In 1974 Stephen Hawking made an important discovery: black holes emit thermal radiation. Until then, black holes were thought to be inert, the last stage of a dying heavy star.

Scientists from the University of Sussex have shown that in fact they are even more complex thermodynamic systems, not only with temperature, but also with pressure.

Xavier Calmet and Folkert Kuipers were taken aback by the additional figure presented in the equations as they worked on quantum gravitational corrections to the entropy of a black hole.

While discussing this curious result, the realization came that what they were seeing was behaving like pressure. After further calculations, they confirmed their exciting discovery that quantum gravity can lead to pressure in black holes.

Xavier Calmet, professor of physics at the University of Sussex, said: “Our discovery that Schwarzschild black holes have pressure as well as temperature is even more exciting given that it was a complete surprise. I am pleased that our research on quantum gravity will contribute to a broader understanding of the nature of black holes in the scientific communities. Hawking's remarkable intuition that black holes are not black, but have a spectrum of radiation very similar to that of a black body, makes black holes an ideal laboratory for studying the interactions between quantum mechanics, gravity, and thermodynamics.

If we consider black holes only in the framework of General Relativity, it can be shown that they have a singularity at the centers where the laws of physics, as we know them, must be violated. It is hoped that when quantum field theory is incorporated into General Relativity, we may be able to find a new description of black holes.

Our work is a step in that direction, and although the pressure exerted by the black hole we studied is tiny, the fact that it is present opens up many new possibilities encompassing the study of astrophysics, particle physics and quantum physics."

Folkert Kuipers, PhD student at the University of Sussex's School of Mathematical and Physical Sciences, said: “It's a pleasure to be working on a discovery that contributes to our understanding of black holes, especially as a research student. The moment we realized that the cryptic result in our equations suggested that the black hole under study had pressure was encouraging. Our result is a result of cutting-edge research that we are doing in the field of quantum physics at the University of Sussex, and it sheds new light on the quantum nature of black holes."

The discovery was made by Professors Xavier Calmet and Folkert Kuipers at the University of Sussex's Department of Physics and Astronomy and published in Physical Review D.