One of the most profound and unresolved questions in physics lies in understanding how the probabilistic and often counterintuitive world of quantum mechanics gives rise to the deterministic classical world we experience: how does the fuzzy, probabilistic world of quantum wave functions transform into the solid, classical world we experience daily—atoms, steak dinners, or rollercoaster rides?
This question has puzzled physicists for decades. The key lies in understanding how the quantum wave function, which can exist in a superposition of multiple states, collapses into a single, definitive state that aligns with our classical reality. Despite being a central concept in quantum theory, the exact mechanism of this transition remains elusive, prompting decades of research and debate.
A recent study by quantum theorists from Spain, published in Physical Review X, may shed new light on this puzzle. Using advanced numerical simulations, the team demonstrated how features of the classical world can emerge from the inherently strange quantum realm, revealing more about how quantum wave functions may physically collapse.
“Quantum physics is at odds with our classical experience as far as the behavior of single electrons, atoms, or photons is concerned,” Philipp Strasberg of the Autonomous University of Barcelona, the study’s lead author said in a recent statement. “However, if one zooms out and considers coarse quantities that we humans can perceive—for example, the temperature of our morning coffee or the position of a stone—our results indicate that quantum interference effects, which are responsible for weird quantum behavior, vanish.”
Classical World in a Quantum Multiverse
The research draws on the many-worlds interpretation of quantum mechanics, which posits that every quantum event spawns multiple branches of reality. Yet a significant challenge with this interpretation has been reconciling these parallel universes with the singular classical experience we observe.
The team’s simulations tackled this issue by exploring how macroscopic, stable structures—like the world we know—emerge from the countless possible states of a quantum system. Their work involved systems with up to 50,000 energy levels and five time steps, far smaller than what would be required to simulate everyday phenomena.
“In particular, we provide clear evidence that this vanishing [of quantum interference effects] happens extremely fast—to be precise: exponentially fast—with growing system size,” Strasberg said in the statement. “Even a few atoms or photons can behave classically. Furthermore, it is a ubiquitous and generic phenomenon that does not require any fine-tuning: the emergence of a classical world is inevitable.”
Decoding the Arrow of Time
One of the most intriguing aspects of the study is its implications for the concept of time. In their simulations, the researchers observed that macroscopic features like order and structure can arise even in a quantum system that appears chaotic at smaller scales. This echoes the principles of statistical mechanics, where observable phenomena like temperature and pressure emerge from the random motion of particles.
The study also hints at the possibility of worlds with reversed arrows of time, where entropy—often associated with disorder—decreases rather than increases. While such scenarios are unlikely in our universe, they expand the theoretical boundaries of how order and time emerge from chaos.
“Remarkably, we explicitly demonstrate that interesting classical worlds can emerge from a quantum system that is overall in thermodynamic equilibrium,” Strasberg noted. “This demonstrates that order, structure, and an arrow of time can emerge on single branches of a quantum multiverse, which overall looks chaotic, unstructured, and time-symmetric.”
While the team’s simulations are a significant step forward, they represent only the beginning of a deeper exploration. As physicists continue to refine their understanding of quantum mechanics, studies like this offer a fascinating glimpse into how the universe transforms the weirdness of the quantum world into the familiar order of everyday life.
Kenna Hughes-Castleberry is the Science Communicator at JILA (a world-leading physics research institute) and a science writer at The Debrief. Follow and connect with her on BlueSky or contact her via email at kenna@thedebrief.org