| local realism | |
|---|---|
| Name | Local realism |
| Field | Quantum mechanics |
| Introduced | 1935 |
| Notable people | Albert Einstein, Boris Podolsky, Nathan Rosen, John Bell |
| Related | Quantum entanglement, Bell's theorem |
local realism
Local realism is a principle combining locality and realism that asserts physical properties exist with definite values and that causal influences cannot travel faster than the speed of light. It matters in Quantum physics because violations of local realism, as indicated by experiments, challenge classical intuitions upheld by institutions of science and national research programs and underpin technologies such as quantum computing and quantum cryptography.
Local realism fuses two commitments: that systems possess pre-existing properties independent of observation (realism) and that physical effects propagate no faster than light (the principle of locality articulated in special relativity). The modern debate began with the 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen (EPR paradox), which argued that Quantum mechanics might be incomplete. Subsequent responses included Niels Bohr's Copenhagen interpretation defense and later work by David Bohm proposing pilot wave theory as a realist alternative. The tension between local realism and quantum predictions became central to foundations research in the 20th century, involving institutions such as Cavendish Laboratory, Bell Labs, CERN, and universities like University of Oxford and California Institute of Technology.
In quantum theory, states are represented by vectors in Hilbert space and measurements by operators; this formalism allows entangled states that do not factor into local subsystems. Local realism implies the existence of underlying hidden variables that determine outcomes. Prominent hidden-variable models include Bohmian mechanics and early proposals by Louis de Broglie. The incompatibility of local hidden-variable models with quantum statistics was formalized by John Bell in Bell's theorem, building on earlier work by David Bohm and (historic) EPR. The mathematical tools used include probability theory, statistical mechanics, and operator algebra; key results connect to concepts in quantum information theory such as Bell inequalities and entanglement measures.
Bell's theorem demonstrates that no local hidden-variable theory can reproduce all predictions of Quantum mechanics. Bell derived inequalities (e.g., CHSH inequality named for John Clauser, Michael Horne, Abner Shimony, and Richard Holt) that local realist models must satisfy. Experimental tests began with the work of John Clauser and Stuart Freedman (1972) and were advanced by Alain Aspect's experiments in the 1980s, which closed some loopholes. Later notable tests include those by Anton Zeilinger's group, experiments at University of Vienna, work by Gregory Weihs, and loophole-free Bell tests by teams led by B. Hensen at Delft University of Technology, Sae Woo Nam (NIST), and groups at University of Geneva. These experiments address detection, locality, and freedom-of-choice loopholes and typically use systems such as photons, trapped ions (e.g., at Institut für Quantenoptik), and superconducting qubits developed by companies like IBM and Google.
Violations of Bell inequalities confirm the existence of quantum entanglement and imply forms of quantum nonlocality that defy classical local realist accounts. Entanglement resources power protocols in quantum key distribution (e.g., BB84 protocol adaptations) and quantum teleportation. The operational distinction between "nonlocal correlations" and "signalling" preserves compatibility with special relativity, since quantum nonlocality does not allow faster-than-light communication. The study of nonlocality has fostered cross-disciplinary links to computer science (notably complexity theory and device-independent quantum cryptography), and experimental platforms from optical laboratories to national facilities like JILA and Max Planck Institute for Quantum Optics.
Challenges to local realism have profound implications for interpretations of quantum theory. Realist accounts include Bohmian mechanics and objective collapse theories (e.g., Ghirardi–Rimini–Weber theory), while anti-realist or instrumentalist views include traditional Copenhagen interpretation and modern QBism proponents. Some researchers invoke many-worlds interpretation (Everettian view) to preserve locality at the cost of branching ontology. Debates engage philosophers and scientists such as Karl Popper, Tim Maudlin, and Bas van Fraassen. The tension touches on concepts of causality, determinism, and the role of measurement, and informs policy and funding priorities in national science agencies like the National Science Foundation and European Research Council.
Contemporary work explores tighter Bell tests, scalable entanglement generation for quantum networks, and device-independent protocols that rely only on observed correlations. Laboratories at MIT, Harvard University, University of Cambridge, ETH Zurich, and corporate research by Microsoft Quantum and Rigetti Computing pursue implementations relevant to quantum sensing, secure communications, and computation. Fundamental research investigates extensions of Bell-type inequalities (e.g., multipartite and high-dimensional systems), connections to quantum nonlocal games studied in theoretical computer science, and the role of locality in proposed quantum gravity frameworks such as loop quantum gravity and string theory. The empirical refutation of local realism in many settings has guided conservative stewardship of scientific institutions to prioritize robust, reproducible experiments and national strategies for technological resilience in quantum infrastructure.