An answer to this question could include the following:
The principle of relativity states that it is not possible to detect motion with uniform velocity while in one frame of reference without referring to another frame of reference. Thus, the principle only applies for non-accelerating steady motion, which occurs in an inertial frame of reference. Within an inertial frame of reference, experiments or observations cannot be performed which would indicate whether an object is stationary or moving with constant velocity. The speed of light, c, is constant in inertial frames of reference, which leads to time dilation, etc.
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