Acoustics provides the primary route for the study of emergent phenomena

Sound easily propagates through a gas where the thermal noise is nine orders of magnitude larger than the velocity amplitude. Acoustics then answers the key question of emergent theory: how many macroscopic variables are a complete set? It has never been proven theoretically, but everyone who studies turbulence starts with five-variable hydrodynamics as compared to first principles kinetic theory. Uhlenbeck said “the frontiers of physics are all around us” and “acoustics is the royal road.” Acoustics is a leading route to nonlinear, multi-scale phenomena. The passage of sound through a fluid can stimulate the expansion and collapse of a bubble which concentrates the acoustic energy density by 12 orders of magnitude to create a light-emitting strongly coupled plasma. Our recent study of the dispersion law for this plasma has led to a unifying theory of SCP that also has five variables as a complete set but a nonlocal equation of state. Ferroelectric crystals can have a spontaneous polarization of 100 kV; strong enough to cause nuclear fusion in deuterium gas. Rudnick used hydrodynamics to discover a quantum phase transition in angstrom-thick films. We have used acoustic radiation pressure to create the only ground-based model for the study of spherical convection.