The troubled song of the sand dunes
Feature: November 2006
Matthew Chalmers exposes the fierce controversy behind attempts to explain the mystery of “singing� sand dunes, which provides a rare insight into how physics is done
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The current controversy surrounding this mechanism can be traced back to 2000, when Stéphane Douady, then at the laboratory for statistical physics at the Ecole Normale Supérieure (ENS) in Paris, heard a geologist talking about his research into the shape and motion of sand dunes. After visiting some dunes in the US, Douady secured funding to work on the problem along with his former PhD student Bruno Andreotti, and the pair was joined by new PhD student Pascal Hersen.
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After seven long days in the desert the team returned to its Moroccan hotel, where Andreotti thrashed out a more detailed explanation for how the motion of sand grains might produce sound. Based on some earlier work he and Douady had carried out on grain avalanches, he expected that the frequency of the sound produced would be inversely proportional to the square root of the grain diameter. But when he plugged in the data, he found that the dunes the team had been studying did not seem to follow this simple model.
An open question
An open question
The plot thickened when the researchers returned to Paris.
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Over the next year or so, the French researchers undertook their own field trips: Douady with Hersen, who had inevitably been drawn into the fight, and Andreotti with a new Moroccan PhD student Hicham Elbelrhiti.
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“Because of the bad relationships, I called a group meeting in spring 2004 during which the overall ENS group leader suggested I write a paper about the song of the dunes with all our names on it,� recalls Douady. “But it soon became clear that this wasn’t going to work.� According to Douady, he found out during this meeting that Andreotti had been making measurements of the velocity of elastic waves during his field trips that contradicted Douady’s synchronization mechanism, although Andreotti insists that his work was no secret. So Douady wrote a draft paper without any mention of Andreotti’s measurements.
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These days Douady and Andreotti tend to avoid one another, which is not easy when working in such a small field. “This episode has destroyed several years of my life,� says Douady. “But it has also taught me that science can be done in very different ways – either by sudden intuitive jumps that appear to be unjustified, or cautiously and methodically.� Andreotti has also learned from the experience. “I am now more confident than ever that peer review is the best, or least worst, system in which to work,� he says. “When scientists start to use the media to make scientific claims, things start to get troublesome� .
But the stormy tale of the singing dunes does not end here. In fact with two or three other groups about to publish their own explanations of the effect, none of which requires the sand grains to be synchronized, the controversy may well be about to blow up again.
“Neither Douady’s nor Andreotti’s analysis explains why some dunes do not sing,� says Melany Hunt at the California Institute for Technology (Caltech), who together with co-workers has made extensive measurements of singing dunes using techniques such as radar.
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Sand-dune science may not dominate the research-funding agenda, but unravelling the mystery of the singing dunes offers a valuable insight into how science is done.
