Local Realism

Local realism combines two fundamental intuitions about the nature of reality that were challenged by the emergence of quantum mechanics. The concept represents the merger of two distinct principles: locality and realism.

Realism in this context refers to the belief that physical properties of objects exist independently of measurement or observation. According to this view, particles possess definite properties (like position, momentum, and spin) regardless of whether anyone measures them. This aligns with our everyday experience of the classical physics world and formed the basis of scientific thinking prior to the quantum revolution.

Locality asserts that objects can only be influenced by events in their immediate surroundings, and that this influence cannot propagate faster than the speed of light. This principle is fundamental to special relativity and reflects our intuitive understanding of causality.

The tension between local realism and quantum mechanics became explicit through the Einstein-Podolsky-Rosen paradox, proposed by Einstein, Podolsky, and Rosen in 1935. Einstein, believing in local realism, argued that quantum entanglement implied quantum mechanics was incomplete, famously describing entanglement's apparent violation of locality as "spooky action at a distance."

The debate was largely philosophical until John Stewart Bell developed his famous Bell's theorem in 1964. Bell derived mathematical inequalities that would have to be satisfied if local realism were true. Subsequent experimental violations of Bell's inequalities have demonstrated that at least one of the assumptions of local realism must be false:

1. Either locality is violated (allowing for instantaneous influences between distant particles)
2. Or realism is violated (physical properties don't exist until measured)
3. Or both principles fail

The experimental rejection of local realism represents one of the most profound challenges to our intuitive understanding of reality. It suggests that the quantum world operates according to principles that fundamentally conflict with our classical expectations, leading to ongoing debates about the nature of quantum measurement and quantum interpretation.

Modern developments in quantum information theory and quantum computing actively exploit the violation of local realism through phenomena like entanglement, turning what was once a philosophical puzzle into a technological resource.

The concept continues to be central to discussions of scientific realism and the philosophy of physics, highlighting the tension between our intuitive understanding of reality and the mathematical formalism that accurately describes quantum phenomena. It serves as a reminder that fundamental physics often requires us to abandon deeply held assumptions about the nature of reality.

Determinism interpretations of quantum mechanics, such as the de Broglie-Bohm theory, maintain realism but explicitly violate locality, while others like the Copenhagen interpretation essentially abandon both locality and realism in favor of a more instrumental approach.