Physicists at the University of Birmingham have created a laboratory ‘mini-universe’ that will bring scientists one step closer to answering one of the biggest questions in physics: ‘What is time?’
In a study published in physical review studyProfessor Giovanni Barontini has demonstrated that it is possible to measure the passage of time without relying on clocks. Instead, this experiment shows that versions of time can emerge naturally from the behavior of the quantum system itself.
Why some physicists think time is not fundamental
Some theories of modern physics suggest that time may not exist as a built-in feature of the universe. One example is the Wheeler-DeWitt equation, which describes the universe as a single quantum state with no external clock. In this diagram, the particles exhibit both wave-like and particle-like behavior, and the familiar flow of time must arise from relationships between different parts of the system, rather than from independent ticking clocks.
To investigate this idea experimentally, Professor Barontini created a simplified quantum “universe” using a cloud of 24,000 ultra-cold atoms cooled to just a few billionths of a degree above absolute zero. The atoms were sealed in an isolated system and separated by a thin barrier created by two laser beams of different frequencies. This produced two regions: one that was observed (“bright”) and one that was not observed (“dark”).
A small universe with its own sense of time
Inside this miniature universe, bright regions expanded and contracted repeatedly, resembling a simplified version of the Big Bang followed by the Big Crunch, a hypothetical event in which the expansion of the universe would eventually reverse.
Because the system was completely isolated, researchers were able to reconstruct the sequence of events using only information from inside the mini-universe itself, without reference to an external laboratory clock.
The results showed that time does not exist as an independent background that is always moving forward, but can emerge from changes that occur within quantum systems.
How entropy created time
This experiment revealed that “time” arises from changes in the disorder, or entropy, of atoms as they move between bright and dark regions. Apart from this movement, the system remained isolated from the outside world.
As the distribution of particles in the bright region increased or decreased, the system effectively moved forward in time. When the particle distribution stopped changing, time itself effectively stopped.
Professor Barontini calls this concept “entropic time.” In our experiments, times in this format look like this:
- Flows in one consistent direction, creating a clear “arrow of time”
- Order events correctly even when the mini-universe grows or shrinks
- It can be faster or slower depending on how the entropy is redistributed
Professor Barontini said: “In some cosmological theories, especially quantum gravity, time does not appear as a built-in feature. But in everyday life, time flows from the past to the future. Why is this so, when most of the fundamental laws of physics work the same way forward and backward?”
“This work provides the first controlled experimental evidence that ‘time’ can be defined by changes within a system, rather than as an external ‘ticking clock’ that we think of as time. This work provides new insights into the nature of time in quantum gravity that can be used to describe dynamics as effectively as classical time.”
Testing quantum gravity in the laboratory
The researchers also discovered that a version of the Schrödinger equation, the fundamental equation of quantum mechanics, can be expressed using entropy time. This means that scientists can predict how the “probability cloud” of a quantum system will evolve over time, even if time is defined by internal changes rather than an external clock.
This research addresses a long-standing problem in physics. If a particular theory is correct and the universe has no built-in clock, how can events be arranged in the correct order? This experiment suggests that the answer may lie in the internal evolution of the system itself.
Professor Barontini has shown that while the miniature universe obeys the standard laws of quantum mechanics, ideas about the nature of time that are normally limited to theories that describe the entire universe can be tested under controlled laboratory conditions.
Towards experiments on the big bang and black holes
Mini-universes provide a valuable experimental platform for testing ideas in quantum cosmology and quantum gravity. Rather than relying solely on mathematical models, scientists may be able to investigate concepts related to the early universe through laboratory experiments.
The researchers say the same approach could eventually be extended to more complex quantum systems, opening the door to experiments exploring the physics of the Big Bang and the “Big Crunch,” black hole simulations, and competing theories about how time itself emerges.
