The passage of time might just be a phantom effect, emerging from quantum interactions among disparate parts of the cosmos – at least, this holds true for a simplified model of the universe. An experimental setup could offer clues about the nature of time within our own reality. A recent publication in Physical Review Research details this study.
Giovanni Baronetti of the University of Birmingham (UK) began contemplating time while observing his 6-year-old son’s playtime. “He was constructing his own little universe, and it struck me as quite similar to what we do in our labs when building a system with ultracold atoms,” he remarked. “But then I considered that it’s also a rather dull universe, as not much is happening, and if nothing transpires, it’s as if time isn’t progressing.”
To investigate whether time is indeed an illusion in such systems, Baronetti employed lasers and electromagnetic forces to cool approximately 20,000 rubidium atoms to temperatures approaching absolute zero. He then divided this miniature universe’s atoms into two regions, labeling one as “bright” and the other as “dark,” drawing a parallel to dark matter.
Initially, this constructed universe was essentially timeless and unchanging. However, Baronetti then used lasers to induce an exchange of atoms between the two regions, thereby facilitating quantum-level interactions. This process altered the universe’s entropy, or disorder. Crucially, we observe that in our universe, time advances in the direction of increasing entropy. Consequently, Baronetti was able to define an intrinsic time for his simulated universe. Furthermore, he was able to incorporate this newfound time into the Schrödinger equation, which governs the evolution of quantum systems, to calculate the quantum states of the atoms and found that the results aligned with his experimental observations.
There’s a precedent for viewing time as an emergent phenomenon stemming from quantum correlations or interactions, rather than an inherent property. This concept was first posited in atomic physics by Neville Mott in the 1930s and has been explored theoretically since. It wasn’t until 2013 that Marco Genovese of Italy’s National Metrological Institute and his colleagues provided the first experimental evidence of its feasibility, utilizing entangled particles of light. In that instance as well, the sensation of time arose from quantum correlations.
“The current work advances this notion, showing considerable progress,” commented Genovese. Specifically, the cold atom universe is more complex than the one constructed using light, and Baronetti succeeded in making the Schrödinger equation function with the system’s intrinsic time, a feat not previously accomplished.
Klaus Kiefer of the University of Cologne (Germany) notes that this toy universe experiment relates to the broader challenge of unifying gravity theory and quantum theory into a single framework applicable to our universe across all scales. This question remains unresolved, but some physicists hypothesize that such a unified theory would be characterized by an absence of time at the most fundamental level. The new experiment mimics this scenario, though Kiefer points out distinctions – for example, when the ultracold atoms migrate between regions, they do not interact in the intricate manner anticipated in a larger universe.
However, Carlo Rovelli of Aix-Marseille University (France) suggests that experiments like these cannot unveil novel insights about time, as they are founded upon physics we already comprehend. Nevertheless, their simulation within the context of major unresolved issues could inspire approaches to studying unknown physics, akin to the notoriously elusive problem of quantum gravity.
For Baronetti, the new research serves as experimental validation for long-standing ideas, thereby demonstrating that they are not entirely speculative. Yet, he stresses, it does not confirm that time actually operates this way across all scales.
Cosmologists who study the entire universe, rather than laboratory-created models, will likely express reservations about this work, according to Baronetti. Nevertheless, he aims to continue investigating the ultracold mini-universe, perhaps by utilizing lasers to create regions from which atoms cannot escape, mimicking the gravitational pull of a black hole.
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