Have been reading the "Impossibility of exceeding c" notes in the Lorentz Transformation physics forum page: www.physicsforums.com/library.php?do=view_item&itemid=19 Where it states: "It is often said that nothing can be accelerated to the speed of light because its mass increases as it gets faster. However, the fundamental reason is simply that "adding" speeds only adds tanh-1(speed/c), and so no amount of adding can make (speed/c) equal to (or greater than) 1." Is this right - I understood that one of the posits of the Lorentz Transformations was that: Both frames, the transforms are applied to, agree on the speed that a light beam is travelling at, and this is c, as this is required by special relativity. This restriction was introduced after the results of the Michaelson Moorley interferometer experiment. So when the transforms are derived for frames observing a light beam we have already restricted the transforms to this! Is'nt the use of rapidities just a re-representation to make it easier to use the transforms?