The unification of relativity and quantum mechanics is the main problem of modern physics. It is still unsolved. The root of this problem could perhaps be the relativistic postulate of the invariance of the speed of light. In countless experiments it was found that the speed of light does not depend neither on the velocity of the light source (c 1) nor on the velocity of the observer (c 2), but in special relativity this dual constancy of light (c 1, c 2) is not understood well. In Einstein’s theory these two parameters c 1 and c 2 are simply supposed as two fundamental principles of nature. In this paper it is shown how this dual constancy of light (c 1, c 2) could be considered as an expression of the quantum mechanical wave-particle-dualism. This quantum-mechanical interpretation implies not only a dual parametrization of c, but the existence of a sort of ether (probably the vacuum) as well. As the result of the dual parametrization of c an etherdrift is predicted, that is significantly smaller than all the values which were expected in previous times. Even if the velocity of Earth of approx. 390 km/s with respect to the cosmic microwave background is taken, the observable ether drift would only be of D = 0.0003 km/s.

Keywords: wave-particle-dualism, fundamental constant, light, special relativity