Scientists may now have found a solution to Stephen Hawking’s famous black hole paradox, which has puzzled experts since the 1970s.

A team of researchers from the University of Sussex, the University of Bologna, and Michigan State University have published two studies that propose that black holes exhibit something called “quantum hair,” which allows them to break out of this decades-old conundrum that poses potential inconsistencies between them showed Einstein’s general theory of relativity and quantum mechanics.

This new work attempts to better integrate these two systems by using new mathematical formulas developed by researchers over the past decade. Should the notion of “quantum hairs” hold true, it would be a significant finding for theoretical physics, while removing the need to radically rethink how we see the universe — at least for now.

“It was widely believed in the scientific community that resolving this paradox would require a major paradigm shift in physics, forcing the potential reformulation of either quantum mechanics or general relativity,” said Xavier Calmet, Professor of Theoretical Physics at the University of Sussex , in a statement. “What we have found – and I find it particularly exciting – is that this is not necessary.”

## “Hairy” black holes

According to the laws of quantum mechanics, information that exists in our universe cannot be destroyed, and this preservation of “quantum information” is fundamental to the universe. However, black holes pose a challenge to these laws because black holes are regions of spacetime where gravity is so strong that nothing – not even light – can escape them. So where does the information sucked into these (supposedly) inevitable black holes go? This question is essentially at the heart of Hawking’s black hole information paradox.

The researchers’ first publication, titled “Quantum Hair from Gravity,” was recently published in the journal *Physical Verification Letters*addresses part of this question by showing that there are in fact more black holes than previously assumed in classical physics.

Rather than just being simple objects with a given mass, velocity, and rotation, as defined in classical physics’ so-called “no hair theorem,” the team’s new findings suggest that black holes are actually more complex and “hairier” than the general theory of relativity could imagine.

That’s because when a collapsing black hole sucks matter in, it leaves an almost imperceptible imprint – a “quantum hair” – in its gravitational field. It is this quantum imprint that is the mechanism for holding information at the quantum level.

The team used their calculations to compare two theoretical stars that form from different initial chemical compositions, which then collapse into two black holes with the same mass and radii. In terms of classical physics, it would be considered impossible to go back in time to distinguish between the two stars, given the similar final states of the two black holes.

However, the team’s new calculations show that while these two black holes might appear the same at the macroscopic level, they would exhibit subtle differences in their gravitational fields at the microscopic quantum level. Information about what black holes were originally made of is stored in gravitons, a hypothetical elementary particle that acts as an intermediary between the gravitational forces operating in the realm of quantum gravity.

According to the team, it was quantum gravity that allowed them to detect these discrepancies in the gravitational field — and create a kind of “memory” in the gravitational field of the black hole’s initial state.

“It turns out that black holes are actually good children, preserving the memory of the stars that gave birth to them,” Calmet said.

## Entangled “Quantum Hairs”

The researchers’ second follow-up paper, published separately in *physics letters* *B*, demonstrates how Hawking’s black hole information paradox is solved by this “quantum hair” mechanism. The team’s findings show that classical physics’ previous notions about the inevitable event horizon of black holes are more complicated when examined more closely under the lens of quantum mechanics.

At the quantum level, there are intricate entanglements between the matter inside the black hole and the state of the gravitons outside the black hole. It is this subtle quantum entanglement that makes it possible to ‘encode’ quantum information in the thermal radiation (also known as Hawking radiation) emitted by the event horizons of such black holes. Thus, it is shown that quantum information is preserved even when a black hole collapses, since the Hawking radiation from a black hole is entangled with the quantum state of spacetime itself.

However, the team is currently unable to empirically test their theory using our current astronomical technology, as such minute gravitational fluctuations would elude the tools now available. Nonetheless, the team’s findings represent a more consistent way to perform calculations for black holes without having to reinvent both classical and quantum physics.

Read more in the team’s articles: Quantum Hair from Gravity and Quantum Hair and Black Hole Information.

Image: Aman Pal via Unsplash.