Effect of gravity on the Rayleigh linewidth near the critical point

The influence of Earth's gravity on measurements of the Rayleigh linewidth $\Gamma$ of a simple fluid near its critical point is investigated in detail. Special attention is given to the scaled linewidth ${\Gamma }^{*}$. From a number of different viewpoints it is found that gravity significantly affects linewidth measurements in the critical region. Small errors in beam centering render available experimental data useless for the purposes of distinguishing between modern theories, as variations in beam height split the universal curve of the scaled linewidth into a single-parameter family. The mode-mode coupling and decoupled-mode theories, which in the gravity-free case predict slightly different results for $\Gamma$ and ${\Gamma }^{*}$, predict qualitatively different behavior in the dependence of $\Gamma$ on height and scattering angle when gravity is taken into account. When the relative deviation of ${\Gamma }^{*}$ from its gravity-free uniform fluid value is plotted versus the reduced correlation length, it exhibits distinct patterns characterizing the different theories used to compute it. By comparing these patterns with the results of carefully controlled experiments, gravity should prove to be a useful tool for eventually selecting a correct theory.