1st Q~ if a man in a spaceship travelling at say 0.5 c measures two beams of light outside the spaceship that are in opposite direction. (the two beams are from sources outside the ship) he would measure both light's speed as c as the einstein says. however, the distances for the two beams he measured would not be same.(or would they be?) if that's the case, he would have two sets of time. can he choose which one he'd like to use? 2nd Q~ if c is indeed constant as the relativity says, would say 0.5c also be constant? from what i learnt in math, i know if c is constant,0.5c should be, too. but it doesnt seem to be a math matter. If it is not constant, then people in relative motions would measure it differently. if a spaceship travelling at 0.6c measures another ship travelling at 0.5c(measured by 3rd person) but in opposite direction, what speed of the 2nd ship would be measured in the 1st ship? again from basic math,i reckon it'd be 1.1c,which is faster than c. is that right? If it is constant. then i could derive many interesting conclusions from it. if 0.5c is constant(i mean the velocity of an object which is 0.5c), there's no reason why 0.05c is not constant.(or is there?) if i walk with a speed of 1m/s, that is (3.33 * 10^-9) c. if that is constant, then people around me standing or running would agree on the speed of me,but not the distance i travelled. they would have different sets of time. and then back to the 1st question, they would have tons of sets of time. how do they choose? u tell me.