Thermally Induced Fluctuations below the Onset of Rayleigh-Bénard Convection

We report quantitative experimental results for the intensity of noise-induced fluctuations below the critical temperature difference $\Delta T_c$ for Rayleigh-Bénard convection. The structure factor of the fluctuating convection rolls is consistent with the expected rotational invariance of the system. In agreement with predictions based on stochastic hydrodynamic equations, the fluctuation intensity is found to be proportional to $1/\sqrt{-\epsilon }$, where $\epsilon \equiv \Delta T/\Delta T_c-1\mathrm{}$. The noise power necessary to explain the measurements agrees with the prediction for thermal noise.