Hi, I am just trying to understand Special Relativity by reading http://en.wikipedia.org/wiki/Introduction_to_special_relativity All is ok except where in "The Minkowski formulation: introduction of spacetime", the introduction of -(ct)^2 term is not clearly justified (or so it seems to me) in the article. Will someone explain to me, or guide me to some reference where I can find answers to: 1. Why is it negative instead of positive? 2. Why is speed of light chosen? On another related note, I have read about the claim/observation/proposition that speed of light is constant for all observers in their own reference frames. But it is not clear whether this is the consequence of experimental result or the inherent characteristic of modelling the space-time coordinate using speed of light as one of the parameters. If it is an experimental observation, what is the experimental set up like? i.e. How is the measurement taken? Any help is appreciated. thanks!