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Science 6 July 2007:
Vol. 317. no. 5834, pp. 105 - 108
DOI: 10.1126/science.1140414
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Reports

Long-Lived Giant Number Fluctuations in a Swarming Granular Nematic

Vijay Narayan,1* Sriram Ramaswamy,1,2 Narayanan Menon3

Coherently moving flocks of birds, beasts, or bacteria are examples of living matter with spontaneous orientational order. How do these systems differ from thermal equilibrium systems with such liquid crystalline order? Working with a fluidized monolayer of macroscopic rods in the nematic liquid crystalline phase, we find giant number fluctuations consistent with a standard deviation growing linearly with the mean, in contrast to any situation where the central limit theorem applies. These fluctuations are long-lived, decaying only as a logarithmic function of time. This shows that flocking, coherent motion, and large-scale inhomogeneity can appear in a system in which particles do not communicate except by contact.

1 Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India.
2 Condensed Matter Theory Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India.
3 Department of Physics, University of Massachusetts, Amherst, MA 01003, USA.

* To whom correspondence should be addressed. E-mail: vj{at}physics.iisc.ernet.in

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