Using thousands of atoms cooled to temperatures barely above absolute zero, Professor Giovanni Barontini demonstrated that the passage of time can be measured without relying on an external clock. Instead, a form of time emerges naturally from the behavior of particles within the experimental system.

The findings, published in Physical Review Research, provide experimental evidence for an intriguing idea in theoretical physics: Time may not be an essential ingredient of reality. It could arise from the relationships and changes occurring within the universe itself.

Why Some Physicists Think Time May Not Be Fundamental

We usually think of time as something that continuously moves forward, regardless of what happens around us. Seconds pass, planets orbit stars, and the universe evolves. But some theories of physics suggest that time may work very differently at the most fundamental level.

One example is the Wheeler-DeWitt equation, which appears in attempts to combine quantum mechanics with gravity. In this framework, the universe can be described as a single, unchanging quantum state, with no external clock marking the passage of time. Quantum particles can exhibit both wave-like and particle-like properties, but the universe as a whole does not necessarily have a built-in time variable.

This creates a major puzzle. If the universe has no external clock, how can anything happen in a particular order? What determines which events come first and which happen later?

One possibility is that time emerges from the changing relationships between different parts of the universe. Barontini designed an experiment to investigate whether such a process could be observed in the laboratory.

Creating a Miniature Universe With 24,000 Atoms

To build his miniature cosmos, Barontini used a cloud of approximately 24,000 atoms cooled to just a few billionths of a degree above absolute zero. At these extraordinarily low temperatures, quantum effects become especially important, allowing researchers to study behaviors that are normally difficult to observe.

The atoms were confined inside 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 ('bright') region and an unobserved ('dark') region.

Although the overall system was isolated from the outside world, atoms could move between these two regions.

The bright region underwent repeated cycles of expansion and contraction, resembling simplified versions of two dramatic cosmological events. The expansion was analogous to the Big Bang, while the contraction resembled a Big Crunch, a hypothetical fate of the universe in which cosmic expansion eventually reverses and everything collapses inward.

The experiment did not recreate these actual cosmic events. Instead, it provided a simplified quantum model in which researchers could investigate how a sequence of events might be reconstructed without referring to a clock outside the system.

How Time Emerged Without a Clock

The key to the experiment was entropy, a concept in physics associated with the spread or disorder (entropy) of particles and energy.

Rather than using seconds or minutes, Barontini developed a way to track time through changes in how atoms were distributed within the miniature universe.

As atoms moved between the bright and dark regions, the distribution of particles in the bright region changed. These changes provided a way to define the system's progression through time, even when the bright region alternated between expansion and contraction.

When the particle distribution increased or decreased in its spread, the system continued moving forward according to this internal measure of time. When the distribution remained unchanged, the progression of this particular form of time effectively stopped.

Importantly, this did not mean that time literally stopped in the laboratory. It meant that the internal measure of time ceased advancing when the relevant changes in the system disappeared.

Barontini called this concept "entropic time." The experiment showed that this form of time has several important properties. It moves consistently in one direction, establishing what physicists call the "arrow of time," the distinction between past and future. It also places events in the correct sequence, even as the miniature universe expands and contracts.

Perhaps most surprisingly, entropic time does not always advance at the same rate. Its progression can speed up or slow down depending on how entropy changes within the system.

A New Clue to One of Physics' Biggest Mysteries

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 distinction between everyday time and the underlying laws of physics is especially important. Many fundamental physical equations work equally well whether time is mathematically reversed or allowed to move forward. Yet our experience of the world has a clear direction: We remember the past, not the future, and events unfold in a recognizable sequence.

Entropy has long played a central role in explaining this difference. Barontini's experiment takes that connection further by demonstrating how changes in entropy can provide an operational measure of time inside a controlled quantum system.

The findings also go beyond simply arranging events in the correct order.

Barontini demonstrated that a version of the Schrödinger equation, one of the foundational equations of quantum mechanics (Schrödinger), can be expressed using entropic time. This equation describes how the quantum state of a system evolves, including changes in the probability of finding particles in particular locations or states.

In other words, the internally generated measure of time could still be used to make predictions about the quantum system's behavior, much as conventional time is used in standard quantum mechanics.

Could a Tiny Universe Help Explain the Big Bang?

For decades, questions about the origin and fundamental nature of time have largely belonged to theoretical physics, particularly quantum cosmology and quantum gravity. These fields attempt to understand how the universe behaves when quantum effects and gravity must be considered together, including under the extreme conditions associated with the early cosmos.

Barontini's experiment offers a way to investigate some of these ideas using physical systems that scientists can create, manipulate, and observe.

The miniature universe continued to obey the established equations of quantum physics while providing a different way to describe its evolution. That makes it a potentially valuable 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 complicated quantum systems, allowing researchers to explore models of the Big Bang and the Big Crunch in greater detail. Similar techniques might eventually help simulate aspects of black holes or distinguish between competing explanations of how time emerges.

The experiment does not establish that time is an illusion or prove that our universe lacks fundamental time. Instead, it demonstrates something that was previously difficult to test: A quantum system can possess a meaningful internal measure of time without depending on an external clock.

By creating a miniature universe from just 24,000 atoms, researchers have gained a new experimental tool for investigating a question that has challenged physicists and philosophers for centuries: What makes time flow?