Birmingham Physicists Build a 24,000-Atom Mini-Universe to Test Time

A University of Birmingham team has built a hermetically sealed “mini-universe” out of 24,000 ultracold rubidium atoms and used it to show that the flow of time can be measured from inside a quantum system, with no reference to any external clock. The experiment, described in an open-access paper published 11 June 2026, is what the authors call the first controlled laboratory test of “entropic time,” a candidate answer to the long-standing “problem of time” in quantum gravity.

Time is one of the few quantities in physics that everyone uses and almost no fundamental theory explains. Most laws of physics run equally well forwards and backwards, yet we experience a clear past and future. The new work treats that asymmetry as a property of entropy inside a closed system, and reproduces the same idea in a tabletop Bose-Einstein condensate. The “problem of time” has mostly been a chalkboard exercise since the 1960s, and the Birmingham paper is described by its authors as a first attempt to test one candidate answer to it with cold atoms.

The Components of the Mini-Universe

The setup is a cloud of about 2.4×10^4 atoms of rubidium-87, cooled to a few billionths of a degree above absolute zero and held in a magneto-optical trap inside a glass vacuum cell. A photo caption from the University of Birmingham puts the temperature at “~0.0001 degrees above absolute zero.” Once the atoms form a condensate, they sit in a conservative, radially symmetric optical dipole trap built by crossing two infrared laser beams, one at 1070 nm and one at 1550 nm, giving final trapping frequencies of about 2π×(25, 70, 70) Hz.

Inside that trap, a third beam at 675 nm, shaped by a digital micromirror device, paints a thin potential barrier roughly 8 μm wide across the centre of the cloud. The barrier cuts the condensate in two. Atoms can still tunnel across, but on the 100-millisecond timescale of the experiment the system is effectively closed, with a time-independent Hamiltonian and no measurable dissipation, the authors write.

One side of the barrier is treated as the “bright” or observed sector. The other is the “dark” or unobserved sector. Atoms shuttle back and forth across the barrier, and the bright sector periodically fills up and empties out, a pattern the paper labels as a “big bang” (when atoms first populate the bright side) and a “big crunch” (when they recross back to the dark side). The two sectors together form the “mini-universe” of the title.

How Time Emerges from Entropy

The team, led by Professor Giovanni Barontini of the University of Birmingham’s School of Physics and Astronomy, treats the system as a mini-universe in the minimal sense of an effectively closed many-body system. The total Hamiltonian is split into bright, dark, and coupling parts, and the bright sector is described with an effective non-Hermitian Hamiltonian that tracks atoms gained and lost across the barrier.

From there, the team constructs a coarse-grained entropy of the bright sector, defined from how atoms are distributed on the unobserved side, and turns it into a clock. When that entropy rises or falls, the system is treated as moving forward in time. When the distribution stops changing, time effectively stops. The paper shows that this internal variable can robustly order the bright sector’s events across many expansion-and-recollapse cycles, with no reference to the laboratory’s wall clock.

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 forward and backward?

That framing comes from Barontini himself, speaking to the original report on the experiment. The same framework lets the team rewrite the Schrödinger equation. Using entropic time as the parameter, they derive an effective Schrödinger equation that reproduces the measured evolution of the condensate, an indication that standard quantum mechanics still works when the “t” in your equation is no longer an external tick but an internal property of the system.

Three Properties of the New Time

Phys.org’s summary of the paper highlights three properties of the constructed time. It flows in a consistent direction, giving a clear arrow of time. It correctly orders events even inside a system that expands and contracts, the way a mini cosmos would. It also speeds up or slows down depending on how entropy moves around inside the system.

Each row traces back to a sentence in the open-access preprint of the paper or its press release. Conventional time is included only as a contrast, not because the paper benchmarks the two head to head. The point of the table is to show what changes when the clock is internal rather than external.

The Wheeler-DeWitt Theory It’s Testing

The deeper context is the Wheeler-DeWitt equation, the central equation of an approach to quantum gravity developed by Bryce DeWitt and John Archibald Wheeler in the 1960s. The equation implies that the universe, at its most fundamental, has no external time parameter. It exists as a single, unchanging quantum state, and any sense of past and future has to come from internal relationships between parts of that state. That is the “problem of time” the paper sets out to probe.

Barontini’s group does not claim to have solved it. What they show is that the condensate can be set up in a way that is “structurally analogous” to a Wheeler-DeWitt minisuperspace model, the simplest class of models in cosmology, in which a small number of variables stand in for the whole universe. In the condensate, the centre-of-mass coordinate of the bright sector plays the role of a clock field, and the radial spread of the bright sector plays the role of a scale factor.

The paper addresses where the analogy stops. The sign of the kinetic term differs from the canonical Wheeler-DeWitt case, and the match holds at the level of relational variables, not the exact equation. Even so, the authors argue that the platform “establishes a controlled experimental setting in which relational-time constructions can be quantitatively tested,” which is the central claim the paper stakes out. Readers who want the historical background can consult the canonical 1967 paper that introduced the Wheeler-DeWitt equation.

From Lab Toy to Cosmology Simulator

Read in isolation, this is one cold-atom paper in one journal. Read against the background of the last decade, it is part of a wider pattern. Ultracold-atom platforms have been quietly promoted from lab curiosities to quantum simulators for high-energy and cosmological questions, ones that used to live only on chalkboards. A popular write-up of the same experiment places it in the same lineage.

The paper itself catalogues the trend in its introduction. Bose-Einstein condensates have been used to detect spontaneous Hawking radiation from analogue black hole horizons. A supersonically expanding ring condensate has been used to emulate a Friedmann-Robertson-Walker universe in the lab. Programmable Rydberg arrays and trapped ions have been used to image the analogue of string breaking. Ultracold gases have been used to watch bubble nucleation and Schwinger-like pair production in a controlled false-vacuum decay.

Barontini’s group has been a steady contributor. He is a co-author on prior work on optical potentials for ultracold atoms, Bloch oscillations along synthetic dimensions, and Fermi acceleration with cold atoms, all of which appear in the reference list of the new paper. His publication record at the University of Birmingham documents the run-up to this experiment. Treating a Bose-Einstein condensate as a mini-universe is the natural next move in that lineage, and the authors are explicit that the same hardware can be retuned to probe black-hole analogues, the early universe, and competing ideas about how time emerges in quantum gravity.

Two things will be worth tracking. First, whether other labs reproduce the entropic ordering on a different atomic species, since 87Rb is one choice among many. Second, whether the entropic-time Schrödinger equation holds up outside the specific parameter range the Birmingham group explored, since the paper’s claim is that the equation reproduces the measured evolution, not that it is universal. Barontini’s own short list, as relayed by phys.org, includes extending the platform to Big Bang and Big Crunch physics, simulating black holes in the lab, and testing competing theories of how time emerges, all of which the open-access data set on Zenodo makes available to other groups. Broader commentary on the result has already appeared in online physics discussions and in a wider news syndication of the findings.

Frequently Asked Questions

What did the University of Birmingham actually build?

A Bose-Einstein condensate of about 24,000 rubidium-87 atoms, cooled to a few billionths of a degree above absolute zero and held in an optical dipole trap. A thin barrier cut the condensate into a bright observed sector and a dark unobserved sector, and the bright sector was made to expand and recollapse in repeated cycles the authors call mini Big Bangs and Big Crunches.

How is time being measured if there is no clock?

The team uses a coarse-grained entropy calculated from the distribution of atoms in the bright sector. When that entropy changes, the system is treated as moving forward in time. When it stops changing, time effectively stops. This entropic time can be used in place of the laboratory’s wall clock to order events inside the bright sector.

What is the problem of time in quantum gravity?

It is the difficulty that fundamental equations like the Wheeler-DeWitt equation, which sits at the heart of one approach to quantum gravity, do not contain an external time variable. If the universe at its deepest level is described by such an equation, any sense of past and future has to come from internal relationships between parts of the system, not from a universal clock.

Is this a model of the real universe?

It is an analogue, not a model. The paper describes the condensate as a mini-universe in the minimal sense of an effectively closed many-body system. The mathematical shape of the bright sector’s effective Hamiltonian is structurally similar to a Wheeler-DeWitt minisuperspace model, but the kinetic term has the opposite sign, and the analogy is at the level of relational variables, not exact equations.

Where can I read the paper?

The paper, “Testing the problem of time with cold atoms,” is published in Physical Review Research 8, L022047, on 11 June 2026. The DOI is 10.1103/1h9j-df4k, and the article is open access under a Creative Commons Attribution 4.0 license. The underlying data is also available on Zenodo.