Scientist creates ‘mini‑universe’ to measure time without a clock
image:
Part of the apparatus to trap and cool rubidium atoms close to absolute zero (~-273.15 degrees Celsius).
view moreCredit: University of Birmingham
New experiment provides powerful testbed for ideas in quantum cosmology and gravity - theories relating to the early universe can now be tested in the laboratory.
A University of Birmingham scientist has built a 'mini universe' that takes a step towards answering one of science’s biggest questions: ‘what is time?’
Publishing his findings today in Physical Review Research, Professor Giovanni Barontini shows how it is possible to measure the flow of time without using a clock at all. The new findings provide a scientific model where a version of time emerges from the experiment itself.
Some theories of physics, such as the Wheeler–DeWitt equation suggest that, at its deepest level, the universe has no built‑in time, but exists as a single, unchanging quantum state where particles exhibit both wave-like and particle-like properties. It treats the universe as a whole with no external clock, and any sense of time must emerge from internal relationships between parts.
Professor Barontini used a cloud of 24,000 ultracold atoms - just a few billionths of a degree above absolute zero - to create a hermetically sealed quantum system that mimics a simple ‘universe’. The particles were trapped and divided with a thin barrier formed with two laser beams of different frequency to create an observed (‘bright’) and an unobserved (‘dark’) region.
The ‘bright’ sector repeatedly expands and collapses, experiencing something like a Big Bang and a Big Crunch - a hypothetical scenario where the expansion of the cosmos eventually reverses. The experiment allows the sequence of events to be reconstructed from within the mini universe itself, without any reference to an external laboratory clock.
The experiment demonstrated that time could emerge from changes happening inside a quantum system, rather than existing as something external that ticks along independently.
Using the ‘mini universe’ demonstrated that ‘time’ could be created from the disorder or spread (entropy) of atoms and how they behaved in a system. Atoms could move between ‘bright’ and ‘dark’ regions, but the system was otherwise isolated from the outside world.
When the spread of particles in the bright sector increased or decreased as atoms moved in or out, the system was ‘moving forward in time’. When this distribution of atoms did not change, time effectively stopped. Professor Barontini called this process ‘entropic time’, after finding that this version of time:
Flows in one consistent direction, giving a clear ‘arrow of time’
Correctly orders events, even in a system expanding and contracting like a mini cosmos
Speeds up or slows down depending on how entropy moves around
Professor Barontini said: “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?
“This study provides 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. It offers new insight into the nature of time in quantum gravity that could be used to describe dynamics just as effectively as conventional time.”
The study also demonstrates that a version of the main equation in quantum mechanics (Schrödinger) can still be written using entropic time – enabling predictions of how the ‘probability cloud’ of a quantum system will change over time.
The experiment addresses a long-standing question in physics - in some theories of the universe, there is no built‑in clock so how do you tell what comes ‘before’ and ‘after’ without external time?
Professor Barontini showed that the system follows the standard equations of quantum physics and demonstrates that deep questions about the nature of time - usually discussed only in theories about the universe as a whole - can be tested in controlled laboratory experiments.
The experiment provides a powerful testbed for ideas in quantum cosmology and gravity, meaning that ideas relating to the early universe can now be tested experimentally in the lab.
The approach could be extended to more complex systems, potentially allowing researchers to probe the physics of the Big Bang and the ‘Big Crunch’. It could also be used to simulate black holes in the lab or test competing theories about how time emerges in the universe.
ENDS
The 'cloud' inside the glass cell is a magneto-optical trap of rubidium atoms at a temperature of ~0.0001 degrees above absolute zero. It is only the first step to "build" the mini-universe.
The University of Birmingham experiment to trap and cool rubidium atoms close to absolute zero.
Giovanni Barontini, Professor of Physics, at the University of Birmingham, using the apparatus to trap and cool rubidium atoms.
Giovanni Barontini, Professor of Physics, at the University of Birmingham, using the apparatus to trap and cool rubidium atoms.
Optics to deliver the lasers on the atoms.
Credit
University of Birmingham
Journal
Physical Review Research
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Testing the problem of time with cold atoms
Article Publication Date
11-Jun-2026
Reversing the arrow of time in heat transfer: A novel operator learning framework for thermal retrodiction
Science China Press
image:
Experimental thermal field retrodiction evolution of the 3D-printed sample.
view moreCredit: ©Science China Press
Thermal diffusion—a fundamental physical process pervasive in applications ranging from electronics to industrial systems—is inherently irreversible under the second law of thermodynamics. As temperature gradients spontaneously smooth out over time, crucial information regarding the initial thermal state is permanently lost due to entropy increase. This fundamental information loss leads to a severe ill-posedness in inverse thermal problems, where infinitesimal errors in the final observation can cause enormous uncertainties in the reconstructed initial state.
Recently, a collaborative research team led by Prof. Ying Li and Prof. Hongsheng Chen from Zhejiang University, alongside Prof. Jiping Huang from Fudan University, has made a significant breakthrough in tackling this longstanding challenge. Published in National Science Review and co-first-authored by master’s graduate Hanqi Chen, as well as PhD students Qiang-Kai-Lai Huang and Yanxiang Wang from Prof. Ying Li’s research group at Zhejiang University, the paper titled “Learning to reverse thermal diffusion” introduces a deep learning framework for inverse diffusion capable of deducing initial thermal states based on final-state observations.
The proposed methodology overcomes the limitations of conventional computational methods, particularly when analyzing heterogeneous materials, by employing two synergistic deep learning components. First, the team introduced the Thermal Field Evolution Network (TE-Net), a finite-difference-based convolutional architecture that accurately estimates spatial material properties—specifically thermal diffusivity—from time-dependent cooling data.
Crucially, this accurately derived diffusivity serves as essential physical prior knowledge for the framework’s core innovation: the Time-Reversal Operator (TRO). Drawing inspiration from generative diffusion models and Fourier Neural Operators, the TRO learns mappings between function spaces rather than relying on traditional point-wise solutions. By synergizing analytical eigenbasis decomposition with frequency-domain operator learning, the TRO effectively filters high-frequency noise and directly projects the final-state thermal field back to its initial distribution.
The research team rigorously validated their approach through comprehensive simulations and real-world infrared imaging experiments using 3D-printed structures and practical semiconductor chips. The operator-driven method demonstrated exceptional performance, achieving a thermal diffusivity estimation error of less than 10% and an astonishing retrodiction numerical error below 0.1%.
This breakthrough establishes a high-fidelity paradigm for spatiotemporal thermal analysis. It holds broad implications for advanced non-destructive testing, defect detection in densely integrated circuits, and next-generation thermal management, with potential applications extending to a wider class of physical phenomena such as mass and charge diffusion.
Journal
National Science Review
Figure 1: Schematic of the diffusion and retrodictive diffusion processes, along with the overall deep learning architecture.
Figure 2: Experimental thermal field retrodiction evolution of the 3D-printed sample and the chip.
Credit
©Science China Press
