Does the speed of light stay the same? What is the reason for the constant speed of light?

The constant speed of light means that the speed of light in a vacuum is a fixed value c (approximate 300,000 kilometers per second) in any reference frame.

After Maxwell completed his theory of electromagnetism, he predicted the existence of electromagnetic waves according to his electromagnetic theory, and at the same time calculated the speed of electromagnetic waves according to the electromagnetic properties of the vacuum (permissivity and permeability), and this calculated speed was consistent with the speed of light measured at that time, so Maxwell believed that light was also an electromagnetic wave. After that, physicists have a question: does the speed of light require a specific frame of reference?

At the beginning, physicists assumed that there was a special medium ether, and that the speed of light relative to the ether was equal in all directions, and the earth moved in the ether, so they believed that the speed of light relative to the earth in all directions was not equal. Through the observation of the phenomenon of optical aberration, people have come to the conclusion that the movement of the earth does not cause the aether to move together (that is, the revolution and rotation of the earth and the movement of the entire solar system in the banking system are independent of the aether, and the earth moves in the aether). So, there has to be a new theory to explain this phenomenon, and this new theory is Einstein's special theory of relativity. According to Einstein's point of view, ether is a dispensable concept, the permittivity and permeability of a vacuum are fixed values, no matter in which frame of reference they are the same, so no matter in which frame of reference, the speed of light in a vacuum is a constant value c, special relativity is a seemingly absurd theory based on such a seemingly absurd premise, but there is no contradiction in this theory itself, and many seemingly absurd predictions made based on this theory have been proven to be correct one after another, so, The physics community accepted such a seemingly absurd theory.

The constant speed of light means that in a vacuum, the propagation speed of light is constant, independent of the state of motion of the light source or the state of motion of the observer. This was proposed by Albert Einstein in his special theory of relativity.

The basic assumption of special relativity is that the form of the laws of physics is the same in all reference frames, and that the speed of light is the same constant in all reference frames, i.e., 299,792,458 meter seconds. This is known as the "principle of invariance of the speed of light".

Albert Einstein proved the principle of the invariance of the speed of light through experiments and theoretical derivations, which he used as the basis for the special theory of relativity. This theory explains why the speed of light remains the same in high-speed moving objects, and brings many other relativistic effects, such as time dilation and length contraction.

In conclusion, the principle of invariance of the speed of light is a very important concept in modern physics, and it is one of the foundations of our understanding of the natural world.