Black holes may have their own exotic version of pressure that is different from the sort found everywhere else in the universe. Calculations of how quantum mechanics affects gravity at the edge of black holes indicate that those regions may have pressure, a finding that was completely unexpected to physicists.
The question of how quantum mechanics and gravity fit together is one of the biggest mysteries in modern physics, and the edge of a black hole is one of the few regions with extreme enough conditions for the effects of both to be simultaneously relevant. and at the University of Sussex in the UK used a framework called quantum field theory to explore what happens when quantum mechanics and gravity meet at the edge of a black hole.
They calculated how minuscule quantum fluctuations would create effects not taken into account by our standard equations of gravity. These calculations revealed a surprising variable, which seems to suggest that fluctuations of quantum particles at the edge of a black hole should give the black hole pressure.
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“It was fully unexpected,” says Calmet. When black holes were first hypothesised, physicists thought that they should be extremely simple. Later work by physicist Stephen Hawking and others showed that they do emit particles in a process now known as Hawking radiation, which means that they must have a temperature. That in itself was a surprise. Now, the addition of pressure means that black holes are even more complicated, says Calmet.
However, the researchers haven’t yet figured out what this pressure might mean in a physical sense. The everyday concept of pressure involves molecules pushing against an object and bouncing off it – but the edge, or event horizon, of a black hole is nearly empty, so there isn’t much to push against.
“The source of the pressure here has to be 100 per cent purely quantum fluctuations,” says at Michigan State University. Quantum fluctuations create virtual particles, which could, in theory, drive the pressure. “It’s not the sort of pressure that we’re used to,” he says.
If you imagine a black hole’s event horizon like a balloon, the pressure isn’t coming from the interior or exterior to shrink or expand the balloon, it is coming from within the balloon’s material itself. “One can imagine the horizon as a quite peculiar surface, and the pressure will therefore push it inward (if negative) or outward (if positive), which correspond to a reduction or growth of the black hole mass, respectively,” says at the University of Bologna in Italy.
The researchers found that the pressure was negative, so it should correspond with the black hole shrinking over time, like a leaky balloon. This is consistent with other work that suggests that black holes get smaller as they undergo Hawking radiation. The two phenomena might be connected, but right now this is unclear.
It may take a long time to figure out exactly where this pressure comes from and what its consequences are for our understanding of black holes, says Hsu. But because it comes from quantum fluctuations, learning more about it could be a step towards understanding quantum gravity.
“Any new feature we discover about black holes on the quantum level can give us pointers on how to merge gravity and quantum mechanics, and what features this underlying theory must have,” says Calmet.
Physical Review D
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