[Vern]

I'm not sure that it makes sense to consider a macroscopic, massless, rigid object traveling at the speed of light.  The internal forces that make objects rigid propagate at the speed of light: i.e. bits of the object "talk to" each other at the speed of light and "tell" each other to stay rigid.  If the entire object is already moving at the speed of light, this would be a problem.

Very well stated !! You have just described the true cause of relativity phenomena. []

I first saw this treatment of relativity phenomena described by H. Ziegler in an article by Einstein.