1905, Einstein published a basic paper on the special relativity of electrodynamics of moving objects. Regarding the basic principle of special relativity, he wrote: "The following considerations are based on the principle of relativity and the principle of invariance of light speed. We stipulate these two principles as follows:

1. The laws followed by the state changes of physical systems have nothing to do with which of the two coordinate systems with uniform motion is used to describe these state changes.

2. Any light moves at a certain speed c in the "stationary" coordinate system, no matter whether it is emitted by a stationary or moving object. "

The first is the principle of relativity, and the second is the constant speed of light (artificial assumption). The whole special theory of relativity is based on these two basic principles.

Einstein's philosophy is that nature should be harmonious and simple. In fact, his theory usually has a remarkable feature: simplicity and profundity. Special relativity is a system with this characteristic. The two basic principles of special relativity seem to be "simple facts" that are not difficult to accept, but their inferences have fundamentally changed the physics foundation since Newton.

We will start this inference later.