Resonant cavity techniques for accurate measurements of the ratio of the speed of sound to the speed of light

In principle, measurements of the resonance frequencies of both the acoustic and the microwave modes of a single cavity can determine the ratio of the speed of sound u of a monatomic gas to the speed of light c. Such measurements, carried out with high accuracy, could determine the universal gas constant R and the thermodynamic temperature T with unprecedented accuracy. The realization of these possibilities can be greatly facilitated by judicious choices of cavity geometry and resonance modes. The present state of the art suggests that the ratio u/c can be measured to parts per million accuracy using cavities whose geometry is known only to parts per thousand. Recent experimental and theoretical results will be presented.