Einstein broke through the shackles of traditional thinking, put forward new concepts and established the theory of relativity. This new concept is () A. The speed of light in vacuum.

A, Einstein broke through the shackles of traditional thinking and thought that the speed of light in vacuum is the limit of speed in nature, so A is correct.

B, Newton's law is the basis of classical physics, only applicable to low-speed and macro fields. Therefore, b and c are wrong.

D, the moving clock slows down, the moving ruler shortens, and the mass of the moving object increases. These are several important effects of special relativity, so D is wrong.

So choose a.