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arxiv: 2606.00456 · v1 · pith:HPHBZJTXnew · submitted 2026-05-30 · 🌀 gr-qc

A perturbative geometric approach for photon spheres, massive particle surfaces and black hole shadows with mass variations

Oscar Lasso Andino , Fernando G. Veloz This is my paper

Pith reviewed 2026-06-28 18:46 UTC · model grok-4.3

classification 🌀 gr-qc
keywords photon spheresblack hole shadowsmassive particle surfacesperturbative geometrygeodesic curvatureGaussian curvaturemass variationstime-like geodesics
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The pith

A perturbative method based on intrinsic curvatures calculates photon sphere and massive particle surface radii under black hole mass changes.

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

The paper develops a perturbative geometric technique that relies on geodesic curvature and Gaussian curvature to locate photon spheres, massive particle surfaces, and black hole shadow radii. It recovers existing null-geodesic results at leading order while supplying the previously uncalculated perturbation radius for time-like geodesics. The same framework tracks how small mass variations shift these radii, producing a new quantitative relation. A reader would care because the approach supplies a coordinate-light way to estimate observable black-hole parameters in regimes where full analytic solutions are unavailable. The authors note that the resulting estimates could be compared against next-generation observations.

Core claim

The central claim is that a perturbative expansion grounded in intrinsic curvatures determines the radii of photon spheres and massive particle surfaces, including the leading-order shift induced by mass variations. For null geodesics the method reproduces known perturbative outcomes; for time-like geodesics it yields the perturbation radius itself, a result not previously obtained. The calculation also shows that mass changes directly alter both the photon-sphere radius and the massive-particle-surface radius.

What carries the argument

Perturbative expansion in geodesic curvature and Gaussian curvature that tracks how mass variations displace the locations of photon spheres and massive particle surfaces.

If this is right

  • The method supplies the perturbation radius for time-like geodesics at leading order.
  • Mass variations produce explicit shifts in both photon-sphere and massive-particle-surface radii.
  • The framework recovers prior null-geodesic perturbative results.
  • The resulting radii supply theoretical estimates that can be tested against future observations.
  • The approach furnishes tools for modeling regions near extremely massive objects.

Where Pith is reading between the lines

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

  • The curvature-based expansion may extend to spacetimes lacking exact closed-form solutions, allowing approximate radii where traditional methods stall.
  • Linking the mass-variation shifts to shadow-size measurements could constrain accretion models in a manner not addressed in the paper.
  • The same leading-order curvature tracking might apply to related geodesic quantities such as innermost stable circular orbits.

Load-bearing premise

The leading-order terms of the curvature-based expansion remain accurate for both null and time-like geodesics without higher-order contributions or coordinate choices altering the extracted radii.

What would settle it

An exact solution for the photon-sphere radius in a known metric with a controlled small mass perturbation that deviates numerically from the leading-order prediction of the curvature expansion.

Figures

Figures reproduced from arXiv: 2606.00456 by Fernando G. Veloz, Oscar Lasso Andino.

Figure 1
Figure 1. Figure 1: We have plotted the photon sphere radius [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: The plot shows the radius of the timelike circular orbit (continuous [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 2
Figure 2. Figure 2: Geodesic curvature for time-like geodesics of the Reissner-Nordström [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
read the original abstract

The spacetime behavior at very extreme conditions, such as the regions near a black hole, can be very difficult to modelize. In this work we introduce a new geometric method that allows to calculate the parameters of photon spheres, massive particle surfaces and shadow radius of black holes. We build upon a perturvative approach but bases in intrinsic curvatures, such as the geodesic and Gaussian curvatures. At leading order, the method allows to find the radius of the perturbation in the time-like case, which has not been studied in the literature. In the null case we are able to recover the results found by the perturvative method only. We also study the mass variations and how they influence the photon sphere radius and the massive particle surface radius, leading to a new and powerful result, that could provide new different research directions. The approach presented here will provide a set of tools that will help to modelize gravity near extremely massive objects and help to improve the theoretical estimations of parameters than can be tested in the next generation experiments.

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 / 1 minor

Summary. The paper introduces a perturbative geometric method based on intrinsic curvatures (geodesic and Gaussian) to compute radii of photon spheres, massive particle surfaces, and black hole shadows. It claims to recover known null-geodesic results from existing perturbative approaches while providing a new leading-order result for the time-like case (previously unstudied) and a new result on how mass variations affect both photon-sphere and massive-particle-surface radii.

Significance. If the leading-order curvature-based expansion is shown to be accurate, the approach would supply a coordinate-independent geometric tool for near-horizon geodesic analysis that could be applied to a range of black-hole metrics and tested against next-generation shadow observations. The claimed novelty for the time-like radius and mass-variation effects would constitute a genuine extension beyond existing perturbative literature.

major comments (2)
  1. [Abstract] Abstract: the central claim that the leading-order curvature perturbation yields a new, previously unstudied radius for the time-like case rests on the unverified assumption that higher-order geodesic/Gaussian curvature terms (or coordinate artifacts) do not shift the reported radius; no explicit comparison to exact solutions or next-to-leading-order terms is indicated to secure this truncation.
  2. [Abstract] Abstract: the new result on mass variations influencing photon-sphere and massive-particle-surface radii is presented as load-bearing, yet the manuscript provides no derivation or consistency check against known static limits, leaving the perturbative treatment of mass dependence unanchored.
minor comments (1)
  1. [Abstract] Abstract contains repeated spelling errors ('perturvative', 'modelize') that should be corrected for clarity.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their insightful comments, which help improve the clarity and rigor of our work. We address each major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that the leading-order curvature perturbation yields a new, previously unstudied radius for the time-like case rests on the unverified assumption that higher-order geodesic/Gaussian curvature terms (or coordinate artifacts) do not shift the reported radius; no explicit comparison to exact solutions or next-to-leading-order terms is indicated to secure this truncation.

    Authors: We agree that additional validation is necessary to support the leading-order result for the time-like case. In the revised version, we will include comparisons with exact analytic solutions for standard black hole metrics such as Schwarzschild and Kerr, as well as an explicit calculation of the next-to-leading-order corrections in the curvature expansion. This will demonstrate that the reported radius remains stable under these checks. revision: yes

  2. Referee: [Abstract] Abstract: the new result on mass variations influencing photon-sphere and massive-particle-surface radii is presented as load-bearing, yet the manuscript provides no derivation or consistency check against known static limits, leaving the perturbative treatment of mass dependence unanchored.

    Authors: We acknowledge the need for a more detailed derivation and validation of the mass-variation effects. The revised manuscript will provide the full perturbative derivation for how mass variations enter the curvature-based expressions and will include consistency checks in the static limit (vanishing mass variation) to recover the known results. This will anchor the treatment of mass dependence. revision: yes

Circularity Check

0 steps flagged

Derivation self-contained; recovers known null results and extends independently to time-like and mass-variation cases

full rationale

The paper introduces a perturbative method based on intrinsic curvatures (geodesic and Gaussian) to compute photon sphere, massive particle surface, and shadow radii. It states that the null-case results are recovered from existing perturbative methods while the time-like radius and mass-variation effects constitute new leading-order findings. No load-bearing step is shown to reduce by construction to a fitted parameter, self-citation chain, or definitional equivalence; the central claims retain independent content beyond any prior work. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review provides no explicit free parameters, axioms, or invented entities; ledger left empty.

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discussion (0)

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Reference graph

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