A quantum “mini universe” made from ultracold atoms revealed that time can emerge from entropy changes inside a system, offering experimental insight into one of physics’ deepest mysteries.
A scientist at the University of Birmingham has created a “mini universe” that could help answer one of science’s most fundamental questions: What is time?
In a study published in Physical Review Research, Professor Giovanni Barontini demonstrates that it is possible to track the passage of time without relying on a clock. The research introduces a model in which a form of time emerges naturally from the behavior of the system being studied.
Some physics theories, including the Wheeler–DeWitt equation, suggest that time is not a fundamental property of the universe. Instead, the universe may exist as a single quantum state that does not change, with particles displaying both wave-like and particle-like behavior. In this view, there is no external clock, and the experience of time arises from relationships between different parts of the system.
A Clockless Universe and the Nature of Time
To explore this idea, Barontini created a sealed quantum system using a cloud of 24,000 ultracold atoms, cooled to just a few billionths of a degree above absolute zero. The atoms were trapped and separated by a thin barrier created with two laser beams of different frequencies, forming an observed “bright” region and an unobserved “dark” region.
The bright region repeatedly expands and contracts, resembling a Big Bang followed by a Big Crunch, a hypothetical scenario in which the universe’s expansion eventually reverses. Because the sequence of events can be reconstructed from inside the system, the experiment does not require any reference to an external clock.
The results show that time can emerge from changes taking place within a quantum system rather than existing as an independent external feature.
The mini universe also revealed that time can arise from entropy, the spread or disorder of particles within a system. Atoms were able to move between the bright and dark regions while the system remained isolated from the outside world.
Building a Mini Universe From Ultracold Atoms
As atoms moved in and out of the bright region, the distribution of particles changed. When that distribution increased or decreased, the system effectively moved forward in time. When the distribution remained unchanged, time effectively came to a halt. Barontini describes this process as “entropic time” because it:
- Flows in a single direction, creating a clear “arrow of time”
- Correctly orders events, even in a system that expands and contracts like a miniature universe
- Speeds up or slows down depending on how entropy is redistributed
Entropic Time and the Arrow of Time
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 shows that a version of the Schrödinger equation, one of the central equations of quantum mechanics, can be expressed using entropic time. This allows researchers to predict how a quantum system’s “probability cloud” evolves over time.
Experimental Evidence for Emergent Time
The research tackles a long-standing question in physics: if certain theories of the universe contain no built-in clock, how can events be placed in order without an external measure of time?
Barontini showed that the system still follows the standard laws of quantum physics, demonstrating that questions about the nature of time, once limited to theories of the entire universe, can now be investigated through controlled laboratory experiments.
The experiment provides a valuable testing ground for ideas in quantum cosmology and gravity, making it possible to examine concepts related to the early universe under laboratory conditions.
The method could eventually be applied to more complex systems, allowing scientists to study the physics of both the Big Bang and the Big Crunch. It may also help researchers simulate black holes in the laboratory and evaluate competing theories about how time emerges in the universe.
Reference: “Testing the problem of time with cold atoms” by Giovanni Barontini, 11 June 2026, Physical Review Research.
DOI: 10.1103/1h9j-df4k
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