dear all, I have a question about Electromagnetic field tensor. As I was reading introduction to relativity by Hobson, I saw this sentence: "in order that the rest mass of a particle is not altered by the action of the erforce we require the latter to be a pure force, so: u.f=0 1) what does it mean? The other question is about the rest of this text: in the component form this reads: f_μ u^μ=q F_μσ u^μ u^σ=0 so that F must be an anti symmetric tensor. 2) why?!!!!!