Special relativity seems to harmonize Maxwell's equation with Galileo's principle of relativity. Maxwell established a systematic electromagnetic field theory, predicted the existence of electromagnetic waves, and pointed out that the propagation speed of electromagnetic waves in vacuum is constant, which is always the speed of light C, which is inconsistent with Newtonian mechanics. At that time, it was assumed that light was a wave, and ordinary waves must propagate in the medium. At that time, a medium was assumed: "ether".

The Michelson-Morey experiment was also designed to measure the speed change of light relative to the "ether", but no change was found. This experiment proves that ether does not exist. Later, Lorentz proposed Lorentz transformation, but it was considered as a mathematical aid. Later, Einstein completely abandoned the hypothesis of ether, re-deduced Lorenz transform and put forward a new view of time and space. Both time and space are related to the relative motion of inertial reference frame, so that the constant speed of light in vacuum does not conflict with Galileo's principle of relativity, and special relativity is born. Later, in order to integrate the theory of universal gravitation and special relativity, acceleration and gravitational equivalence were deduced from the equality of gravitational mass and inertial mass. The laws of physics have the same form in all reference frames. "Gravity" is interpreted as a mass object bending the space-time around the object and making other objects around it move along the curve. General relativity was founded.