The butterfly effect is today commonly identified with the sensitive dependence of deterministic chaotic systems upon initial conditions. However, this is only one facet of the notion of unpredictability pioneered by Lorenz, who actually predicted that multiscale fluid flows could spontaneously lose their deterministic nature and become intrinsically random. This effect, which is radically different from chaos, have remained out of reach for detailed physical observations. Here we show that this scenario is inherent to the elementary Kelvin–Helmholtz hydrodynamical instability of an initially singular shear layer. We moreover provide evidence that the resulting macroscopic flow displays universal statistical properties that are triggered by, but independent of specific physical properties at micro-scales. This spontaneous stochasticity is interpreted as an Eulerian counterpart to Richardson’s relative dispersion of Lagrangian particles, giving substance to the intrinsic nature of randomness in turbulence.

如今，蝶形效应通常被认为是确定性混沌系统对初始条件的敏感依赖性。但是，这只是Lorenz提出的不可预测性概念的一个方面，后者实际上预测多尺度流体流动可能会自发地丧失其确定性并本质上变得随机。这种影响与混乱有根本的不同，对于详细的物理观察而言，仍然遥不可及。在这里，我们证明了这种情况是初始奇异剪切层的基本Kelvin-Helmholtz流体动力学不稳定性所固有的。此外，我们提供的证据表明，所产生的宏观流动显示出普遍的统计特性，这些特性是由微观尺度上的特定物理特性触发的，但与它们无关。