$\newcommand{\h}{{\mathcal H}}$

We now come to the central concept of the scattering theory. What does it mean when we say that a particle moving under the influence of a potential $V$ looks like a free particle in the remote past?

Let us consider two systems. One, the actual system, with hamiltonian $H$. Its states are represented by vectors $\psi$ etc. in a Hilbert space $\h$. The other is a fictitious, free system, with hamiltonian $H_0$, and states represented by vectors in the same common Hilbert space.

Let $\psi$ be the state of the particle at some given time, say $t=0$. Then the state at time $t$ is

\begin{eqnarray*} \psi(t)=U(t)\psi\qquad U(t)=\exp(-iHt/\hbar) \end{eqnarray*}

A vector $\phi \in \h$ would represent the sate of a free particle at $t=0$ if, at any other time $t$, it were given by

\begin{eqnarray*} \phi(t)=U_0(t)\psi\qquad U_0(t)=\exp(-iH_0t/\hbar). \end{eqnarray*}

Now suppose that $\psi(t)$ is such that it becomes {\em indistinguishable} from $\phi(t)$ (for some $\phi$) for large negative $t$ then we can say that the particle indeed behaves like a free particle in remote past, that is, if

\begin{eqnarray*} \lim_{t\to -\infty}\psi(t)=\lim_{t\to -\infty}\phi(t). \end{eqnarray*}