space-time crystals

A space-time crystal is a dynamical system that is periodic in time and space, analogous to a crystal which is periodic in space. Some physicists think that actually atomic scale physical systems can take this form, and be stable long enough to explore their properties. Or perhaps stable enough to survive the heat death of the universe!

"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.]