This Demonstration simulates the quantum dynamics of two interacting nonrelativistic particles. The goal is to demonstrate inelastic scattering in which a particle hits a target (a bound particle), excites the internal dynamics of the target and flies away with its state changed. The challenge is to arrange things in such a way that the essential features of inelastic scattering become visible in accessible observation times. We must make a considerable reduction in state-space dimension while conserving basic scattering geometry.

The solution presented here is the "crossway system," which I devised in 2006. Here the particles "live" in one-dimensional discrete finite spaces. Each particle has its own space, its "biotope" as it may be called. The two biotopes are arranged in Euclidean space normal to one another, resembling two crossing roads. The particles interact through a potential depending on the distance between the particles. The potential has finite range, so that interaction becomes effective only if both particles are close to the intersection point. Apart from the interaction between particles, particle 1 moves freely and particle 2 moves under the influence of a parabolic potential that has its minimum just at the intersection point. Particle 2 is thus a finite-dimensional version of a harmonic oscillator. In order to keep the particles in the system, we give the two biotopes a toroidal topology by declaring the next neighbor of the last point to be the first point.

There are controls for sizes of the biotopes, numbers of their discretization points, masses of the particles and strengths of oscillator potential and interaction potential. Further controls set the initial condition: an oscillator eigenstate for particle 2 and a bell-shaped position distribution together with a "boost" velocity for particle 1. The velocity with which particle 1 is sent toward the crossing is set primarily by setting the oscillator level, which is just reachable by absorbing all kinetic energy of particle 1. Detailed explanation of all control parameters is given in Tooltips that appear as you mouse over them. To read the annotations, you may need to pause the program by unchecking the "run" box.

The "run" box triggers time evolution of the two-particle wavefunction, which is shown as a color distribution on a rectangular display area. In this display, colors correspond to complex numbers, pixel position along the horizontal direction to discrete position of particle 1, and pixel position along the vertical direction to discrete position of particle 2. Thus each color pixel encodes a quantum amplitude for a situation in which the discrete position of each particle has a definite value. The display area is set such that the path crossing is the center of the area. Taking this as the origin of a conventional coordinate system, particle 1 runs along the axis and particle 2 along the axis.

The following Snapshots all deal with the same system (the standard system in the bookmarks) for two values for the interacting coupling constant .

Snapshot 1: here we have : strong repulsive interaction

Snapshot 2: : strong attractive interaction

Snapshot 3: Again , but in a representation mode that is explained in the annotation to the selected show box. Here the black-and-white mode is used, since it improves the visibility of the dark stripe patterns, which are reminiscent of photographs of stellar spectra.

The motivation for considering the crossway system is more fully described in [1, Section 5]. It should be noted that the default integrator of this Demonstration is different from the one employed in [1]. It is the method DALF of [2]. The numbers in this program may refer to any system of units for length, time and mass, setting .