>>4634539OK, for a theory describing actual matter, consider this extreme oversimplification: replace the bar with two point particles, each starting a distance r from one of the portals, and thus a total distance of 2r apart. Call the particle on the orange side orange and the blue side blue. The orange portal starts to move towards the orange particle, and the distance decreases until that particle reaches the portal. It then emerges with speed v = the portal's instantaneous speed, and distance r away from the blue particle, which remains motionless. The orange particle flies off to infinity, and the total COM velocity will be v/2.
Now, allow there to be a weak spring force between the two particles (ignore the portal's effect on the spring--we just have F=-k(x-2r). This is like a simple model of a hydrogen atom. The orange particle will now be pushed away as the distance to the blue particle drops, but the blue particle will be accelerated equally in the opposite direction. Total momentum will thus be conserved, and after the orange particle passes through the portal the COM velocity will again be v/2. (v may be slightly different here, since it's the portal velocity when the orange particle passes through, which may be different due to the spring force.) In principle, one can extend this basic method to cover any solid object, working it out atom by atom. Again, this gives answer B to the OP.
