Black holes aren’t as single-minded as we thought. It turns out their quantum behaviour adds a new, recalcitrant dimension to their monstrous character.
In a first-of-its-kind discovery, UK physicists have found that black holes exert quantum pressure on their environment.
We know black holes are voracious cosmic monsters that operate at nature’s very extremes. They exert a gravitational pull so strong that they gobble up anything – even light – that comes too close. The ‘point of no return’ is known as the event horizon (or, in more technical terms: the Schwarzschild radius). Beyond this point, the laws of physics as we understand them break down.
Physicists have long used black holes as a way to probe the edges of the known – including the quantum nature of gravity, in an ongoing quest to link Einstein’s general relativity with quantum mechanics.
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In 1974, Stephen Hawking took our understanding of black holes to a new level when he theorised that they emit thermal radiation. Specifically, he predicted that relativistic quantum effects near the event horizon would cause a black hole to radiate off heat. This means that black holes aren’t just growing indefinitely – they actually lose mass, too, and will eventually evaporate.
Now, physicists from the University of Sussex in the UK have found that black holes not only emit thermal radiation – they also exert pressure on their surrounding environment.
“Our finding that Schwarzschild black holes have a pressure, as well as a temperature, is even more exciting given that it was a total surprise,” says physicists Xavier Calmet, lead author of the paper published in Physical Review D.
Calmet and his colleague, Folkert Kuipers, made the discovery when running equations on quantum gravitational corrections to the entropy of a black hole (that is, how a black hole becomes more disordered over time).
An extra figure kept appearing in their equations, and they soon realised that it was behaving as a pressure. Further calculations showed that quantum gravity could lead to a black hole exerting pressure.
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Calmet says that black holes are an ideal laboratory to study the links between quantum mechanics, gravity and thermodynamics.
“If you consider black holes within only general relativity, one can show that they have a singularity in their centres where the laws of physics as we know them must breakdown,” he explains. “It is hoped that when quantum field theory is incorporated into general relativity, we might be able to find a new description of black holes.”
This new study is a step in this direction. The pressure black holes exert is only tiny, but it’s another clue that will help physicists understand the true quantum nature of black holes.