How Polarization of Light Reveals the Spin of Black Holes ✛ ✎ ✦
Context
The spin of a black hole is a dimensionless parameter that, along with mass, fully describes an isolated stationary black hole within the framework of general relativity. In astrophysical systems, spin affects how matter moves near the event horizon, the efficiency of energy extraction through the accretion disk, and the formation of powerful relativistic jets. Because angular momentum accumulates during accretion and mergers, the distribution of spins carries information about galaxy evolution. However, spin cannot be measured directly — it manifests indirectly through its influence on magnetized plasma and radiation in a strong gravitational field.
Methods
To identify polarization signatures of spin, the authors used numerical simulations of general relativistic magnetohydrodynamics (GRMHD) and polarized radiative transfer, comparing the results with data from radio interferometry with very long baseline (VLBI) by the Event Horizon Telescope (EHT). A key tool was the decomposition of complex linear polarization into azimuthal harmonics $β_m$, introduced in the context of studying energy extraction by R. Penrose. Spin influences the observed polarization through three channels: photon propagation in the Kerr metric, boundary conditions at the horizon, and the dynamics of magnetized plasma. The simulations made it possible to separate the effects of gravitational lensing from those of the accretion flow.
Results
Analysis of polarization images of M87* and Sagittarius A* obtained by the EHT in 2017–2021 showed that both sources have ordered magnetic fields and are in a magnetically arrested disk (MAD) state, where magnetic pressure dominates. For M87*, the resolved linear polarization reaches 15%, and the electric vector position angle (EVPA) is close to azimuthal, corresponding to a $β_2$ parameter with a phase of about –160°. However, the spin information is complex: different models predict different a* values for the same data. For example, the $β_2$ phase in the force-free approximation is related to the ratio of toroidal to radial magnetic field components, but accounting for plasma inertia and Faraday rotation shifts the estimates. For Sgr A*, polarization loops on the Q–U diagram indicate clockwise rotation, and rapid variability requires careful treatment of Faraday effects. Overall, polarimetry most reliably constrains the magnetic flux and orbital inclination, rather than the magnitude or sign of the spin.
Implications
These results deepen the understanding of the physics of accretion and jets in strong gravitational fields. While directly measuring spin remains challenging, polarimetry enables mapping of magnetic fields near the horizon, which is crucial for testing energy extraction mechanisms like the Blandford–Znajek process and for building a unified picture of jet formation. The consistency of data from horizon to jet scales in M87* supports a model with an ordered magnetic field connecting the black hole to the large-scale outflow.
Future development
In the future, space-based VLBI missions such as the Black Hole Explorer (BHEX) will play a key role, providing ultra-long baselines to resolve the photon ring and sub-rings. This will allow direct testing of general relativity in the strong gravity regime. Multi-wavelength polarimetry at 86, 230, and 345 GHz will help separate the source contributions from propagation effects, and dynamic imaging and variability analysis will distinguish turbulent fluctuations from persistent geometric signatures.
Impact
The results will impact black hole astrophysics, accretion physics, and gravitational-wave astronomy, since black hole spin is a key parameter for interpreting mergers detected by LIGO and Virgo. Moreover, polarimetric diagnostic methods can be applied to other classes of objects, such as blazars and quasars, to study their jet structure.
Next steps
Next steps include expanding the libraries of numerical simulations with variation of all parameters, including inclination, electron thermodynamics, and positron content, as well as performing joint Bayesian analysis of polarization data from the EHT, ALMA, and future instruments. A key step will be testing consistency between diagnostics sensitive to different aspects of the system.
Key open problems
The article is directly connected to unsolved problems in physics: the Blandford–Znajek mechanism and rotational energy extraction from black holes, the nature of magnetically arrested accretion, the formation and collimation of relativistic jets, and the origin of magnetic fields in accretion flows. Polarimetric constraints on spin will help advance our understanding of these issues.
The knowledge machine's view
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 paper analyzes polarization images of M87* and Sgr A* from EHT data to isolate signatures of black hole spin. Using a decomposition of complex linear polarization into azimuthal harmonics and GRMHD simulations, the authors separated the effects of the Kerr metric from the influence of magnetized plasma. They show that polarimetry constrains the magnetic flux and orbital inclination more reliably than the spin itself.
Where its strength is
The strength of the work lies in its methodological separation of contributions from gravitational lensing, horizon boundary conditions, and plasma dynamics to polarization signatures. This provides a basis for cleaner extraction of spin effects in future observations.
Where it could go next
- It might be interesting to see how the polarization pattern changes when considering alternative metrics beyond the Kerr solution. Nearby work uses the black hole shadow to test modified gravity.based on: A Black Hole's Shadow as a Mirror of Unknown Gravity arXiv:2606.15727
- It could be productive to compare the obtained constraints on magnetic flux and spin with models of rotational energy extraction via the Blandford–Znajek mechanism. Our archive has an analysis of how black holes release energy back into space.based on: How Black Holes Spit Out Energy arXiv:2607.28919v1
- It would be interesting to test how sensitive the $$\beta_m$$ harmonics are to the dark matter distribution in the galactic center, which could affect the effective spin. A neighboring work examines how dark matter 'spins up' black holes.based on: Dark Matter Spins Black Holes arXiv:2606.15381
Worth trying
- Apply harmonic decomposition to synthetic black hole images in theories with non-minimal coupling to identify characteristic phase shifts.
- Combine the analysis of Sgr A* polarization loops with estimates of jet energy flux from multi-year EHT data to test the correlation between spin and jet power.
- Use Bayesian inference to jointly constrain spin and dark matter profile from polarimetric and dynamical data.
Why this may matter
If polarimetry can be turned into a reliable spin indicator, it would open the way to measuring the angular momenta of supermassive black holes on a large scale, and consequently to reconstructing their growth history and feedback with galaxies.
Nearby works in our archive
- Black Hole Spin Paints a Polarization Patternsimilarity 0.69
- How a Black Hole Warps Timesimilarity 0.69
- A Black Hole's Shadow as a Mirror of Unknown Gravitysimilarity 0.67
- A Black Hole in Polarized Glassessimilarity 0.67
- A Black Hole Remembers Only Its Spinsimilarity 0.67
- The Shadow of a Black Hole: How Bent Light Paints a Portrait of the Invisiblesimilarity 0.66
- How Black Holes Spit Out Energysimilarity 0.66
- Dark Matter Spins Black Holessimilarity 0.66
The closest work in the archive sits at 0.69 by meaning — the usual level for our corpus.
Interestingly, Sagittarius A*, the supermassive black hole at the center of our Milky Way, evolves about 1500 times faster than M87* because its mass is 1500 times smaller. Therefore, its polarization pattern changes over minutes, while M87* requires hours and days.
In Christopher Nolan's film Interstellar (2014), the rotating black hole Gargantua takes center stage, visualized with gravitational lensing and frame-dragging effects, which directly echoes the theme of the article: it is the black hole's spin that creates a 'river' of spacetime, dragging the planet Miller along.
Key numbers
- EHT angular resolution at 230 GHz: ~20 microarcseconds
- M87* ring diameter: 43.3 microarcseconds
- Sgr A* ring diameter: 51.8 microarcseconds
- Peak linear polarization of M87* in 2017: ~15%
- Dimensionless spin parameter a*: from –1 to 1