How a "Mini Universe" of 24,000 Atoms Proved Time is Just a Quantum Illusion

On June 11, 2026, a team of physicists at the University of Birmingham in the United Kingdom published a landmark paper in Physical Review Research that transformed one of the most abstract debates in theoretical physics into a concrete laboratory reality. Led by Professor Giovanni Barontini, the researchers successfully constructed a hermetically sealed "mini-universe" composed of approximately 24,000 rubidium-87 atoms cooled to temperatures measuring just a few billionths of a degree above absolute zero.

By partitioning this microscopic cosmos with a thin barrier of laser light and observing it without an external clock, the team demonstrated that time is not a fundamental, built-in background parameter of our universe. Instead, they proved that what we perceive as the flow of time is an emergent illusion born entirely of quantum correlations and thermodynamic changes within a closed system.

For decades, the question of whether is time an illusion has hovered on the border between mathematical physics and philosophy. By capturing the physical evolution of this atomic cloud at precise 2-millisecond intervals over a total experimental window of 120 milliseconds, Barontini's team provided the first controlled, macroscopically scaled empirical data demonstrating how an internal "arrow of time" can be generated purely from the redistribution of entropy between entangled subsystems. From the outside, the global system remains static and timeless; from the inside, a sequential, dynamic history unfolds.

                 +---------------------------------------------+
                 |            THE MINI-UNIVERSE SYSTEM         |
                 +---------------------------------------------+
                                        |
                 +----------------------+----------------------+
                 |                                             |
                 v                                             v
     +-----------------------+                     +-----------------------+
     |   OBSERVED SECTOR     |   Atomic Exchange   |  UNOBSERVED SECTOR    |
     |   ("Bright" Region)   | <=================> |    ("Dark" Region)    |
     |                       |  Entropy Transfer   |                       |
     |  Expands/Collapses    |                     |   Acts as Reservoir   |
     |  (Big Bang/Crunch)    |                     |   and Quantum Clock   |
     +-----------------------+                     +-----------------------+
                 |                                             |
                 +----------------------+----------------------+
                                        |
                                        v
                 +---------------------------------------------+
                 |            ENTROPIC TIME (τ)                |
                 |      Emerges from relational change         |
                 |  τ advances as entropy is exchanged (dS > 0)|
                 |   τ stalls when exchange ceases (dS = 0)   |
                 +---------------------------------------------+

The Battle of the Constants: Why Time Stalls the Theory of Everything

To comprehend the significance of the 24,000-atom experiment, one must first confront the fundamental impasse that has stalled modern physics for nearly a century. Our two most successful and rigorously tested descriptions of reality—quantum mechanics and Albert Einstein’s theory of general relativity—rely on completely incompatible definitions of time.

In quantum mechanics, time ($t$) is treated as an absolute, classical background parameter. It is not a quantum observable; there is no "time operator" in standard quantum theory analogous to the operators for position ($\hat{x}$) or momentum ($\hat{p}$). Instead, time is a static, universal scaffolding against which the states of quantum systems evolve according to the Schrödinger equation:

$$i\hbar \frac{\partial}{\partial t} |\psi(t)\rangle = \hat{H} |\psi(t)\rangle$$

Here, the parameter $t$ ticks uniformly, completely unaffected by the physical processes occurring within the system. It is an external, god-like coordinate that must be assumed a priori for the mathematics of quantum theory to function.

Conversely, general relativity treats time as dynamic, local, and thoroughly integrated with spatial dimensions into a four-dimensional pseudo-Riemannian manifold. Spacetime is malleable; it stretches, dilates, and warps in the presence of mass and energy according to the Einstein field equations:

$$G_{\mu\nu} + \Lambda g_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}$$

In this framework, there is no absolute clock. Time is relative; a clock at a lower gravitational potential ticks slower than one at a higher potential, a phenomenon verified to more than 14 decimal places of accuracy by modern optical lattice clocks. Furthermore, in general relativity, time is simply a coordinate label ($x^0$), completely equivalent to the spatial coordinates ($x^1, x^2, x^3$).

When physicists attempt to merge these two frameworks into a unified theory of quantum gravity, this conceptual conflict erupts mathematically in the Wheeler-DeWitt equation. Formulated in 1967 by John Archibald Wheeler and Bryce DeWitt as a quantum version of general relativity's Hamiltonian constraint, the equation describes the quantum state of the entire universe, represented by the wavefunction of the universe, $\Psi$:

$$\hat{H} \Psi = 0$$

The implications of this equation are mathematically stark: the right-hand side is exactly zero. There is no time parameter $t$ anywhere in the equation.

Because the universe as a whole has no external "outside" clock to measure it, the global quantum state of the cosmos must be completely static, stationary, and frozen. This is the famous "frozen formalism" or the "problem of time" in quantum cosmology.

If the fundamental equation of quantum gravity contains no time, then at the deepest level of reality, the question is time an illusion must be answered in the affirmative. Yet, this mathematical reality stands in glaring contrast to our everyday macroscopic experience, where we observe sequence, change, and an unyielding progression from yesterday to tomorrow.

The Page-Wootters Mechanism: The Mathematical Engine of Emergent Time

In 1983, theoretical physicists Don Page and William Wootters proposed a brilliant solution to this paradox, now known as the Page-Wootters (PaW) mechanism. They suggested that while the universe as a whole is globally static, unchanging, and timeless, time can emerge locally for an observer who is inside the universe.

The core of the PaW mechanism is quantum entanglement. Page and Wootters argued that we must conceptually partition the timeless global universe into two subsystems:

A quantum clock subsystem ($C$), which possesses degrees of freedom that change dynamically.
The rest of the universe or the system of interest ($S$).

While the total state of the combined system $|\Psi\rangle\rangle$ is an eigenstate of the total Hamiltonian with eigenvalue zero (satisfying the Wheeler-DeWitt constraint $\hat{H}_{total} |\Psi\rangle\rangle = 0$), the clock and the system are highly entangled. The total Hamiltonian is written as:

$$\hat{H}_{total} = \hat{H}_C \otimes \mathbb{I}_S + \mathbb{I}_C \otimes \hat{H}_S$$

Because of this entanglement, the global state is a coherent superposition of all possible times simultaneously:

$$|\Psi\rangle\rangle = \int dt |t\rangle_C \otimes |\psi(t)\rangle_S$$

Here, $|t\rangle_C$ represents the state of the clock reading the "time" $t$, and $|\psi(t)\rangle_S$ is the correlated state of the system.

To an hypothetical external "super-observer" looking at the universe from the outside, nothing ever changes. The entire cosmos is a single, frozen, timeless block.

However, to an internal observer who is part of the system and correlates their measurements with the clock $C$, time dynamically emerges. If the internal observer projects the global state onto a specific clock reading $|t\rangle_C$, the conditional state of the system "snaps" into the corresponding physical state $|\psi(t)\rangle_S$:

$$|\psi(t)\rangle_S = {}_C\langle t | \Psi\rangle\rangle$$

By applying the global constraint equation $\hat{H}_{total} |\Psi\rangle\rangle = 0$ and projecting it onto the clock state, we can derive the dynamic evolution of the system:

$${}_C\langle t| \left( \hat{H}_C \otimes \mathbb{I}_S + \mathbb{I}_C \otimes \hat{H}_S \right) |\Psi\rangle\rangle = 0$$

Assuming an ideal, non-interacting clock where the clock Hamiltonian is canonically conjugate to the time operator (meaning $\hat{H}_C = -i\hbar \frac{\partial}{\partial t}$ in the clock's representation), the projection yields:

$$-i\hbar \frac{\partial}{\partial t} \left( {}_C\langle t | \Psi\rangle\rangle \right) + \hat{H}_S \left( {}_C\langle t | \Psi\rangle\rangle \right) = 0$$

Substituting $|\psi(t)\rangle_S$ back into the expression recovers the standard, time-dependent Schrödinger equation for the system:

$$i\hbar \frac{\partial}{\partial t} |\psi(t)\rangle_S = \hat{H}_S |\psi(t)\rangle_S$$

This mathematical derivation is a profound revelation: it demonstrates that the time parameter $t$ in the Schrödinger equation is not an absolute, fundamental property of the universe, but rather a coordination coordinate arising from the quantum entanglement between the system and the clock.

=========================================================================================
                               THE PAGE-WOOTTERS MECHANISM
=========================================================================================

  EXTERNAL VIEW (Super-Observer)                  INTERNAL VIEW (Internal Observer)
  ------------------------------                  ---------------------------------
  * Stands outside the system.                    * Entangled within the system.
  * Measures global state |Ψ>>.                   * Projectively measures clock state |t>_C.
  * No external clock exists.                     * System state collapses to |ψ(t)>_S.
  
  Mathematically:                                 Mathematically:
  H_total |Ψ>> = 0                                iħ ∂/∂t |ψ(t)>_S = H_S |ψ(t)>_S
  
  +----------------------------+                  +----------------------------+
  |    STATIC, FROZEN STATE    |                  |    DYNAMIC EMERGENCE OF    |
  |     (Timeless Universe)    |                  |      TIME AND SEQUENCE     |
  +----------------------------+                  +----------------------------+
=========================================================================================

While this theory has been mathematically consistent since 1983, testing it in a physical laboratory has been an overwhelming challenge. Early attempts at validating the Page-Wootters mechanism were limited to highly simplified toy models.

In 2013, researchers at the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy, performed a pioneering test using just two entangled photons. One photon acted as a clock (its polarization state indicating "time"), while the other acted as the system.

While successful, this experiment was restricted to the quantum behavior of single particles. It could not address whether relational time could scale up to many-body systems, nor could it demonstrate how a macroscopically observable "arrow of time" emerges from microscopic static states.

This is where Professor Giovanni Barontini’s 24,000-atom "mini-universe" steps in, scaling the validation of emergent time by four orders of magnitude and introducing thermodynamics into the quantum temporal equation.

Engineering the Mini-Universe: Under the Hood of the 24,000-Atom Cold Lab

To build a controlled physical analog of a timeless, closed universe, Barontini and his research team at the University of Birmingham designed an extraordinarily complex optoelectronic apparatus. The experiment required isolating a collection of atoms so thoroughly from the external environment that no thermal or electromagnetic information could leak in or out during the experimental run. Any such leakage would act as an external "measurement," introducing a classical ticking clock from the outside laboratory and ruining the self-contained nature of the "mini-universe".

The construction of this self-contained cosmos followed a highly precise, multi-stage cooling and trapping protocol:

Step 1: Magneto-Optical Trapping (MOT)

The process began by loading a gaseous vapor of rubidium-87 ($^{87}\text{Rb}$) isotopes into a ultra-high vacuum glass cell maintained at an extreme base pressure of less than $10^{-11}\text{ mbar}$. A set of six orthogonal, circularly polarized laser beams tuned slightly below the $D_2$ transition frequency of rubidium ($\lambda \approx 780.24\text{ nm}$) intersected at the center of a magnetic quadrupole field generated by a pair of anti-Helmholtz coils. This magneto-optical trap (MOT) successfully captured and pre-cooled approximately $10^7$ atoms to a temperature of 100 microkelvins ($\mu\text{K}$).

Step 2: Sub-Doppler cooling and Magnetic Transport

To bypass the Doppler cooling limit, the lasers were dynamically detuned further from resonance while transitioning the optical configuration into a gray molasses phase. This brought the atomic cloud's temperature down to 10 microkelvins.

The cold cloud was then magnetically transferred over a physical distance of several centimeters into a secondary, ultra-clean vacuum region optimized for long quantum coherence times.

Step 3: Evaporative Cooling in an Optical Dipole Trap

The atoms were loaded into a far-off-resonance optical dipole trap (ODT) formed by a single, tightly focused infrared laser beam operating at a wavelength of $\lambda = 1064\text{ nm}$. This wavelength is far red-detuned from any atomic transitions in rubidium, ensuring that the trapping potential is purely conservative and does not induce heating through spontaneous photon scattering.

By systematically reducing the power of the 1064 nm laser, the team performed forced evaporative cooling. The most energetic, "hottest" atoms escaped the shallowing trap, allowing the remaining atoms to re-thermalize through elastic collisions.

Once the temperature crossed the critical threshold of approximately 100 nanokelvins ($100\text{ nK}$), the atomic cloud underwent a phase transition into a Bose-Einstein Condensate (BEC).

Step 4: The 24,000-Atom Monolith

By carefully calibrating the final evaporation parameters, Barontini's team resolved a highly stable BEC containing precisely $24,000 \pm 500$ rubidium-87 atoms. In this exotic fifth state of matter, the individual atoms lost their distinct classical identities. Their de Broglie wavelengths overlapped, causing all 24,000 atoms to collapse into a single, macroscopic quantum wave function:

$$\Psi(\mathbf{r}) = \sqrt{\rho(\mathbf{r})} e^{i\theta(\mathbf{r})}$$

where $\rho(\mathbf{r})$ represents the spatial density distribution of the condensate and $\theta(\mathbf{r})$ is its global phase. This highly coherent, hermetically sealed BEC acted as the physical medium of the "mini-universe".

+-------------------------------------------------------------------------+
|                  BEC PREPARATION AND ISOLATION FLOW                     |
+-------------------------------------------------------------------------+
|  1. Vapor of Rubidium-87 in UHV Glass Cell (10^-11 mbar)               |
|                           |                                             |
|                           v                                             |
|  2. Magneto-Optical Trap (MOT) cools 10^7 atoms to 100 microkelvins     |
|                           |                                             |
|                           v                                             |
|  3. Gray Molasses phase cools cloud further to 10 microkelvins          |
|                           |                                             |
|                           v                                             |
|  4. Load into conservative Optical Dipole Trap (1064 nm infrared laser) |
|                           |                                             |
|                           v                                             |
|  5. Forced Evaporation: Red-detuned power lowered, hot atoms escape     |
|                           |                                             |
|                           v                                             |
|  6. Phase Transition (T < 100 nK): Macroscopic Bose-Einstein Condensate |
|                           |                                             |
|                           v                                             |
|  7. Static, Timeless Monolith of 24,000 Rubidium-87 atoms               |
+-------------------------------------------------------------------------+

Step 5: Projecting the Barrier

To implement the Page-Wootters partition, the team projected a thin, blue-detuned optical potential barrier directly through the center of the BEC. A blue-detuned laser has a frequency higher than the atomic transition, meaning that atoms are repelled from the regions of high intensity.

By combining two laser beams of slightly different frequencies, the team shaped a flat, sheet-like potential barrier with a typical spatial width of roughly $2.5\text{ }\mu\text{m}$.

This barrier divided the BEC into two distinct spatial regions:

The Bright Sector ($S$): The observed region of the mini-universe. It was repeatedly imaged using weak, non-destructive absorption probes to track spatial and density variations.
The Dark Sector ($C$): The unobserved region. Left completely unmeasured, it acted as the quantum environment/clock reservoir.

The barrier height ($V_0$), measured in units of the chemical potential of the BEC ($\mu$), was dynamically tunable from $0.5\mu$ up to $5.0\mu$. When $V_0$ was low, atoms could quantum-tunnel across the barrier, allowing a free exchange of matter and information between the bright and dark sectors. When $V_0$ was high, the sectors were isolated, preventing any exchange.

Entropic Time: Turning Thermodynamics into a Temporal Metric

Once the 24,000-atom mini-universe was constructed, Barontini's team faced their most profound experimental hurdle: how do you measure the passage of time inside a closed quantum system without relying on an external, ticking laboratory clock?

If they used the laboratory's master atomic clock—which ticks continuously via microwave transitions in cesium atoms—they would violate the very premise of the experiment. They had to show that a temporal coordinate could be constructed entirely from within the bright sector itself, using only the relationships and distributions of the rubidium atoms.

The team realized that any standard "clock" variable must satisfy two strict criteria:

It must be monotonic, meaning it must flow in a single, consistent direction to establish a clear "arrow of time".
It must be able to sequence events uniquely, even when the physical system is expanding and contracting.

Initially, they analyzed mechanical variables, such as the spatial center of mass ($x_{com}$) of the atoms in the bright sector. However, because the atomic cloud in the bright sector repeatedly expanded and collapsed (resembling a cyclic cosmos moving through Big Bangs and Big Crunches), the center of mass oscillated back and forth.

A variable that goes back and forth cannot act as a universal timekeeper, because a single value of $x_{com}$ would correspond to multiple different states in the expansion-contraction cycle. It lacked the property of uniqueness.

To solve this, Barontini turned to the second law of thermodynamics: entropy.

=========================================================================================
                                   ARROW OF TIME COMPARISON
=========================================================================================

  MECHANICAL CLOCK (Center of Mass)               ENTROPIC CLOCK (Atomic Dispersion)
  ---------------------------------               ----------------------------------
  * Oscillates back and forth.                    * Increases monotonically.
  * Non-monotonic.                                * Strictly monotonic.
  * Cannot uniquely sequence events.              * Creates a clear, irreversible arrow.
  
  +---------------------------------+             +---------------------------------+
  |        CYCLE: A -> B -> A       |             |        FLOW: A -> B -> C        |
  |   (Ambiguous Time Parameters)   |             |    (Unique Temporal Order)      |
  +---------------------------------+             +---------------------------------+
=========================================================================================

As atoms tunnel back and forth across the optical barrier, they redistribute quantum information between the observed bright sector and the unobserved dark sector. By performing high-resolution absorption imaging on the bright sector every 2 milliseconds, the team mapped the spatial density profile, $\rho(x, z)$, of the atomic cloud.

From these density profiles, they calculated an experimentally defined, coarse-grained entropy ($S_{cg}$). The coarse-grained entropy was computed by partitioning the bright sector's coordinate space into discrete micro-bins of volume $\Delta V$ and evaluating the classical Shannon entropy of the probability distribution of the atoms:

$$S_{cg} = -k_B \sum_{i} p_i \ln p_i$$

where $p_i = \int_{\Delta V_i} \rho(\mathbf{r}) d^3\mathbf{r} / N_{bright}$ is the probability of finding an atom in the $i$-th spatial bin, and $N_{bright}$ is the total number of atoms in the bright sector at that moment.

Because the unobserved dark sector acted as a massive quantum reservoir (analogous to the unobservable dark energy and dark matter in our macroscopic universe), the exchange of atoms across the barrier naturally led to a progressive, irreversible increase in the coarse-grained entropy of the bright sector.

The team defined a new, internal temporal metric called entropic time ($\tau$):

$$\tau = \alpha \left[ S_{cg}(\tau_{lab}) - S_{cg}(0) \right]$$

where $\alpha$ is a scaling constant.

The experimental data for this entropic time revealed extraordinary properties:

1. Monotonicity and the Arrow of Time

Unlike any mechanical coordinate, the entropic time $\tau$ grew monotonically throughout the 120-millisecond run. Even when the physical cloud in the bright sector was actively contracting (collapsing back toward the barrier in a "Big Crunch" phase), its internal coarse-grained entropy continued to rise. This provided an unyielding, one-way "arrow of time" that successfully ordered every single image slice in its proper chronological sequence.

2. Tunable Velocity

By varying the physical height of the optical barrier ($V_0$), the researchers could directly control the rate at which entropic time flowed relative to the laboratory's master clock ($t_{lab}$).

When the barrier was low ($V_0 = 0.8\mu$), the rate of atomic tunneling was high, resulting in a rapid exchange of entropy. Under these conditions, the internal entropic clock ran fast: the slope $d\tau/dt_{lab}$ was steep.
When the barrier was raised to a high potential ($V_0 = 4.2\mu$), tunneling was heavily suppressed. The rate of entropy exchange dropped dramatically, and the internal flow of time slowed to a crawl.

3. The Pause of Time

Most astonishingly, when the optical barrier was raised to a point of complete isolation ($V_0 \ge 5.0\mu$), the exchange of atoms and entropy between the bright and dark sectors was completely severed.

As the distribution of atoms in the bright sector ceased to change, the coarse-grained entropy stalled ($dS_{cg} = 0$). At this exact moment, internal entropic time came to a complete halt.

To an observer living inside that bright sector, the universe was frozen. Time had stopped. Yet, outside in the laboratory, the master atomic clock continued to tick continuously, recording the passage of tens of milliseconds.

This was the first direct, experimental demonstration of the core premise of relational quantum gravity: time is not an external fluid that flows at a constant, unyielding rate. It is a relational parameter that only exists when there is active, physical interaction and entropy exchange between the subsystems of a closed universe.

Rewriting Quantum Dynamics with Relational Time

To prove that this internally defined "entropic time" was not just a clever mathematical metaphor, but could actually function as a true temporal parameter in the laws of physics, Barontini’s team attempted a daring mathematical experiment. They sought to completely eliminate the classical laboratory time parameter $t$ from the foundational equation of quantum mechanics—the Schrödinger equation—and replace it with their newly derived entropic time $\tau$.

The standard, time-dependent Schrödinger equation for the wave function of the bright sector, $|\psi(t)\rangle$, is given by:

$$i\hbar \frac{\partial}{\partial t} |\psi(t)\rangle = \hat{H}_{eff}(t) |\psi(t)\rangle$$

where $\hat{H}_{eff}(t)$ is the effective, time-dependent Hamiltonian of the bright sector under the influence of the external optical trapping potentials.

The researchers performed a change of variables. By utilizing the chain rule, they expressed the temporal derivative with respect to laboratory time $t$ in terms of the internal entropic time $\tau$:

$$\frac{\partial}{\partial t} = \frac{d\tau}{dt} \frac{\partial}{\partial \tau}$$

Because the rate of change of entropic time, $d\tau/dt$, is directly proportional to the rate of entropy exchange, they could rewrite the Schrödinger equation as:

$$i\hbar \left( \frac{d\tau}{dt} \right) \frac{\partial}{\partial \tau} |\psi(\tau)\rangle = \hat{H}_{eff}(\tau) |\psi(\tau)\rangle$$

Dividing both sides by the dynamic scaling factor $d\tau/dt$ yielded a brand-new, self-contained entropic-time Schrödinger equation:

$$i\hbar \frac{\partial}{\partial \tau} |\psi(\tau)\rangle = \left[ \frac{1}{\left( \frac{d\tau}{dt} \right)} \hat{H}_{eff}(\tau) \right] |\psi(\tau)\rangle = \hat{H}_{\tau} |\psi(\tau)\rangle$$

where $\hat{H}_{\tau}$ is the modified Hamiltonian scaled by the instantaneous velocity of entropic time.

=========================================================================================
                             EQUATION OF MOTION COMPARISON
=========================================================================================

  STANDARD SCHRÖDINGER EQUATION                     ENTROPIC-TIME SCHRÖDINGER EQUATION
  -----------------------------                     ----------------------------------
  * Relies on absolute lab time (t).               * Relies on internal entropic time (τ).
  * Assumes an external ticking clock.              * Generated entirely within the system.
  
  Equation:                                         Equation:
  iħ ∂/∂t |ψ(t)> = H_eff(t) |ψ(t)>                  iħ ∂/∂τ |ψ(τ)> = H_τ |ψ(τ)>
  
  +---------------------------------+             +---------------------------------+
  |      EXTERNAL DEPENDENCY        |             |      RELATIONAL EMERGENCE       |
  |  (Violates Closed Universe)     |             |   (Validates Timeless WDW)      |
  +---------------------------------+             +---------------------------------+
=========================================================================================

The team then ran a highly detailed numerical simulation. They solved this entropic-time Schrödinger equation to predict how the spatial probability cloud of the 24,000 rubidium atoms would expand and contract within the bright sector.

If entropic time were a mere mathematical illusion, the predictions of this equation would quickly diverge from the physical reality of the experiment.

The quantitative results of this test were stunningly precise. The team compared the predicted spatial width of the atomic cloud, $w(\tau)$, derived from the entropic-time Schrödinger equation, against the actual, physical widths measured via absorption imaging.

The correlation metrics were extraordinary:

Coefficient of Determination ($R^2$): The simulated evolution of the atomic cloud matched the experimental data with an $R^2$ value of $0.987 \pm 0.005$ across all experimental runs.
Predictive Accuracy: The entropic-time equation correctly predicted the exact moment of the "Big Crunch" (when the expanding atomic cloud reversed its trajectory and collapsed back toward the optical barrier) to within a margin of error of less than $1.2\text{ milliseconds}$.
Robustness across Potential Heights: This high predictive accuracy held true regardless of whether the optical barrier was set to $1.2\mu$, $2.5\mu$, or $3.8\mu$. The mathematics of the entropic Schrödinger equation dynamically compensated for the altering flow of time, yielding a flawlessly consistent physical description.

This was the ultimate proof: a version of the Schrödinger equation, the central equation of quantum mechanics, can be written and solved using an internally generated, entropic temporal coordinate. The physical dynamics of a many-body system can be predicted just as effectively using relational quantum time as they can with the conventional, external clocks of our laboratories.

The Cosmological Parallel: Big Bangs, Big Crunches, and Dark Sectors in a Glass Cell

The 24,000-atom experiment at the University of Birmingham is more than an exercise in ultracold atomic engineering; it is a highly sophisticated, laboratory-scale quantum simulator of the entire cosmos. By trapping a closed, highly isolated system, Professor Barontini successfully constructed a physical analog to some of the most profound concepts in quantum cosmology.

                 +---------------------------------------------+
                 |             COSMOLOGICAL ANALOGY            |
                 +---------------------------------------------+
                                        |
                 +----------------------+----------------------+
                 |                                             |
                 v                                             v
     +-----------------------+                     +-----------------------+
     |   THE METAPHORICAL    |                     |     THE PHYSICAL      |
     |     LABORATORY        |                     |      COSMOLOGICAL     |
     +-----------------------+                     +-----------------------+
     | 24,000 Rubidium Atoms | <=================> |  The Entire Universe  |
     |                       |                     |                       |
     | Laser Potential Sheet | <=================> | Event Horizon/Boundary|
     |                       |                     |                       |
     |     Bright Sector     | <=================> | Observable Matter/    |
     | (Expands/Contracts)   |                     | Spacetime             |
     |                       |                     |                       |
     |     Dark Sector       | <=================> | Unobservable Dark     |
     |                       |                     | Sector (Dark Matter)  |
     +-----------------------+                     +-----------------------+

The behavioral parallels between the atomic cloud in the glass cell and our macroscopic universe are deeply striking:

1. The Cyclic Cosmos

When the blue-detuned barrier was projected and the atoms were perturbed, the atomic cloud in the observed bright sector began to oscillate. It rapidly expanded outward into the available trapping volume, reached a maximum point of dispersion, and then collapsed back toward the barrier under the influence of the trapping potential.

Barontini described this recurring motion as a laboratory analog of a cyclic universe. The rapid expansion represents the Big Bang, while the subsequent recollapse mimics the Big Crunch—a hypothetical cosmological scenario where the expansion of our actual universe eventually reverses due to gravitational attraction, collapsing back into an infinitely dense singularity.

2. The Unobserved Dark Sector

A critical feature of the experiment was the partition of the BEC into an observed bright region and an unobserved dark region.

Barontini drew a direct physical parallel between the dark sector of his experiment and the unobservable 95% of our macroscopic universe—namely, dark matter and dark energy.

Just as we cannot directly observe dark matter, we can only infer its existence through its gravitational influence on visible matter. Similarly, the internal observer in the bright sector cannot directly see the dark sector.

Yet, it is the quantum exchange of atoms and information across that invisible barrier that generates the internal "entropic clock" of the bright sector. This suggests that our own cosmic time may be generated by quantum correlations between the observable matter around us and the vast, unobservable dark sectors of the cosmos.

3. The Stationary Global Wavefunction

If we look at the entire 24,000-atom BEC as a single, undivided system, it possesses a completely time-independent Hamiltonian. Its global quantum state, $|\Psi\rangle\rangle$, is stationary.

To an external observer looking at the entire vacuum apparatus, the global system is timeless and frozen. There is no change, no flow of time, and no sequence.

Yet, inside the glass cell, when we focus solely on the bright sector and its relational entanglement with the dark sector, a vivid, dynamic history unfolds: an arrow of time emerges, events are sequentially ordered, and the laws of quantum motion govern the expanding and collapsing atomic cloud.

This is the ultimate realization of the Page-Wootters mechanism: is time an illusion? Yes, on a global, universal scale, time is indeed a static illusion.

The universe does not evolve in time. Rather, time is an emergent, local perspective that arises from the intrinsic, quantum relationships and correlations between the constituent parts of a timeless, stationary whole.

Technical Appendix: Key Parameters of the University of Birmingham Experiment

For those looking to analyze the quantitative boundaries of this experiment, the following table compiles the critical physical, optical, and thermodynamic parameters utilized by Professor Giovanni Barontini's team in their June 2026 paper:

Parameter	Symbol / Formula	Experimental Value / Range
Total Number of Atoms	$N_{total}$	$24,000 \pm 500$ rubidium-87 isotopes
Bose-Einstein Condensate Temperature	$T_{BEC}$	$15 \text{ to } 35\text{ nanokelvins}$ (nK)
UHV Base Pressure	$P_{base}$	$< 8 \times 10^{-12}\text{ mbar}$
Trapping Laser Wavelength	$\lambda_{ODT}$	$1064\text{ nm}$ (infrared, red-detuned)
Partition Laser Wavelength	$\lambda_{barrier}$	$760\text{ nm}$ (optical, blue-detuned)
Barrier Potential Height	$V_0$	$0.5\mu \text{ to } 5.0\mu$ (where $\mu$ is the chemical potential)
Barrier Spatial Width	$w_{barrier}$	$2.5\text{ }\mu\text{m} \pm 0.2\text{ }\mu\text{m}$
Imaging Interval	$\Delta t_{image}$	$2.0\text{ milliseconds}$ (ms)
Total Experimental Run Duration	$t_{run}$	$120\text{ milliseconds}$ (ms)
Coarse-Grained Entropy Range	$S_{cg}$	$1.2\text{ }k_B \text{ to } 4.8\text{ }k_B$
Entropic Time Correlation Fit	$R^2$	$0.987 \pm 0.005$
Predictive Singularity Timing Error	$\Delta \tau_{error}$	$< 1.2\text{ milliseconds}$ (ms)

The Horizon of Quantum Cosmology: What We Watch For Next

The publication of "Testing the problem of time with cold atoms" marks a watershed moment in experimental physics, shifting the study of quantum gravity and canonical cosmology out of the realm of pure mathematical speculation and into the domain of empirical, laboratory-controlled science. By proving that time can emerge as a relational thermodynamic coordinate within a closed many-body system, Barontini's team has opened up a plethora of exciting new research frontiers.

+-------------------------------------------------------------------------+
|                        FUTURE RESEARCH ROADMAP                          |
+-------------------------------------------------------------------------+
|  1. SCALE THE SYSTEM: Increase atom count to 10^6 - 10^7               |
|     - Observe classical transition and decoherence effects.             |
|                           |                                             |
|                           v                                             |
|  2. MULTIPLE PARTITIONS: Implement 3+1D entangled networks              |
|     - Model the simultaneous emergence of spatial dimensions.           |
|                           |                                             |
|                           v                                             |
|  3. SIMULATE SINGULARITIES: Create sonic black hole analog horizons      |
|     - Measure relational time dilation and Hawking radiation.          |
|                           |                                             |
|                           v                                             |
|  4. SOLVE COHERENCE BOUNDARY: Map quantum-to-classical emergence limit  |
|     - Track the emergence of smooth, continuous relativistic spacetime.   |
+-------------------------------------------------------------------------+

As the physics community digests these findings, several key milestones and unresolved questions will dominate the research landscape over the coming years:

1. Scaling the "Mini-Universe"

The Birmingham experiment proved relational time works for 24,000 atoms.

The next major milestone is to scale these traps to contain hundreds of thousands or even millions of atoms.

As the number of atoms increases and the system approaches the classical macroscopic limit, researchers want to observe how quantum decoherence begins to smooth out the discrete, step-like progression of entropic time into the continuous, fluid coordinate of our daily lives. Tracking this transition will provide crucial clues about the exact boundary between the quantum and classical worlds.

2. Simultaneous Emergence of Space and Time

A theoretical extension of the Page-Wootters mechanism, published by researchers in late 2025, suggests that quantum entanglement can underpin the emergence of not just time, but space itself.

By creating complex, multi-partitioned networks of entangled qubits, physicists hope to design experiments where a full 3+1 dimensional spacetime emerges relationally from a globally static, "positionless" and "timeless" quantum state.

If successful, this would provide empirical validation for the holographic principle and theories of emergent gravity, such as the AdS/CFT correspondence.

3. Simulating Black Holes and Hawking Radiation

The ability to precisely shape optical potentials and control the flow of entropic time allows researchers to simulate the extreme environments of black hole event horizons inside a laboratory glass cell.

By accelerating the flow of entropic time in one region of the BEC while stalling it in another, physicists can create a "sonic horizon".

Measuring how phonons (sound-quantums) behave at this artificial horizon will allow scientists to test theories of gravitational time dilation, black hole information loss, and Hawking radiation under highly controlled conditions.

   FAST TIME REGION (Low Barrier, High dS/dt)
   +---------------------------------------+
   |   Atoms moving quickly, τ advancing   |
   +---------------------------------------+
                      |
                      v   <--- Sonic Event Horizon (Velocity boundary)
                      |
   SLOW/FROZEN REGION (High Barrier, dS/dt -> 0)
   +---------------------------------------+
   |   Tunneling blocked, τ ground to halt |
   +---------------------------------------+

4. The Unified Theory of Everything

Ultimately, proving that the question is time an illusion has a definitive "yes" at the quantum scale provides a new pathway toward the elusive "Theory of Everything".

By showing that the time parameter in the Schrödinger equation can be replaced with relational entropic time, physicists may finally have the mathematical toolset needed to reconcile the frozen, timeless formalism of the Wheeler-DeWitt equation with the dynamic, coordinate-based spacetime of Einstein’s general relativity.

The clock in Professor Giovanni Barontini's lab did not tick because of some external mechanical gear. It ticked because 24,000 cold rubidium atoms were talking to each other, sharing information, and generating disorder in a quiet vacuum cell.

By proving that time is an emergent property of quantum relationships, this mini-universe has taken us one massive step closer to understanding the true, timeless nature of the grand cosmos we inhabit.

References

[1] Barontini, G. "Testing the problem of time with cold atoms." Physical Review Research, vol. 8, no. L022047, June 11, 2026. DOI: 10.1103/PhysRevResearch.8.L022047