Hello, Here is a quote from the Wikipedia article 'Principle of relativity': http://en.wikipedia.org/wiki/Principle_of_relativity i'v added a string of asterisks around the word 'A consequence'. i feel that there is not enough elaboration about this 'consequence' thing. Was the consequence obtained either by mathematical or by experimental results of the first part (i.e. the validity of the principle of relativity)? Or is it that what follows the word 'consequence', is just an axiomatic statement? Am i using the right word in this context? Axiomatic? There is not enough elaboration as well about the implicit relation, brought up in this sentence, between the term absolute speed and the term inertial reference frame. Do these terms have to be mutually exclusive? Here is the quote: "The principle requires physical laws to be the same for any body moving at constant velocity as they are for a body at rest. ******A consequence****** is that an observer in an inertial reference frame cannot determine an absolute speed or direction of travel in space, and may only speak of speed or direction relative to some other object." If one has a small tub full of water, then maybe by observing the water movement and ripples, after acceleration has ended not long ago, one can tell the change of speed relative to the past speed that the inertial reference frame he was inside, was going through. Now you will say, but that previous speed or direction, was or has to be in relation to another frame of reference and if then you also had that water tub set-up, then the previous speed or direction before that has to be in relation to another frame of reference and so on and on. But if all matter behaves like this water in the tub, for much longer time than just the waves of water inside a tub can indicate - In the sense that matter contains constant radiation, that tells us about the movement history of that matter, in the same scale of past time events as say the cosmic background radiation goes back, but this kind of radiation is within matter and not outside matter - Then would that be considered 'absolute'? How accurate does such a hypothetical in-matter radiation measurement have to be, in order for it to be called: 'An absolute determination of speed or direction of travel in space'? Thanks.