Identical twins have nothing in the black holes. Twins can grow with the same genetic backgrounds, 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 these values are the same for any black hole, it is impossible to distinguish one twin from another. They say black holes have no hair.
“In classical general relativity, they would be exactly identical,” said Paul Chesler, a theoretical physicist at Harvard University. “Can’t differentiate.”
Still, scientists have begun to wonder if the “non-hair theorem” is strictly true. In 2012, a mathematician named Stefanos Aretakis — then at Cambridge University and now at the University of Toronto — suggested that some black holes could have instabilities in their event horizons. These instabilities would effectively give some regions of the horizon of a black hole a stronger gravitational pull than others. This would distinguish identical black holes.
However, their equations only showed that this was possible for so-called extreme black holes, which have a maximum possible value for their mass, rotation, or load. And as far as we know, “these black holes can’t exist, at least exactly, in nature,” Chesler said.
But, what if you had an almost extreme black hole, that would approach those extreme values but not reach them at all? This black hole should be able to exist, at least in theory. Could it have detectable violations of the non-hair theorem?
An article published late last month shows that yes. In addition, this hair could be detected by gravitational wave observatories.
“Aretakis basically suggested that there was some information 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 opens up the possibility of measuring this hair.”
In particular, scientists suggest that remnants of black hole formation or subsequent perturbations, 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 normal non-extreme black holes,” Khanna said.
If black holes have hair — so they retain some information about their past — this could have implications for the famous black hole information paradox proposed by the late physicist Stephen Hawking, said Lia Medeiros, an astrophysicist. Princeton Institute for Advanced Study, New 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 assumptions [of the information paradox], it is possible that you can solve the paradox itself “, said Medeiros. “One of the assumptions is the non-hair theorem.”
Its ramifications could be broad. “If we can show that the real space-time of the black hole outside the black hole is different than we expect, I think it will have really huge implications for general relativity,” said Medeiros, who co-authored a paper in The Month of ‘October was about whether the observed geometry of the black holes is consistent with the predictions.
Perhaps the most exciting aspect of this latest article, however, is that it could provide a way to combine the observations of black holes with fundamental physics. Detecting hair in black holes — perhaps the most extreme astrophysics laboratories in the universe — could allow us to probe ideas such as string theory and quantum gravity in a way that had never been done 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 remotely, it’s amazing.”
However, there are important obstacles. It is not certain that there are near-extreme black holes. (The best simulations right now typically produce black holes that are 30 percent from being extreme, Chesler said.) And even if they do, it’s unclear if gravitational wave detectors would be sensitive enough to detect them. these hair instabilities.