  We describe and present the first observational evidence that light
propagating near a rotating black hole is twisted in phase and carries orbital
angular momentum. The novel use of this physical observable as an additional
tool for the previously known techniques of gravitational lensing allows us to
directly measure, for the first time, the spin parameter of a black hole. With
the additional information encoded in the orbital angular momentum, not only
can we reveal the actual rotation of the compact object, but we can also use
rotating black holes as probes to test General Relativity.
