This looks interesting: "Symmetry groups are undeniably basic but why must spacetime have a group structure? The answer to this question is delightfully simple. The Poincaré group follows from the homogeneity of time. If you're interested in a revolutionary new approach to special relativity, inspect my proof carefully. I derive special relativity without postulating the existence of any group structure. My axioms are intuitive: Time is to be defined with motion and all time computations can be performed with homogeneous functions." "See the updated http://www.everythingimportant.org/relativity/special.pdf "