The speed of light is postulated to be constant in all inertial frames. but why is it called a postulate actually? looking at the SI definition of the meter that is specifically defined by the propagation of light i notice it makes it constant by definition. so isn't this actually just a choice, a fixing - like a coordinate system - rather then a physical fact? i mean one could define a meter via propagation of sound waves instead (a bat could come up with that) - just for the sake of showing its possible (at least here on earth) rather then for any practical purpose. such a definition has a major impact on the speed of light obviously and for the space time geometry. So aren't the speed of light, the meter and second actually simply degrees of freedom that can be fixed at each point of space differently?