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

Professor Giovanni Barontini and his team at the University of Birmingham have constructed a tabletop "mini-universe" that offers a surprising answer to one of physics' deepest questions: what is time? By cooling roughly 24,000 rubidium-87 atoms to just a few billionths of a degree above absolute zero, forming a Bose-Einstein condensate, and splitting the cloud with a thin laser barrier, the researchers created a closed quantum system with two distinct sectors—a bright (observed) sector and a dark (hidden) sector. The experiment, published in Physical Review Research, provides the first controlled laboratory evidence that a coherent arrow of time can emerge from within a completely isolated system, based entirely on entropy exchange between its parts, without any external clock.

The Setup: A Hermetically Sealed Quantum Cosmos

The experiment confronts a long-standing theoretical puzzle in quantum gravity known as the "problem of time." In the Wheeler–DeWitt framework—a canonical approach to unifying quantum mechanics and general relativity—the universe as a whole is described by a time-independent equation. This suggests the cosmos has no built-in external clock. If time is real, the thinking goes, it must emerge relationally from within the system. Barontini's team set out to test this concept physically.

They created a Bose-Einstein condensate—a state of matter where ultra-cold atoms act as a single coherent quantum object—and held it in a time-independent conservative trap. A carefully tuned optical laser wall then partitioned this cloud into an observed "bright" sector and an unobserved "dark" sector, with atoms able to tunnel or cross the barrier between them. Crucially, the global system's fine-grained entropy remains constant, a defining feature of a closed, isolated system. Any change in the bright sector's coarse-grained entropy, therefore, must be due to an exchange of entropy with the hidden dark sector.

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.