I've got a simple question I can't find any quick answer to. I understand that if various observers with different relative kinetic energy (velocity) are to measure the speed of light of the same event the same (c), time and space values must be different for them. But how do we know which of the variables changes and by what fraction? I believe that the Lorentz transformation take the fraction to be 50:50 for both time and space values. Why not that time slows by 1/3 and space shortens by 2/3 or any other arbitrary fraction?