What is Einstein's main theoretical content?

Einstein's theory of relativity involves a lot, but the basic idea can be summarized as one sentence:

Research method of physical laws based on the principle of invariance of light speed and relativity.

But in this sentence, you may still feel that it is not specific, or you may not know exactly what relativity means.

Many people doubt the theory of relativity because they don't understand the "constant speed of light". In addition, some conclusions drawn from the theory of relativity seem to be inconsistent with people's intuitive experience, which leads to some people's rejection.

But the core of relativity is not the change and invariability of light speed, but the application of relativity principle. From this point of view, relativity is just an empathy method in classical physics.

Warp physics is based on the premise of absolute space-time, that is, when the default time passes evenly, the space is infinitely uniform and straight. People think that motion is absolute, so there should be a way to judge and measure this absolute motion. This is the motion used as a reference object. But the problem is that ether has no exact detection method to capture it, so absolute motion has lost its practical significance.

However, it has been proved theoretically and experimentally that the speed of light in vacuum does not change with the speed of the light source, so it is not a problem to catch the ether, because it can be replaced by the speed of light, that is, light can be used as a reference.

However, light is different from other substances after all. We cannot assume that the speed of light is zero, so we can only assume that the speed of light is a constant value. Only in this way can it be used as a reference.

Taking the speed of light as a reference means that the speed of light cannot be changed, just as we assume that the speed of a river can only be zero under the premise of being stationary.

Another reason for misunderstanding is the Memo experiment, because the results of the Memo experiment show that the interference fringes of light do not change with the speed of the closed system. This means that as long as the relative position of the light source relative to the light receiver remains unchanged, the interference fringes of light will not change. It is a very serious misunderstanding that people mistake this phenomenon for the speed of light relative to the inertial system. This misunderstanding almost led to the complete collapse of classical physics. If interested, we can find another opportunity to discuss why the interference fringes don't change separately.

The principle of the invariance of light speed in relativity is not directly related to the results of Memmo experiment, but it is Memmo experiment that reminds Einstein of the idea of replacing ether with light. Because light is taken as the reference object, the change and invariance of the speed of light relative to the inertial system will no longer have an impact on physical research, thus successfully avoiding the difficulties brought by Memmo experiment to physics.

Because light is the reference object, the relationship between space-time and space and motion has changed from geometric expression (Einstein learned geometry very well).

For the simplest example, when we look at an object with a magnifying glass, we will feel that the object is getting bigger. According to classical physics, the expression that light is refracted, but the speed of light is constant (the size and direction are the same) must mean that the object becomes smaller (relative to the size we see). From the perspective of relativity, this statement is logically correct, but I feel unwilling to accept it. Why do we see objects smaller with a magnifying glass? This is totally out of line with people's daily experience.

I think after saying this, I may have a clearer understanding of the theory of relativity.