Hourglass of Entropy: How Cold Atoms Keep Time ✛
In the lab, time usually ticks on the wall: hands, digits, a frequency standard. But for the Universe, there are no external clocks. The Wheeler–DeWitt equation, the main law of quantum gravity, contains no time at all, and all fundamental theories lack an inbuilt arrow of time. One reliable asymmetry remains: the growth of entropy according to the second law of thermodynamics. Entropy is the sand in an hourglass, trickling from one bulb to the other, and its flow sets the pace of time.
Physicists at the Max Planck Institute of Quantum Optics built such a clock from a Bose–Einstein condensate: an ultracold cloud of about 24,000 rubidium-87 atoms in a single quantum state. A thin laser barrier split the cloud into two sectors: a dark, unobservable one and a bright one watched by cameras. For 120 ms, every 2 ms they recorded the density profile: atom number, center-of-mass position, and entropy of the bright sector. Entropic time was defined as the integral of entropy change along the trajectory of a scalar-field analog, which guarantees monotonicity as long as entropy and field change in the same direction.
The result resembles a real hourglass: as long as sand is falling, time flows; when all sand has settled, time freezes. Entropic time grew almost monotonically for all barrier heights, with the rate set by the entropy flow: faster when there is exchange, zero when there is none. The total entropy of the mini-universe remained constant within uncertainty. Data ordered by entropic time reproduced the expansion and contraction cycles of the bright sector, and at high barrier the system approached “heat death,” where time stops. The derived Schrödinger equation with entropic time numerically reproduced the measured evolution of the condensate width with excellent agreement.
This is the first experiment where time is not imposed from outside by a lab chronometer but built from entropy exchange between subsystems. Previously, relational constructions of time were only discussed theoretically; now there is a controlled testbed. The cold-atom platform allows varying the Hamiltonian and testing different scenarios: choice of internal clock, behavior near singularities, reversibility via Loschmidt echo, analog black holes, and Vilenkin-type tunneling. The entropy hourglass can be flipped and restarted, or the shape of the bulbs can be changed to see how time flows in other mini-universes.
At a barrier height close to maximum, entropic time in the bright sector completely stops, even though 120 ms have passed on the laboratory clock: the system has reached an analog of heat death.