Modern physical theory is not a complete theory, especially the overall self- consistency. The incompleteness of the theory is not limited to the quantum mechanics that describes the microstructure, but also includes the macroscopic theory based on "relativity". For example, light theory, space theory, etc. In the theory of relativity, "the speed of light is constant" is put forward as a basic assumption. In fact, the "constant speed of light" is due to the Doppler redshift of light. The reason for the Doppler redshift of light is that "the speed of light does not change". This article will take you to understand the physical principle of this mechanism. Through cosmological redshift, we can draw the conclusion that vacuum space is