Neighboring papers on the arXiv investigate the shadow of M87*: how its shape depends on spin, charge, dark matter, and quantum corrections. All rely on the same observed image: a bright ring around a dark depression. This work stands apart: it looks not at the shadow but at the light that creates it. Instead of the silhouette's geometry, it analyzes the orbital angular momentum of the radiation, a quantity that until recently had not been measured in astrophysics. This is a first step toward an independent method for probing strong gravity, one not tied to the shadow's size.
The method is based on an analogy between rotating spacetime and an inhomogeneous anisotropic medium. The Kerr–Newman black hole is treated as an effective q-plate, an optical element that converts circular polarization of light into orbital angular momentum. The known formula for the average orbital angular momentum is generalized to include electric charge. Then the observed value of the average orbital angular momentum for M87*, previously obtained from Event Horizon Telescope data with a relative uncertainty of about 3%, is used. Requiring that the charge correction not exceed this uncertainty yields an inequality for the charge-to-mass ratio.
An upper limit Q/M ≲ 0.39 is obtained for the electric charge of M87*, assuming the black hole is described by the Kerr–Newman solution. This is stricter than the constraint Q/M ≲ 0.90 from shadow size analysis and comparable to a recent result Q/M ≲ 0.96 under different assumptions. The constraint is derived in a linear-in-spin approximation and under the idealized assumption that the black hole acts as a q-plate. Despite the simplicity of the model, the average orbital angular momentum is an observable quantity extracted from data via phase-retrieval methods, not a model-dependent inference.
This work differs from its arXiv neighbors: instead of the shadow's size or shape, it uses the orbital angular momentum of light. This is a different observational channel, sensitive to the same spacetime parameters but through the phase structure of the radiation rather than the geometry of its paths. If the method is confirmed by numerical simulations, it will provide an independent check of shadow-based constraints and help break degeneracies between charge, spin, and modifications of gravity. For now, the work falls within the general effort to test strong gravity, but it opens a new observational axis.
Numerical simulations of light propagation in a realistic accretion disk geometry could refine the connection between orbital angular momentum and black hole parameters. Including higher-order spin terms and dropping the q-plate analogy would make the constraints more robust. The method can be extended to other types of charges, such as scalar charge in theories with extra fields, or to non-singular solutions. Dynamical lensing effects and accretion in alternative theories of gravity may also introduce corrections to the orbital angular momentum spectrum.
The method could be useful for interpreting future observations by the Event Horizon Telescope and other interferometric arrays. It adds a new observable quantity to the toolkit for testing strong gravity.
Run numerical simulations of light propagation near a charged rotating black hole to test the analytical formula. Compare predictions with the full orbital angular momentum spectrum extracted from EHT data.
In black hole astrophysics, the question remains open as to how much real compact objects deviate from Kerr solutions. Electric charge, even if small, serves as a test parameter for the no-hair theorem. Orbital angular momentum provides a new way to search for such deviations, without relying solely on the shadow shape.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors used Event Horizon Telescope data on the twisted light from M87* to estimate the black hole's electric charge through an analogy with a q-plate. They generalized the formula for the average orbital angular momentum to the case of a charged Kerr-Newman black hole and, comparing with the observed value, obtained an upper limit of Q/M ≲ 0.39.
Where its strength is
This work opens a new observational avenue: the orbital angular momentum of light, which is sensitive to spacetime parameters through its phase structure rather than through the shadow geometry. This provides an independent channel for probing strong gravity and could potentially break degeneracies between charge, spin, and modifications to gravity.
Where it could go next
Worth trying
Why this may matter
If the method is confirmed, it will give astrophysicists a fundamentally new tool for testing strong gravity, independent of the shadow model. This could help resolve degeneracies between parameters and advance our understanding of the nature of supermassive black holes.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.57, against a usual 0.64. It looks like few have drilled nearby.
Light from M87* is twisted like a corkscrew: its wavefront has a helical structure due to the black hole's rotation. This effect was predicted back in 2019 and first measured in 2021.
In M. John Harrison's novel 'Light', twisted light is used to transmit information through curved space. Here, the same idea, light as a carrier of information about geometry, is applied to a real black hole.