Consider a phenomenon
that's not coupled with the system. To examine the motion of a phenomenon
relative to the outside observer of the system (which is the observer living in
the laboratory), we do not need to examine that phenomenon relative to the
system.
But if the phenomenon is coupling with the system, then to examine the
system by any other observer, we need to study the phenomenon relative to the
system (it means, we need system information about the phenomenon).
For
example, considering the light is not coupled with the train (you can ask why),
If a light flash on the moving train turns on, The observer in the station can
talk about this light without needing the train information (or internal
observer's information).
But if a sound wave is generated on the same
train, considering the sound wave is coupled with the train (you can still ask
why), Then the observer in the station needs the internal observer's
information about this sound wave to examine that.
We (in the APG) call this
concept coupling physics.
To examine any phenomenon relative to any observer
(or system), we must consider that the phenomenon is coupling with the system
or not (relative to the observer or the system under study), and this is what
we call (in the APG) very special relativity. (M.A.Sh)
