Physicists at the University of Birmingham have used about 24,000 atoms cooled to just above absolute zero to create a miniature quantum universe in which time can be measured without an external clock. Led by Professor Giovanni Barontini, the experiment provides controlled evidence for the idea that time could emerge from changes and relationships within a physical system rather than existing as a fundamental, independently ticking feature of the universe.
Why some physicists think time may not be fundamental
We normally think of time as something that moves continuously forward, regardless of what happens around us. Seconds pass, planets orbit stars and the universe evolves. At the most fundamental level, however, some theories of physics suggest that time may work very differently.
One example is the Wheeler-DeWitt equation, which appears in attempts to combine quantum mechanics and gravity. In this framework, the universe can be described as a single, immutable quantum state with no external clock to mark the passage of time. Quantum particles can display both wave-like and particle-like properties, but the universe as a whole does not necessarily contain a built-in time variable.
This creates a fundamental puzzle: if there is no external clock, how can events happen in a particular order? What determines which events occur first and which occur later?
One possibility is that time emerges from changing relationships between different parts of the universe. Barontini designed an experiment to investigate whether such a process could be observed in the laboratory.
A miniature universe of 24,000 atoms
Barontini used a cloud of about 24,000 atoms cooled to a few billionths of a degree above absolute zero to construct the miniature universe. At such low temperatures, quantum effects become especially important, allowing researchers to study behavior that is normally difficult to observe.
The atoms were confined in a carefully controlled quantum system designed to remain isolated from its surroundings. Two laser beams operating at different frequencies created a thin barrier that divided the system into two regions: an observed, or “bright,” region and an unobserved, or “dark,” region.
Although the entire system was isolated from the outside world, atoms could move between the two regions.
The bright region went through cycles of expansion and contraction, resembling simplified versions of two dramatic cosmological events. Its expansion was similar to the Big Bang, while its contraction was similar to the Big Crunch, a hypothetical fate in which the universe’s expansion eventually reverses and everything collapses inward.
The experiment did not reproduce either cosmic event. Instead, it provided a simplified quantum model for investigating how sequences of events can be reconstructed without reference to a clock outside the system.
How time emerged without an external clock
The key to the experiment was entropy, a concept related to the spread and disorder of particles and energy.
Rather than using seconds or minutes, Barontini developed a way to track time through changes in the distribution of atoms within the miniature universe. As atoms moved between the bright and dark regions, the distribution of particles in the bright region changed. Those changes provided a way to define the system’s evolution as the bright region repeatedly expanded and contracted.
As the spread of the particle distribution increased or decreased, the system continued to advance according to this internally defined time measurement. If the distribution stopped changing, this particular form of time would effectively stop progressing.
This does not mean that time literally stopped in the laboratory. It means that the internal time scale would stop advancing once the relevant changes in the system disappeared.
Barontini called the concept “entropic time.” Experiments showed that this form of time has several important properties. It consistently moves in one direction, establishing what physicists call the “arrow of time”—the distinction between past and future. It also places events in the correct order even when the miniature universe expands or contracts.
Entropic time does not always advance at the same rate. Its progress speeds up or slows down depending on changes in entropy within the system.
Clues to one of physics’ biggest mysteries
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. Most fundamental laws of physics work the same way forward and backward, so why is this the case?”
“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 traditional time.”
The distinction between everyday time and the underlying physical laws is particularly important. Many basic physical equations work the same way whether time is mathematically reversed or moves forward. Our experience of the world, however, has a clear direction: we remember the past rather than the future, and events unfold in a recognizable order.
Entropy has long played a central role in explaining this difference. Barontini’s experiments took that relationship further by demonstrating how changes in entropy can provide an operational measure of time within controlled quantum systems.
The result involves more than simply arranging events in the correct order. Barontini demonstrated that a version of the Schrödinger equation, one of the fundamental equations of quantum mechanics, can be expressed using entropic time. The equation describes how a quantum system evolves, including changes in the probability of finding a particle in a particular location or state.
In other words, internally generated time measurements could still be used to make predictions about the behavior of quantum systems, just as conventional time is used in standard quantum mechanics.
Could a small universe help explain the Big Bang?
For decades, questions about the origin and fundamental nature of time have belonged primarily to theoretical physics, particularly quantum cosmology and quantum gravity. These fields seek to understand how the universe behaves when quantum effects and gravity must be considered together, including under the extreme conditions associated with the early universe.
Barontini’s experiment provides a way to investigate some of these ideas using a physical system that scientists can create, manipulate and observe.
The miniature universe continued to follow established equations of quantum physics, providing another way to describe its evolution. It could therefore serve as a testing ground for theories in which time is not assumed to exist independently of the system being studied.
Future experiments could extend this approach to more complex quantum systems, allowing researchers to explore models of the Big Bang and Big Crunch in greater detail. Similar techniques may eventually help simulate aspects of black holes and distinguish between competing explanations for how time emerges.
The experiment does not prove that time is an illusion, nor does it show that our universe lacks fundamental time. Instead, it demonstrates something that was previously difficult to test: a quantum system can maintain a meaningful internal time scale without relying on an external clock.
By creating a miniature universe from about 24,000 atoms, the researchers have developed a new experimental tool for investigating a question that has challenged physicists and philosophers for centuries: what makes time flow?
Source: www.sciencedaily.com
