A method is proposed to constrain the electric charge of the supermassive black hole M87* by analyzing the orbital angular momentum of its radiation. Using an analogy between rotating spacetimes and inhomogeneous optical media, a simple analytical formula links the average orbital angular momentum of observed light to the charge-to-mass ratio. Applying it to existing data gives an upper bound of Q/M ≲ 0.39 for M87*. The analysis focuses on electric charge as a theoretical example, since astrophysical black holes are presumed neutral, but the method is general and can be extended to constrain other types of charges — degrees of freedom that define different black hole solutions. The results highlight the potential of orbital angular momentum as a new fundamental observable, serving as an independent complement to shadow-based methods for probing the properties of rotating compact objects and testing gravity in the strong-field regime.
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.