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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

Leiming Chen , Patrick Jentsch , Chiu Fan Lee , Ananyo Maitra , Sriram Ramaswamy , John Toner This is my paper · ORCID
classification cond-mat.softcond-mat.stat-mech
keywords flocksdensityexactexponentsscalingdemonstratedimensionsextra
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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 6 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. New universality classes govern the critical and multicritical behavior of an active Ising model

    cond-mat.stat-mech 2025-07 conditional novelty 7.0 of 10

    The critical behavior of a density-coupled active Ising model is governed by a new, stable renormalization-group fixed point rather than the Wilson-Fisher fixed point.

  2. Only the Ambidextrous Can Flock: Two-dimensional Chiral Malthusian Flocks, Time Cholesterics, and the KPZ Equation

    cond-mat.soft 2025-06 conditional novelty 7.0 of 10

    Chiral Malthusian flocks in 2D map onto the (2+1)-dimensional KPZ equation, making their equal-time velocity correlations short-ranged.

  3. Hydrodynamic Theory of Two-dimensional Chiral Malthusian Flocks

    cond-mat.soft 2025-07 conditional novelty 6.0 of 10

    Two-dimensional chiral Malthusian flocks map to (2+1)-dimensional KPZ growth, so any chirality destroys long-range order but leaves a weak-chirality cascade of quasi-ordered regimes.

  4. True and Quasi Long-Range Order in Malthusian Flocks

    cond-mat.soft 2026-08 conditional novelty 5.0 of 10

    A nonperturbative renormalization group calculation finds the ordered-state exponents of Malthusian flocks and a nonuniversal quasi-long-range-ordered phase whose endpoint differs from the BKT transition.

  5. Large Spin-Wave Fluctuations Suppress Activity in Malthusian Flocks

    cond-mat.soft 2026-08 conditional novelty 5.0 of 10

    Spin-wave fluctuations in 2D Malthusian flocks can screen out activity, producing an equilibrium-XY phase separated from the active Malthusian phase by a BKT-like critical point.

  6. Response to the comment on The inconvenient truth about flocks by Chat\'e and Solon

    cond-mat.soft 2025-05 conditional novelty 3.0 of 10

    A rebuttal to the Chaté-Solon comment, restating the claim that orientational Goldstone modes in flocks can have non-conserved dynamics, with nematic liquid crystals as the key counterexample.

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