Also, the system one considers in those cases is a single particle state, not a particle.

All I can do in my ... particle state, you see, is encourage and suggest.

The three particle state shown above thus becomes the following four-term superposition:

But this also means that the outgoing state isn't a one particle state.

Instead one must follow Gibbs, and consider the ensemble of states of the system as a whole, rather than single particle states.

Other particle states originate from strings beginning and ending on the same D-brane.

The more you measure one aspect of a particle state, the less you know about another.

The amplitude for scattering is the sum of each possible interaction history over all possible intermediate particle states.

This means that these particle states have negative norm-they are not physical states.

Single particle states describe an object with a finite mass, a well defined momentum, and a spin.