Water in a kettle, as it boils, clearly separates into liquid and steam. But under enormous pressure, the boundary vanishes: the substance enters a supercritical state — neither one nor the other, but something in between.
Black holes can do this too. In the 1970s, Stephen Hawking and Jacob Bekenstein discovered that they have a temperature and entropy (a measure of disorder). Now physicists have applied to them a method originally developed for fluids. It uses imaginary numbers to reveal hidden transitions, like a photographic negative slowly filling with detail. In this invisible realm, a black hole displays not one supercritical zone but three: liquid-like, gas-like, and completely indistinguishable.
Previously, that region was thought to be unique. This new finding helps us understand the quantum nature of gravity and possibly the structure of neutron stars.
as written by the authors arXiv:2604.04968
Ising symmetry typically emerges in the critical domain between liquid-gas phases. Universality of this property imposes strong constraints on the behavior of thermodynamic crossovers for supercritical fluids. In this work, we develop a novel approach to investigate the crossover lines for supercritical AdS black holes using Lee-Yang phase transition theory. We analytically continue Lee-Yang zeros into the complex plane within the supercritical region by keeping a modular pressure real. Consequently, we obtain a pair of complex crossover lines, which exhibit universal scalings and manifest the emergent Ising symmetry in the complex phase space. The real crossover lines are defined by projecting the complex crossovers onto the real phase space. As a result, the phase diagram above the critical point is divided into three distinct regimes: liquid-like, indistinguishable and gas-like states, in sharp contrast to scenarios featuring only a single crossover line.
Supercritical water is used for eco-friendly disposal of toxic waste: it seeps like a gas and dissolves like a liquid.