In 1974, Stephen Hawking made the seminal discovery that black holes emit thermal radiation. Previous to that, black holes were believed to be inert. In new research, a duo of researchers from the Department of Physics and Astronomy at the University of Sussex shows that black holes are in fact even more complex thermodynamic systems, with not only a temperature but also a pressure. University of Sussex’s Professor Xavier Calmet and Ph.D. student Folkert Kuipers were perplexed by an extra figure that was presenting in equations that they were running on quantum gravitational corrections to the entropy of a Schwarzschild or static black hole. During a discussion on this curious result, the realization that what they were seeing was behaving as a pressure dawned. Following further calculations they confirmed their exciting finding that quantum gravity can lead to a pressure in Schwarzschild black holes. “Our finding that Schwarzschild black holes have a pressure as well as a temperature is even more exciting given that it was a total surprise,” Professor Calmet said. “I’m delighted that the research that we are undertaking into quantum gravity has furthered the scientific communities’ wider understanding of the nature of black holes”. “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.” “If you consider black holes within only general relativity, one can show that they have a singularity in their centers where the laws of physics as we know them must breakdown.” “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.” “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.” “It is exciting to work on a discovery that furthers our understanding of black holes,” Kuipers said. “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.” “Our result is a consequence of the cutting-edge research that we are undertaking into quantum physics and it shines a new light on the quantum nature of black holes.”

Xavier Calmet & Folkert Kuipers