The claim that the classical world is an "illusion" arising out of quantum processes is a perspective held by some physicists, particularly those influenced by the interpretation of quantum mechanics known as the "Copenhagen interpretation" or other interpretations emphasizing the role of measurement and observer effects.
According to these interpretations, quantum mechanics provides a fundamental description of the physical world, encompassing both microscopic and macroscopic systems. At the microscopic level, the behavior of particles and other quantum entities is described by wave functions that exhibit wave-particle duality and superposition, where particles can exist in multiple states simultaneously. However, when we observe or measure these quantum systems, their wave functions collapse into a definite state, yielding the familiar classical properties we perceive in everyday experience.
From this perspective, the claim that the classical world is an illusion arises because the behavior of quantum systems is fundamentally different from our classical intuition. Classical physics, which includes Newtonian mechanics and electromagnetism, provides an effective description of macroscopic objects in terms of continuous variables like position, momentum, and energy. However, at the fundamental level, these continuous variables are believed to emerge as statistical averages or coarse-grained descriptions of the underlying quantum behavior.
The claim of the classical world being an illusion emphasizes that our everyday experience and intuition are based on classical physics, which is an approximation of the underlying quantum reality. It suggests that the classical world is a result of the way our observations and measurements interact with the quantum realm, effectively obscuring its true nature.
It's important to note that this perspective is not universally accepted among physicists, and there are alternative interpretations of quantum mechanics that offer different views on the nature of the classical world. These include the many-worlds interpretation, pilot-wave theory, and various forms of quantum decoherence, among others. These interpretations seek to explain the transition from the quantum to the classical realm in different ways, but they all acknowledge the fundamental differences between the two and the challenges in reconciling them.
Ultimately, the nature of reality at the quantum level and its relationship to the classical world is still an active area of research and debate among physicists.