British Scientists Confirm Black Holes Exert Tiny Bits Of Pressure

Black holes are getting weirder by the day. When scientists first confirmed the behemoths existed back in the 1970s, we thought they were pretty simple, inert corpses.

Photo Insert: That black holes - once thought to be inert corpses - exert pressure has come as a total surprise.

Then, famed physicist Stephen Hawking discovered that black holes aren't exactly black and they actually emit heat. And now, a pair of physicists have realized that the sort-of-dark objects also exert pressure on their surroundings, Paul Sutter reported for Live Science.

The finding that such simple, non-rotating "black holes have a pressure as well as a temperature is even more exciting given that it was a total surprise," co-author Xavier Calmet, a professor of physics at the University of Sussex in England, said in a statement.

Calmet and his graduate student Folkert Kuipers were examining quantum effects near the event horizons of black holes, something that is fiendishly hard to pin down. To tackle this, the researchers employed a technique to simplify their calculations. As they were working, a strange term appeared in the mathematics of their solution.

After months of confusion, they realized what this newly discovered term meant: It was an expression of the pressure produced by a black hole. Nobody had known this was possible before, and it changes the way scientists think about black holes and their relationships with the rest of the universe.

In the 1970s, Hawking became one of the first physicists to apply quantum mechanics to try to understand what happens at the event horizon — the area around a black hole beyond which nothing, not even light, can escape.

Calmet and Kuipers were exploring the thermodynamics of black holes using EFT in the vicinity of the event horizon when they noticed a strange mathematical term pop up in their equations. At first, the term completely stumped them — they didn't know what it meant or how to interpret it.

But that changed during a conversation on Christmas day, 2020. They realized that the term in the equations represented a pressure. An actual, real pressure. The same pressure that the hot air exerts inside of a rising balloon, or pressure on a piston inside the engine of your car.

"The pin-drop moment when we realized 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," recalled Kuipers.

That pressure is almost absurdly tiny, less than 10^54 times smaller than standard air pressure on the Earth. But it's there. They also found that the pressure can be positive or negative, depending on the particular mix of quantum particles near the black hole. A positive pressure is the kind that keeps a balloon inflated, while a negative pressure is the tension you feel in a stretched rubber band.

Since the 1970s, various physicists have tried their luck both at developing a theory of quantum gravity and at applying those theories to the physics of the event horizon. The latest attempt comes from this new study by Calmet and Kuipers, published in September in the journal Physical Review D.

"Although the pressure exerted by the black hole that we were studying is tiny, the fact that it is present opens up multiple new possibilities, spanning the study of astrophysics, particle physics, and quantum physics."

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