REVIEW 2 major objections 2 minor 49 references
Prospects for Improving the Theoretical Uncertainty for Tests of General Relativity with the EHT
T0 review · 2 major / 2 minor · reviewed 2026-07-01 · grok-4.3
Pith's one-line read The theoretical uncertainty in the bright ring to black hole shadow size relationship can be reduced substantially with future EHT observations at 345 GHz and targeted constraints.
desk verdict A broad simulation campaign that ranks practical ways to tighten the ring-shadow calibration, but the gains rest on how well the models match real flows. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The calibrated relationship between bright ring size in simulated images and analytic black hole shadow size, used to convert observed ring diameters into mass or GR-test inferences.
What would settle it
A set of real 345 GHz EHT images, after application of the listed geometric and astrophysical constraints, that show ring-to-shadow discrepancies lying outside the reduced uncertainty range reported in the simulations.
Extended reading notes
Core claim
The relationship between the observed bright ring diameter and the analytic black hole shadow diameter, calibrated across a large suite of high-resolution general relativistic magnetohydrodynamic simulations with varied accretion-flow models and initial conditions, exhibits a theoretical uncertainty that decreases relative to current levels under higher-frequency observations, geometric image constraints, astrophysical alignment constraints, and the assumption of a magnetically arrested disk. The uncertainty is also projected to decrease further with additional observations in subsequent years when dwell-time filtering is used to evaluate persistence of geometric measurements.
Load-bearing premise
The simulated images with varied accretion-flow models faithfully reproduce the ring-shadow relationship that will appear in actual EHT observations of M87 and Sgr A*.
Editorial extensions
If this is right
- Observing at 345 GHz reduces the uncertainty in the ring-shadow relationship compared with existing measurements.
- Applying geometric constraints such as ring width from the PRIMO algorithm further reduces the uncertainty.
- Incorporating the constraint that the M87 black hole spin axis aligns with the large-scale jet reduces the uncertainty.
- Adopting the assumption of a magnetically arrested accretion flow configuration reduces the uncertainty.
- The uncertainty is expected to decrease as more observations are obtained over time, with dwell-time filtering providing an additional improvement path.
Reading between the lines
- If the reduced uncertainty holds, independent mass measurements combined with EHT ring sizes could yield tighter bounds on deviations from the Kerr metric.
- Dwell-time filtering could be tested on existing multi-epoch EHT data sets to check whether it improves calibration stability before new observations arrive.
- The same constraint strategy might be adapted to other compact-object imaging efforts that rely on ring or shadow features.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper calibrates the relationship between the bright ring size in simulated EHT images and the analytic black hole shadow size using a large library of high-resolution GRMHD simulations that vary accretion-flow modeling approaches, system parameters, and initial conditions. It claims that the theoretical uncertainty in this calibration (relevant for mass measurements and GR tests) can be reduced substantially relative to current values by (i) observing at 345 GHz, (ii) applying geometric constraints such as PRIMO ring-width inference, (iii) imposing astrophysical constraints such as M87 spin-jet alignment, and (iv) restricting to magnetically arrested disk (MAD) models; it further projects continued improvement with additional epochs and dwell-time filtering.
Significance. If the simulation-based uncertainty reductions are representative of real EHT data, the work would directly improve the precision of shadow-based GR tests and mass measurements for M87* and Sgr A*. The broad simulation campaign spanning multiple modeling approaches is a methodological strength that allows an empirical rather than purely analytic estimate of scatter.
major comments (2)
- [simulation campaign description and results sections] The central claim that the quoted uncertainty reductions apply to real observations rests on the assumption that the simulation library brackets the ring-shadow relationship present in actual M87/Sgr A* data. The manuscript does not include explicit tests or discussion of additional physics (two-temperature effects, non-thermal electron distributions, or time-dependent reconnection) known to alter image morphology in EHT-relevant regimes; without such tests the projected reductions remain conditional on the chosen GRMHD setups.
- [results on uncertainty reduction] The quantitative uncertainty values before and after each constraint (345 GHz, PRIMO, jet alignment, MAD) are presented only as relative decreases; the manuscript should report the absolute calibrated mean and scatter (with error budgets) for each case so that the improvement can be assessed against the existing measurement baseline.
minor comments (2)
- Notation for the ring and shadow radii should be defined once at first use and used consistently; occasional switches between descriptive phrases and symbols reduce readability.
- [future observations discussion] The dwell-time filtering proposal would benefit from a short quantitative example showing how persistence over multiple epochs reduces scatter in the calibration sample.
Simulated Author's Rebuttal
We thank the referee for their constructive comments. We address each major comment below, indicating where revisions will be made to the manuscript.
read point-by-point responses
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Referee: [simulation campaign description and results sections] The central claim that the quoted uncertainty reductions apply to real observations rests on the assumption that the simulation library brackets the ring-shadow relationship present in actual M87/Sgr A* data. The manuscript does not include explicit tests or discussion of additional physics (two-temperature effects, non-thermal electron distributions, or time-dependent reconnection) known to alter image morphology in EHT-relevant regimes; without such tests the projected reductions remain conditional on the chosen GRMHD setups.
Authors: We agree that the uncertainty reductions we report are conditional on the GRMHD setups in our library. While the campaign spans multiple accretion-flow modeling approaches, initial conditions, and system parameters, it does not incorporate two-temperature effects, non-thermal electron distributions, or time-dependent reconnection. In the revised manuscript we will add explicit discussion of these limitations in the methods and conclusions sections, stating that the quoted reductions apply within the context of the current simulation suite and that additional physics could modify the calibration. We will also insert a clarifying sentence in the abstract. revision: partial
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Referee: [results on uncertainty reduction] The quantitative uncertainty values before and after each constraint (345 GHz, PRIMO, jet alignment, MAD) are presented only as relative decreases; the manuscript should report the absolute calibrated mean and scatter (with error budgets) for each case so that the improvement can be assessed against the existing measurement baseline.
Authors: We appreciate this recommendation. The revised manuscript will include a new table (and updated figures) reporting the absolute mean ring-to-shadow ratio and the standard deviation (scatter) across the simulation ensemble for the baseline and each constrained case, together with the associated error budgets. This will enable direct comparison against the current EHT measurement baselines. revision: yes
Circularity Check
No significant circularity; calibration is independent of target claims
full rationale
The paper measures the ring-to-shadow size relationship directly from a library of high-resolution GRMHD simulations that vary accretion models, parameters, and initial conditions. This empirical calibration is performed first and independently; the subsequent claims about uncertainty reduction (via 345 GHz, PRIMO constraints, jet alignment, or MAD assumptions) are obtained by subsetting or filtering the same pre-existing library rather than by redefining the relationship in terms of the reductions themselves. No equation equates a derived quantity to its own input by construction, no fitted parameter is relabeled as a prediction, and no load-bearing premise rests on a self-citation whose content is unverified. The derivation chain is therefore self-contained against the simulation benchmarks.
Assumptions & free parameters
free parameters (2)
- accretion-flow modeling parameters
- geometric constraint parameters
assumptions (2)
- domain assumption Simulated images accurately capture the ring-shadow mapping present in real EHT data
- domain assumption Spin axis of M87 is aligned or anti-aligned with the large-scale jet
Cite this review
Pith. "Pith review of Prospects for Improving the Theoretical Uncertainty for Tests of General Relativity with the EHT." pith.science (2026). https://pith.science/paper/XWDJMVHS
@misc{pith2026260630753,
author = {Pith},
title = {Pith review of: Prospects for Improving the Theoretical Uncertainty for Tests of General Relativity with the EHT},
year = {2026},
howpublished = {\url{https://pith.science/paper/XWDJMVHS}},
note = {Machine review of arXiv:2606.30753}
}
read the original abstract
We characterize the relationship between the size of the bright ring observed in simulated black hole images and the size of the analytic black hole shadow. Calibrating this relationship is crucial for mass measurements and, when independent mass measurements are available, for tests of general relativity using Event Horizon Telescope (EHT) images. We perform this calibration using a large set of high-resolution simulated images generated with different accretion-flow modeling approaches and spanning a wide range of system parameters and initial conditions. We show that the theoretical uncertainty in this relationship can be reduced significantly through future observations, improved imaging techniques, and the application of astrophysical or model-based constraints. In particular, the uncertainty decreases compared to existing measurements when (i) observing at 345 GHz, (ii) applying geometric image constraints, such as the ring width inference from the PRIMO image reconstruction algorithm, (iii) incorporating astrophysical constraints such as the black hole spin axis in M87 being aligned (or anti-aligned) with the large-scale jet observed at longer radio wavelengths, and (iv) assuming that the accretion flow can be described by a magnetically arrested field configuration. Finally, we quantify how the uncertainty is expected to decrease as additional observations are obtained in subsequent years and identify dwell-time filtering, i.e., evaluating the persistence of a geometric measurement over time, as a promising avenue for improving the precision of the calibration.
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Reviewed July 1, 2026 · model on record in the stance chip above.
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