<p>Could a system track its own passage through time, without an outside clock? A ScienceDaily report, based on material provided by the University of Birmingham, describes an experiment by physicist Giovanni Barontini that explores this question with a controlled quantum system. The report says the work offers experimental evidence that an internal measure of time can be defined by changes within a system.</p>
<p>The question matters because some approaches to quantum gravity do not treat time as a built-in feature of the universe. The report points to the Wheeler-DeWitt equation, which appears in attempts to combine quantum mechanics and gravity. In that framework, the universe can be described as a single, unchanging quantum state without an external clock. That raises a puzzle: how can events have an order if the universe has no outside timekeeper?</p>
<p>Barontini’s model used a cloud of about 24,000 atoms cooled to a few billionths of a degree above absolute zero. The atoms were held in a system designed to be isolated from its surroundings. Two laser beams with different frequencies created a thin barrier between two regions, described in the report as “bright” and “dark.” Atoms could still move between them.</p>
<p>The bright region repeatedly expanded and contracted. The report compares those cycles to simplified versions of the Big Bang and a possible Big Crunch, when expansion might reverse. It stresses that the experiment did not recreate either cosmic event. Instead, it provided a laboratory model for studying how events in a changing system might be ordered without referring to an external clock.</p>
<p>The proposed internal measure, which Barontini calls “entropic time,” follows changes in how particles are distributed. The report describes entropy as associated with the spread or disorder of particles and energy. As atoms moved between the bright and dark regions, the distribution in the bright region changed; those changes supplied a way to mark the system’s progression.</p>
<p>According to the report, this measure can continue to advance as the distribution’s spread increases or decreases. When the relevant distribution remains unchanged, this particular measure of time effectively stops advancing. That does not mean time itself literally stopped in the laboratory: it means the internal measure no longer registered the changes on which it depends.</p>
<p>The report says entropic time gives events a consistent direction, the “arrow of time,” and places them in sequence even while the modeled region expands and contracts. Its rate need not be uniform: it can speed up or slow down as entropy changes. The work therefore explores how a system’s own evolving relationships might provide a practical way to describe change, rather than assuming a clock outside the system.</p>
<p>The report also says a version of the Schrödinger equation can be expressed using entropic time. That equation describes the evolution of a quantum state, including the probabilities of finding particles in particular locations or states. In the reported result, the internal measure could still be used to describe and predict the system’s behavior, much as conventional time is used in standard quantum mechanics.</p>
<p>The journal reference identifies the paper as “Testing the problem of time with cold atoms,” by Giovanni Barontini, published in <em>Physical Review Research</em>. The report presents the experiment as a way to investigate ideas usually discussed in theoretical physics with a system researchers can create and observe. That makes it a potential testing ground for models that do not assume time exists independently of the system being studied.</p>
<p>The result has defined limits. It does not show that time is an illusion or establish that our universe lacks fundamental time, and the laboratory system is a simplified model rather than a miniature version of the actual cosmos. The report says future experiments could extend the approach to more complex quantum systems and explore models involving the Big Bang, the Big Crunch, or aspects of black holes. For now, the contribution is narrower: a controlled quantum system can have a meaningful internal measure of time without relying on an external clock.</p>
<p><a href="https://www.sciencedaily.com/releases/2026/10/261028234703.htm">Source: ScienceDaily report on the cold-atom time experiment</a></p>