I'm struggling with the physical meaning/consequence of Einstein's first postulate of Special Relativity, which states that
all physical laws are the same (invariant) in all inertial frames.
Any textbook on special relativity will then continue to say something along the lines:
"Postulate 1 implies that no physical experiment can be used to measure the absolute motion."
I fail to draw this very conclusion myself. I know it has something to do with Newton's second law, $F=ma$, in the sense that Newton's law doesn't apply in a reference frame that is accelerating, but I think I fail to see the bigger picture.
How did they come to the conclusion that we can't measure absolute motion? I understand that it means that we can't determine who is actually moving and who would be standing "absolutely" still.
What is the reasoning? Are we going to assume absolute motion exists, and then derive a contradiction? I understand that if absolute motion were to exist (or at least, to be physically relevant), then we should be able to measure it, but then there would be a "special" reference frame, which is in contradiction with the first postulate.
This line of reasoning is exactly what I don't understand: why do laws need to be different in order to measure something different/something "special"? Can't it just be that the "absolutely still" reference frame were so special, that we would actually measure different things, while the laws were obeyed?