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We present the first observational evidence that light propagating near arotating black hole is twisted in phase and carries orbital angular momentum(OAM). This physical observable allows a direct measurement of the rotation ofthe black hole. We extracted the OAM spectra from the radio intensity datacollected by the Event Horizon Telescope from around the black hole M87* byusing wavefront reconstruction and phase recovery techniques and from thevisibility amplitude and phase maps. This method is robust and complementary toblack-hole shadow circularity analyses. It shows that the M87* rotatesclockwise 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 toa magnetic arrested disk with inclination $i=163^\circ\pm2^\circ$. From ouranalysis we conclude, within a 6 $\sigma$ c.l., that the M87* is rotating.

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