A team of researchers from the University of Sussex, in the United Kingdom, and Michigan State University, in the United States, seem to have solved the famous paradox of the ‘hair’ of black holes raised by Stephen Hawking, a mystery that has baffled researchers. scientists for almost fifty years.

The answer, the researchers write in two papers just published in Physical Review Letters and in Physics Letters B, could lie in a kind of ‘mark’ left in a black hole’s gravitational field by matter falling into it. him, something called ‘quantum hair’. Black holes appear to be more complex than originally thought and have gravitational fields that contain information about how they formed. And ‘quantum hair’ resolves the Hawking black hole information paradox.

In 1975, Stephen Hawking showed that black holes ‘are not so black’ and that in fact they slowly evaporate, emitting radiation that has taken the name of the great British physicist. A year later, Hawking suggested that as black holes evaporate, they forever destroy the information they contain, which goes against a fundamental law of quantum mechanics that states that any process in physics can be mathematically reversed.

Much earlier, in the 1960s, physicist John Archibald Wheeler, discussing black holes’ lack of observable characteristics beyond their total mass, spin, and charge, coined the phrase ‘black holes are hairless’, i.e. , no differential ‘physical trait’.

But the ‘quantum hair’ discovered in this study provides a way to preserve information when a black hole collapses, and in doing so solves one of modern science’s most famous dilemmas.

“For a long time – explains Xavier Calmet, lead author of both studies – black holes have been considered the perfect laboratory to study how to merge Einstein’s general theory of relativity with quantum mechanics. In general, the scientific community assumes that resolving this paradox would require a major paradigm shift in physics, forcing the potential reformulation of quantum mechanics or general relativity. But what we’ve found, and I think this is particularly exciting, is that that won’t be necessary.”

Black holes are formed by the collapse of compact objects and then, according to quantum theory, there is no absolute separation between the inside and outside of the black hole. “In the classical theory -explains Roberto Casadio, co-author of the studies-, the event horizon acts as a perfect unidirectional membrane that does not let anything out and, therefore, the exterior is the same for all black holes. of a given mass. This is the classic hairless theorem.”

“However -concludes the researcher-, in quantum theory, the state of matter that collapses and forms the black hole continues to affect the state of the outside, although in a way that is compatible with current experimental limits. This is what is known as ‘quantum hair’.”