Picture a planet with oceans, sunlight and all the right chemistry for life — except every fluid on it behaves like tar. Blood would barely creep through veins. Nutrients would crawl, rather than rush, into cells. Nothing would metabolize fast enough to live.

This is the strange universe that physicist Kostya Trachenko, of Queen Mary University of London, has been imagining mathematically, and his conclusion is unsettling: it might not take much to turn our world into that one. In a study published in Science Advances, Trachenko builds on work from 2020, in which he and his colleagues discovered something odd about viscosity, the property that makes honey slow and water quick. There appears to be an absolute lower limit to how easily any liquid can flow, and that limit is set not by chemistry but by the fundamental constants of physics — things like the electron's charge and the Planck constant, the tiny number that governs the quantum world. These constants are the same everywhere, from Tokyo to Buenos Aires, and nobody really knows why they carry the exact values they do.

What Trachenko has now shown is that this viscosity limit is not just a curious footnote. It intersects directly with biology. Life depends on molecules diffusing through watery environments inside and between cells, a process that only works within a narrow range of fluid viscosity. Nudge the fundamental constants even slightly, he argues, and liquids across the cosmos could thicken past the point where cellular machinery functions at all — regardless of whether water, carbon and warmth are all present.