It is true that if two particles are far away from each other,

the interaction can be considered nil. However the state that they are in tends to be conserved in the motion, this state is set by their earlier interaction. That is they may be in a two particle wave function that cannot be factored. ie. nothing

drastic in the evolution forces a change of state. That is how I think about it.

Since this is a spiny topic , there may be other ideas...

regards, juan

Thanks Dr. Juan,

If the far away particles interaction is nil, then it is just the contributed initial conditions that influences their dynamics. In fact, of course maybe, it is initial conditions that cannot be factored for the case of entangled particles. It is because the creation time is the same.

All fundamental interactions are local. Non-local correlations are created by interaction and propagation.

The two particles of which you speak, interacted when you prepared the entanglement. After separating, the particles do not interact anymore.

As to SPDC, this interaction consists in the transformation of a high energy photon, usually ultra-violet, into two photons whose total energy is that of their parent photon. After separating in time, the two children-photons don't interact among themselves anymore.

They separate, the Hamiltonian is no longer relevant, but the important point is that the entangled state exists yet by continuity from the previous.