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SUMMARY:Natural swarms in 3.99 dimensions - Andrea Cavagna (Institute for 
 Complex Systems\, CNR\, Rome)
DTSTART:20240305T130000Z
DTEND:20240305T140000Z
UID:TALK209752@talks.cam.ac.uk
CONTACT:Sarah Loos
DESCRIPTION:Collective behaviour is found in a startling variety of biolog
 ical systems\, from clusters of bacteria and colonies of cells\, up to ins
 ect swarms\, bird flocks\, and vertebrate groups. A unifying ingredient is
  the presence of strong correlations: experiments in bird flocks\, fish sc
 hools\, mammal herds\, insect swarms\, bacterial clusters and proteins\, h
 ave found that the correlation length is significantly larger than the mic
 roscopic scales.  In the case of natural swarms of insects another key hal
 lmark of statistical physics has been verified\, namely dynamic scaling: s
 patial and temporal relaxation are entangled into one simple law\, so that
  the relaxation time scales as a power of the correlation length\, thus de
 fining the dynamical critical exponent\, z. Within statistical physics\, s
 trong correlations and scaling laws are the two stepping stones leading to
  the Renormalization Group (RG): when we coarse-grain short-scale fluctuat
 ions\, the parameters of different models flow towards one common fixed po
 int ruling their large-scale behaviour. RG fixed points therefore organize
  into few universality classes the macroscopic behaviour of strongly corre
 lated systems\, thus providing parameter-free predictions of the collectiv
 e behaviour. Biology is vastly more complex than physics\, but the widespr
 ead presence of strong correlations and the validity of scaling laws can h
 ardly be considered a coincidence\, and they rather call for an exploratio
 n of the correlation-scaling-RG path also in collective biological systems
 . However\, to date there is yet no successful test of an RG prediction ag
 ainst experimental data on living systems. In this talk I will apply the r
 enormalization group to the dynamics of natural swarms of insects. Swarms 
 of midges in the field are strongly correlated systems\, obeying dynamic s
 caling with an experimental exponent z=1.37 + / -\n 0.11\, significantly s
 maller than the naive value z = 2 of equilibrium overdamped dynamics. I wi
 ll show that this anomalous exponent can indeed be reproduced by an RG cal
 culation to one-loop\, provided that off-equilibrium activity and inertial
  dynamics are both taken into account\; the theory gives z=1.35\, a value 
 closer to the experimental exponent than any previous theoretical determin
 ation and perfectly in line with the numerical value\, z=1.35 +/- 0.04. Th
 is successful result is a significant step towards testing the core idea o
 f the RG even at the biological level\, namely that integrating out the sh
 ort-scale details of a strongly correlated system impacts on its large-sca
 le behaviour by introducing anomalies in the dimensions of the physical qu
 antities. In the light of this\, it is fair to hope that the renormalizati
 on group\, with its most fruitful consequence -- universality -- may have 
 an incisive impact also in biology.
LOCATION:Center for Mathematical Sciences\, Lecture room MR4
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