The rotating spacetime around black holes behaves like a non-uniform lens, twisting the light that passes through. By measuring this twist — the orbital angular momentum of the radiation — we can estimate the black hole's electric charge. Applying this approach to M87* observations yielded an upper limit on the charge-to-mass ratio: no more than 0.39. Although astrophysical black holes are expected to be neutral, the technique can be applied to any type of 'charge' that defines different solutions to gravitational equations. Thus, the orbital angular momentum of light becomes a new tool for independently testing gravity theories in strong fields, complementing traditional shadow observations.
Light passing by a rotating black hole twists into a spiral, like water swirling around a drain. The black hole’s electric charge acts as an extra twist: the stronger it is, the tighter the beams coil. Astrophysicists have found a way to use this spiraling to measure the charge of a black hole — without direct contact, simply by analyzing the light’s twist. Previously, this phenomenon was only studied in labs; now it’s become a cosmic tool.
Researchers applied the method to observations of the galaxy M87, home to the famous hole M87*, whose shadow was first photographed in 2019. The analysis showed: the charge-to-mass ratio doesn’t exceed 0.39 — the object is almost electrically neutral, as expected. But the real breakthrough isn’t the number, it’s the method. Now curved spacetime can be probed through the degree of beam twisting, providing an independent check of strong gravity. In the future, this could allow searches for new fields or extra dimensions inaccessible by other means. A surprising twist: the same light-spiraling principle is used in quantum communications to pack multiple streams of information into a single beam.
🎯 M87* became famous in 2019 when astronomers captured its first image — that iconic orange ring. Its shadow is three times wider than Pluto’s orbit.
🎬 In science fiction, charged black holes are rare — writers and filmmakers are much more interested in their spin (as in ‘Interstellar’) or the possibility of travel through wormholes.