A model that describes the microscopic origin of black hole entropy

Black holes are fascinating astronomical objects whose gravitational pull is so strong that it prevents objects and even light from escaping. While black holes have been the subject of numerous astrophysical studies, their origins and underlying physics remain largely a mystery.

Researchers from the University of Pennsylvania and the Centro Atómico Bariloche recently presented a new model of black hole microstates that investigates the origin of entropy (that is, the degree of disorder) in black holes.

This model was presented in an article published in Physical examination lettersoffers an alternative perspective on black holes that could inform future astrophysics research.

“The Bekenstein-Hawking entropy formula, which describes the thermodynamics of black holes, was discovered in the 1970s,” Vijay Balasubramanian, co-author of the paper, told Phys.org. “This formula suggests that black holes have an entropy that is proportional to the area of ​​their horizon.

“According to statistical physics, as developed by Boltzmann and Gibbs in the late 19th century, the entropy of a system is related to the number of microscopic configurations that have the same macroscopic description.

“In a quantum mechanical world like ours, entropy arises from the quantum superpositions of ‘microstates’, i.e. microscopic components that provide the same observable characteristics on large scales.”

Physicists have been trying to provide a credible account of black hole entropy for decades. In the 1990s, Andrew Strominger and Cumrun Vafa used a hypothetical property called “supersymmetry” to develop a method for counting the microstates of a special class of black holes, whose mass is equal to the electromagnetic charge, in universes with additional dimensions and multiple species of black holes electric and magnetic fields.

To explain the origin of black hole entropy in universes like ours, Balasubramanian and his colleagues had to create a new theoretical framework.

“Despite previous attempts, there is no report yet that applies to the types of black holes that form from stellar collapse in our world,” Balasubramanian said. “Our goal was to provide such an account.”

The main contribution of this recent work was to introduce the new model of black hole microstates, which can be described as collapsing dust shells inside the black hole. In addition, the researchers developed a technique to count the possibilities of quantum mechanical superposition of these microstates.

“The key finding of our work is that very different spacetime geometries, which appear to correspond to different microstates, can mix with each other due to the subtle effects of quantum mechanical ‘wormholes’ that connect distant regions of space,” said Balasubramanian.

“After accounting for the effects of these wormholes, our results showed that for any universe containing gravity and matter, the entropy of a black hole is directly proportional to the area of ​​its event horizon, as Bekenstein and Hawking suggested.”

Recent work by Balasubramanian and his colleagues introduces a new way of thinking about black hole microstates. She describes her model specifically as quantum superpositions of simple objects that are well described by classical physical theories of matter and space-time geometry.

“This is very surprising, because the community had expected that a microscopic explanation of black hole entropy would require the entire apparatus of a quantum theory of gravity, such as string theory,” Balasubramanian said.

“We also show that universes that differ from each other on macroscopic, even cosmic, scales can sometimes be understood as quantum superpositions of other, macroscopically different universes. This is a manifestation of quantum mechanics on the scale of the entire universe, which is surprising given that we typically associate quantum mechanics with small-scale phenomena.”

The newly introduced theoretical framework could pave the way for further theoretical work to explain the thermodynamics of black holes. In the meantime, the researchers plan to expand and enrich their description of black hole microstates.

“We are now investigating to what extent and under what circumstances an observer outside the event horizon can determine which microstate a black hole is in,” Balasubramanian added.

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