# Black holes can have “hair”. Is Einstein wrong?

According to Einstein’s general theory of relativity, **black holes have only three observable properties: mass, spin (angular momentum) and charge**… Additional characteristics, or, as physicists call them, “hair”, do not exist.

To explain the idea, imagine identical twins. They have the same genotype, they are genetic copies, but even such twins will differ in many things: from temperament to hair style. 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 any two black holes, then they are identical, it will be impossible to distinguish one from the other. Black holes have no hair.

“According to classical general relativity, such black holes would be absolutely identical,” notes Paul Chesler, a theoretical physicist from Harvard University.

However, scientists are wondering if the no-hair theorem is true. In 2012 mathematician Stefanos Aretakis, then at Cambridge University and now at the University of Toronto, suggested that some black holes may have instabilities (instabilities) on the event horizon.

Instabilities would give some parts of the black hole’s horizon a stronger gravitational pull than others. It turns out that in this case even identical black holes will be distinguishable…

However, the Aretakis equations showed that this is possible only for the so-called **extreme black holes** – those that have the maximum possible value for mass, spin or charge. And, according to Chesler, such black holes cannot exist in nature.

But let’s say there is an almost extreme black hole that approaches the maximum values, but does not reach them. Such a black hole could exist, at least in theory. Would that disprove the no hair theorem?

IN reportpublished in late January shows that this is possible.

Moreover, terrestrial gravitational wave detectors can pick up such hair.

“Aretakis suggested that there is some information that remains on the horizon,” commented Gaurav Hanna, a physicist at the University of Massachusetts and the University of Rhode Island, one of the co-authors of the study.

Scientists speculate that evidence of black hole formation or later event horizon disturbances (such as material falling into a black hole) may create gravitational instability at or near the near-extreme black hole’s event horizon.

“We assume that the gravitational signal that we detect will be very different from ordinary black holes, which are not extreme,” says Hanna.

If black holes have hair, then some information about their past is stored, and this will also affect the famous **black hole information paradox**which was formulated by Stephen Hawking, as noted Leah Medeiros, an astrophysicist at the Institute for Advanced Study at Princeton.

This paradox reveals a fundamental conflict between general relativity and quantum mechanics, the two pillars of 20th century physics.

If we refute one of the conditions of the information paradox, we can solve the paradox itself. One of the conditions is the no hair theorem.

The implications of this discovery will be significant. “If we can prove that the real spacetime of a black hole outside the black hole is different from what we expect to see, then I think it will make a really huge difference to general relativity,” said Medeiros, co-author of the October report, which focuses on whether the observed geometry of black holes matches the assumptions.

However, perhaps the most exciting part of the study is that it opens a way for how to integrate black hole observations and fundamental physics. Finding hairs on black holes in perhaps the most extreme astrophysical laboratories in the universe could allow ideas such as string theory and quantum gravity to be explored in ways that have never been possible before.

It turns out that Einstein’s equations are so complex that we discover new properties every year.

Paul Chesler

“One of the big problems with string theory and quantum gravity is that these assumptions are difficult to test,” Medeiros says, “so if we have something that can be verified even remotely, it’s amazing.”

However, there are also serious obstacles. There is no certainty about the existence of nearly extreme black holes. According to Chesler, the best models at the moment tend to form black holes that are 30% different from extreme values. And even if near-extreme holes do exist, it’s not entirely clear if gravitational wave detectors are sensitive enough to detect instability in hair.

Moreover, hair is assumed to be extremely fleeting, lasting a fraction of a second.

But the report itself looks solid. “I don’t think anyone in the community doubts that,” Chesler said.

The next step is to see what signals we will detect using gravitational wave detectors: we are currently working with LIGO and Virgo, but new tools are being launched, for example, LISA, joint experiment of the European Space Agency and NASA on the study of gravitational waves.

“Now we should rely on their work and really calculate what the frequency of gravitational radiation will be. It is important to understand how we can measure and identify it, ”notes Helvi Vitek, astrophysicist at the University of Illinois, Urbana-Champaign.

Although the chances of finding hair are not so great, such a discovery would cast doubt on Einstein’s theory of general relativity and prove the existence of near-extreme black holes.

“We would like to know if nature allows such a beast to exist,” says Hannah.