Dear Forum, I have discovered by accident the method developed by Feynman and Stuekelberg to explain the origin of antiparticles ( I have the little book "Elementary particles and the laws of physics). As you might be all aware by using basic perturbation theory and assuming that negative energies are not possible because as Feynman quoted "we could dump particles into this negative energy and run the world with the extra energy" it is found that particles traveling faster that the speed of light contribute to the amplitude. The perturbation expansion calculates the amplitude of a particle to go state from θ to state θ by means of a perturbation by a potential U1 at t1 (change state to p) and second perturbation at t2 by potential U2 that brings the particle back to state θ. I am having problem to understand the following part of the argument. Then it is assumed that such particles correspond to a different relativistic framework by which the order of two events: t1 particle scattered by potential U1 to state p from state θ and event t2 from state p back to state θ happen in reverse order. Under the new framework we will see that t2 correspond to the creation of a particle - antiparticle and t1 annihilation of the original particle. First question is: what is the physical origin of the potentials U1 and U2? Do they create and destroy particles? Second question :Could we claim that antiparticles are just particles traveling back in time because of simple relativistic consideration? Third question: What is the place of Stuekelberg theory in modern particle physics?