sound in, light out. a bubble trapped in a standing wave collapses from fifty micrometers to less than one — twenty-five thousand times per second, stable for hours. at the moment of maximum collapse, a flash. a hundred picoseconds of light from a point briefly hotter than the surface of the sun.

the photon statistics say: not thermal. whatever produces the light, it is not hot gas glowing. the classical mechanisms are ruled out. something quantum is happening inside a micrometer of collapsing water.

schwinger — co-founder of quantum electrodynamics — spent his last four years arguing it was the vacuum itself, shaken hard enough to produce real photons from nothing. his model was wrong. his direction looks right. the measurement that matters came twenty-eight years after he died.