Abstract

Considering that photons travel at the velocity of light c in the fundamental frame, we expect logically that these photons travel at velocity c-v (or c+v) with respect to a frame moving at velocity v. We know that the observed velocity is measured as c. However, that logical consequence has never been explained. Using Newton's physics and conventional logic, we explain how the velocity of light APPEARS constant in the two way measurement of the velocity of light, while it is c-v (and c+v) in the Sagnac effect. We answer the question: "With respect to what does light move?" This paper gives a physical explanation how the velocity of light is really (c-v) with respect to the observer, even if the observer's tools always measure a velocity represented by the number c. We explain how this problem is crucial in the Global Positioning System (GPS) and in clocks synchronization. The Lorentz' transformations become quite useless. This apparent constant velocity of light is the most fascinating illusion in science.