1. Does anyone know why for an isotropic distribution function, pressure tensor reduces to a scalar pressure? For instance, for a Maxwellian distribution P=A ∫ vx vy exp-(vx2 + vy2 + vz2) dvx dvy dvz is not zero. I think everybody should realize how bogus some of the authors are. Google this problem, as I did, and you will see hundreds of books, repeating the same line " for isotropic, tensor is diagonal ....". You would think there would be at least one book that might show some derivation.. nope. It's almost like many of them write the books without knowing the equations and just repeating the other authors.