Every phisical quantity is tensor quantity (special cases of tensors are vectors and scalars). There are transformation rules for tensors. For example for scalar quantity F transformation rule is F'(x')=F(x). Why energy is not Lorentz invariant. Is this becauce is energy coordinate of 4 momentum ? Ok, mass is conserved quantity this is scalar. For me relativistic mass is wrong concept, because we have E=gamma*m0*c^2=mr*c^2 and we have that all energy contained in mass. Where is energy of moving(kinetic energy) ? Is right my concept ?