Published: 21 October 2026
Symmetric equations, asymmetric records
Many microscopic equations work under time reversal, yet measurements leave records and everyday processes do not spontaneously undo themselves. Quantum experiments can quantify this asymmetry trajectory by trajectory.
Weak measurement as a movie
A weak measurement extracts partial information while disturbing a system gradually. Repeated readouts form a trajectory. Researchers compare how probable that record is under a forward process and an appropriately defined reverse process.
Information defines an arrow
In experiments with ultracold atomic spins, the statistical weight favouring the forward record grows with measurement strength. The average arrow is positive even though rare individual trajectories can appear to run backward.
Fluctuation theorems
Small systems fluctuate enough to temporarily move against the typical direction of entropy production. Fluctuation relations predict how rapidly the probability of such events falls. They connect microscopic reversibility with macroscopic irreversibility without forbidding rare exceptions.
What is not reversed
A backward-looking quantum trajectory does not make memories unform or clocks run in reverse. It is a statistical statement about a carefully defined measurement record. The thermodynamic environment and stored information still matter.
Discussion question
If a microscopic trajectory can look time-reversed, why do you think human-scale experience never does?
Sources and further reading
Quantum measurement arrow of time in ultracold atoms ↗Reversing heat flow using quantum correlations ↗Arrow of time and its reversal on a quantum computer ↗Share your answer
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