All physical laws (except mechanics, electromagnetism and other dynamic laws of interaction) are valid in all inertial systems; In other words, under the Lorentz transformation, the equations of all physical laws (except gravity) remain unchanged. Experiments conducted at different times give the same laws of physics, which is the experimental basis of relativity principle.
Principle of constant speed of light:
Light always travels at a certain speed c in vacuum, and the speed has nothing to do with the motion state of the light source. In all directions in a vacuum, the propagation speed of optical signals (that is, the unidirectional speed of light) is the same (that is, the speed of light is isotropic).
The speed of light has nothing to do with the motion state of the light source and the observer's inertial system. This principle is incompatible with classical mechanics. With this principle, we can accurately define the simultaneity of different places.
Extended data:
The establishment of special relativity has greatly promoted physics and penetrated into the scope of quantum mechanics. It has become an indispensable theory for studying high-speed particles and has achieved fruitful results. But there are two fundamental problems in principle that have not been solved.
1, first of all, the difficulty of defining inertial system. Due to the negation of "absolute space-time", inertial reference system (inertial system) has become an undefined concept. If the inertial system refers to the frame of reference in which Newton's second law is established, then Newton's second law can only be established in the inertial system, thus falling into a "logical cycle", and the whole theory is like building on the beach.
2. The second is the difficulty caused by gravity. The law of universal gravitation is closely related to "absolute space-time" and must be revised, but it is not covariant under Lorentz transformation, so it cannot be included in the framework of special relativity. It was not until the establishment of general relativity that this problem was completely solved.
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