Experimental Demonstration of Gravitational Impact on the Speed of Light

A cornerstone of Einstein’s special relativity is the postulation that light travels at a constant velocity in vacuum throughout spacetime, independent of the direction, location, time and photon-energy. The constancy and universality of the speed of light play an important role in modern physics for understanding the universe, and their validation has been demonstrated by various experiments and observations. Here we present an experiment to demonstrate that photons could interact with the Earth gravitational field and consequently the speed of light would be affected by the Earth gravity. The experimental results show that the speed of light increases with the gravitational field and the variation fits with a test model proposed in this work. The finding of this work would open a new research direction on the interaction between photons and the gravitational field and could lead to explore novel gravito-optic effects and photonics-based gravity detection technologies.