Light, Time, Mass, and Length

Abstract

Because the speed of light is absolute, space, time, and mass itself must be relative: We have to specify the speed of rods or clocks when we determine space and time around us. Rods moving relative to us become shorter when lying in direction of their speed, but not when lying at right angles. Moving clocks go slower. Events at different places may happen “at the same time” for someone, but not at the same time for someone else traveling at different speed. We show how this works out, how to deal with moving rods and clocks, and that there is really no contradiction. What is more, we use these facts to show how much mass increases with speed, and how to add speeds correctly.