Blocking such an example and the failure of the experiment trying to prove the movement of the earth relative to "smooth coal" have caused a conjecture: the concept of absolute rest does not conform to the characteristics of phenomena not only in mechanics, but also in electrodynamics. On the contrary, it should be considered that all coordinate systems suitable for mechanical equations are also suitable for the above electrodynamics and optical laws, which has been proved for the first-order trace. We upgrade this conjecture (whose content will be called "relativity principle" later) to a postulate, and introduce another postulate that seems incompatible with it on the surface: light always propagates at a certain speed c in vacuum space, regardless of the motion state of the emitter. Starting from these two postulates, according to Maxwell's static body theory, it is enough to get a simple and contradictory dynamic body electrodynamics. The formulation of "optical ether" will prove to be redundant, because according to the viewpoint to be expounded here, it is neither necessary to introduce an "absolute static space" with special properties nor to specify a velocity vector for each point in the vacuum space where the electromagnetic process takes place. The theory to be explained here-like other electrodynamics-is based on the kinematics of rigid bodies, because any such theory is about the relationship between rigid bodies (coordinate systems), clocks and electromagnetic processes. Inadequate consideration of this situation is the root of the difficulties that must be overcome in dynamic electrodynamics at present. A kinematic part 1 has a coordinate system in which Newton's mechanical equations are valid. In order to make our expression more rigorous, and to literally distinguish this coordinate system from other coordinate systems to be introduced later, we call it "static system". If a particle is stationary relative to this coordinate system, its position relative to the latter can be determined by rigid measuring rod according to the method of Aureli geometry, and can be expressed by Cartesian coordinates. If we want to describe the motion of a particle, we will give its coordinate values as a function of time. Now we must remember that such a mathematical description has physical significance only after we know exactly what "time" means here. We should take into account that all our judgments that work all the time are always about simultaneous events. For example, when I say "that train arrived here at 7 o'clock", it probably means "the short hand of my watch points to 7, which is the same event as the arrival of the train." Some people may think that it is possible to overcome all the difficulties in defining "time" by replacing "time" with "the position of the short hand of my watch". In fact, if the problem is only to define a time where this table is located, then such a definition is enough; However, if the problem is to connect a series of events in different places in time, or-the result is still the same-to determine the time when those events occurred far away from this table, then this definition is not enough. Of course, we may be satisfied with measuring the time of an event in the following way, that is, let the observer be at the coordinate origin of the watch. When every light signal indicating the occurrence of an event reaches the observer through an empty space, he will correspond the position of the hour hand with the time when the light arrives. But this correspondence has a disadvantage, as we already know from experience, which is related to the position of the observer and the table. Through the following consideration, we get a much more practical judgment method. If a clock is placed at point A in space, the observer at point A can determine the time of events near it by finding out the position of the hour hand that appears at the same time as these events. If a clock is placed at point B in space, we will add "This is the same as the clock placed at point A". Then, the observer at point B can also find the time of the event near point B. However, if there is no further regulation, it is impossible to compare the events at A with those at B in time. So far, we have only defined "time A" and "time B", but not the time shared by A and B. Only when we define that the time required for light to travel from A to B is equal to the time required for light to travel from B to A, can we define the time of A and B, which is set at "time A" tA, and a beam of light is emitted from A to B. It reflects from B to A and returns to A at "time A" ... If TB-TA = T'A-T 'B, then by definition, these two clocks are synchronous. Assuming that this definition of synchronicity can be consistent and applicable to any number of points, the following two relationships are generally true: 1. If the clock at B is synchronized with the clock at A, the clock at A is synchronized with the clock at B. 2. If the clock at A is synchronized with the clock at B and the clock at C, then the two clocks at B and C are also synchronized with each other. In this way, with the help of some (hypothetical) physical experience, we define what synchronization means for clocks that are stationary in different places, thus clearly obtaining the definitions of "simultaneity" and "time". The "time" of an event means that the stationary clock at the place where the event occurred is synchronized with the event, and that the clock is synchronized with a specific stationary clock, and all time measurements are synchronized with the specific clock. According to experience, we also take the following value 2|AB|/(t'A-tA)=c as a universal constant (the speed of light in vacuum). The key is that we use a clock that is stationary in a stationary coordinate system to define time. Because it belongs to the static coordinate system, we call the time defined in this way "static time".