I am just beginning graduate self-study of Special and General Relativity, so forgive me if my question seems niave. I have found the beautiful line " The essence of Special Relativity is that the laws of physics are Poincaré invariant" - Modern Mathematical Physics, Szekeres. The space time interval s is invariant under Poincaré transformations. So then, s must appear everywhere in physics forumlae, right? Is that true? I haven't come across any as of yet, but that's what I get from this. And what kinds of formulae does it appear in? What are some good examples?