Black holes are becoming weirder by the day. When scientists first confirmed behemoths existed back in 1970s, we thought, they were pretty simple, inert corpses.
Then, famed physicist Stephen Hawking discovered that black holes are not exactly black and they actually emit heat. And now, physicists realized that the sort of dark objects also exert a pressure on their surroundings.
The finding that such simple, non-rotating “black holes have a pressure also as a temperature is even more exciting given that it was a total surprise,” co-author Xavier Calmet, a professor of physics at University of Sussex in England, said in a statement.
Calmet & his graduate student Folkert Kuipers were examining quantum effects near the event horizons of black holes, something that’s fiendishly hard to pin down. To tackle this, researchers employed a technique to simplify their calculations.
As they were working, a weird term appeared in mathematics of their solution. After months of confusion, they realized, what this newly discovered term meant: it was an expression of pressure produced by a black hole. Nobody had known this was possible before and it changes the way scientists believe black holes & their relationships with the rest of the universe.
In 1970s, Hawking became one among the first physicists to apply quantum physics to try to understand what happens at the event horizon (the area around a black hole beyond which nothing, not even light, can escape). Prior to this work, everyone just assumed that black holes were simple objects.
Consistent with general theory of relativity, theory of gravity that first-suggested black holes could exist, there’s nothing at all remarkable about the event horizon.
The event horizon is “boundary” of a black hole, defining the region, where exiting the black would require traveling faster than light. But it was just an imaginary line in space, if you happen to cross it, you would not even know you did, until you tried to turn around & leave.
Hawking changed all that. He realized that quantum foam which refers to a sea of particles constantly popping into & out of existence in the vacuum of space-time, can affect that simplistic view of the event horizon.
Sometimes, pairs of particles appear spontaneously from the empty vacuum of space-time, then annihilate one another in a flash of energy, returning the vacuum to its original state.
But when this happens too-close to a black hole, one among the pair can get trapped behind the event horizon & the other escapes. The black hole is left holding the energy bill for the escaped particle, then it’s to lose mass.
This process is now-known as Hawking radiation and it is through these calculations that we discovered that black holes are not entirely, 100 percent black. They glow a little. This glow, referred to as “blackbody radiation,” means they even have heat & entropy (also called “disorder”) and all the other terms we usually apply to far more mundane objects like refrigerators & car engines.
An effective technique
Hawking focused on, how quantum physics affected the vicinity of a black hole. But that is not the entire story. Quantum physics does not include the force of gravity and an entire description of what is happening near event horizons will have to include quantum gravity, or a description of how strong gravity acts at teeny tiny scales.
Since 1970s, various physicists have tried their luck both at developing a theory of quantum gravity and at applying those theories to physics of the event horizon. the newest attempt comes from this new study by Calmet & Kuipers, published in September in the journal Physical Review D.
“Hawking’s landmark intuition that black holes aren’t black but have a radiation spectrum that’s very similar to that of a black body makes black holes an ideal laboratory to investigate the interplay between quantum physics, gravity & thermodynamics,” Calmet said.
Without a full theory of quantum gravity, duo used an approximation technique called effective theory or EFT.
This theory assumes gravity at quantum level is weak with an assumption that permits you to make some progress in the calculations without everything falling apart, as happens, when gravity in the quantum regime is modeled as extremely strong. While these calculations will not reveal the complete picture of the event horizon, they may deliver insights around & inside the black hole.
“If you consider about black holes within only general theory of relativity, one can show that they have a singularity in their centers where laws of physics as we all know them must break down,” explained Calmet.
“It is hoped that when quantum field theory is incorporated into general relativity, we’d be ready to find a new description of black holes.”
Here comes the pressure
Calmet & Kuipers were exploring thermodynamics of black holes using EFT within the vicinity of the event horizon, when they noticed a weird mathematical term pop-up in their equations. At first, the term completely stumped them, they did not know what it meant or the way to interpret it. But that changed during a conversation on Christmas Day, 2020.
They realized, the term in the equations represented a pressure. An actual, real pressure. The same pressure that the hot-air exerts inside 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 nearly absurdly tiny, but 10^54 times smaller than standard air pressure on the Earth. But it is there. They also found that the pressure can be often positive or negative, depending on the particular mix of quantum particles near black hole. A positive pressure is that the type that keeps a balloon inflated, while a negative pressure is the tension you feel in a stretched elastic rubber band.
Their result extends the idea of black holes as thermodynamic entities that haven’t just temperature & entropy, but also pressure. Because their work only model weak quantum gravity & neglects strong gravity, it cannot completely explain the behavior of black holes, but it’s a crucial step.
“Our work is a step in this direction and although the pressure exerted by the black hole that we were studying is small, the fact that it’s present opens up multiple new possibilities, spanning the study of astrophysics, high energy physics & quantum physics,” Calmet concluded.