Rajan Menon’s letter “Nobels neglect fluid dynamics” (Physics Today, January 2021, page 10) correctly points out how the importance of physicists’ real-world contributions have been undervalued in the physics community. Many physicists apparently have the misunderstanding that finding elementary forces among particles solves the world’s problems. The reality is far from it.
Since Isaac Newton’s time, it’s been well known that the three-body system cannot be solved analytically, and numerical approaches can lead to chaos. The real world consists of infinitely many–body systems whose temporal evolution is intrinsically unsolvable. Even for the simplest hydrodynamics systems, the Navier–Stokes equation, which is only an approximate model, is not solvable. Plasmas are much more rich in their time evolution, and a large number of fundamental discoveries in that area have not been properly appreciated in the physics community. Some unexpected discoveries in nonlinear continuous media certainly deserve higher valuation in terms of their real-world contributions; conspicuous examples include the applications of optical solitons in high-speed transcontinental communications and the influence that self-organization of plasma turbulence has had on fusion confinement.
So far the only reliable universal law of physics in nonlinearly interacting many-body systems remains the second law of thermodynamics, which states that the entropy of an isolated system will not spontaneously decrease. That law’s unique stature simply shows that a real physical system is unpredictable, so any new discoveries that go against the entropy law for at least a limited period of time deserve more attention. For example, in quasi-two-dimensional hydrodynamics systems, such as planetary atmospheres, a quantity called the enstrophy (the squared vorticity) is conserved in addition to the total energy, and the entropy can be defined with regard to either the uncertainty in the energy spectrum or the uncertainty in the enstrophy spectrum. The maximum-entropy state of enstrophy can lead to ordered structure in the energy spectrum and to nontrivial states such as zonal flows observed on the Jovian surface.
