Quote:
se5a said:what narf is saying, is if you use said torch, and mesure the amount of time it takes for light to get from the torch to an known distance, you should be able to work out how fast, and in what direction the torch is moving, and therefore be able to say what "still" is.
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What I'm saying is that this exercise will just tell you that whatever you had already decided was "still" for purposes of calculation was, in fact, correct. Simply stating that something is moving at speed X inherently requires a definition of what is at rest. The point of relativity is that this definition can be absolutely anything and the speed of light will be the same regardless.
For example:
Suppose you are on Earth trying to carry out that torch experiment. You carefully measure the distance between the torch and a mirror, you light the torch, and you time how long it takes for the reflected light to get back to you. You will calculate from this data that you (actually your measuring device) are at rest.
Next, you move to Mars and repeat the whole experiment. You will again calculate that your measuring device is at rest even though it is clearly moving at a different velocity than it was on Earth. Consideration of this simple fact is how the entirety of the Theory of Relativity was derived.