Nozzles {Laval nozzle}, such as rocket nozzles, can have narrowing, in which fluid exceeds sound speed but makes no shock wave. Narrowing pushes sound going upstream back. Original sound wavelength is distance between molecules. Above boundary, pushing back increases wavelength. Below boundary, pushing back makes sound faster than it can travel. At boundary, at near-zero temperature, sound emits thermal-phonon pairs. One pair member can go up flow, and one down flow. At near-zero temperature, narrow region acts like black-hole event horizon.
Physical Sciences>Physics>Quantum Mechanics>Theory>Quantum Relativity
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Date Modified: 2022.0224