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Black holes are getting weirder every day. When scientists first confirmed that behemoths existed in the 1970s, we thought they were fairly simple and inert corpses. Then the famous physicist Stephen Hawking discovered that black holes aren’t exactly black and that they actually emit heat. And now a pair of physicists have realized that somehow dark objects put pressure on their surroundings as well.
The observation that “black holes having pressure and temperature is all the more exciting since it was a total surprise, ”said co-author Xavier Calmet, professor of physics at the University of Sussex in England, in a statement.
Related: 8 ways we know black holes really exist
Calmet and his graduate student Folkert Kuipers were examining the near-horizon quantum effects of black hole events, something that is devilishly elusive. To remedy this, the researchers used a technique to simplify their calculations. While they were working, a strange term appeared in the math 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. No one knew this was possible before, and it is changing the way scientists think about black holes and their relationship to the rest of the universe.
Hawking’s engine
In the 1970s, Hawking became one of the first physicists to apply Quantum mechanics to try to figure out what’s happening on the event horizon – the area around a black hole beyond which nothing, not even light, can escape. Before this work, everyone had assumed that black holes were simple objects. According to general relativity, the theory of gravity which first suggested that black holes might exist, there is nothing remarkable about the event horizon. The event horizon is the “boundary” of a black hole, defining the region where out of black would require traveling faster than light. But it was just an imaginary line in space – if you crossed it, you wouldn’t even know it, until you tried to turn around and go.
Related: 8 ways to see Einstein’s theory of relativity in real life
Hawking changed all that. He realized that quantum foam, which refers to a sea of particles that constantly arise and disappear in the vacuum of space-time, can affect this simplistic view of the event horizon. Sometimes pairs of particles appear spontaneously from the empty vacuum of space-time, then annihilate in a flash of energy, returning the void to its original state. But when this happens too close to a black hole, one can get trapped behind the event horizon and the other can escape. The black hole keeps the energy bill of the escaped particle and must therefore lose mass.
This process is now known as Hawking radiation, and it was through these calculations that we discovered that black holes are not 100% completely black. They shine a little. This glow, known as “black body radiation”, means they also have heat and entropy (also called “disorder”) and all the other terms we usually apply to much more objects. mundane like refrigerators and car engines.
An effective technique
Hawking focused on how quantum mechanics affected the neighborhood of a black hole. But that’s not the whole story. Quantum mechanics does not include the force of gravity, and a full description of what happens in the horizons of nearby events should include quantum gravity, or a description of how gravity acts on tiny scales.
Since the 1970s, various physicists have tried their luck both in developing a theory of quantum gravity and in applying these 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 examination D.
“Although the pressure exerted by the black hole we are studying is minimal, the fact that it is present opens up multiple new possibilities, spanning the study of astrophysics, particle physics and quantum physics.”
Xavier Calmet
“Hawking’s historical intuition that black holes are not black but have a radiation spectrum very similar to that of a black body makes black holes an ideal laboratory for studying the interaction between quantum mechanics and gravity. and thermodynamics, ”Calmet said.
Without a full theory of quantum gravity, the duo used an approximation technique called effective field theory, or EFT. This theory assumes that gravity at the quantum level is weak – an assumption that allows you to move forward with calculations without everything collapsing, as happens when gravity in the quantum regime is modeled as extremely strong. While these calculations do not reveal a complete picture of the event horizon, they can provide information around and inside the black hole.
“If you look at black holes only in the context of general relativity, we can show that they have a singularity in their centers where the laws of physics as we know them must collapse,” Calmet explained. “We hope that when quantum field theory becomes part of general relativity, we may be able to find a new description of black holes.”
Here is the pressure
Calmet and Kuipers were exploring the thermodynamics of black holes using EFT near the event horizon when they noticed a strange mathematical term in their equations. At first, the term completely baffled 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 pressure. Real, real pressure. The same pressure as hot air exerts inside a rising balloon, or the pressure on a piston inside your car’s engine.
“The moment we realized that the mysterious result of our equations told us the black hole we were studying was under pressure – after months of struggling with it – was exhilarating,” Kuipers recalled.
This pressure is almost absurdly tiny, less than 10 ^ 54 times smaller than the norm air pressure on the ground. But it is there. They also found that the pressure can be positive or negative, depending on the particular mixture of quantum particles near the black hole. Positive pressure is the one that keeps a balloon inflated, while negative pressure is the tension you feel in a stretched rubber band.
Their result extends the idea of black holes as thermodynamic entities that have not only temperature and entropy, but also pressure. Because their work only models weak quantum gravity and neglects strong gravity, it can’t fully explain the behavior of black holes, but it’s an important step.
“Our work is a step in this direction, and although the pressure exerted by the black hole we are studying is minimal, the fact that it is present opens up multiple new possibilities, covering the study of astrophysics, particle physics and quantum physics, “concluded Calmet.
Originally posted on Live Science.
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