make the Webb result less tenuous by the day. If he and his team are going to re-iterate their claim that the fine structure constant has changed in the past, it’ll be interesting to see how they do it.

Ref: arxiv.org/abs/0904.1663: Testing the Stability of the Fine Structure Constant in the Laboratory

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