Doppler effect

Interactions

drag the train (or whatever is on the line) at any speed and direction: the rings and the recorder strip respond by geometry alone
click the listener: he turns and walks closer to the track, or back, so the pitch glide on the strip becomes sharper or gentler
click a galaxy: its three spectral lines slide back to rest for comparison, then return to their redshift
watch the supersonic dart: when its Mach cone sweeps the listener the pen kicks, sparrows lift from the hedge and his hat jumps