A physicist at the University of Birmingham has created a laboratory “mini universe” that brings scientists a step closer to answering one of the biggest questions in physics: What is time?
In a study published in Physical Review Research, Professor Giovanni Barontini demonstrates that it is possible to measure the passage of time without relying on a clock. Instead, the experiment shows that a version of time can emerge naturally from the behavior of a quantum system itself.
Why Some Physicists Think Time May Not Be Fundamental
Several 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 picture, particles display 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 an independent ticking clock.
To investigate this idea experimentally, Professor Barontini created a simplified quantum “universe” using a cloud of 24,000 ultracold atoms cooled to just a few billionths of a degree above absolute zero. The atoms were sealed inside an isolated system and separated by a thin barrier created with two laser beams of different frequencies. This produced two regions: an observed (“bright”) region and an unobserved (“dark”) region.
A Tiny Universe With Its Own Sense of Time
Inside this miniature universe, the bright region repeatedly expanded and contracted, resembling a simplified version of a Big Bang followed by a Big Crunch, a hypothetical event in which the expansion of the universe eventually reverses.
Because the system was completely isolated, researchers could reconstruct the sequence of events using only information from inside the mini universe itself, without referring to any outside laboratory clock.
The results showed that time could emerge from changes taking place within the quantum system rather than existing as an independent background that always moves forward.
How Entropy Created Time
The experiment revealed that “time” arose from changes in the disorder, or spread (entropy), of the atoms as they moved between the bright and dark regions. Aside 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 came to a halt.
Professor Barontini refers to this concept as “entropic time.” In the experiment, this form of time:
- Flows in one consistent direction, producing a clear “arrow of time”
- Correctly orders events, even as the mini universe expands and contracts
- Can speed up or slow down depending on how entropy is redistributed
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.”
Testing Quantum Gravity in the Laboratory
The researchers also found that a version of the Schrödinger equation, the fundamental equation of quantum mechanics, can be expressed using entropic time. This means scientists can still predict how the “probability cloud” of a quantum system evolves over time even when time is defined by internal changes rather than an external clock.
The work tackles a long standing problem in physics. If certain theories are correct and the universe has no built in clock, how can events be placed in the correct order? The experiment suggests that the answer may lie in the system’s own internal evolution.
Professor Barontini showed that the miniature universe follows the standard laws of quantum mechanics while allowing ideas about the nature of time, which are normally confined to theories describing the entire universe, to be tested under controlled laboratory conditions.
Toward Experiments on the Big Bang and Black Holes
The mini universe provides a valuable experimental platform for testing ideas in quantum cosmology and quantum gravity. Instead of relying only on mathematical models, scientists may now be able to investigate concepts related to the early universe through laboratory experiments.
The team says the same approach could eventually be expanded to more complex quantum systems, opening the door to experiments that explore the physics of the Big Bang, the “Big Crunch,” simulated black holes, and competing theories about how time itself emerges.