In the theory of special relativity when two observers are in relative motion with each other then each observer measures an increase in mass, a slower clock rate and a contraction of length in the other observer's frame of reference. How are we to interpret this according to the theory of special relativity? Is this actually happening to both frames or is this simply an effect of the relative motion. In other words is it considered real or simply a distortion of reality? Edit: highlighted text to clarify the questions.