The question of whether purely classical systems exist in the real, uncontrolled world is a topic of debate among physicists. It is true that experiments demonstrating quantum behavior at the macroscopic scale typically require very controlled and carefully isolated conditions to observe and manipulate quantum effects effectively. These experiments, often conducted in highly controlled laboratory settings, involve cooling systems to extremely low temperatures or using other techniques to minimize interactions with the environment.
However, it is important to note that the distinction between classical and quantum systems is not always black and white. Quantum mechanics provides a framework for describing the behavior of particles and systems at a fundamental level, while classical physics describes macroscopic objects with well-defined properties and deterministic behavior. In reality, most macroscopic objects are composed of an enormous number of quantum particles, and their behavior is often effectively described by classical physics due to the phenomenon of quantum decoherence.
Quantum decoherence refers to the interaction of a quantum system with its surrounding environment, which leads to the suppression of quantum interference and the emergence of classical-like behavior. As systems become larger and more complex, interactions with the environment cause quantum effects to become less pronounced and eventually indistinguishable from classical behavior. This is often referred to as the "classical limit" or the "emergence of classicality."
While it is difficult to directly observe quantum behavior in macroscopic systems outside of controlled laboratory conditions, there is evidence to suggest that quantum effects play a role in various natural processes. For example, photosynthesis in plants, bird navigation, and certain chemical reactions are thought to exploit quantum phenomena. Additionally, there are ongoing efforts to observe quantum effects in larger and more complex systems, such as mechanical oscillators or superconducting circuits, even at room temperature.
The claim that there are no purely classical systems in the real world is based on the understanding that all physical systems ultimately have quantum properties at a fundamental level. However, it is acknowledged that the distinction between classical and quantum behavior becomes less relevant as systems become larger and more complex, and classical physics provides an excellent approximation for many practical purposes.
It is worth noting that the precise boundary between classical and quantum behavior is an active area of research, and the quest to understand the transition from quantum to classical behavior in complex systems is an ongoing scientific endeavor.