Hidden Symmetries and Gyromagnetic Ratio of Kerr-Newman Black Holes in f(R) Gravity
Pith reviewed 2026-05-10 12:45 UTC · model grok-4.3
The pith
Kerr-Newman black holes in f(R) gravity retain the gyromagnetic ratio g=2 due to preserved hidden symmetries.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By deriving the Killing and Killing-Yano tensors, the role of hidden symmetries in the spacetime structure of Kerr-Newman black holes in f(R) gravity is established. The gyromagnetic ratio retains its universal value of g=2, consistent with all four-dimensional black holes. This highlights the connection between hidden symmetries and the separability of the Hamilton-Jacobi equation in this modified gravity setting.
What carries the argument
Killing and Killing-Yano tensors that encode the hidden symmetries and enable separation of the Hamilton-Jacobi equation while fixing the gyromagnetic ratio at 2.
If this is right
- The Hamilton-Jacobi equation remains separable for these black holes in f(R) gravity.
- The gyromagnetic ratio stays at the universal value g=2 without additional constraints from the f(R) modification.
- These results apply to electrically charged rotating black holes and extend the consistency seen in general relativity.
- Implications arise for theoretical studies of black holes in modified gravity and observational aspects like gravitational wave analyses.
Where Pith is reading between the lines
- If the metric form holds for other f(R) functions, the same symmetries and ratio would apply more broadly.
- This preservation might indicate a deeper geometric reason independent of the specific gravity theory.
- Future work could test this by examining the motion of charged particles or the black hole's response in f(R) cosmologies.
Load-bearing premise
The standard Kerr-Newman metric form and the usual method for building Killing and Killing-Yano tensors continue to work without changes imposed by the particular f(R) function chosen.
What would settle it
Explicitly computing the gyromagnetic ratio using a specific f(R) model where the spacetime metric differs from the Kerr-Newman form and finding it deviates from 2 would falsify the claim.
read the original abstract
We explore hidden symmetries in electrically charged, four-dimensional rotating Kerr-Newman black hole within $f(R)$ gravity. By deriving the Killing and Killing-Yano tensors, we establish their role in the spacetime structure. The gyromagnetic ratio is calculated and shown to retain its universal value of $g = 2$, consistent with all four-dimensional black holes. These findings show that the gyromagnetic ratio remains consistent in this modified gravity setting. Moreover, they highlight the connection between hidden symmetries and the ability to separate the Hamilton-Jacobi equation in $f(R)$ gravity. This work advances the study of black holes in modified gravity, offering implications for both theoretical frameworks and observational cosmology, such as gravitational wave analyses.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript explores hidden symmetries of the electrically charged rotating Kerr-Newman black hole in f(R) gravity. It derives the Killing and Killing-Yano tensors, uses them to establish the spacetime structure and separability of the Hamilton-Jacobi equation, and computes the gyromagnetic ratio, reporting that it retains the universal value g=2 found for all four-dimensional black holes.
Significance. If the derivations hold and the metric is shown to solve the f(R) equations, the result would establish that hidden symmetries and the gyromagnetic ratio g=2 are robust against this class of modifications to general relativity. This would strengthen the case for universality of these properties across four-dimensional black-hole solutions and could inform both analytic separability techniques and observational probes such as gravitational-wave ringdown.
major comments (1)
- [Abstract and metric/tensor sections] The central claim that the Kerr-Newman metric and its Killing-Yano tensor can be used unchanged to obtain g=2 rests on the unverified assumption that this geometry solves the f(R) field equations for the chosen f. Outside the horizon R=0 identically, so the trace of the field equations reduces to the constraint 3□f'(R)−2f(R)=0. This is not satisfied for arbitrary f(R) and forces f to obey a specific differential relation (e.g., f''(R)=0). The manuscript applies the standard GR construction of the tensors and gyromagnetic ratio without demonstrating that the adopted f(R) satisfies the constraint or that the metric solves the full fourth-order equations. (Abstract; metric ansatz and tensor derivation sections.)
minor comments (1)
- [Abstract] The abstract repeats the statement that the gyromagnetic ratio remains consistent; a single concise sentence would improve readability.
Simulated Author's Rebuttal
We thank the referee for the thorough review and valuable comments on our manuscript. The major concern regarding the status of the Kerr-Newman metric as a solution of the f(R) field equations is addressed point by point below. We agree that clarification is needed and will revise the manuscript accordingly.
read point-by-point responses
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Referee: [Abstract and metric/tensor sections] The central claim that the Kerr-Newman metric and its Killing-Yano tensor can be used unchanged to obtain g=2 rests on the unverified assumption that this geometry solves the f(R) field equations for the chosen f. Outside the horizon R=0 identically, so the trace of the field equations reduces to the constraint 3□f'(R)−2f(R)=0. This is not satisfied for arbitrary f(R) and forces f to obey a specific differential relation (e.g., f''(R)=0). The manuscript applies the standard GR construction of the tensors and gyromagnetic ratio without demonstrating that the adopted f(R) satisfies the constraint or that the metric solves the full fourth-order equations. (Abstract; metric ansatz and tensor derivation sections.)
Authors: We appreciate the referee highlighting this important point. The Kerr-Newman metric is considered in our work as an exact solution within the class of f(R) gravity models for which the Ricci scalar vanishes identically (R=0), as is the case for the Kerr-Newman spacetime both inside and outside the horizon. Under this condition the trace equation reduces to 3□f'(R)−2f(R)=0. Because R is a constant, f'(R) is constant and □f'(R)=0, so the equation is satisfied provided f(0)=0. This is a standard requirement for f(R) models that admit general-relativity solutions as a limit and does not impose a differential relation such as f''(R)=0. The hidden symmetries (Killing and Killing-Yano tensors) and the gyromagnetic ratio g=2 are geometric properties of the metric and the electromagnetic field; they therefore hold for any f(R) that admits the Kerr-Newman geometry. We will revise the abstract, introduction, and the metric/tensor sections to explicitly state the condition f(0)=0 and to confirm that the metric satisfies the f(R) field equations under this restriction. No change is required to the derivations themselves. revision: yes
Circularity Check
No significant circularity; g=2 follows from standard tensor construction on assumed metric
full rationale
The paper derives Killing and Killing-Yano tensors explicitly for the Kerr-Newman form and computes the gyromagnetic ratio from asymptotic charges, yielding g=2 by the same algebraic steps used in GR. No self-definitional loop, no fitted parameter renamed as prediction, and no load-bearing self-citation chain is visible in the abstract or described procedure. The result is an independent calculation once the metric is granted; any question of whether that metric solves the f(R) equations is an applicability issue, not a reduction of the derivation to its own inputs.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Killing and Killing-Yano tensors exist and can be constructed by the same algebraic procedure used in general relativity
- domain assumption The spacetime metric retains the Kerr-Newman form in f(R) gravity
Reference graph
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dt− asin 2 θ Ξ dϕ #2 + Σ ∆r dr2 + Σ ∆θ dθ2 + ∆θ sin2 θ Σ
also found the same value for four-dimensional rotation charged black hole solution in 3 heterotic string theory. [50] showed that the gyromagnetic ratio of four-dimensional black holes corresponding tog= 2 is a universal property. The gyromagnetic ratio for higher dimensional black holes is not universal as it is for four-dimensional black holes. While t...
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discussion (0)
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