In most special relativity primer book, length contraction section, the length is always defined or measured by the time of a light flashing at one end of a rod, traveling to the other end, reaching a mirror and reflecting back to the first end. Please read this example if I did not make myself clear. http://www.berkeleyscience.com/relativity.htm My question is, why cannot we define or meature the length by a one-way trip of the light flash, either from left end to right end, or from right to left? And if we define this way, the result will be different than L=L'/γ. The length seems to be more contracted towards the moving direction of O' frame L=L'/γ * (c / (c+v)). The length also seems to be less contracted opposite the moving direction L=L'/γ * (c / (c-v)) So, could anyone explain to me: 1. Why is length defined this way? 2. How to explain to results of one-way light trip definition?