Physicists at the University of Birmingham have used a cloud of about 24,000 ultracold atoms to explore whether time could emerge from changes within a system rather than depend on an external clock. Led by Professor Giovanni Barontini, the experiment created a controlled quantum “miniature universe” in which the distribution of particles provided an internal measure of time.
Why time may not be fundamental
In everyday life, time appears to move continuously from the past to the future. Seconds pass, planets orbit stars and the universe evolves. However, some theories of fundamental physics suggest that time may not be built into reality at the most basic level.
One example is the Wheeler–DeWitt equation, which appears in attempts to reconcile quantum mechanics with gravity. In this framework, the universe can be represented as a single, unchanging quantum state without an external clock to mark its evolution. Although quantum particles can behave as both waves and particles, the universe as a whole may not contain an intrinsic time variable.
This creates a fundamental puzzle: if there is no external clock, how can events occur in a particular order? What determines which events happen first and which happen later?
One proposed answer is that time emerges from changing relationships between different parts of the universe. Barontini and his colleagues designed an experiment to investigate whether this process could be observed in the laboratory.
A miniature universe of 24,000 atoms
The researchers used a cloud of approximately 24,000 atoms cooled to a few billionths of a degree above absolute zero. At such low temperatures, quantum effects become especially prominent, making it possible to study behavior that is difficult to observe under ordinary conditions.
The atoms were confined in a carefully controlled quantum system designed to remain isolated from its surroundings. Two laser beams with different frequencies created a thin barrier that divided the system into two regions: an observed “bright” region and an unobserved “dark” region.
Although the system was isolated from the outside world, atoms could still move between the two regions. The bright region underwent cycles of expansion and contraction, providing simplified analogues of two dramatic cosmological scenarios: the Big Bang and the hypothetical Big Crunch, in which the universe’s expansion eventually reverses and everything collapses inward.
The experiment did not reproduce either cosmic event. Instead, it created a simplified quantum model for studying how a sequence of events can be reconstructed without referring to a clock outside the system.
How “entropic time” emerged
The key to the experiment was entropy, a concept associated with the distribution and disorder of particles and energy. Rather than measuring time in seconds or minutes, the researchers tracked changes in how the atoms were distributed between the bright and dark regions.
As atoms moved between the two regions, the particle distribution in the bright region changed. Those changes provided an internal way to define the system’s progression, even as the bright region repeatedly expanded and contracted.
When the spread of the particle distribution increased or decreased, the system continued to progress according to this internal time measure. If the relevant distribution stopped changing, that particular internal time scale would also stop advancing. This did not mean that time literally stopped in the laboratory; it meant that the system’s internally defined time no longer progressed once the relevant changes had disappeared.
Barontini called this concept “entropic time.” The experiments indicated that it has several important properties. It consistently moves in one direction, producing what physicists call the “arrow of time”—the distinction between past and future. It also orders events correctly, even when the miniature universe expands or contracts.
Entropic time does not necessarily advance at a constant rate. Its apparent pace speeds up or slows down depending on how entropy changes within the system.
Implications for quantum physics
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 laws of physics is important. Many fundamental equations work the same way whether time is mathematically reversed or moves forward. Yet human experience has a clear direction: we remember the past rather than the future, and events unfold in an identifiable order.
Entropy has long been central to attempts to explain this difference. Barontini’s experiments extended that relationship by showing how changes in entropy can provide an operational measure of time in a controlled quantum system.
The result involved more than simply placing events in the correct sequence. The researchers 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 position or state.
This suggests that internally generated time measures could be used to predict the behavior of quantum systems, much as conventional time is used in standard quantum mechanics.
Could the approach help study the early universe?
For decades, questions about the origin and fundamental nature of time have been studied mainly through theoretical physics, particularly quantum cosmology and quantum gravity. These fields examine 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 offers a way to investigate some of these ideas using a physical system that scientists can create, control and observe. The miniature universe continued to obey established quantum-physics equations while also allowing its evolution to be described through an internally generated time scale.
Future experiments could apply the approach to more complex quantum systems and explore models of the Big Bang and Big Crunch in greater detail. Similar techniques might eventually help researchers simulate aspects of black holes and compare different explanations for how time emerges.
The experiment does not prove that time is an illusion, nor does it show that the universe lacks a fundamental time. It demonstrates instead that 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