An illustration of an active black hole, one that accretes matter and accelerates a portion of it outwards in two perpendicular jets. The normal matter undergoing an acceleration like this describes how quasars work extremely well. All known, well-measured black holes have enormous rotation rates, and the laws of physics all but ensure that this is mandatory. (Credit: University of Warwick/Mark A. Garlick)

Why black holes spin at nearly the speed of light

Black holes aren’t just the densest masses in the Universe, but they also spin the fastest of all massive objects. Here’s why it must be so.

Whenever you take a look out there at the vast abyss of the deep Universe, it’s the points of light that stand out the most: stars and galaxies. While the majority of the light that you’ll first notice does indeed come from stars, a deeper look, going far beyond the visible portion of the electromagnetic spectrum, shows that there’s much more out there. The brightest, most massive stars, by their very nature, have the shortest lifespans, as they burn through their fuel far more quickly than their lower-mass counterparts. Once they’ve reached their limits and can fuse elements no further, they reach the end of their lives and become stellar corpses.

These corpses come in multiple varieties: white dwarfs for the lowest-mass (e.g., Sun-like) stars, neutron stars for the next tier up, and black holes for the most massive stars of all. These compact objects give off electromagnetic emissions spanning wavelengths from radio to X-ray light, revealing properties that range from mundane to absolutely shocking. While most stars themselves may spin relatively slowly, black holes rotate at nearly the speed of light…