Change is no longer in the air. Or in space, for that matter. Three years ago, a team of astronomers surprised the scientific community with evidence that a fundamental constant of nature--one related to the speed of light--was changing slowly over time. If true, a changing fine-structure constant would force physicists to rethink some of their fundamental assumptions about the immutability of the laws of nature. Now, astronomers in France and India have presented new data that seriously undermine the case for an inconstant constant.
The case for a changing constant comes from observations of bright, ancient celestial objects known as quasars. Gas clouds between the quasars and Earth absorb certain wavelengths of light, leaving dark bands in the object's spectrum. These bands are like a fingerprint that tell astronomers precisely what elements are in the cloud. However, in 2001, Australian and U.S. astronomers saw a curious pattern in the light--the bands seemed to be somewhat out of place relative to one another. This effect might be caused by a slow change in a fundamental number known as the fine-structure constant; this constant, denoted by the Greek letter alpha, is related to the strength of the electromagnetic force, which, in turn, determines the spacing in the bands. The astronomers concluded that the fine-structure constant might have been a tiny fraction of a percent smaller in the early universe (ScienceNow, 15 August 2001).
Now a new set of observations with the Very Large Telescope in Chile casts doubt on the 2001 conclusions. Like the earlier group, the team looked at the absorption lines of quasars, but the new observations concentrated on getting high-quality data about relatively few quasars rather than lower-quality data about lots of quasars, like the 2001 team. The better data enabled the team to make a precise measurement of whether the fine-structure constant was different in the distant past, says team leader Patrick Petitjean, an astronomer at the Institute for Astrophysics in Paris. The answer appears to be no. "In my opinion, there's no variation of alpha," he says. The new study is published in Physical Review Letters and Astronomy & Astrophysics.
"I find it convincing enough to rate it as a very important result," says Jason Prochaska, one of the authors of the 2001 paper and an astronomer at Lick Observatory in California. Yet, he's not ready to throw out his earlier work. Prochaska suspects that the discrepancy between the two results is due to a systematic error in one or the other group's work; hopefully, a third telescope will be able to settle the difference.