Identical twins have nothing on black holes. Twins can 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 properties – mass, rotation and charge. If these 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 not see the difference.”
Yet scientists have begun to wonder if the ‘statement without hair’ is strictly 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 may have instability in their horizons. These instabilities will effectively give some regions of the horizon of a black hole a stronger attraction than others. This would otherwise make identical black holes distinguishable.
However, his comparisons only showed that it was possible for so-called extreme black holes – those that have the maximum value for their mass, turn or charge. And as far as we know, “these black holes, at least exactly, cannot exist in nature,” Chesler said.
But what if you have an almost extreme black hole, one that approaches these extreme values but has not quite achieved them? At least in theory such a black hole should be able to exist. Could it have observable violations of the statement without hair?
From an article published late last month, it appears that it may. In addition, these hairs can be detected by gravity wave observations.
“Aretakis basically suggested that information be 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 paper offers the possibility to measure this hair.”
In particular, the scientists suggest that remnants of the formation of the black hole or of later disturbances, such as matter falling into the black hole, can cause gravitational instabilities on or near the event horizon of an almost extreme black hole. “We would expect the gravitational signal we would see to be completely different from ordinary black holes that are not extreme,” Khanna said.
If black holes do have hair – and thus retain information about their past – it could have implications for the famous black hole information paradox 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 great pillars of 20th-century physics. ‘If you violate any of the assumptions [of the information paradox], you may be able to solve the paradox yourself, ”Medeiros said. “One of the assumptions is the statement without hair.”
The consequences can be far-reaching. “If we can prove that the actual space-time of the black hole outside the black hole differs from what we expect, then I think it’s going to have very big implications for general relativity,” Medeiros said. October in which the question of whether the observed geometry of black holes agrees with predictions.
Perhaps the most exciting aspect of this latest article, however, is that it can provide a way to combine observations of black holes with fundamental physics. If we detect hair on black holes – perhaps the most extreme astrophysical laboratories in the universe – we can explore ideas such as string theory and quantum gravity in a way that has never been possible before.
“One of the biggest issues with string theory and quantum gravity is that it’s really difficult to test the predictions,” Medeiros said. “So if you have something that can be tested even remotely, it’s great.”
However, there are major obstacles. It is not certain that there are almost extreme black holes. (The best simulations at the moment usually yield black holes that are 30 percent off extremity, Chesler said.) And even if they are, it is not clear whether gravity wave detectors would be sensitive enough to get this instability out of the hair. do not see.