Black holes are becoming stranger by the day. As far back as the 1970s, when scientists first discovered the behemoths, they believed they were just simple, inert corpses. Stephen Hawking, a famed physicist, discovered black holes aren’t black, because they emit heat. A group of physicists has recently discovered that so-called dark objects also exert pressure on their surroundings.
It was a total surprise to discover that the 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.
It is extraordinarily hard to determine quantum effects near the event horizons of black holes, but Professor Calmet explored them with his graduate student Folkert Kuipers. Researchers simplified calculations in order to address this issue. An odd term appeared as they were working on their solution. A black hole is an expression of the pressure it produces. The term was discovered after months of confusion. The discovery changes the way scientists view black holes and their relationship to the rest of the universe, as nobody had ever before imagined it was possible.
Toward understanding what happens just beyond a black hole’s event horizon – that zone beyond which nothing, not even light, can escape – Hawking conducted experiments with quantum mechanics in the 70s. The black hole was thought to be a simple object until this study. There is nothing unusual about the event horizon of a black hole if one considers general relativity, the theory of gravity that led to the discovery of black holes. The event horizon is a region inside a black hole that must be crossed by travelling faster than light in order to exit it. You would not even realize you crossed it until you tried to turn around and leave it – it was just an imaginary line in space.
All that changed with Hawking. The event horizon is a simplistic view of a sea of particles constantly bursting in and out of existence in space-time. Thus, he realized that quantum foam can alter this simplistic view of the event horizon. The space-time vacuum appears sometimes with pairs of particles that arrive from nowhere, then explode in a flash of energy, resetting the vacuum. It is possible for one of the pairs to get trapped behind the event horizon when this happens too close to a black hole, while the other one is able to escape. Due to the escaped particle’s energy bill, the black hole must lose mass.
We discovered that black holes aren’t entirely, 100% black through this process now known as Hawking radiation. Little glows emanate from them. As well as “blackbody radiation,” they have heat, entropy (also known as “order”), and all the other terms we apply to more mundane objects like refrigerators and cars.
The most effective technique
What Hawking used, was based on the effects of quantum mechanics on black holes. However, that’s not the whole story. The force of gravity is not included in quantum mechanics, so any description of what’s going on near event horizons has to include quantum gravity, or how strong gravity operates at small scales.
Various physicists have been working toward developing a theory of quantum gravity as well as applying it to the study of event horizons since the 1970s. According to this new study by Calmet and Kuipers published in Physical Review D in September, it is possible to create a multiverse.
“Hawking’s landmark intuition that black holes are not black but have a radiation spectrum that is very similar to that of a black body makes black holes an ideal laboratory to investigate the interplay between quantum mechanics, gravity and thermodynamics,” Calmet said.
“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.”Xavier Calmet
They used a technique known as effective field theory, or EFT, to approximate quantum gravity without a full quantum gravity theory. Using this theory, gravity is modeled as weak at the quantum level, allowing us to make some progress with the calculations without everything coming apart. This is the case if gravity is modeled as extremely strong at the quantum level. Even though these calculations cannot give us a complete picture of the event horizon, they could provide insights about the black hole’s surroundings and interior.
“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,” explained Calmet. “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.”
Reaching the pressure point!
In their analysis of black holes using EFT, Calmet and Kuipers noticed a weird mathematical expression that popped up in their equations. The term was unfamiliar at first – they had no idea what it meant. However, on Christmas day of 2020, something changed.
Their understanding was that the pressure was represented in the equation. Pressure that actually exists. Similar to the pressure a balloon experiences when it rises, or the pressure inside an engine of your car’s piston.
After months of grappling with it, Kuipers remembered that it was a “pin-drop moment” when they realized the mystery result in their equations revealed that the black hole they were studying carried a pressure.
In comparison to Earth’s standard air pressure, that pressure is almost absurdly small. The problem still exists. Additionally, depending on the quantum particles near the black hole, the pressure can be positive or negative. An inflated balloon has a positive pressure, while a stretched rubber band has a negative pressure.
As a consequence, black holes are now understood not just as thermodynamic entities, but as objects with pressure as well. Although the authors ignore strong gravity in their work, it is an important step in modeling weak quantum gravity, which cannot explain the behavior of black holes.
“Our work is a step in this direction, and 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,” Calmet concluded.