Hi! I can't understand something in field theory and need your assistance: I wish to understand why a particle of mass m^2<0 can't exist. For a massive particle, in its reference frame, one would write: [itex]p_\mu=(m,0,0,0)[/itex]. I understand that. But for m=0, why is: [itex]p_\mu=(p,0,0,p)[/itex] And for [itex]m^2<0[/itex] Why is: [itex]p_\mu=(0,0,0,m)[/itex] ? Thank you...!