Silent Flocks: Constraints on Signal Propagation Across Biological Groups

CAVAGNA, A.; GIARDINA, I.; GRIGERA, T. S.; JELIC, A; LEVINE, D; RAMASWAMY, S; VIALE, M.

PHYSICAL REVIEW LETTERS

AMER PHYSICAL SOC

Lugar: New York; Año: 2015 vol. 114 p. 218101 - 218101

0031-9007

Experiments find coherent information transfer through biological groups on length and time scalesdistinctly below those on which asymptotically correct hydrodynamic theories apply. We present here anew continuum theory of collective motion coupling the velocity and density fields of Toner and Tu to theinertial spin field recently introduced to describe information propagation in natural flocks of birds. Thelong-wavelength limit of the new equations reproduces the Toner-Tu theory, while at shorter wavelengths(or, equivalently, smaller damping), spin fluctuations dominate over density fluctuations, and second-soundpropagation of the kind observed in real flocks emerges. We study the dispersion relation of the new theoryand find that when the speed of second sound is large, a gap in momentum space sharply separates first-from second-sound modes. This gap implies the existence of silent flocks, namely, of medium-sizedsystems across which information cannot propagate in a linear and underdamped way, either under the formof orientational fluctuations or under that of density fluctuations, making it hard for the group to achievecoordination.