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Science 2 May 2008:
Vol. 320. no. 5876, p. 612
DOI: 10.1126/science.1153456
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Technical Comments

Comment on "Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic"

I. S. Aranson,1* A. Snezhko,1 J. S. Olafsen,2 J. S. Urbach3

Narayan et al. (Reports, 6 July 2007, p. 105) reported giant number fluctuations attributed to curvature-driven active currents specific for nonequilibrium nematic systems. We present data demonstrating that similar results can be found in systems of spherical particles due either to inelastic clustering or persistent density inhomogeneity, suggesting two alternative explanations for their results.

1 Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA.
2 Department of Physics, Baylor University, One Bear Place, Box 97316, Waco, TX 76798–7316, USA.
3 Department of Physics, Georgetown University, Washington, DC 20057, USA.

* To whom correspondence should be addressed. E-mail: aronson{at}msd.anl.gov

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Science. ISSN 0036-8075 (print), 1095-9203 (online)