Owen Duffy wrote:

I can see that we can deal mathematicly with two or more coherent
components thought of as travelling in the same direction on a line (by
adding their voltages or currents algebraicly), but it seems to me that
there is no way to isolate the components, and that questions whether
they actually exist separately.

I reckon that if you can't see them, measure them separate them etc,
then they don't exist.
It's different to being in a null between two antennae. The signals
don't appear to exist where you are, but move directly away from either
antenna, and there they are. So they exist at the null even though you
can't see them there.
Alan