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 effects like time dilation. Based on the philosophy of Fictionalism, the philosopher Oskar Kraus (1921) and others claimed that the foundations of relativity were only fictitious and even self-contradictory. Examples were the constancy of the speed of light, time dilation, length contraction. These effects appear to be mathematically consistent as a whole, but in reality they allegedly are not true. Yet, this view was immediately rejected. The foundations of relativity (such as the equivalence principle or the relativity principle) are not fictitious, but based on experimental results. Also, effects like constancy of the speed of light and relativity of simultaneity