Time evolution of the crossway system.

Ulrich Mutze 2022-12-15

The system under consideration is described in The direct midpoint method as a quantum mechanical integrator ('the article' for short). The computer run which created the frames of the present video differs from the one which created Figures 2-5 of the article mainly by covering a larger time span (12500 steps) and by taking place in a larger part of 2-dimensional discrete space (1600 x 64 positions). There are also two technical differences: In the present video the particle moving along the x-axis (the projectile particle) has no periodic boundary conditions but reflecting ones. Since the wave function does not gets into relevant contact with the boundaries this makes no difference. The graphical gamma is 0.4 (it is 0.5 in the Figures of the article) so that small brightness variations in the video frame correspond to large factors for the wave function. So, the part of the wave function left of the central white bar, the back-scattered part of the wave function, looks quite noticeable, although the probability for back-scattering is about 1%. The integrator is not the one of the article but the ALF-integrator, which was not yet invented when the article was written. Both integrators have the same stability properties and both are reversible. How the video graphic is related to the wave function of a two-particle system each of the particles moving along a discretized line, where the to lines form a right angle and intersect forming a crossing of their respective ways --- all this is explained in the article. I hope that replacing a series of 4 Figures by a video of 501 frames and following the particles till quite a large distance from the crossing points allows to get a clearer picture of what scattering is all about: The projectile particle's wave function fans out in groups of different velocities with different target particle wave functions associated with the group. This suggests an understandable mechanism for the emergence of measuring results: Any measurement of the momentum (or velocity) of the project particle will involve selecting a place and a point in time for operating a momentum analyzer (such as a bubble chamber in a magnetic field) somewhere in the path of the projectile particle. From the extended stream which the wave function provides, the analyzer cuts out a short piece and always finds a well defined value of momentum. If he finds a low value for momentum the reason is that the target particle became excited. The beauty of our simple system is that the wave function of the target particle at 'collision time' can be read of the two-particle wave function. One is able to count the knots of this wave function and thus gets directly the level of excitation.

The total computation time for creating the video frames was 1678 s. (The execution time of 58.3 s, reported in the article refered to a run in which all graphical activities were disabled). These computing tools were used: CPU 4.464 GHz; RAM 16 GB; OS Ubuntu; SW C++20, OpenGL, FreeGlut, Code::Blocks, ffmpeg (for video creation from ppm-files), C+- (self-made C++ library); Compiler GNU.