Quantum mechanics fascinates and intrigues both scientists and the general public. This fundamental field of physics, which explores the behavior of particles at the subatomic scale, has enabled spectacular technological advances, such as the invention of lasers and quantum computers. But it also asks profound questions about the nature of reality. Among these questions, one of the most intriguing remains: how do we move from a strange and uncertain quantum world to our familiar classical world, where objects are solid and events clearly defined?
The strange world of quantum mechanics
In the quantum domain, the rules we know on the macroscopic scale cease to apply. At the heart of this oddity is the notion of wave function. A wave function is not a simple description of where a particle is, but rather a probabilistic map showing all the possibilities of where it could be. This superposition of states is one of the most counterintuitive concepts in quantum mechanics.
Take the famous example of Schrödinger's cat. Imagine a cat locked in a box with a quantum mechanism that can release poison. As long as the box remains closed, the cat is both alive and dead — a superposition of states. Only when the box is opened and we look inside does the cat take on a definite state: alive or dead. This transition from quantum vagueness to a well-defined reality is called the collapse of the wave function. But why and how does this collapse occur?
Physicists' tools to explain the emergence of the classical
To explain this passage, physicists relied on several concepts. The first is the Born's rulewhich states that when we make a measurement, the probability of observing a particular state is proportional to the square of the amplitude of the wave function at that state. In other words, the wave function does not give a precise result, but only probabilities.
Another crucial concept is that of quantum decoherence. When a quantum system interacts with its environment, its different superpositions of states quickly become incompatible. This means that we only observe a consistent classical state — for example, a living cat or death — rather than a combination of the two. This idea is central to the so-called Copenhagen interpretation, but it does not explain everything.
Alternative interpretations have also emerged, such as that of multiple worlds. According to this view, all possible states of the wave function continue to exist, but in parallel universes. For example, in one universe the cat is alive, and in another it is dead. But if these worlds exist, why do we never see them directly?
A recent discovery: the emerging classical world
Spanish physicists have recently shed new light on this problem. Their research, published in Physical Review X, shows that features of the classical world emerge naturally from complex quantum systems. In other words, our macroscopic world is not in contradiction with quantum physics — it follows inevitably.
Imagine a bag of water with holes in it. Although the water molecules inside move chaotically and unpredictably, the water flowing from the holes forms regular, predictable streams. Similarly, research shows that quantum systems, although complex and chaotic, generate stable classical structures when observed at large scales.
The team simulated quantum evolutions on unprecedented scales, involving up to 50,000 energy levels. They discovered that quantum interference effects, responsible for strange behavior on small scales, disappear very quickly as the size of the system increases. This phenomenon occurs exponentially and universally, without the need for special conditions. Thus, even systems consisting of a few atoms can begin to behave in classical ways.
What this means for our understanding of reality
This research sheds new light on the question of the emergence of the classical world. It shows that our observable reality is not an anomaly, but a natural consequence of physical laws. This could also explain why time seems to flow in only one direction — an arrow of time emerges from some branches of quantum evolution, while other branches could theoretically have a reverse arrow of time.
By linking this work to statistical mechanics, the researchers show how concepts like temperature and pressure emerge from the interaction of countless microscopic particles. This reinforces the idea that order and structure can emerge from a globally chaotic and temporally symmetrical universe.
And the other universes?
The idea of multiple worlds remains fascinating. According to this theory, our universe is just one branch among countless others. Recent work provides a framework for understanding how these branches can coexist while producing coherent and stable worlds. Although we do not have direct access to these other universes, their existence could explain the richness and complexity of our own world.
