Birmingham Physicist Builds a Cold-Atom 'Mini-Universe' to Test the Problem of Time, Showing Time Can Emerge From Internal Entropy

Overview

A physicist at the University of Birmingham has used a cloud of ultracold atoms to test one of the most stubborn puzzles in theoretical physics: the so-called “problem of time.” In a paper titled “Testing the problem of time with cold atoms,” published in Physical Review Research on 11 June 2026, Professor Giovanni Barontini reports building a self-contained quantum system in which the passage of time can be reconstructed entirely from the internal disorder of its particles, with no reference to an external clock, according to a University of Birmingham announcement and a release distributed via EurekAlert!.

Barontini, a Professor of Physics, describes the result as “the first controlled experimental evidence that ‘time’ can be defined by changes within a system rather than as the external ‘ticking clock’ we think of as time,” according to the University of Birmingham.

What We Know

The “problem of time” arises from a tension between quantum gravity and everyday experience. As Barontini framed it, “In some theories of the universe, especially quantum gravity, time doesn’t appear as a built‑in feature. Yet in everyday life, time flows from past to future – why is this so, when most basic laws of physics work the same way forwards and backwards?”, according to EurekAlert!. One such theory, the Wheeler–DeWitt equation, suggests that at its deepest level the universe has no built‑in time but exists as a single, unchanging quantum state, according to the University of Birmingham.

To probe that idea experimentally, Barontini built what the team describes as a “mini‑universe” from 24,000 ultracold rubidium atoms held a few billionths of a degree above absolute zero, according to EurekAlert!. Two laser beams of different frequency formed a thin barrier that divided the atoms into an observed “bright” region and an unobserved “dark” region, according to EurekAlert!. Atoms were allowed to move between the regions while the overall system stayed isolated, according to Quantum Zeitgeist.

Within that isolated system, the bright sector repeatedly expanded and collapsed, behaving like cycles of a Big Bang and a Big Crunch, according to The Debrief and Quantum Zeitgeist. Rather than reading time off a laboratory clock, Barontini defined it from the disorder, or entropy, of the atoms and how they were distributed between the two regions — a quantity he named “entropic time,” according to the University of Birmingham.

This internally defined time behaved like ordinary time in three respects, according to the University of Birmingham: it flowed in one consistent direction, giving a clear arrow of time; it correctly ordered events even in a system expanding and contracting like a mini cosmos; and it sped up or slowed down depending on how entropy moved around. Barontini reported that the standard Schrödinger equation remained valid when written in terms of this entropic time, according to Quantum Zeitgeist.

Why It Matters

The “problem of time” has long lived in the abstract reaches of quantum cosmology, where time is treated as something that may emerge from relationships between parts of a system rather than as a fundamental backdrop. Barontini’s contribution is to move that question from theory into a controlled benchtop experiment. He argued the work “offers new insight into the nature of time in quantum gravity that could be used to describe dynamics just as effectively as conventional time,” according to EurekAlert!.

The broader significance is methodological. If a relational, entropy-based notion of time can be implemented and measured in a tabletop cold-atom platform, then ideas previously confined to equations describing the whole universe become testable in the laboratory. The full technical treatment, including the formal construction of entropic time, appears in the Physical Review Research paper (DOI 10.1103/1h9j-df4k), summarized by the University of Birmingham.

What We Don’t Know

The announcements describe a single-investigator demonstration and do not report independent replication. How far the entropic-time construction generalizes beyond this specific cold-atom configuration — to larger systems, different particle species, or other models of quantum gravity — is not addressed in the available summaries. Nor do the releases quantify how precisely entropic time tracks conventional laboratory time, or characterize the regimes in which the two might diverge. Those details rest in the primary publication, which carries the experiment’s quantitative claims.