Editing Superluminal motion (section)

====Signal velocity====
The model identifies a difference between the information carried by the wave at its signal velocity ''c'', and the information about the wave front's apparent rate of change of position. If a light pulse is envisaged in a wave guide (glass tube) moving across an observer's field of view, the pulse can only move at ''c'' through the guide. If that pulse is also directed towards the observer, he will receive that wave information, at ''c''.  If the wave guide is moved in the same direction as the pulse, the information on its position, passed to the observer as lateral emissions from the pulse, changes.  He may see the rate of change of position as apparently representing motion faster than ''c'' when calculated, like the edge of a shadow across a curved surface.  This is a different signal, containing different information, to the pulse and does not break the second postulate of special relativity. ''c'' is strictly maintained in all local fields.