In another thread that was locked, Chris Hillmann told me: Understanding time still makes me scratch my head Don't we believe that the twin paradox is true, i.e. if I go on a long journey with high enough speed and come back that I am less old than the twin brother I left behind. This is not just 'times kept by observers'. We are not just talking about mechanical or other clocks. I am biologically provable younger than my brother. If I would have taken along radioactive material, it would have decayed less than the same amount of the same material left behind. Everything that went along aged less than similar things left behind. May I at least say that SR shows that things age slower on a speedy journey? Well, this is what I see by comparison when I come back from the journey. But then, how is time different from a measure of age? (Please no arguments that GR is needed for the twin paradox, see http://www.math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html) Still puzzled, Harald.