Photon Ring Polarization Demonstrates Odd Kerr Spacetime Imprints, Enabling Degree-Scale Analysis

The swirling light around black holes holds a wealth of information about the extreme gravity and spacetime distortions they create, and scientists are developing new ways to decode these signals, offering a unique probe of fundamental physics. M. Baran Ökten from Sabancı University, along with colleagues, demonstrates that the subtle twisting of light polarisation around a black hole, specifically, a property called the Kerr EVPA, reveals crucial details about the black hole’s spin and orientation. This research establishes a new method for measuring these properties using observations of light’s polarisation, and importantly, the technique is inherently achromatic, meaning it is independent of wavelength, and protected against common observational errors. The team’s approach promises to unlock more precise measurements of black holes like those at the centre of our galaxy and the M87 galaxy, potentially confirming predictions of general relativity or revealing unexpected physics.

Gravitational Holonomy and Light Polarization Signatures

This research details a novel method for detecting and characterizing black holes by analyzing the polarization of light surrounding them. The study centers on the concept of gravitational holonomy, where extreme gravity twists the polarization of light in a predictable manner, creating a unique fingerprint that telescopes can potentially detect. Through numerical simulations, the team modeled light paths around black holes and calculated the resulting polarization patterns, identifying specific observational signatures for telescopes like the Event Horizon Telescope (EHT) to seek. The identified signatures include a characteristic pattern of polarization rotation around the black hole’s shadow, notable for being achromatic and possessing a specific symmetry property. This method offers a new way to study black holes, complementing existing methods like shadow imaging and gravitational wave detection, and could allow astronomers to independently measure a black hole’s mass and spin. Precise measurements of the polarization pattern could also provide a stringent test of Einstein’s theory of general relativity in the strong-gravity regime, potentially revealing new physics if deviations occur.

Rotating Black Hole Polarization via Geodesic Transport

Scientists have developed a technique to detect the gravitational imprint of rotating black holes on the polarization of light. The method simplifies the complex problem of parallel transport in curved spacetime by recasting it into a single scalar evolution law, enabling precise calculation of how light polarization rotates due to frame dragging, a key prediction of general relativity. The team validated their method through three independent calculations, confirming agreement at high precision. Numerical simulations demonstrate that the amplitude of this polarization signal grows consistently with both the black hole’s spin and the observer’s viewing angle.

To enhance detectability, scientists defined a new estimator, designed to isolate the geometric signal from other effects, which is achromatic after accounting for wavelength variations. This estimator compresses the signal into a few dominant modes, enabling a minimal template for analyzing horizon-scale rings in sources like M87 and Sagittarius A*. The research suggests that biases in the signal remain small for typical telescope configurations, and that combining data from multiple wavelengths further increases the signal-to-noise ratio.

Polarization Reveals Kerr Spacetime Spin and Tilt

Researchers achieved precise measurements of the gravitational imprint on the polarization of light around Kerr spacetime, focusing on the region near the photon ring. They recast the mathematical description of light paths within a specialized coordinate system, yielding a single equation governing how the polarization angle evolves. Analysis reveals a distinct pattern on the observer’s screen, strictly dependent on the black hole’s spin, appearing on a narrow annulus. Three independent methods confirm the results, validating the underlying theoretical framework and numerical techniques. The team developed a new estimator, designed to isolate the geometric signal from other effects, which is achromatic after accounting for wavelength variations.

This estimator compresses the signal into a few dominant modes, enabling a minimal template for analyzing horizon-scale rings in sources like M87 and Sagittarius A*. Measurements confirm that the signal-to-noise ratio can reach significant levels at high spin and inclination, allowing for precise determination of the black hole’s spin or inclination. This research delivers a powerful new tool for probing the strong-gravity regime around black holes.

Frame Dragging Reveals Light Polarization Signatures

This research demonstrates a new method for detecting the effects of strong gravity near rotating black holes through the measurement of linear polarization of light. Scientists have shown that the rotation of spacetime around a black hole, known as frame dragging, leaves a unique, predictable imprint on the polarization of light rays orbiting the event horizon. This effect is captured by a newly defined observable, which represents the accumulated rotation of polarization along the path of light. The team achieved a robust theoretical framework by independently verifying this observable through three distinct calculations, yielding consistent results.

Importantly, the resulting signal is demonstrably dependent on the black hole’s spin, effectively cancelling out even-parity contaminants commonly found in astronomical observations. Numerical simulations reveal that the magnitude of this polarization signal grows predictably with both the black hole’s spin and the observer’s viewing angle. This research culminates in a minimal template, based on spin and inclination, that can be used to analyze existing and future observations of black hole shadows.