A black hole pulls in anything that gets too close. But if you toss in not one object but a pair, one of them can fly back out, carrying off some of the hole's rotational energy. This trick, devised by Roger Penrose in 1969, works only because a black hole has a horizon — a boundary beyond which nothing returns. Now it turns out the horizon isn't necessary.
Take a rapidly spinning blob of matter, star-like but with no solid surface. Around it lies a region where space itself is dragged along by the rotation, like water in a whirlpool. A particle entering that region can split into two: one gets negative energy and stays swirling inside, while the other escapes with more energy than the original had. Previously, it was thought such a feat required a horizon to 'swallow' the negative fragment. It turns out it's enough for that fragment to simply get stuck in the rotating region, like a splinter in a whirlpool.
The authors tested the idea on a concrete example — a so-called boson star. This is a cloud of ultralight particles held together by its own gravity, with no sharp boundary. Calculations showed that a particle can decay, one fragment gets trapped in the rotating region, and the other flies away with extra energy. And it escapes for sure if the star is symmetric about its equator.
So, extracting rotational energy doesn't require a black hole. A compact, rapidly spinning object without a horizon is enough. This changes our view of how active galactic nuclei and other sources of intense radiation work. Perhaps some of them are not black holes but something more exotic.
The fastest known pulsar spins more than 700 times per second: its surface moves at about a quarter of the speed of light.