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Black holes with 鈥渉air鈥� will quickly go bald. A key theoretical prediction about black holes called the no-hair theorem states that an isolated black hole can be described by just three numbers 鈥� its mass, spin and charge 鈥� and any other properties, or 鈥渉air鈥�, are irrelevant. Now, a set of detailed simulations has shown how black holes can shed a magnetic field to comply with the no-hair theorem.

When a black hole forms from a magnetised star, it is born with a magnetic field. How long that magnetic field sticks around is an open question, and some previous work has suggested that the plasma surrounding black holes may keep the magnetic field around for longer than expected, which would be a violation of the no-hair theorem. at Columbia University in New York and his colleagues used a set of detailed computer simulations to solve this problem.

鈥淚n the magnetic domain, this is the most realistic test of the no-hair theorem to date,鈥� says at Trinity College Dublin, Ireland. The researchers simulated a rotating black hole surrounded by magnetised plasma, and found that the lines of the magnetic field converged at the black hole鈥檚 equator and turned into closed loops in a process called magnetic reconnection.

These magnetic loops filled with plasma dissipated the magnetic field that had previously permeated the black hole, freeing up space at the equator for more field lines to converge. 鈥淔ield lines keep flowing in and making these loops, and that just keeps happening until they鈥檙e all gone,鈥� says Bransgrove. 鈥淪ome of these loops fly away into space, and some fall into the black hole.鈥�

This process happened relatively quickly, allowing the black hole to follow the no-hair theorem. 鈥淥f course the original no-hair theorem is for a black hole in a vacuum, but we鈥檙e trying to move that into something more realistic,鈥� says Bransgrove. For now, it seems that the theorem works even for black holes that are surrounded by plasma, as many are in the real world.

This result isn鈥檛 particularly surprising, says at the University of Lisbon in Portugal. 鈥淲e had predictions for practically everything that was reported now 鈥� but those predictions were back-of-the-envelope estimates, and we all know that the devil hides in the details,鈥� he says. 鈥淣ow, this work worked out the details, [and] there was no devil, which is reassuring!鈥�

However, it does help us understand the magnetic behaviour of black holes, he says. Understanding this process of shedding magnetic fields could help researchers find and identify newly formed black holes

鈥淲e can鈥檛 say for sure yet whether we see this in observations, but there are hints,鈥� says Bransgrove. Some huge black holes send out powerful flares of X-rays that may be powered by magnetic reconnection, but astronomers haven鈥檛 yet spotted the magnetic loops full of plasma that these simulations suggest. Observing the X-rays coming from the hot plasma surrounding the biggest black holes may allow us to confirm that this process is taking place, moving the no-hair theorem one step closer to describing the real world.

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