Peak temperature and density inside a collapsing sonoluminescent bubble
In plain words
A single bubble trapped in a sound field collapses once per cycle and emits a flash of light lasting less than a billionth of a second. Nobody has measured directly how hot and how dense its center becomes.
Precise statement
For a stably driven argon or air bubble in water (ambient radius $R_0 \sim 5e-4\,\mathrm{cm}$, drive frequency $\sim 2.5e4\,\mathrm{Hz}$, acoustic pressure amplitude $P_a \sim 1.2\ \text{to}\ 1.5\,\mathrm{bar}$), determine the spatially resolved peak temperature $T_{\max}$ and gas density at collapse. Spectral fits in water give $T$ of order $6e3\ \text{to}\ 2e4\,\mathrm{K}$, spectroscopy in concentrated sulfuric acid shows plasma signatures, and some models predict converging shocks heating a small core above $1e5\,\mathrm{K}$. An answer is a measured $T_{\max}(r)$ with error bars, or a simulation that matches spectra, absolute photon number and pulse width simultaneously without free parameters.
What would settle it
A model-independent temperature diagnostic of the collapse core (for example time-resolved line broadening or X-ray or neutron yield limits) consistent with a parameter-free bubble-dynamics simulation.
Status in the literature
Unverified note
Temperature estimates in water remain model-dependent and differ by factors of several; no direct measurement of the core temperature as of 2026 (not re-verified for 2024-2026).