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Extreme black holes probably have 'hair'

Artist's illustration showing a black hole emitting jets of fast-moving plasma above and below it, as matter swirls around in an orbiting disk.
Artist's illustration showing a black hole emitting jets of fast-moving plasma above and below it, as matter swirls around in an orbiting disk. (Image credit: NASA's Goddard Space Flight Center)

Black holes may not be so simple after all.

According to a leading idea known as the "no hair" or "black hole uniqueness" theorem, black holes can be fully characterized using just three data points — their mass, spin and electric charge. There's no other observable information to be had about these light-gobbling behemoths, which therefore seem to be sleekly and uniquely "bald."

But a new study casts doubt on the no-hair idea, or at least its universal application: Computer simulations suggest that "extreme" black holes — the ones whose spin or electrical charge is fully maxed out — do sport a few wispy hairs here and there.

Related: 10 huge black hole findings of 2020

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"This new result is surprising, because the black hole uniqueness theorems are well established, [as is] their extension to extreme black holes," lead author Lior Burko, of Theiss Research in La Jolla, California, said in a statement. "There has to be an assumption of the theorems that are not satisfied to explain how the theorems do not apply in this case."

The inferred black-hole hairs manifest as "a quantity that can be constructed from the space-time curvature at the black hole horizon that is conserved, and measurable by a distant observer," according to the statement. This quantity depends on details of the black hole's formation, so it goes beyond the bare-bones trio of mass, spin and charge.

Excitingly, such hairs may not remain purely theoretical forever. Scientists might be able to spot them using gravitational-wave detectors such as the Laser Interferometer Gravitational-Wave Observatory, according to the new study, which was published online Tuesday (Jan. 26) in the journal Physical Review D.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

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