What is it about?
Water molecules constantly spin and drift around, posing a chicken-and-egg question: which motion triggers the other? Using computer simulations and a statistical test that checks whether one motion helps predict another, researchers found that at room temperature, spinning slightly drives drifting. In water chilled far below freezing and squeezed under high pressure, yet still liquid, this flips: each molecule is boxed in by its neighbours and can only turn once they move, much like in molten glass, thick and slow-flowing.
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Why is it important?
Until now, scientists could see that these motions are linked, but standard tools, which only measure how properties vary together, could not tell which one prevails. This work reads that answer straight from simulated “molecular movies,” and sheds new light on the strange behaviour of liquid water cooled far below its freezing point, a hot topic in physics. Beyond pure water, the same tool could reveal how molecular motions trigger chemical reactions, help proteins fold, or cause them to clump together. Its predictions could also be tested with ultrafast laser experiments, which nudge molecules directly and watch how they respond.
Perspectives
For me, the most striking outcome of the study is the flip in the direction of influence between room temperature and supercooled water. At room temperature, the signals between spinning and drifting were so faint that we had to build a dedicated dataset to be sure they were real, whereas at low temperature they were clear and pointed the other way. This shows that our tool, the Imbalance Gain (“IG” for the initiated), can pick up subtle changes in how a liquid moves, and even differences between computer models of water. That sensitivity makes it a powerful probe of the dynamics of a wide range of phases. I believe its applications are vast, from chemical reactions and proteins to glasses, materials, and beyond.
Léon HUET
the Abdus Salam International Centre for Theoretical Physics (ICTP)
Read the Original
This page is a summary of: Causality in liquid water as a hallmark of emergent glassy dynamics, Proceedings of the National Academy of Sciences, October 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2616852123.
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