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Yes, Oli, I understand all that- what I do not get is by what trick of mathematics from that notion of fundamental observers which per your description sounds like something perfectly arbitrary, relative, hypothetical and imaginary, the contention that there is a single time-line possessing a definite point of beginning called the Big Bang with a strict calendar of sequential events that is valid for the whole infinite process called the universe is derived? How anything that is supposed to possess a physical reality is extrapolated from those universal fluids and fundamental observers?
Well the answer to that question is a whole graduate course on General Relativity. However let me try to give some answers.
Okay, so we notice a few things:
(a)The laws of physics don't really depend on position or velocity. (The insight of Newton)
I'll just say a little bit more about (a). Newton's famous equations F=ma tells you that the force on a body, F, causes an acceleration, a (proportional to the mass). So objects will all react to the same force in the same way, even if they encounter it at different locations and at different speeds.
Let's call objects which aren't accelerating (a=0) "inertial observers".
Now Maxwell's equations predict:
(b) For a given inertial frame, light has a constant speed that never changes.
So by the spirit of Newton's insight this means that every inertial observer should see light moving at a constant speed.
Unfortunately the transformations that Newtonian physics gives for moving between frames (which are based on our intuition), change the speed light, so that different inertial observers see the speed of light differently. Hence a prediction of Maxwell's equations seems to depend on velocity, unlike anything else in physics.
Einstein didn't like this (neither did experiment), so he had the idea that perhaps the transformations to go from one observer to another should be altered from our intuition to new transformation laws which keep the speed of light the same.
He wrote down these new transformation laws. The new transformation laws say that time will dilate and space contract as you move between observers (as you speed up or slow down). It seems odd, but it is the only way to keep the speed of light the same for all observers, so that the laws of physics don't depend on your velocity.
Of course it leads to strange things, the same transformation laws imply that mass is related to energy, e.t.c. However these things have been confirmed. So if you live near a nuclear power station your house is "powered by" resolving a conflict between Maxwell's equations and Newton's insight.
Soon after this, Minkowski realised that these new transformation laws are simply the transformation laws for a four dimensional world where time is an extra dimension and this four dimensional world is now called Minkowski space.
This is Special Relativity.
What about gravity?
Einstein was initially having trouble working gravity into relativity, until he had a fundamental insight:
the equivalence principle. A person who is falling feels no force. If you fell toward a planet with your eyes closed and the planet had no atmosphere, you would feel nothing (well except when you hit the ground). Since force is related to acceleration, observers who are falling are like those with no acceleration, a=0. Or to put it another way, falling observers are inertial observers.
If you just put a planet in Minkowski space the trajectories of inertial observers do not bend towards the planet, so falling observers will not be inertial. To correctly model the scenario you need another spacetime, one whose inertial trajectories are falling trajectories. In fact you'll need a different spacetime for each arrangement of matter. "Falling" in these trajectories will then just be following an inertial path, something that requires no force. So gravity is not a force, just the result of following the contours of spacetime and the shape of spacetime depends on the matter present.
This is General Relativity.
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