Special relativity I: spacetime diagrams and the invariance of the interval
In this post I will explain, to my liking, (parts of) two papers which I highly recommend: Mermin, An introduction to space–time diagrams; American Journal of Physics 65, 476 (1997) Brill, D., Jacobson, T. Spacetime and Euclidean geometry . Gen Relativ Gravit 38, 643–651 (2006). Both of them aim to give purely geometric derivations of the essential facts of special relativity, by the use of spacetime diagrams. Mermin's paper is particularly helpful to elucidate the meaning of spacetime diagrams, and that is where we start. The main postulate of relativity is that all laws of physics look the same in every inertial frame of reference . A useful, but not entirely necessary , second postulate is that there is a thing called 'light' which is easy to produce and which travels in every (inertial) frame of reference at exactly the same speed $c$. An event is just a name for a point in spacetime. It is something, or perhaps nothing, that happens at a particular location at...