Dynamic scaling in natural swarms

CREATO, CHIARA; GRIGERA, TOMAS S.; VIALE, MASSIMILIANO; CAVAGNA, ANDREA; DEL CASTELLO, LORENZO; MELILLO, STEFANIA; CONTI, DANIELE; GIARDINA, IRENE; PARISI, LEONARDO

NATURE PHYSICS

NATURE PUBLISHING GROUP

Año: 2017 vol. 13 p. 914 - 918

1745-2473

Collective behaviour in biological systems presents theoretical challenges beyond the borders of classical statistical physics.The lack of concepts such as scaling and renormalization is particularly problematic, as it forces us to negotiate details whoserelevance is often hard to assess. In an attempt to improve this situation, we present here experimental evidence of theemergence of dynamic scaling laws in natural swarms of midges. We find that spatio-temporal correlation functions in differentswarms can be rescaled by using a single characteristic time, which grows with the correlation length with a dynamical criticalexponent z ≈ 1, a value not found in any other standard statistical model. To check whether out-of-equilibrium effects may beresponsible for this anomalous exponent, we run simulations of the simplest model of self-propelled particles and find z ≈ 2,suggesting that natural swarms belong to a novel dynamic universality class. This conclusion is strengthened by experimentalevidence of the presence of non-dissipative modes in the relaxation, indicating that previously overlooked inertial effects areneeded to describe swarm dynamics. The absence of a purely dissipative regime suggests that natural swarms undergo anear-critical censorship of hydrodynamics.