Special relativity to a first observer O, and frame S′ (pronounced "S prime") belongs to a second observer O′. Since there is no absolute reference frame in relativity theory, a concept of 'moving' doesn't strictly exist, as everything is always moving with respect to some other reference frame. Instead, any two frames that move at the same speed in the same direction are said to be "comoving". Therefore, "S" and "S"′ are not "comoving". The principle of relativity, which states that physical laws have the same form in each inertial reference frame, dates back to Galileo, and was incorporated into Newtonian physics.
Criticism of the theory of relativity many philosophical critics that they had insufficient knowledge of the mathematical and formal basis of relativity, which led to the criticisms often missing the heart of the matter. For example, relativity was misinterpreted as some form of relativism. However, this is misleading as it was emphasized by Einstein or Planck. On one hand it's true that space and time became relative, and the inertial frames of reference are handled on equal footing. On the other hand, the theory makes natural laws invariant—examples are the constancy of the speed of light, or the covariance of Maxwell's equations. Consequently, Felix Klein (1910)