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arxiv: 2604.10168 · v1 · submitted 2026-04-11 · ❄️ cond-mat.soft

Spectral Signatures of Active Fluctuations in Semiflexible Polymers

Love Grover , Anil Kumar Dasanna , Abhishek Chaudhuri This is my paper

Pith reviewed 2026-05-10 15:50 UTC · model grok-4.3

classification ❄️ cond-mat.soft
keywords semiflexible polymersactive bathfluctuation spectrumactive Brownian particlesmode reorganizationnonequilibrium noisecontour length renormalization
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The pith

Active baths reorganize semiflexible polymer fluctuations by boosting low modes and shifting spectral weight to longer wavelengths.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper derives an effective model of temporally persistent and spatially correlated noise from the statistics of forces exerted by an explicit bath of active Brownian particles. It tests this against simulations of both explicit-bath and implicit-noise models and finds that activity reorganizes polymer fluctuations in a mode-dependent way rather than uniformly. Increasing active force strength predominantly enhances the lowest modes, while increasing persistence shifts the spectral weight toward progressively longer wavelengths. The effective theory captures this reorganization and accounts for the qualitative match between the two bath models, but it underestimates global size measures because it assumes fixed contour length.

Core claim

Starting from explicit active forces, an effective description in terms of temporally persistent and spatially correlated noise captures how activity reorganizes the polymer's fluctuation spectrum at the mode level, with force boosting low modes and persistence shifting weight toward longer wavelengths, though it underestimates global sizes due to absent contour-length renormalization.

What carries the argument

Effective temporally persistent and spatially correlated active noise derived from active Brownian particle bath statistics.

If this is right

  • The polymer's mode spectrum acts as a multiscale probe that resolves the temporal and spatial structure of nonequilibrium forcing.
  • Explicit and implicit active bath models show strong qualitative correspondence over a broad parameter range.
  • Fixed-contour-length theories miss systematic activity-induced effects on global polymer size measures.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Semiflexible polymers could be used in experiments to distinguish different active forcing structures by reading their mode spectra.
  • Allowing variable contour length in the effective model would likely remove the underestimation of radius of gyration and related global measures.

Load-bearing premise

The polymer contour length remains fixed under activity, ignoring bond stretching and renormalization.

What would settle it

Compare measured radius of gyration in explicit active-bath simulations that allow bond stretching against predictions from the fixed-contour theory; the gap should close if contour-length effects are the source of the underestimate.

Figures

Figures reproduced from arXiv: 2604.10168 by Abhishek Chaudhuri, Anil Kumar Dasanna, Love Grover.

Figure 1
Figure 1. Figure 1: FIG. 1. Schematic representation of a semiflexible polymer [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2 [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4 [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p012_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6 [PITH_FULL_IMAGE:figures/full_fig_p018_6.png] view at source ↗
read the original abstract

We study how an active bath is transduced into the internal fluctuation spectrum of a semiflexible polymer. Starting from the statistics of active forces exerted by an explicit bath of active Brownian particles, we derive an effective description in terms of temporally persistent and spatially correlated noise, and test it against simulations of both explicit-bath and implicit-noise models. We find that activity reorganizes polymer fluctuations spectrally rather than uniformly: increasing the active force predominantly enhances the lowest modes, while increasing persistence shifts the spectral weight toward progressively longer wavelengths. The theory captures this mode-level reorganization well and explains the strong qualitative correspondence between explicit and implicit active baths over a broad parameter range. In contrast, global size measures such as the radius of gyration are systematically underestimated, which we trace to activity-induced bond stretching and contour-length renormalization absent from the present fixed-contour theory. Our results show that a semiflexible polymer acts as a multiscale probe of active matter, resolving the temporal and spatial structure of nonequilibrium forcing through its mode spectrum.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript derives an effective description of active fluctuations in a semiflexible polymer as temporally persistent and spatially correlated noise, starting from the explicit statistics of an active Brownian particle bath. It tests this model against simulations of both explicit-bath and implicit-noise realizations, reporting that activity reorganizes the fluctuation spectrum by preferentially enhancing the lowest modes with increasing active force strength and shifting spectral weight toward longer wavelengths with increasing persistence. The effective theory is said to capture this mode-level reorganization well and to explain the qualitative correspondence between explicit and implicit baths over a broad parameter range, while global size measures (e.g., radius of gyration) are systematically underestimated due to the fixed-contour-length approximation that neglects activity-induced bond stretching and contour renormalization.

Significance. If the central spectral claims hold, the work is significant for establishing semiflexible polymers as multiscale probes that resolve the temporal and spatial structure of nonequilibrium active forcing. The first-principles derivation from explicit active-particle statistics, followed by direct comparison to separate simulations of explicit and implicit models, provides a concrete strength that supports the reported qualitative agreement and the explanation for bath correspondence. This advances the field beyond uniform scaling pictures of activity and offers a framework for interpreting fluctuation spectra in active matter contexts.

major comments (2)
  1. [Abstract] Abstract: the statement that the effective theory 'captures this mode-level reorganization well' is presented without any quantitative fit metrics (e.g., mean-squared deviation, correlation coefficient, or error bars on the mode amplitudes) for the spectral comparisons across active force and persistence values; given the acknowledged discrepancy in global size measures, the absence of such metrics leaves the strength of the 'well' agreement difficult to assess.
  2. [Theory/derivation section] Theory/derivation section: the fixed-contour-length assumption is correctly identified as the source of underestimated global sizes, yet no analysis is provided of whether (or how) this approximation propagates into the derived effective noise correlations or biases the reported low-mode enhancements and wavelength shifts; because the central claim concerns spectral reorganization, a short sensitivity test or bounding argument on contour fluctuations would be required to confirm the approximation does not undermine the mode-level results.
minor comments (2)
  1. Figure captions and legends should explicitly list the numerical parameter values (active force magnitude, persistence time, polymer length, etc.) used for each curve or panel to facilitate direct comparison with the stated broad parameter range.
  2. The manuscript would benefit from a brief methods paragraph or supplementary note detailing the simulation protocols (e.g., integration timestep, ensemble size, equilibration criteria) for both explicit-bath and implicit-noise runs, as these are essential for reproducibility of the reported qualitative matches.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive evaluation and the recommendation for minor revision. We address each major comment below and will incorporate the suggested improvements in the revised manuscript.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the statement that the effective theory 'captures this mode-level reorganization well' is presented without any quantitative fit metrics (e.g., mean-squared deviation, correlation coefficient, or error bars on the mode amplitudes) for the spectral comparisons across active force and persistence values; given the acknowledged discrepancy in global size measures, the absence of such metrics leaves the strength of the 'well' agreement difficult to assess.

    Authors: We agree that quantitative metrics would allow a more objective assessment of the agreement. In the revised manuscript we will report correlation coefficients and mean-squared deviations between the effective-theory predictions and the simulation data for the mode amplitudes, across the active-force and persistence values shown in the figures. These metrics will be added to the figure captions or a supplementary table. revision: yes

  2. Referee: [Theory/derivation section] Theory/derivation section: the fixed-contour-length assumption is correctly identified as the source of underestimated global sizes, yet no analysis is provided of whether (or how) this approximation propagates into the derived effective noise correlations or biases the reported low-mode enhancements and wavelength shifts; because the central claim concerns spectral reorganization, a short sensitivity test or bounding argument on contour fluctuations would be required to confirm the approximation does not undermine the mode-level results.

    Authors: The referee correctly identifies that we have not explicitly examined how the fixed-contour-length approximation affects the derived noise correlations or the reported spectral reorganization. In the revision we will add a short bounding argument in the theory section: we will estimate the magnitude of activity-induced contour-length fluctuations from the explicit-bath simulations and show that, for the parameter range considered, these fluctuations remain small enough that they do not qualitatively change the relative low-mode enhancements or the wavelength shifts. The effective noise correlations themselves are derived from the bath statistics and are independent of contour length to leading order. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The derivation begins from the explicit statistics of active Brownian particles and produces an effective temporally persistent, spatially correlated noise model that is then validated against independent simulations of both explicit-bath and implicit-noise polymer models. The reported spectral reorganization (preferential enhancement of low modes with active force, wavelength shift with persistence) is presented as an output of this derivation and is compared to simulation data rather than being fitted or defined in terms of the target observables. Global size discrepancies are explicitly attributed to the fixed-contour-length approximation, which is flagged as a limitation rather than hidden. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations appear in the provided abstract or reader summary; the central claims remain independently testable against external simulation benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review supplies insufficient detail to enumerate specific free parameters, axioms, or invented entities; the effective noise is described as temporally persistent and spatially correlated but no explicit functional form or fitting procedure is given.

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    Transverse contributionR 2 g,⊥ Integrating the tangent expansion gives r⊥(s) = Z s 0 ds′ u⊥(s′) = X n≥1 an r 2 L sin(qns) qn .(G3) The transverse radius of gyration is R2 g,⊥ ≡ 1 L Z L 0 ds D r⊥(s)−r ⊥,cm 2E ,(G4) wherer ⊥,cm ≡ 1 L R L 0 dsr ⊥(s).Using Eq. (G3), mode de- coupling, and 1 L R L 0 dssin 2(qns) = 1 2, we obtain 1 L Z L 0 ds⟨|r ⊥(s)|2⟩= X n≥1 ...

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    (G8) andR 2 g,∥ from Eq

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