Phase steerable arrays for things like radar are beautiful machines. You have a matrix of small radio transmitter aerials and, by controlling the phase of each, you can point the beam in any direction in the same way that a diffraction grating splits a single beam into multiple copies at angles. It's all done by constructive and destructive interference.
You have a device that can move a beam anywhere without itself moving, so its speed of change of direction is not constrained by mechanical inertia.
Digital micromirror devices are also beautiful machines. You have a matrix of tiny mirrors that are illuminated by a bright light. Each mirror can be caused to deflect by an electric charge. The result is a mirror matrix of pixels, some of which can go dark because they are not reflecting at you.
But suppose, instead of each little mirror flipping through an angle to deflect its reflection, it was just moved back or forth in parallel. Now you'd have a flat mirror that still looked like a mirror whatever state it was in.
But if you illuminated it with coherent monochromatic light, the phase of the reflected light from each pixel would depend on how much it had moved. You would have made a phase steerable array for light.
Penultimately, why bother with mirrors at all? It is possible to synchronise the phase of laser diodes, so you could simply replace the mirror pixels with light sources that were all in phase, then move them back and forwards to steer the beam.
However, finally, if you can control the phase of the diodes (which is necessary if you are to synchronise them), you don't need the mechanical movement at all. We are back to the phase steerable radar array, but now with light rather than microwaves...
Postscript
@mastrack@noc.social sent me this interesting link.
