Last thread that I initiated was titled `Simultaneity is directional while Time Dilation is not', which presented a scenario for the Non-directional property of Time Dilation in STR. Which I think, needs to be reconsidered. And I believe this is the reason why `STR can only be worked out when we consider one reference frame at a time only'. Being Non-directional implies: Time dilation would occur to each and everyone, NO matter which direction they tend to move. Let us try to realize the above statement by considering a simple thought experiment : Let us suppose that, there exist two very distant stars, of which sun lies at the middle of the line joining these two stars. Naming them star A and star B, two twin astronauts Adam and Bob in their identical and respective rockets blast off from the Earth at the same time. Destination of Adam is star A whereas for Bob it is star B. They return to earth let's say after 10 years completing their identical trip in reference frame of Earth. Now, the question is `Will the brothers still be of the same age, i.e age of Adam = age of Bob?' since, according to Adam, Bob is getting younger due to relative motion, whereas, according to Bob, Adam is getting younger due to relative motion. Moreover, if your answer is `yes', then undoubtedly, Earth is a preferential reference frame for your calculations, which violates the `Principle of relativity'. Note: gravitational effects should be neglected.