A University of Birmingham scientist has built a “mini-universe” that takes a step toward answering one of science’s biggest questions: “What is time?” Publishing his findings 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 in which 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 in which 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.
How the mini-universe works
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 frequencies 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 in which 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.
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.
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The University of Birmingham experiment to trap and cool rubidium atoms close to absolute zero. Credit: University of Birmingham
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Giovanni Barontini, Professor of Physics, at the University of Birmingham, using the apparatus to trap and cool rubidium atoms. Credit: University of Birmingham
Entropic time in action
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. 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
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 forward and backward?
“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?
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 test bed 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.
Publication details
Giovanni Barontini, Testing the problem of time with cold atoms, Physical Review Research (2026). DOI: 10.1103/1h9j-df4k
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University of Birmingham
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Scientist creates ‘mini‑universe’ to measure time without a clock (2026, June 12)
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