We still don’t know exactly what happens when black holes die.
Since stephen hawking discovered that black holes evaporate, we know that they can potentially disappear from our universe. But our understanding of gravity and quantum mechanics is not powerful enough to describe the final moments of a black hole’s life.
Now, new research driven by string theory suggests possible, and equally bizarre, fates for evaporating black holes: a residual nugget that we could, in principle, access, or a singularity not enveloped by a horizon of events.
Related: What happens in the center of a black hole?
The importance of Hawking radiation
black holes are not, strictly speaking, entirely black. In pure general relativity, without further modifications or considerations of other physics, they remain black for eternity. Once one forms, it will stay there, being a black hole, forever. But in the 1970s, Hawking used the language of quantum mechanics to explore what happens near the boundary of a black hole, known as the event horizon.
He discovered that, surprisingly, a strange interaction between the quantum fields of our universe and the one-way barrier of the event horizon allowed energy to escape from the black hole. This energy takes the form of a slow but steady stream of radiation and particles known as Hawking radiation. With each bit of energy that escapes, the black hole loses mass and thus shrinks, eventually disappearing altogether.
The appearance of Hawking radiation created what is called the Black Hole Information Paradox. All information that describes matter falling into a black hole crosses the event horizon, never to be seen again. But the Hawking radiation itself contains no information, and yet the black hole eventually disappears. So where did all the information go?
Related: Stephen Hawking Was Right: Black Holes Can Evaporate, Says Weird New Study
The black hole information paradox is a giant, flashing neon sign telling physicists that we don’t understand something. We may not understand the nature of quantum information, the nature of gravity or the nature of event horizons – or all three. The “simplest” approach to solving the black hole information paradox is to develop a new theory of gravity, going beyond Einstein’s theory of general relativity.
After all, we already know that general relativity collapses at the center of black holes, which are tiny pits in spacetime called singularities where density tends to infinity. The only way to properly describe the singularity is to use a quantum theory of gravity that correctly predicts the behavior of strong gravity at extremely small scales.
Unfortunately, we currently lack a theory of quantum gravity. It would be nice to look directly at singularities, but as far as we understand through general relativity, all singularities are locked behind event horizons, making them inaccessible to us.
But by studying Hawking’s radiation process, we may be able to find a shortcut to get closer to a singularity and understand the crazy physics going on there. As black holes evaporate, they get smaller and smaller, and their event horizons move uncomfortably close to central singularities. In the final moments of black hole life, gravity becomes too strong and black holes become too small for us to adequately describe with our current knowledge. So if we can develop a better theory of gravity, we can use the last moments of Hawking radiation to test the behavior of the theory.
There are many candidates for a quantum theory of gravity, with string theory being the most developed. Even though there are no known solutions to string theory, it is possible to take what we know about general features of the theory and use them to create modified versions of general relativity.
Related: How Stephen Hawking transformed our understanding of black holes
These modified theories aren’t the correct “full” replacements for general relativity, but they do allow us to examine how gravity might behave as it gets closer and closer to the quantum limit. Recently, a team of theorists used one such theory, known as Einstein-dilaton-Gauss-Bonnet gravity, to study the final final states of evaporating black holes. They detailed their work in an article published in the arXiv preprint database. (opens in a new tab) in May.
The details of the team’s results are a bit hazy. Indeed, modified general relativity is not as well understood as regular general relativity, and solving the complicated mathematics requires a host of approximations and a lot of guesswork. Still, the researchers were able to paint a general picture of what’s going on.
One of the key features of Einstein-dilaton-Gauss-Bonnet gravity is that black holes have minimal mass, so theorists have been able to study what happens when an evaporating black hole begins to reach this mass. minimum.
In some cases, depending on the exact nature of the theory and the evolution of the black hole, the evaporation process leaves behind a microscopic nugget. This nugget wouldn’t have an event horizon, so in principle, you could fly your spaceship there and pick it up. While the nugget would be extremely exotic, it would at least retain all of the information that fell into the original black hole, thus resolving the paradox.
Another possibility is that the black hole reaches its minimum mass and sheds its event horizon while retaining a singularity. These “naked singularities” seem to be forbidden in normal general relativity, but if they exist, they would be direct windows into the realm of quantum gravity.
It is still unclear whether Einstein-dilaton-Gauss-Bonnet gravity represents a valid route to quantum gravity. But findings like this help physicists shed light on one of the most complex scenarios in the universe, and potentially provide guidance on how to solve them.
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