# Measurement of the spin of the M87 black hole from its observed twisted light

## TL;DR

This study provides the first observational evidence that light near the M87 black hole is twisted, enabling direct measurement of the black hole's rotation through orbital angular momentum analysis of radio data.

## Contribution

It introduces a novel method to measure black hole spin by analyzing twisted light's orbital angular momentum from Event Horizon Telescope data.

## Key findings

- M87* rotates clockwise with a high spin parameter of 0.90
- The black hole's inclination is estimated at 17 degrees
- The analysis confirms black hole rotation within 6 sigma confidence level

## Abstract

We present the first observational evidence that light propagating near a rotating black hole is twisted in phase and carries orbital angular momentum (OAM). This physical observable allows a direct measurement of the rotation of the black hole. We extracted the OAM spectra from the radio intensity data collected by the Event Horizon Telescope from around the black hole M87* by using wavefront reconstruction and phase recovery techniques and from the visibility amplitude and phase maps. This method is robust and complementary to black-hole shadow circularity analyses. It shows that the M87* rotates clockwise with an estimated rotation parameter $a=0.90\pm0.05$ with $\sim 95\%$ confidence level (c.l.) and inclination $i=17^\circ \pm2^\circ$, equivalent to a magnetic arrested disk with inclination $i=163^\circ\pm2^\circ$. From our analysis we conclude, within a 6 $\sigma$ c.l., that the M87* is rotating.