How a 'Mini-Universe' of 24,000 Atoms Just Showed Time Can Emerge From Entropy

Observing the Arrow of Time

With the system sealed and partitioned, the team tracked the bright sector's evolution and discovered a remarkable series of cosmological analogs.

Big Bang and Big Crunch Cycles

The bright sector repeatedly expanded and then recollapsed as atoms crossed the laser barrier, a cycle that mimics a cosmological bounce. The moment atoms first populated the bright sector was interpreted as a "Big Bang," while their complete return to the dark sector marked a "Big Crunch." This bouncing cycle repeated many times, creating a miniature, repeating cosmic history within the lab.

Entropic Time as an Internal Clock

From this ebb and flow of atoms, the researchers defined an "entropic time." Because the total system's entropy is conserved, the directional movement of atoms between sectors created a measurable, one-way flow of entropy in the bright sector. This flow served as a reliable internal clock that exhibited several striking properties:

• A consistent arrow: The entropic time flowed in one consistent direction, correctly ordering events across multiple expansion–recollapse cycles.
• Variable speed: The clock sped up or slowed down depending on the rate of entropy exchange between the sectors.
• Accurate prediction: Barontini derived an effective Schrödinger-like equation parameterized by this internal, entropic time—not conventional lab time—and showed it accurately reproduced the measured evolution of the bright sector.

Heat Death and the End of Time

When the distribution of atoms between the bright and dark sectors eventually stabilized and stopped changing, entropy exchange halted. At this point, from the perspective of the observed sector, time effectively stopped—an analog of the heat death predicted for our own universe.

Why This Matters for Our Understanding of the Universe

The experiment matters because it moves a foundational question from theoretical speculation into the realm of experimental physics. By partitioning a closed quantum system and watching time emerge from entropy dynamics alone, the team provided the first controlled testbed for relational-time constructions. Their findings support the idea that time is not a fundamental, external backdrop, but rather a thermodynamic phenomenon that arises when an observer distinguishes a subsystem—much like the distinction between the bright and dark sectors—from a larger, timeless whole. This tabletop miniature universe now offers a new empirical window for exploring the physics of the actual cosmos.