In this paper, consider a rod A (inertial frame A) and rod B (inertial frame B) moving at constant velocity relative to each other. Assume that the lengths of two rods are equal when they are stationary. According to the STR, when length in the direction of motion of rod B, moving at constant velocity, is measured from inertial frame A, the rod contracts in the direction of motion. Also, the time which elapses on clock in inertial frame B is delayed compared to the time which elapses on clock in inertial frame A. If, conversely, inertial frame A is measured from inertial frame B, rod A contracts in the direction of motion, and the time which elapses on clock is delayed. However, according to classical common sense, if rod B contracts when measured from inertial frame A, then rod A measured from rod B must be longer than rod B. Thus, this paper discusses the symmetry of rod contraction, and elucidates this problem. It is found, based on the discussion in this paper, that the contraction of a rod includes true physical contraction, and relativistic contraction obtained due to measurement using the method indicated by Einstein. However, in the STR, any two inertial frames are equivalent, and therefore is not possible to accept points such as the fact that reasons for contraction are different. This paper concludes that STR is not a theory which describes the objective state of reality.
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