Quantum gravity solves the biggest riddle left by Stephen Hawking

Authors of a new study claim to have solved the information paradox, one of the most difficult theoretical problems in physics in recent decades. But there is a caveat: the solution relies on the graviton, a quantum gravity particle whose existence has yet to be proven.

The information paradox

Proposed by Stephen Hawking in the 1970s, the information paradox is a consequence of the evaporation of black holes, an idea also put forward by this same physicist. This question proved so challenging that Hawking himself died 40 years later without reaching a conclusion.

Black holes, according to Hawking, evaporate because they emit radiation in the form of thermal energy, which became known as Hawking radiation. If the energy released implies a loss of mass (according to the formula E=mc²), it means that the black hole will shrink until, one day, it disappears.

However, Hawking radiation, due to its thermal nature, cannot carry information. If all that’s left of a black hole after it evaporates is this radiation, then all information about the black hole’s previous state is lost.

This is a major violation of quantum laws, which say the following: a particle’s information cannot be destroyed, and an object’s final state must always reveal clues about its initial state. For example, we might discover that a handful of ashes was a sheet of paper before it was burned.

If Hawking radiation is real — and it probably is — where does the information from the particles “devoured” by the black hole go throughout its history? Where is the information that will say that the initial state of the black hole is a massive star?

Solution to the problem

One of the possible answers to the paradox is that the black hole would have some way of keeping the information somewhere safe. But where? Some theorists have proposed a radiation around black holes, dubbed “quantum hair”, capable of storing information.

It turns out that, for Albert Einstein’s theory, black holes can only carry three types of information: mass, spin and charge. In other words, there is no hair, as Professor John Archibald Wheeler demonstrated in the 1960s with the “no hair theorem” (yes, the physicists’ dilemma is whether black holes are bald or hairy).

Stephen Hawking’s last work, written with his colleagues like Hawking, Sasha Haco and Andrew Strominger and published after his death, defends another proposition: that of “soft hair”. Now, the new study develops this idea further.

By 2022, the team was already arguing that black holes have hairs in the form of a unique quantum “print” — that is, one black hole’s hair will always be different from another’s hair — in the gravitational fields that surround them. In the most recent study, the authors used the hypothetical graviton, the particle of quantum gravity.

According to Xavier Calmet, professor of physics at the University of Sussex and lead author of the study, these “quantum gravitational corrections” are “crucial for black hole evaporation”. With the graviton in play, the study shows how Hawking radiation is modified “in such a way that this radiation becomes non-thermal”.

With this result, everything about Hawking radiation can be seen in a different way, with particles that can, yes, carry information. This is really impressive and may resolve the paradox once and for all.

Unfortunately, despite the optimism of the authors, there is still a very important step to prove this conclusion. Researchers have been trying to observe the graviton for some time, as the information paradox would not be the only problem to be solved — all incompatibility between general relativity and quantum mechanics will finally be overcome.

The study article was published in Physical Review Letters.

Source: Physical Review Letters

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