author: Diggory Blake
This puzzle consists of a gif with a number (616) of pixels or ants moving at constant velocity with cyclic boundary conditions for 100 frames. The title, 'all you zombies', is a reference to a short story involving travelling backwards in time. Thus, we have only to reverse the trajectories of each ant and trace them back in time.
An example solution (Will Earley) can be computed with two python scripts, track.py and trace.py. track.py looks for nearby ants in subsequent frames and works out the most likely trajectories for each ant in the initial frame by fitting them to a linear function,
x(t) = x_0 + v t for free parameters
v. After analysing 50 of the 100 frames, trace.py can be used to plot the trajectories (using a data.p pickle file that track.py produces), and we find that the ants converge at around t=-250 frames:
Thus the answer is 'T-symmetry'.