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 the general covariance of his 1915 theory of gravity ensured implementation of a generalized relativity principle. This view was challenged by Erich Kretschmann (1917), who argued that every theory of space and time (even including Newtonian dynamics) can be formulated in a covariant way, if additional parameters are included, and thus general covariance of a theory would in itself be insufficient to implement a generalized relativity principle. Although Einstein (1918) agreed with that argument, he also countered that Newtonian mechanics in general covariant form would be too complicated for practical uses. Although it is now understood that Einstein's response to