Gas passing through a narrow valve can heat up or cool down: this is the Joule–Thomson effect, which powers refrigerators and air liquefiers. It turns out black holes can do the same. The authors considered a charged black hole in anti-de Sitter space (a world with constant negative curvature, like a saddle), adding corrections from modified gravity and nonlinear electrodynamics (where fields are so strong that Maxwell's laws stop being linear).
The main takeaway: the black hole's charge black hole is the most powerful regulator of its thermal behavior. By changing the charge, you can make the hole cool or heat at the same pressure. Additional parameters, modified gravity and the nonlinear electromagnetic field, only slightly adjust the picture, like fine-tuning knobs. At distances comparable to interstellar scales, the geometry of spacetime curved spacetime is almost indistinguishable from the predictions of standard theory, allowing the model to be applied to real astrophysical objects.
So a black hole becomes like a heat engine with a main regulator — electric charge. Change the charge, and you switch the mode from heating to cooling. This same charge brings us back to the familiar picture of distant space: at large distances, the hole looks like an ordinary massive object, and only up close do the quirks of modified gravity show up.
as written by the authors arXiv:2607.06704
We herein study both the Joule-Thomson (JT) expansion process and the geodesic properties of a charged anti-de Sitter (AdS) black hole arising in modified gravity with nonlinear electrodynamic (NLED) sources. Our thermodynamic study reveals that the black hole charge has the most pronounced impact on the JT behaviour. The nonlinear electromagnetic sector together with the modified gravity parameters introduces further corrections to the inversion temperature and the associated cooling characteristics. At astrophysically relevant distances, the geometry closely reproduces expected outcomes.
The Joule–Thomson effect was discovered in the mid-19th century by forcing air through a porous plug, and now it's found in black holes that are billions of times more massive than the Sun.
In Dan Simmons' novel 'Hyperion', black holes were used as energy sources and portals — controlling their thermal properties could be the basis for such technology.