Not much pressure, to be sure – but it's a finding that's fascinatingly consistent with Stephen Hawking's prediction that black holes emit radiation and therefore not only have a temperature, but slowly shrink over time, in the absence of accretion.

"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," said physicist and astronomer Xavier Calmet of the University of Sussex in the UK.

"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 break down.

"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."

When they made their discovery, Calmet and his University of Sussex colleague, physicist and astronomer Folkert Kuipers, were performing calculations using quantum field theory to try and probe the event horizon of a black hole.

Specifically, they were trying to understand the fluctuations at the event horizon of a black hole that correct its entropy , a measure of the progression from order to disorder.

While they were performing these calculations, Calmet and Kuipers kept running across an additional figure that appeared in their equations, but it took a while for them to recognize what they were looking at – pressure.

"The pin-drop moment when we realised that the mystery result in our equations was telling us that the black hole we were studying had a pressure – after months of grappling with it – was exhilarating," Kuipers said .

It's unclear what's causing the pressure, and according to the team's calculations, it's very small. Moreover, it's negative – expressed as -2E^-46 bar for a black hole the mass of the Sun, compared to Earth's 1bar at sea level.

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https://www.sciencealert.com/physicists-just-accidentally-made-a-new-discovery-about-black-holes