Physicists may have just solved one of the universe’s most troubling paradoxes by discovering that black holes are not entirely featureless, but in fact possess a characteristic called "quantum hair". Such a revelation could finally resolve the black hole information paradox proposed by Stephen Hawking back in the 1970s, and may help to connect the theories of general relativity and quantum mechanics.
According to Einstein’s theory of general relativity, black holes can only have three discernible characteristics: mass, charge, and angular momentum (otherwise known as spin). Any two black holes that possess equal values for each of these attributes can’t be differentiated between as they lack any distinguishing features, or "hair".
Quantum mechanics, however, begs to differ. According to this fundamental theory, the particles that make up any object are packed with quantum information, which is preserved indefinitely even if the item itself is destroyed. This means that the quantum information pertaining to the star that collapsed to form a black hole, as well as any objects that may have been sucked into that black hole, must still be present somewhere. In other words, black holes must have "hair".
It has been suggested that all this quantum information may remain intact beyond the event horizon, which is the boundary of a black hole beyond which nothing can escape, and thus be observed. While this hypothesis neatly explains why we can’t detect any of this hair, it was smashed to smithereens by Stephen Hawking back in the 1970s.
Hawking’s calculations indicated that black holes are all slowly evaporating, but that the particles they emit contain no trace of the quantum information contained within them. This means that the quantum information that falls into a black hole does not escape as radiation but simply vanishes, thereby violating the rules of quantum mechanics.
Known as Hawking radiation, these tiny evaporating particles sent panic rippling through the scientific community by implying that either general relativity or quantum mechanics – the two theories on which we base our entire understanding of reality – may be flawed. However, after almost half a century of existential dread, researchers have finally come up with a solution for this so-called black hole information paradox.
Publishing their work in the journal Physics Letters B, the study authors explain that all matter that collapses into a black hole leaves an imprint on that black hole’s gravitational field. In this way, the quantum information pertaining to that matter is preserved, giving the black hole its hair.
To reach this conclusion, the researchers used a series of mathematical equations to explain what happens when two stars of equal size and mass but different compositions collapse into black holes. In doing so, they demonstrated that while the resultant black holes may be identical in charge, mass, and spin, their gravitational fields differ, maintaining an imprint of the quantum information contained within the stars from which they formed.
Commenting on these remarkable findings, study author Professor Xavier Calmet from the University of Sussex, UK, explained that “it was generally assumed within the scientific community that resolving this paradox would require a huge paradigm shift in physics, forcing the potential reformulation of either quantum mechanics or general relativity.”
“What we found – and I think is particularly exciting – is that this isn’t necessary," he said in a statement sent to IFLScience. "Our solution doesn’t require any speculative idea, instead our research demonstrates that the two theories can be used to make consistent calculations for black holes and explain how information is stored without the need for radical new physics.”
“It turns out that black holes are in fact good children, holding onto the memory of the stars that gave birth to them.”