Having identical twins nothing about black holes. Gemini may grow from the same genetic blueprints, but they can differ in a thousand ways – from temperament to hairstyle. According to Albert Einstein’s theory of gravity, black holes can have only three characteristics: mass, spin, and charge. If those values are the same for two black holes, it is impossible to distinguish one twin from the other. Black holes, they say, have no hair.
“In classical general relativity, they would be exactly identical,” said Paul Chesler, a theoretical physicist at Harvard University. “You can’t tell the difference.”
Still, scientists are starting to question whether the “no-hair theorem” is true. In 2012, a mathematician named Stefanos Aretakis – then at the University of Cambridge and now at the University of Toronto – suggested that some black holes could have instability on their event horizon. In fact, these instabilities would make some parts of a black hole’s horizon more attractive than others. That could distinguish otherwise identical black holes.
However, his equations only showed that this was possible for so-called extreme black holes – which have a maximum value for either their mass, spin or charge. And for all we know, “these black holes can’t exist, at least exactly, in nature,” Chesler said.
But what if you had a near-extreme black hole, one that approached these extreme values but didn’t quite reach them? Such a black hole should be able to exist, at least in theory. Could there be detectable violations of the theorem without her?
An article published late last month shows it could be. In addition, this hair could be detected by gravitational wave observatories.
“Aretakis actually suggested that some information had been left on the horizon,” said Gaurav Khanna, a physicist at the University of Massachusetts and the University of Rhode Island and one of the co-authors. “Our article opens up the possibility of measuring this hair.”
In particular, the scientists suggest that remnants of the black hole’s formation or later perturbations, such as matter falling into the black hole, may cause gravitational instabilities on or near the event horizon of a near-extreme black hole. “We would expect that the gravitational signal we would see would be very different from regular black holes that are not extreme,” Khanna said.
If black holes have hair – and thus retain some information about their past – it could have ramifications for the famous black hole information paradox put forward by late physicist Stephen Hawking, said Lia Medeiros, an astrophysicist at the Institute for Advanced. Study in Princeton, New Jersey. This paradox distills the fundamental conflict between general relativity and quantum mechanics, the two major pillars of 20th-century physics. ‘If you violate any of the assumptions [of the information paradox], you might be able to solve the paradox yourself, ”said Medeiros. “One of the assumptions is the theorem without hair.”
The consequences could be wide-ranging. “If we can prove that the actual space-time of the black hole outside the black hole is different from what we expect, then I think that will have really big implications for general relativity,” said Medeiros, who co-authored an article In October, it was discussed whether the observed geometry of black holes is consistent with predictions.
Perhaps the most exciting thing about this latest paper is that it could provide a way to merge black hole observations with basic physics. By detecting it on black holes – arguably the most extreme astrophysical labs in the universe – we can explore ideas like string theory and quantum gravity in a way that has never been possible before.
“One of the big problems with string theory and quantum gravity is that it’s really hard to test those predictions,” said Medeiros. “So if you have something that can even be tested remotely, that’s great.”
However, there are major hurdles. It is not certain that near-extreme black holes exist. (The best simulations right now typically produce black holes that are 30 percent away from extremity, Chesler said.) And even if they do, it’s not clear whether gravitational wave detectors are sensitive enough to spot these instabilities in the hair.