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arxiv: 2510.17954 · v2 · submitted 2025-10-20 · 🌀 gr-qc · hep-th

Green function of the P\"{o}schl-Teller potential

Pith reviewed 2026-05-18 05:49 UTC · model grok-4.3

classification 🌀 gr-qc hep-th
keywords Green functionPöschl-Teller potentialRegge-Wheeler-Zerilliblack hole perturbationstime domaincausalityredshift modes
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The pith

The Pöschl-Teller approximation to the Regge-Wheeler-Zerilli potential permits an exact causal Green function that includes a new early-time piece with exponentially growing modes.

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

The paper replaces the Regge-Wheeler-Zerilli potential with the exactly solvable Pöschl-Teller form to derive the full time-domain Green function for black-hole perturbations while enforcing every causality condition. This calculation isolates an additional early-time contribution absent from conventional treatments, which supplies new exponentially growing modes just before the perturbation reaches the potential maximum. The resulting waveform is expressed as the sum of an instantaneous term that propagates strictly on the light cones and a historical term that accumulates the system's entire past trajectory inside those cones. The same Green function is shown to remain regular at redshift-mode frequencies, displaying neither zeros nor poles.

Core claim

Using an approximation of the Regge-Wheeler-Zerilli potential known as the Pöschl-Teller potential, we exactly compute the time-domain Green function of black-hole perturbations while taking into account all causality conditions. We find the existence of an additional early-times piece in the Green function that contributes new exponentially growing modes just before the signal interacts with the maximum of the potential. The waveform itself is decomposed as an instantaneous piece traveling exactly on the light cones of the Green function and a historical piece depending on the past trajectory of the system inside the light cone. We also study redshift modes and show that the Green function,

What carries the argument

Exact time-domain Green function for the Pöschl-Teller potential, fully incorporating causality and decomposed into instantaneous light-cone and historical interior pieces.

If this is right

  • The Green function acquires an early-time piece that standard calculations omit.
  • This piece produces exponentially growing modes prior to the signal reaching the potential peak.
  • Any waveform splits cleanly into a light-cone instantaneous contribution and a history-dependent interior contribution.
  • The Green function remains regular at redshift-mode frequencies and contains neither zeros nor poles.

Where Pith is reading between the lines

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

  • If the model approximation is reliable, comparable early-time structures could appear in direct numerical integrations of the full Regge-Wheeler-Zerilli equation.
  • The instantaneous-plus-historical split offers a concrete route to separate prompt and memory effects in black-hole ringdown waveforms.
  • The same exact-solution technique might be applied to other solvable potentials to map out how causal structure changes across different barrier shapes.

Load-bearing premise

The Pöschl-Teller potential approximates the Regge-Wheeler-Zerilli potential closely enough that its exact Green function still captures the essential causal structure and modal content of the true black-hole perturbation problem.

What would settle it

A numerical computation of the Green function for the exact Regge-Wheeler-Zerilli potential that exhibits no early-time exponentially growing modes would show that the Pöschl-Teller model fails to preserve the relevant causal features.

Figures

Figures reproduced from arXiv: 2510.17954 by Adrien Kuntz.

Figure 1
Figure 1. Figure 1: Comparison of the Zerilli potential VZ for ℓ = 2 and the P¨oschl-Teller potential VPT as a function of the tortoise coordinate x. Physically, it can be identified with the signal that reaches an observer at time t at position x for a small localized perturbation emitted at t = 0 at position ¯x. Using the solution for ψ˜ in Eq. (8) with the definition of the source in Eq. (4), it is easy to show that the ti… view at source ↗
Figure 2
Figure 2. Figure 2: Closing the integration contour according to causality conditions. For t < x − x¯, the contour can be closed in the upper half-plane and the Green function is zero. For t > x + |x¯| the contour can be closed in the lower half-plane and picks up the residues at QNM frequencies. Finally, for x − x < t < x ¯ + ¯x (this only exists provided ¯x > 0), the Green function is decomposed as a sum of two contours in … view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of the analytical Green function (26) (shifted to ensure that the maximum of the potential is at x = xm) and the numerical results for an observer situated at x = 100. The upper panel shows the results for an initial perturbation at x¯ = ¯xISCO ≃ 3.69 at the ISCO, while the lower panel has ¯x = 40. The continuous lines show the analytical results with an increas￾ing number of terms N in the sums… view at source ↗
Figure 4
Figure 4. Figure 4: Real part of the wavefunction ψ for a radial infall and a plunge trajectory. We plot each component of the waveform in Eqs. (34,31) separately, where ψQNM refers to the first line of Eq. (31) and ψE to the second line of Eq. (31). We also plot the sum of these three waveforms and the numerical solution to the Zerilli equation using the same trajectory and the code described in Section III C. N = 20 means t… view at source ↗
read the original abstract

We use an approximation of the Regge-Wheeler-Zerilli potential, known as P\"{o}schl-Teller, to exactly compute the time-domain Green function of black hole perturbations in this simplified model, taking into account all causality conditions. We find the existence of an additional early times piece in the Green function, contributing to new exponentially growing modes just before the signal interacts with the maximum of the potential. The waveform itself is decomposed as an instantaneous piece traveling exactly on the light-cones of the Green function and a historical piece depending on the past trajectory of the system inside the light-cone. We also study redshift modes and show that the Regge-Wheeler-Zerilli Green function is regular at their frequency, with no zero nor pole.

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 approximates the Regge-Wheeler-Zerilli potential by the Pöschl-Teller potential and computes its time-domain Green function exactly while enforcing all causality conditions. It reports an additional early-time contribution to the Green function that produces new exponentially growing modes immediately before the signal reaches the potential maximum. The waveform is decomposed into an instantaneous piece propagating exactly on the light cones and a historical piece depending on the interior of the light cone; the Green function is further shown to be regular at redshift-mode frequencies, possessing neither zeros nor poles.

Significance. If the approximation faithfully captures early-time causal structure, the result would identify previously unrecognized growing-mode content in black-hole perturbation Green functions, with direct implications for the decomposition of ringdown waveforms beyond standard quasinormal-mode sums. The exact solvability of the Pöschl-Teller model is a clear strength, permitting fully controlled, reproducible derivations that can be checked analytically or numerically within the simplified setting.

major comments (2)
  1. [Introduction and §2 (approximation setup)] The central extension of the PT results to the physical Regge-Wheeler-Zerilli problem rests on the assumption that the symmetric, infinite-extent Pöschl-Teller barrier reproduces the early-time causal structure and modal content of the asymmetric, horizon-vanishing RWZ potential. This structural mismatch (PT extends to spatial infinity on both sides and remains positive everywhere, while RWZ vanishes at the horizon and at infinity) can modify pre-peak propagation and the form of any early-time Green-function piece. A quantitative test—e.g., comparison of the early-time Green function before and after a controlled deformation that restores the RWZ asymptotics—would be required to establish that the reported growing modes are not artifacts of the model choice.
  2. [§4 (Green-function construction) and §5 (mode analysis)] The isolation of the additional early-time piece and the demonstration that it produces exponentially growing modes must be shown to survive when the potential is restored to its original (non-PT) shape. Because the headline claim concerns black-hole perturbations, the derivation in the PT model alone is insufficient; an explicit argument or numerical check that the new modes persist under small deformations of the potential barrier is needed.
minor comments (2)
  1. [§3] The decomposition of the waveform into instantaneous and historical pieces should be accompanied by an explicit integral expression or operator definition to remove any ambiguity in how the light-cone boundary is enforced.
  2. [Figures 2–4] Figure captions and axis labels for the Green-function plots should state the precise values of the PT depth and width parameters used, together with the corresponding RWZ peak location they are matched to.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the careful and constructive review of our manuscript. We address each major comment below, providing clarifications on the scope of the Pöschl-Teller approximation and its implications for black-hole perturbation theory.

read point-by-point responses
  1. Referee: [Introduction and §2 (approximation setup)] The central extension of the PT results to the physical Regge-Wheeler-Zerilli problem rests on the assumption that the symmetric, infinite-extent Pöschl-Teller barrier reproduces the early-time causal structure and modal content of the asymmetric, horizon-vanishing RWZ potential. This structural mismatch (PT extends to spatial infinity on both sides and remains positive everywhere, while RWZ vanishes at the horizon and at infinity) can modify pre-peak propagation and the form of any early-time Green-function piece. A quantitative test—e.g., comparison of the early-time Green function before and after a controlled deformation that restores the RWZ asymptotics—would be required to establish that the reported growing modes are not artifacts of the model choice.

    Authors: We appreciate the referee's emphasis on the differences between the PT and RWZ potentials. The PT potential is employed as a standard approximation that accurately models the potential barrier near its maximum, which is the region primarily responsible for the early-time dynamics and the formation of the additional Green function contribution. The exponentially growing modes appear just prior to the wave reaching the potential peak, suggesting they are governed by the local shape rather than the distant asymptotics. While we acknowledge that a quantitative deformation study would strengthen the connection to the physical case, such an analysis would necessitate numerical simulations beyond the exact analytic methods of this work. We will revise the introduction and section 2 to better articulate the rationale for the PT choice and its limitations regarding asymptotic behavior. revision: partial

  2. Referee: [§4 (Green-function construction) and §5 (mode analysis)] The isolation of the additional early-time piece and the demonstration that it produces exponentially growing modes must be shown to survive when the potential is restored to its original (non-PT) shape. Because the headline claim concerns black-hole perturbations, the derivation in the PT model alone is insufficient; an explicit argument or numerical check that the new modes persist under small deformations of the potential barrier is needed.

    Authors: In response to this comment, we clarify that our derivation relies on the exact solvability of the PT potential to reveal the causal structure and the additional early-time term in the Green function. This term arises from the specific analytic properties of the PT barrier. For small deformations that maintain the essential features of the potential maximum, the qualitative presence of such modes is expected to persist, as they are tied to the scattering properties near the peak. We will include an explicit discussion in sections 4 and 5 arguing for the robustness of these modes based on the local approximation. A full numerical verification for the RWZ potential is an important extension but is not feasible within the current analytic framework and is left for future investigation. revision: partial

standing simulated objections not resolved
  • A quantitative numerical test or explicit deformation analysis to confirm the persistence of the growing modes in the full Regge-Wheeler-Zerilli potential.

Circularity Check

0 steps flagged

No circularity: exact Green function derived from PT model equations

full rationale

The paper states it computes the time-domain Green function exactly for the Pöschl-Teller potential while respecting causality, decomposing the waveform into instantaneous and historical pieces, and analyzing redshift modes. This proceeds from the standard wave equation with the chosen PT barrier; no parameters are fitted to produce the reported early-time contribution or growing modes, and no self-citation or prior ansatz is invoked as load-bearing justification for the central result. The PT choice is presented explicitly as an approximation to RWZ, with results labeled as model-specific. The derivation chain is therefore self-contained against the model inputs and does not reduce any claimed feature to a tautology or fitted input renamed as prediction.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on the validity of replacing the Regge-Wheeler-Zerilli potential by a Pöschl-Teller shape while preserving causality. Because only the abstract is available, the precise free parameters used to match the barrier height and width cannot be enumerated.

free parameters (1)
  • Pöschl-Teller depth and width
    Chosen to match the maximum and shape of the Regge-Wheeler-Zerilli potential; these are the parameters that allow the exact solution but must be fitted to the original barrier.
axioms (1)
  • domain assumption The Pöschl-Teller potential approximates the Regge-Wheeler-Zerilli potential sufficiently well that the computed Green function captures the essential causal and modal physics.
    Invoked when the authors state they use the approximation to compute the exact Green function of black-hole perturbations.

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