Since Stephen Hawking discovered that black holes evaporate, it has become clear that they could disappear from our universe.
But our understanding of gravity and quantum mechanics is not strong enough to describe the last moments of a black hole’s life.
New research driven by string theory suggests possible, equally bizarre, fates for black holes to evaporate:
A residual nugget that we can, in principle, access, or a singularity that is not surrounded by a horizon of events.
The importance of Hawking radiation
Black holes are not, strictly speaking, completely black.
In pure general relativity, with no other modifications or considerations of other physics, it remains forever black.
Once one of them forms, it will remain there, being a black hole, forever.
But in the 1970s, Hawking used the language of quantum mechanics to explore what was happening near the boundary of a black hole, known as the event horizon.
He found, surprisingly, that a strange interaction between the quantum fields of our universe and the one-way barrier of the event horizon allowed a pathway for energy to escape from the black hole.
This energy takes the form of a slow but steady stream of radiation and particles that has come to be known as Hawking radiation.
With every bit of energy that escapes, the black hole loses mass and thus contracts, eventually coming out of existence completely.
The appearance of Hawking radiation created what is known as the black hole information paradox.
All information describing the matter falling into it crosses the event horizon, and will never be seen again.
But the Hawking radiation itself does not carry any information with it, and yet the black hole eventually disappears.
So where did all the information go?
The black hole information paradox is a giant flashing neon signal to 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 “easiest” way to solve the black hole information paradox is to develop a new theory of gravity that goes beyond Einstein’s theory of general relativity.
After all, we already know that general relativity breaks down at its centers, which are tiny holes in spacetime known as singularities where density goes to infinity.
The only way to correctly describe the singularity is with a theory of quantum gravity that correctly predicts how strong gravity will behave at very small scales.
Unfortunately, we currently lack a theory of quantum gravity.
It would be nice to look at singularities directly, but as far as we understand through general relativity, all singularities are hidden behind event horizons, making them inaccessible to us.
But by studying the Hawking radiation process, we may be able to find a shortcut to get closer to the singularity and understand the crazy physics that’s going on there.
As black holes evaporate, they get smaller and smaller and their event horizons get uncomfortably close to central singularities.
In the last moments of a black hole’s life, gravity becomes too strong, and itself too small, to describe it correctly 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 theory’s behavior.
There are many candidates for quantum gravity, and string theory is the most developed.
Although there are no known solutions to string theory, it is possible to take what we know about the general features of the theory and use them to make modified versions of general relativity.
These modified theories are not “complete” valid alternatives to general relativity, but they do allow us to examine how gravity might behave as it gets closer and closer to the quantum limit.
More recently, a team of theorists has used one of these theories, known as Einstein-Delaton-Gauss-Bonnett gravity, to investigate the end states of evaporating black holes.
They detailed their work in a paper published in the arXiv database in May.
Details of the team’s results are a bit vague.
This is because modified general relativity is not as well understood as ordinary general relativity, and solving complex mathematics requires a range of estimates and a lot of guesswork.
Nevertheless, the researchers were able to paint a general picture of what is happening.
A key feature of Einstein-Delaton-Gauss-Bonet is that black holes have a minimum mass, so theorists have been able to study what happens when an evaporating black hole begins to reach a minimum mass.
In some cases, depending on the exact nature of the theory and its development, the evaporation process leaves behind a microscopic solid mass.
This solid mass might lack an event horizon, so in principle, you could fly your spaceship to it and catch it.
While the solid mass would be very strange, it would at least retain all the information that fell into the original black hole, thus resolving the contradiction.
Another possibility is that it reaches its minimum mass and gives up its event horizon but still retains its singularity.
These “naked singularities” appear to be forbidden in ordinary general relativity, but if they existed, they would serve as direct windows into the realm of quantum gravity.
It remains unclear whether Einstein-Delaton-Gauss-Bonnett gravity represents a valid path toward quantum gravity.
But results like these help physicists shed light on the most complex scenarios in the universe, and may provide guidance on how to solve them.
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