Identical twins have nothing on the black holes. Gemini can grow from the same genetic background, but they can differ in a thousand ways – from temperament to hairstyle. Black holes, according to Albert Einstein’s theory of gravity, can have only three characteristics – mass, rotation and load. If those values are the same for any two black holes, it is impossible to distinguish one twin from the other. It is said that black holes have no hair.
“In classical general relativity, they would be exactly the same,” said Paul Chesler, a theoretical physicist at Harvard University. “You can’t tell the difference.”
However, scientists have begun to question whether the “hairless theorem” 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 instabilities in their event horizons. These instabilities would effectively give some regions of the horizon of a black hole a stronger gravitational attraction than others. This would differentiate otherwise identical black holes.
However, his equations only showed that this was possible for so-called extreme black holes – those that have a maximum possible value for either mass, rotation or loading. And as far as we know, “these black holes cannot exist, at least exactly, in nature,” Chesler said.
But what if you had an almost extreme black hole, one that came close to these extreme values, but didn’t really touch them? Such a black hole should be able to exist, at least in theory. Could he have detectable violations of the hairless theorem?
A paper published at the end of last month shows that it could. Moreover, this hair could be detected by gravitational wave observatories.
“Aretakis practically suggested that some information was 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 newspaper opens up the possibility of measuring this hair.”
In particular, scientists suggest that the remnants of black hole formation or subsequent disturbances, such as matter falling into the black hole, could create gravitational instabilities on the event horizon of an almost extreme black hole. “We would expect the gravitational signal we would see to be quite different from ordinary non-extreme black holes,” Khanna said.
If black holes have hair – keeping some information about their past – this could have implications for the famous black hole information paradox presented by the late physicist Stephen Hawking, said Lia Medeiros, an astrophysicist at the Institute for Advanced Study in Princeton, New York. Jersey. This paradox distills the fundamental conflict between general relativity and quantum mechanics, the two great pillars of twentieth-century physics. “If you violate one of the hypotheses [of the information paradox], you may be able to solve the paradox itself “, said Medeiros. “One of the hypotheses is the hairless theorem.”
The ramifications of this could be wide. “If we can prove that the real time-space of the black hole outside the black hole is different from what we expect, then I think it will have really huge implications for general relativity,” said Medeiros, who co-authored a paper in October. who addressed whether the observed geometry of black holes is consistent with predictions.
Perhaps the most interesting aspect of this latest work is that it could provide a way to combine the observations of black holes with fundamental physics. Detecting hair on black holes – perhaps the most extreme astrophysics laboratories in the universe – could allow us to research ideas such as 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 very difficult to test these predictions,” Medeiros said. “So if you have something that can be tested even remotely, it’s amazing.”
However, there are major obstacles. It is not certain that there are almost extreme black holes. (The best simulations at the moment usually produce black holes that are 30 percent away from being extreme, Chesler said.) And even if they do, it’s unclear whether gravitational wave detectors would be sensitive enough to detect these instabilities on the hair.