Sugar dissolves in tea, and then with a single motion gathers back into the spoon. In an ordinary kitchen, you won't see that — viscosity and disorder get in the way. But in the world of ultra-slow motions, where inertia vanishes, such tricks become reality.
Physicists took a cloud of cold atoms in a magneto-optical trap — an invisible box that holds particles with light and a magnetic field. They applied a force to the cloud using a smoothly varying magnetic field and watched how the atoms rearranged. Then they reversed the force. Despite the full complexity of three-dimensional movements, the cloud returned exactly to its initial shape. It's like rewinding a film: each particle retraced its path in reverse order.
But reversibility wasn't perfect. At a certain trap setting, the scientists noticed deviations — the system seemed to "stick" and didn't fully return. This resembles hysteresis: a phenomenon where the state depends on history, like a magnetic latch. Such glitches turned out to be a valuable tool for studying systems where energy dissipates quickly.
Let's return to tea with sugar. In an ordinary cup, stirring is irreversible, but in the microworld, where friction plays the main role, you can make particles come back. This experiment showed: even interacting atoms can achieve precise reversibility if conditions are chosen correctly. And when reversibility breaks, we get a chance to study how irreversibility arises in nature.
as written by the authors arXiv:2605.03970
We have investigated kinematic reversibility in a cold atom system under strongly overdamped conditions. In such systems, inertia is negligible, and for noninteracting rigid particles, inverting the external force causes a perfect reversal of individual particle trajectories. We used a magneto-optical trap (MOT) as a model low Reynolds number fluid and show the kinematic reversibility survives in the presence of interparticle interactions. In our experiment, we applied controlled external forces via a linearly ramped magnetic bias field and monitored the resulting cloud dynamics. Despite the complex three-dimensional rearrangement induced by the forces, the system exhibits precise reversibility when the force is reversed, consistent with Purcell's framework for kinematic reversibility in low Reynolds number hydrodynamics. Reversibility was not universal,however-- under certain MOT alignment conditions we have also observed clear deviations associated with system hysteresis. Our work shows that strongly dissipative cold atom fluids are a versatile and rich platform for exploring overdamped dynamics.
A magneto-optical trap cools atoms to temperatures a few millionths of a degree above absolute zero — colder than outer space.
In Ray Bradbury's novel 'The Pharaoh's Contract', a time machine is described that works on the principle of entropy reversal — almost like this experiment, but for the entire Universe.