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The inconvenient truth about flocks

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arxiv 2503.17064 v2 pith:YLOLHYE7 submitted 2025-03-21 cond-mat.soft cond-mat.stat-mech

The inconvenient truth about flocks

Leiming Chen , Patrick Jentsch , Chiu Fan Lee , Ananyo Maitra , Sriram Ramaswamy , John Toner This is my paper
classification cond-mat.soft cond-mat.stat-mech
keywords flocksdensityexactexponentsscalingdemonstratedimensionsextra
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We reanalyze the hydrodynamic theory of "flocks" that is, polar ordered "dry" active fluids in two dimensions. For "Malthusian" flocks, in which birth and death cause the density to relax quickly, thereby eliminating density as a hydrodynamic variable, we are able to obtain two exact scaling laws relating the three scaling exponents characterizing the long-distance properties of these systems. We also show that it is highly plausible that such flocks display long-range order in two dimensions. In addition, we demonstrate that for "immortal" flocks, in which the number of flockers is conserved, the extra non-linearities allowed by the presence of an extra slow variable (number density) make it impossible to obtain any exact scaling relations between the exponents. We thereby demonstrate that several past published claims of exact exponents for Malthusian and immortal flocks are all incorrect.

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Cited by 3 Pith papers

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  1. On the Renormalization Group Flow of Active Flocks

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    All-order RG analysis of Toner-Tu flocks in 2D isotropic diffusion yields a line of fixed points and marginal vertex instability at Δ/κ = 2π separating Gaussian and symmetry-protected interacting gapless phases with o...

  2. Breakdown of Emergent Chiral Order and Defect Chaos in Nonreciprocal Flocks

    cond-mat.soft 2026-05 conditional novelty 7.0

    Chiral order in 2D nonreciprocal flocks is generically unstable and destroyed by defect proliferation, yielding spatiotemporally chaotic dynamics with a diverging correlation length as nonreciprocity vanishes.

  3. Hydrodynamics, Renormalization Group, and Universality Classes Far from Equilibrium

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