Weirdly enough, the term held a much smaller place in science until about 50 years ago. Scientists were well aware, of course, of nonlinear interactions, effects, equations and processes, but few people noticed their profound importance. The famous Fermi–Pasta–Ulam experiment in 1953 showed that nonlinearity threatened the legitimacy of fundamental assumptions in statistical mechanics — namely, the notion of ergodicity, or the idea that a system over time will visit all states equally, so that time averages can be replaced with easier-to-calculate ensemble averages. The rediscovery of the ubiquity of dynamical chaos from the 1960s onwards made nonlinearity a household word — at least in households populated by physicists, mathematicians, meteorologists or ecologists.

But nonlinearity also has other meanings, as I noticed when absorbing the message of a recent paper on human ageing. The title is "Nonlinear dynamics of multi-omics profilesduring human aging" (X. Shen et al. Nat. Aging https://doi.org/10.1038/s43587-024-00692-2; 2024). But in what sense are these dynamics ‘nonlinear’?

奇怪的是，直到大约 50 年前，这个词在科学中的地位要小得多。当然，科学家们很清楚非线性相互作用、效应、方程和过程，但很少有人注意到它们的深刻重要性。1953 年著名的 Fermi-Pasta-Ulam 实验表明，非线性威胁到统计力学中基本假设的合法性——即遍历性的概念，或者系统随时间推移将平等地访问所有状态的想法，因此时间平均值可以被更容易计算的集成平均值所取代。从 1960 年代开始，人们重新发现了无处不在的动力学混沌，使非线性成为家喻户晓的词——至少在物理学家、数学家、气象学家或生态学家的家庭中是这样。