Deffner, Sebastian; Michelle Driscoll; Juergen Kurths; Sid Redner and Greg Voth

What is the hardest problem in physics? While there is definitely no unique answer, pretty much anyone would agree that turbulence is a strong contender. Turbulence is ubiquitous in the universe, comprising a wide range of spatial and temporal scales. In three-dimensional (3D) homogeneous and isotropic turbulence, energy is transferred from large to small scales, the direct cascade. In two dimensions (2D), the opposite is true, the inverse cascade. Many important flows in geo- and astrophysical contexts are anisotropic due to planetary rotation, density stratification, and layer geometry. By varying the degree of anisotropy, one can study transitions between the classical cases of 2D and 3D turbulence. The physical properties of these non-equilibrium transitions, although fundamental to characterizing the nature of turbulence, remain incompletely understood. As the story goes, Werner Heisenberg had once said that, if he were allowed to ask God two questions, they would be, “Why quantum mechanics? And why turbulence?” Supposedly, he was pretty sure God would be able to answer the first question.