The Reciprocality of Relativity.
A Frame of Reference then, maps the whole of Spacetime relative to a single, specific, Spacetime Point, or Event, that we care to define, real or imaginary. And, as a single specific Event in Spacetime is a moment in time, it cannot, therefore, be moving; Spacetime must be fixed relative to that Frame of Reference, i.e. relative to that Event.
Consider Space as we see it from the Earth. It moves in relation to us. The night sky changes throughout the year, while at night the stars, constellations and the moon rise and set. So relative to our Frame of Reference, here on the earth, everything else in the Universe is moving.
Similarly, if we were mapping space from Saturn's moon, Titan, the whole of the universe, including the Earth, would be moving relative to Titan's Frame of Reference.
It follows from this that if the whole of Spacetime can be considered static, relative to any individual Frame of Reference, then the relative movement of two Frames of Reference will each be the reciprocal of the other; i.e. as A moves with respect to B, so B moves with respect to A.
Imagine two Kings on a chess board, facing one another three spaces apart. If either one is moved one space forward, the distance between them, will be reduced to two spaces from either perspective; similarly, if white moves one space to the right, Black will be three spaces away and one space to the left; similarly black will now see white three spaces away and one space to its left. For any such movement, the participants can be reversed and the record of movements will still be valid.
From this it follows that position and movement can only be relative; there can be no absolute position nor movement, for any entity, real or imaginary, may be considered to be stationary. (As it is within its own Frame of Reference. i.e. the Frame of reference centred on that entity).
This raises an interesting conundrum for how may a body possess properties due to its speed or velocity if it is at rest relative to Spacetime? If such attributes or properties can only be measured from within Frames of Reference in which that entity is moving?
Specifically, those two old phantoms, Time Dilation and Length Contraction, only materialize where the entity affected is measured to be moving. They cannot materialize in that entity's own Frame of Reference in which there is no movement, the entity being at rest.
Suffice it to say that Time Dilation and Length Contraction (which are the same phenomenon viewed from different perspectives) are real enough, yet only when measured by a moving observer.
