A ratchet produces directed motion from fluctuations — extracting useful work from noise by breaking a symmetry. The classic Feynman ratchet uses a spatial asymmetry: a sawtooth-shaped potential that is easy to move along in one direction and hard in the other. The fluctuations provide the energy; the asymmetry provides the direction.
Pamulaparthy and Harris (arXiv 2602.23327, February 2026) demonstrate a ratchet that requires no spatial asymmetry at all. The asymmetry is entirely temporal: the distributions of waiting times before forward and backward steps differ, even though they have the same mean. Same average duration between steps in both directions. Different shapes of the distribution — different variances, different tails. The difference is enough to generate a sustained current.
The mechanism is a continuous-time random walk where particles randomly reverse direction. Each reorientation resets the clock, and the time until the next step is drawn from a waiting-time distribution that depends on whether the particle is currently moving forward or backward. If forward steps have a narrower distribution (more predictable timing) and backward steps have a wider one (more variable timing), a net current emerges in the forward direction — even though the mean step rates are identical.
The current arises from the shape of time. Narrower distributions concentrate steps around the mean, producing steady forward progress. Wider distributions spread backward steps over a longer range, sometimes delaying them far beyond the mean. The asymmetry in the higher moments — variance, skewness, tail behavior — breaks the time-reversal symmetry that would otherwise guarantee zero net flow.
The large deviations analysis characterizes how the current fluctuates. The complete rate function — the probability of observing a current far from the mean — is derived via renewal theory. The structure of the rate function determines whether the system exhibits dynamical phase transitions: qualitative changes in the fluctuation behavior at extreme currents.
No potential. No spatial structure. No external force. The direction comes from when things happen, not where.