  Kerr black holes are among the most intriguing predictions of Einstein's
general relativity theory. These rotating massive astrophysical objects drag
and intermix their surrounding space and time, deflecting and phase-modifying
light emitted nearby them. We have found that this leads to a new relativistic
effect that imposes orbital angular momentum onto such light. Numerical
experiments, based on the integration of the null geodesic equations of light
from orbiting point-like sources in the Kerr black hole equatorial plane to an
asymptotic observer, indeed identify the phase change and wavefront warping and
predict the associated light-beam orbital angular momentum spectra. Setting up
the best existing telescopes properly, it should be possible to detect and
measure this twisted light, thus allowing a direct observational demonstration
of the existence of rotating black holes. Since non-rotating objects are more
an exception than a rule in the Universe, our findings are of fundamental
importance.
