Cavitation — the nucleation of vapor bubbles in a liquid — requires negative pressure: tension that pulls the liquid apart faster than its molecules can hold together. The standard route is a rarefaction wave: a region of expanding, decompressing fluid. You need decompression to get bubbles. That's the textbook.
Fiorini and colleagues demonstrated cavitation inside a sub-millimetric droplet using a purely compressive shock wave. No rarefaction. No decompression anywhere in the system. The shock wave enters the droplet as positive pressure and stays positive pressure — yet it generates negative pressure at the focal point, tearing the liquid apart from inside.
The mechanism is the Gouy phase shift, borrowed from optics. When a converging wave — light or sound — passes through its geometric focus, it undergoes a π phase shift. For a pressure wave, this means the peak of compression converts to a trough of tension at the focal point. The wave doesn't decompress in transit. It flips polarity at convergence. The geometry does the sign change, not the source.
High-speed X-ray imaging confirmed homogeneous nucleation at the focus — bubbles forming in pristine liquid, not from pre-existing gas pockets or surface defects. The cavitation was initiated by the geometry of the wave's own convergence, not by anything external.
The structural lesson: a system can generate its opposite at a singular point through convergence alone. Compression everywhere, tension only where the wave focuses. The same mechanism that optics has known for two centuries — the Gouy shift — produces the same effect in acoustics, but with explosive consequences. What focuses also inverts. The bubble is born from the geometry of the squeeze, not from any relaxation of it.