In 1963, the mathematician Roy Kerr found a solution to Einstein’s equations that precisely described the spacetime outside what we now call a rotating black hole. (The term wouldn’t be coined for a few more years.) In the nearly six decades since his achievement, researchers have tried to show that these so-called Kerr black holes are stable. What that means, explained Jérémie Szeftel, a mathematician at Sorbonne University, “is that if I start with something that looks like a Kerr black hole and give it a little bump”—by throwing some gravitational waves at it, for instance—“what you expect, far into the
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