Relative Velocity of Light and Radio Waves in Space

SINCE the velocity of light is one of the most fundamental physical quantities, any measures which indicate its constancy with wave-length are of basic physical interest. Experimental determinations1,2 in the optical region and in the short-wave region indicate a constancy to 1 part in 106. Astronomical measurements, because of the great distances involved, hold tho promise of extremely precise determinations of the relative velocities of electromagnetic waves from the observation of distant events that occur simultaneously or nearly simultaneously at different wave-lengths. The recent observations of optical and radio events in flare stars now provide the basis for such determinations.