Play a video of a colloidal particle diffusing in water. Now play it backwards. You can’t tell the difference — Brownian motion looks the same forward and backward. Time reversal symmetry holds.

Play a video of a colloidal particle inside a living cell. Now play it backwards. You can tell. The cell breaks time reversal symmetry.

The measurement is called mean back relaxation: compare where a particle goes after a time interval with where it was before. In equilibrium, these are statistically identical. In cells, they’re not. The particle’s future and past are different, which means the system is being driven out of equilibrium by active forces.

The source: microtubules. Drug experiments that selectively disrupt actin filaments, myosin motors, or microtubules reveal that depolymerizing microtubules eliminates the time-reversal asymmetry. The other cytoskeletal components contribute to motion, but microtubules are specifically responsible for the arrow of time inside the cell.

This makes physical sense. Microtubule-associated motors (kinesin, dynein) walk directionally along polar filaments, converting ATP energy into directed transport. The directionality — always toward the plus or minus end — is the microscopic origin of the macroscopic time asymmetry. The cell’s arrow of time is literally built from the polarity of its internal tracks.