I've read that they must be calculated in the same frame , and so to calculate them in the easiest frame.. * So for a collision when I compute the momentum before and after I should do this in the easiest frame. Considering a specific collision where we are computing the minimum energy to create some given particles after a collision that occurs between a particle with energy E, colliding into a stationary particle, both particles have mass m. Questions: 1) Isn't the chose of frame fixed by the requirement of minimum energy and so we need a frame in which the created particles are at rest - is there only one such frame? 2) In evaluating the four-momentum before the collision, I get p =(E+mc,p,0,0). (Assuming the collision to occur in 1-d x direction and using natural units.) Where p is the momentum of the not stationary particle, and is unknown. - Here I'm unsure how * applies and what frame we are evaluating in - we have to use a frame where the colliding particle has energy E as this is the only data known? So by choosing a frame in which the final particles are at rest, are we able to evaluate the four-momentum in different frames before or after the collision? Or how would we know we have calculated four-momentum before and after in the same frame? I think I might be confused with the space-time interval susu[/SUB ]which is the same in every frame. Any help greatly appreciated, thank you !