My understanding is that when a particle-antiparticle pair is created, they will have opposite spins and momenta. So the net momentum will be zero. But doesn't the momentum depend on the frame of reference? Say the particles both have mass m and velocities +v and -v. Then the net momentum is +mv-mv=0. What about in a different reference frame that has a velocity boost of u in the positive direction? The velocities in that frame are u+v and u-v. So the net momentum is m(u+v) + m(u-v) = 2mu. So it appears that, viewed in this new reference frame, momentum has been increased by 2mu by the creation of the pair, apparently violating the law of conservation of momentum. No doubt I have made a silly mistake somewhere here. Can someone please explain what it is? Thank you.