I have some questions about time dilation I not really understand to come further (and I find it all very interesting). 1) I understand that the situation between A (standing still) and (moving) B is SRT symmetric, so if they compare clocks they both see the same time. In fact there is no time dilation in this situation to messure (you can not suppose if you are standing still, that your clock is going slower because somebody else is going to move compared with you, let's say an astronaut in space) ? Does this mean that a time is measured in this case included a time dilation (and same of course) ? 2) Only if forces are used, a time dilation is to measure ? 3) But why is a Muon lifetime to measure ? (let's say his time is always 4 seconds if standing still (A), if moving (B) his 4 seconds takes 6 seconds for (A), so we see on our clock 6 seconds, if the Muon could show a clock it should show 4 seconds, where is now the symmetry) 4) Some professor in the SRT says, the most time dilation (A or B) is for whom travels the longest worldline (path in a time space diagram). But it is always symmetric ? 5) Can you express time with light waves between two points in space e.g. point 1 is our reference point Earth and point 2 has a constant speed going into space ?