The development of mathematical tools for understanding many-particle systems was one of the greatest achievements of 19th-century science. Extracting macroscopic properties of a gas by tracking the random motions of each individual atom remains an impossible feat. Statistical mechanics provided a framework for properly averaging over the individual particles’ motions and understanding how they lead to collective macroscopic properties such as temperature and pressure.

But disorder takes many forms. Whereas gas molecules bouncing around a bottle sample the phase space of possible positions and velocities, the molecules that make up that glass or plastic bottle are frozen in disarray, trapped far from equilibrium. Explaining how those disordered molecules ended up stuck in a particular configuration and how that connects to any macroscopic properties is currently beyond the reach of statistical mechanics.

Giorgio Parisi is to receive half of this year’s Nobel Prize in Physics for finding order in seemingly disordered...

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