Physics:Quantum Bell's theorem

From HandWiki
Short description: Fundamental theorem demonstrating incompatibility of quantum mechanics with local hidden-variable theories


← Back to Conceptual and interpretations Bell's theorem is a foundational result in quantum mechanics demonstrating that no theory based on local hidden variables can reproduce all predictions of quantum physics. It establishes that quantum correlations arising from entanglement are fundamentally incompatible with the classical assumptions of locality and realism.[1][2]

In essence, Bell showed that:

→ If a theory is local, it cannot agree with quantum mechanics → If it agrees with quantum mechanics, it must be nonlocal

Illustration of Bell’s theorem: measurements on entangled particles exhibit correlations that violate classical (local realistic) expectations.

Conceptual background

Bell’s theorem builds on the Einstein–Podolsky–Rosen (EPR) paradox, which questioned whether quantum mechanics provides a complete description of reality.[3]

EPR considered pairs of particles in an entangled state:

  • Measuring one particle instantaneously determines the state of the other
  • Even when separated by large distances

This suggests either:

  • Faster-than-light influence (violating locality), or
  • Pre-existing hidden variables determining outcomes

Bell formalized this dilemma mathematically.

Bell inequalities

Bell derived inequalities that any local hidden-variable theory must satisfy. The most widely used version is the **CHSH inequality**, which constrains correlations between measurements:

|⟨A0B0⟩+⟨A0B1⟩+⟨A1B0⟩−⟨A1B1⟩|≤2

This inequality relies on two key assumptions:

  • Locality: no influence propagates faster than light
  • Realism: physical properties exist prior to measurement

Quantum mechanics predicts violations of this bound.

Quantum violation

For entangled states, quantum mechanics predicts stronger correlations. For example, using a maximally entangled Bell state:

|ψ⟩=|0⟩|1⟩−|1⟩|0⟩2

the CHSH expression reaches:

22

This exceeds the classical limit of 2 and is known as the Tsirelson bound.[4]

Thus:

→ Quantum correlations violate Bell inequalities → Local hidden-variable theories cannot reproduce these results

Experimental tests

Bell tests experimentally measure correlations between entangled particles.

Key milestones include:

  • 1972 – First experimental test (Clauser & Freedman)
  • 1982 – Aspect experiments improving locality conditions
  • 2015 – Loophole-free Bell tests

All experiments consistently confirm:

  • Violation of Bell inequalities
  • Agreement with quantum mechanics

These results rule out local hidden-variable theories.[5]

Conceptual implications

Bell’s theorem has profound implications:

  • Nature is not both local and realistic
  • Quantum entanglement implies non-classical correlations
  • Classical intuitions about separability fail

It does not specify which assumption must be abandoned, leading to multiple interpretations.

Relation to other no-go theorems

Bell’s theorem is part of a broader class of results limiting classical interpretations:

Interpretational perspectives

Different interpretations resolve Bell violations differently:

  • Copenhagen: abandons realism or counterfactual definiteness
  • Many-worlds: retains locality but allows multiple outcomes
  • Bohmian mechanics: retains realism but introduces nonlocality
  • Objective collapse: modifies quantum dynamics

No consensus exists on the “correct” interpretation.

Physical significance

Bell’s theorem demonstrates that:

→ Quantum mechanics is fundamentally incompatible with classical worldviews

It underpins modern developments such as:

and is one of the most experimentally tested principles in physics.

See also

Table of contents (184 articles)

Index

Full contents

1. Foundations (11)↑
  1. Physics:Quantum basics
  2. Physics:Quantum Postulates
  3. Physics:Quantum Hilbert space
  4. Physics:Quantum Observables and operators
  5. Physics:Quantum mechanics
  6. Physics:Quantum mechanics measurements
  7. Physics:Quantum state
  8. Physics:Quantum system
  9. Physics:Quantum superposition
  10. Physics:Quantum probability
  11. Physics:Quantum Mathematical Foundations of Quantum Theory
2. Conceptual and interpretations (13)↑
  1. Physics:Quantum Interpretations of quantum mechanics
  2. Physics:Quantum Wave–particle duality
  3. Physics:Quantum Complementarity principle
  4. Physics:Quantum Uncertainty principle
  5. Physics:Quantum Measurement problem
  6. Physics:Quantum Bell's theorem
  7. Physics:Quantum Hidden variable theory
  8. Physics:Quantum nonlocality
  9. Physics:Quantum contextuality
  10. Physics:Quantum Darwinism
  11. Physics:Quantum A Spooky Action at a Distance
  12. Physics:Quantum A Walk Through the Universe
  13. Physics:Quantum The Secret of Cohesion and How Waves Hold Matter Together
  14. Physics:Quantum measurement problem
3. Mathematical structure and systems (13)↑
  1. Physics:Quantum Density matrix
  2. Physics:Quantum Exactly solvable quantum systems
  3. Physics:Quantum Formulas Collection
  4. Physics:Quantum A Matter Of Size
  5. Physics:Quantum Symmetry in quantum mechanics
  6. Physics:Quantum Angular momentum operator
  7. Physics:Quantum Runge–Lenz vector
  8. Physics:Quantum Approximation Methods
  9. Physics:Quantum Matter Elements and Particles
  10. Physics:Quantum Dirac equation
  11. Physics:Quantum Klein–Gordon equation
  12. Physics:Quantum pendulum
  13. Physics:Quantum configuration space
4. Atomic and spectroscopy (14)↑
    Quantum atomic structure and spectroscopy: orbitals, energy levels, and emission and absorption spectra.
  1. Physics:Quantum Atomic structure and spectroscopy
  2. Physics:Quantum Hydrogen atom
  3. Physics:Quantum number
  4. Physics:Quantum Multi-electron atoms
  5. Physics:Quantum Fine structure
  6. Physics:Quantum Hyperfine structure
  7. Physics:Quantum Isotopic shift
  8. Physics:Quantum defect
  9. Physics:Quantum Zeeman effect
  10. Physics:Quantum Stark effect
  11. Physics:Quantum Spectral lines and series
  12. Physics:Quantum Selection rules
  13. Physics:Quantum Fermi's golden rule
  14. Physics:Quantum beats
5. Wavefunctions and modes (9)↑
    A quantum wavefunction showing probability amplitude in space; the square of its magnitude gives the probability density.
  1. Physics:Quantum Wavefunction
  2. Physics:Quantum Superposition principle
  3. Physics:Quantum Eigenstates and eigenvalues
  4. Physics:Quantum Boundary conditions and quantization
  5. Physics:Quantum Standing waves and modes
  6. Physics:Quantum Normal modes and field quantization
  7. Physics:Number of independent spatial modes in a spherical volume
  8. Physics:Quantum Density of states
  9. Physics:Quantum carpet
6. Quantum dynamics and evolution (17)↑
  1. Physics:Quantum Time evolution
  2. Physics:Quantum Schrödinger equation
  3. Physics:Quantum Time-dependent Schrödinger equation
  4. Physics:Quantum Stationary states
  5. Physics:Quantum Perturbation theory
  6. Physics:Quantum Time-dependent perturbation theory
  7. Physics:Quantum Adiabatic theorem
  8. Physics:Quantum Scattering theory
  9. Physics:Quantum S-matrix
  10. Physics:Quantum tunnelling
  11. Physics:Quantum speed limit
  12. Physics:Quantum revival
  13. Physics:Quantum reflection
  14. Physics:Quantum oscillations
  15. Physics:Quantum jump
  16. Physics:Quantum boomerang effect
  17. Physics:Quantum chaos
7. Measurement and information (9)↑
  1. Physics:Quantum Measurement theory
  2. Physics:Quantum Measurement operators
  3. Physics:Quantum Projective measurement
  4. Physics:Quantum POVM
  5. Physics:Quantum Weak measurement
  6. Physics:Quantum Measurement collapse
  7. Physics:Quantum entanglement
  8. Physics:Quantum Zeno effect
  9. Physics:Quantum limit
8. Quantum information and computing (10)↑
  1. Physics:Quantum information theory
  2. Physics:Quantum Qubit
  3. Physics:Quantum Entanglement
  4. Physics:Quantum Gates and circuits
  5. Physics:Quantum Computing Algorithms in the NISQ Era
  6. Physics:Quantum Noisy Qubits
  7. Physics:Quantum random access code
  8. Physics:Quantum pseudo-telepathy
  9. Physics:Quantum network
  10. Physics:Quantum money
9. Quantum optics and experiments (5)↑
    Experimental quantum physics: qubits, dilution refrigerators, quantum communication, and laboratory systems.
  1. Physics:Quantum Nonlinear King plot anomaly in calcium isotope spectroscopy
  2. Physics:Quantum optics beam splitter experiments
  3. Physics:Quantum Ultra fast lasers
  4. Physics:Quantum Experimental quantum physics
  5. Physics:Quantum optics
    6. Template:Quantum optics operators
10. Open quantum systems (9)↑
  1. Physics:Quantum Open systems
  2. Physics:Quantum Master equation
  3. Physics:Quantum Lindblad equation
  4. Physics:Quantum Decoherence
  5. Physics:Quantum dissipation
  6. Physics:Quantum Markov semigroup
  7. Physics:Quantum Markovian dynamics
  8. Physics:Quantum Non-Markovian dynamics
  9. Physics:Quantum Trajectories
11. Quantum field theory (20)↑
    Structural dependency map of quantum field theory.
  1. Physics:Quantum field theory (QFT) basics
  2. Physics:Quantum field theory (QFT) core
  3. Physics:Quantum Fields and Particles
  4. Physics:Quantum Second quantization
  5. Physics:Quantum Harmonic Oscillator field modes
  6. Physics:Quantum Creation and annihilation operators
  7. Physics:Quantum vacuum fluctuations
  8. Physics:Quantum Propagators in quantum field theory
  9. Physics:Quantum Feynman diagrams
  10. Physics:Quantum Path integral formulation
  11. Physics:Quantum Renormalization in field theory
  12. Physics:Quantum Renormalization group
  13. Physics:Quantum Field Theory Gauge symmetry
  14. Physics:Quantum Non-Abelian gauge theory
  15. Physics:Quantum Electrodynamics (QED)
  16. Physics:Quantum chromodynamics (QCD)
  17. Physics:Quantum Electroweak theory
  18. Physics:Quantum Standard Model
  19. Physics:Quantum triviality
  20. Physics:Quantum confinement problem
12. Statistical mechanics and kinetic theory (9)↑
  1. Physics:Quantum Statistical mechanics
  2. Physics:Quantum Partition function
  3. Physics:Quantum Distribution functions
  4. Physics:Quantum Liouville equation
  5. Physics:Quantum Kinetic theory
  6. Physics:Quantum Boltzmann equation
  7. Physics:Quantum BBGKY hierarchy
  8. Physics:Quantum Relaxation and thermalization
  9. Physics:Quantum Thermodynamics
13. Condensed matter and solid-state physics (13)↑
  1. Physics:Quantum Band structure
  2. Physics:Quantum Fermi surfaces
  3. Physics:Quantum Semiconductor physics
  4. Physics:Quantum Phonons
  5. Physics:Quantum Electron-phonon interaction
  6. Physics:Quantum Superconductivity
  7. Physics:Quantum Topological phases of matter
  8. Physics:Quantum well
  9. Physics:Quantum spin liquid
  10. Physics:Quantum spin Hall effect
  11. Physics:Quantum phase transition
  12. Physics:Quantum critical point
  13. Physics:Quantum dot
14. Plasma and fusion physics (8)↑
    Conceptual illustration of plasma physics in a fusion context, showing magnetically confined ionized gas in a tokamak and the collective behavior governed by electromagnetic fields and transport processes.
  1. Physics:Quantum Fusion reactions and Lawson criterion
  2. Physics:Quantum Plasma (fusion context)
  3. Physics:Quantum Magnetic confinement fusion
  4. Physics:Quantum Inertial confinement fusion
  5. Physics:Quantum Plasma instabilities and turbulence
  6. Physics:Quantum Tokamak core plasma
  7. Physics:Quantum Tokamak edge physics and recycling asymmetries
  8. Physics:Quantum Stellarator
15. Timeline (8)↑
  1. Physics:Quantum mechanics/Timeline
  2. Physics:Quantum mechanics/Timeline/Pre-quantum era
  3. Physics:Quantum mechanics/Timeline/Old quantum theory
  4. Physics:Quantum mechanics/Timeline/Modern quantum mechanics
  5. Physics:Quantum mechanics/Timeline/Quantum field theory era
  6. Physics:Quantum mechanics/Timeline/Quantum information era
  7. Physics:Quantum mechanics/Timeline/Quantum technology era
  8. Physics:Quantum mechanics/Timeline/Quiz
16. Advanced and frontier topics (16)↑
  1. Physics:Quantum topology
  2. Physics:Quantum battery
  3. Physics:Quantum Supersymmetry
  4. Physics:Quantum Black hole thermodynamics
  5. Physics:Quantum Holographic principle
  6. Physics:Quantum gravity
  7. Physics:Quantum De Sitter invariant special relativity
  8. Physics:Quantum Doubly special relativity
  9. Physics:Quantum arithmetic geometry
  10. Physics:Quantum unsolved problems
  11. Physics:Quantum Yang-Mills mass gap
  12. Physics:Quantum gravity problem
  13. Physics:Quantum black hole information paradox
  14. Physics:Quantum dark matter problem
  15. Physics:Quantum neutrino mass problem
  16. Physics:Quantum matter-antimatter asymmetry problem

References

  1. ↑ Bell, J. S. (1964). "On the Einstein Podolsky Rosen paradox". Physics Physique Физика 1 (3): 195–200. doi:10.1103/PhysicsPhysiqueFizika.1.195. 
  2. ↑ Mermin, N. David (1993). "Hidden variables and the two theorems of John Bell". Reviews of Modern Physics 65 (3): 803–815. doi:10.1103/RevModPhys.65.803. 
  3. ↑ Einstein, A.; Podolsky, B.; Rosen, N. (1935). "Can quantum-mechanical description of physical reality be considered complete?". Physical Review 47 (10): 777–780. doi:10.1103/PhysRev.47.777. 
  4. ↑ Rau, Jochen (2021). Quantum Theory: An Information Processing Approach. Oxford University Press. 
  5. ↑ Hensen, B. (2015). "Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres". Nature 526: 682–686. doi:10.1038/nature15759. 


Author: Harold Foppele




Retrieved from "https://handwiki.org/wiki/index.php?title=Physics:Quantum_Bell%27s_theorem&oldid=4328067"