Don’t tell Einstein, but black holes might have ‘hair’


Identical twins have nothing about black holes. Twins can grow up from the same genetic patterns, 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, spin 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 the same,” said Paul Chesler, theoretical physicist at Harvard University. “You can’t tell the difference.”

Yet 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 could have instabilities on their event horizons. These instabilities would effectively give certain regions of a black hole’s horizon a stronger gravitational pull than others. This would make it possible to distinguish black holes that are otherwise identical.

However, his equations only showed that this was possible for so-called extremal black holes – those with a maximum possible value for their mass, spin, or charge. 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, which approached those 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 Hairless Theorem?

A article published at the end of last month shows he could. In addition, these hairs could be detected by gravitational wave observatories.

“Aretakis basically suggested that there was information that remained 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, scientists suggest that remnants of black hole formation or subsequent disturbances, such as matter falling into the black hole, could create gravitational instabilities on or near a nearly black hole’s event horizon. extreme. “We would expect the gravitational signal that we are seeing to be very different from ordinary black holes which are not extremal,” Khanna said.

If black holes have hair – thus retaining information about their past – it could have implications for the famous black hole information paradox presented by the late physicist Stephen Hawking, said Lia Medeiros, 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 do not respect any of the assumptions [of the information paradox], you may be able to resolve the paradox itself, ”Medeiros said. “One of the hypotheses is the Hairless Theorem.”

The ramifications of this could be vast. “If we can prove that the real space-time 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, co-author an article in October which aimed to determine whether the observed geometry of black holes was as expected.

Perhaps the most exciting aspect of this latest article, however, is that it could provide a way to merge black hole observations with fundamental physics. Detecting hair on black holes – perhaps the most extreme astrophysical labs in the universe – could allow us to probe ideas like string theory and quantum gravity in ways that never have been possible before.

“One of the big problems with string theory and quantum gravity is that it’s really hard to test these predictions,” Medeiros said. “So if you have something that is even remotely testable, that’s amazing.”

There are, however, major obstacles. It is not certain that there are near extreme black holes. (The best simulations at the moment typically produce black holes that are 30% far from extreme, Chesler said.) And even if they do, it’s not clear whether gravitational wave detectors would be enough. sensitive to detect these hair instabilities.

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