# Stephen Hawking's famous black hole paradox may finally be solved

Black holes are the remnants of massive stars that have reached the end of their life cycles and collapsed into a region of spacetime that is incredibly dense. They consist of masses of matter packed so tight that nothing can escape.

About half a century ago, legendary physicist Stephen Hawking discovered that black holes function in a way that would change the wave-like nature of surrounding quantum fields, allowing heat radiation to be produced. This discovery signaled that black holes would gradually radiate energy, shrink at an increasing rate, and then disappear.

The fact that everything that falls into these celestial objects would go away, however, conflicted with a rule in quantum physics that states that the qualities that comprise a particle are conserved in the universe forever. This was Hawking's paradox, a.k.a. the black hole information paradox.

How can something exist forever but also disappear? This was the puzzle Hawking first began to ponder in the early 1970s.

## Puzzle solved?

Now, some scientists think they may have solved this paradox by discovering that black holes have a property that they refer to as "quantum hair," in a theory they refer to as the "yes hair theorem." Their research was published on Wednesday in the journal *Physical Review Letters*.

"The problem has been cracked!" exclaimed professor Xavier Calmet of the University of Sussex to the BBC.

The theory of quantum hair proposes that information about what goes into a black hole can come out again, without violating any of the important principles of general relativity and quantum mechanics.

"But it is going to take some time for people to accept it," Calmet said. "Hawking came up with the paradox in the year that I was born."

If the quantum hair theory proves to be true, it could finally connect the theories of relativity which relate to gravity and quantum mechanics. "One of the consequences of the Hawking paradox was that general relativity and quantum mechanics were incompatible. What we are finding is that they are very much compatible," concluded Calmet.

Will the new theory be able to stand up to scrutiny, or will it just fade away like matter in a black hole?

## Study abstract:

We explore the relationship between the quantum state of a compact matter source and of its asymptotic graviton field. For a matter source in an energy eigenstate, the graviton state is determined at leading order by the energy eigenvalue. Insofar as there are no accidental energy degeneracies there is a one to one map between graviton states on the boundary of spacetime and the matter source states. Effective field theory allows us to compute a purely quantum gravitational effect which causes the subleading asymptotic behavior of the graviton state to depend on the internal structure of the source. This establishes the existence of ubiquitous quantum hair due to gravitational effects.

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