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 a rigid body (neither in special nor in general relativity)—so the signal velocity of an object never exceeds the speed of light. Another criticism that was raised by both Lenard and Gustav Mie concerned the existence of "fictitious" gravitational fields in accelerating frames, which according to Einstein's Equivalence Principle are no less physically real than those produced by material sources. Lenard and Mie argued that physical forces can only be produced by real material sources, while the gravitational field that Einstein supposed to exist in an accelerating frame of reference has no concrete physical meaning. Einstein responded that, based on