ralfcis, As usual, I can't make much sense of what you've written. You claim SR is different than relativity, you still have problems with length contraction, it's a mess as usual. But if you will indulge me, I can give you some constructive ideas that you can think about in your spare time. All of this will take place in an inertial frame of reference, (that means a non-accelerating reference frame, with no gravity -- I'm sure you know that, but you never seem to mention it, so I am trying to be very clear). Now, you know that the speed of light is constant in that reference frame. So if you were to send a very short pulse of light from one mirror to another mirror a known distance away (both stationary with respect to our inertial reference frame), you would know how much time would be required for the light to travel that distance, and how much time would be required for the light to reflect back to where it came from. That could be considered to be one complete cycle of a clock, let's call this a "light-clock" since it uses light bouncing back and forth between two mirrors to count a repeating time interval. For simplicity, let the light clock be in a vertical orientation, so that if we consider another reference frame that is moving horizontally with respect to the first reference frame, we will not have to consider length contraction of the distance between the mirrors. Makes sense so far? Okay, so let's do exactly that and consider another reference frame that is moving horizontally with respect to the first reference frame. The speed of light is also constant in this second reference frame, but in this new frame the light clock is moving horizontally at constant speed, so as the light inside it travels up and down between the mirrors, it also has to take a diagonal path that is longer than the light path was in the other frame where it only moved straight up and straight down. Here are some crude diagrams made using text: In the first reference frame we have light following this type of path || over and over (because the light-clock is stationary), and in the second reference frame we have that same light in that same light clock following this type of path /\/\/\/\/\/\ (because the light-clock is moving horizontally. You should be able to use the Pythagorean Theorem to determine exactly how much slower the rate of the light clock is in the second reference frame, compared to the first. That is time dilation. Now, earlier in the thread you were claiming that time dilation can be explained by different start and stop times, or some such nonsense that never made any sense. Do you now understand that time dilation is a direct result of the constant speed of light in all inertial reference frames, as shown by the above light-clock exercise? And if so, why did it take you this long to even understand what time dilation is? Is it because you "learned" SR from Brian Green videos and he never mentioned light clocks? If you had wanted us to believe that you have a good understanding of SR, then you should have covered these simple basics long ago. Now do you see why you should probably be studying SR, instead of trying to teach it "your own way"?