"Can matter cycle through shapes eternally?
'Time crystals' idea is challenged but its proponent doubles down."
Here's a 3-D (time x 2-spatial dimensions) space-time crystal, composed of elastic collisions of particles on a plane:
|3-D space-time crystal: A 4x4 checkerboard tiling, where each particle's path is a square. Viewed as a cross sections of continuous fibers, it is a three dimensional knit. [Dynamically modelled and rendered in Processing.]|