Spin-zero-particle decay can make two entangled coupled spin-1/2 particles, one +1/2 and one -1/2, which have one coherent system wavefunction {Einstein-Podolsky-Rosen experiment} {EPR experiment}. After particle-pair production, one particle always has spin opposite to the first, by conservation of angular momentum, but observation has not yet determined which particle has which spin. If an instrument detects one particle's spin direction and collapses the system wavefunction, the other particle immediately has the opposite spin, even over long distances. Einstein, Podolsky, and Rosen said instantaneous information transmission was impossible, so particles changed to the measured spins when the particles separated. Experiments showed that both particles have no definite spin until measured, so particles had superposed states until measured. By quantum mechanics, neither particle has definite spin-axis direction, so particles have superposition of +1/2 and -1/2 states until measured.
Experimenters must choose direction around which to measure spin and can measure in any direction. If they measure opposite direction, they can observe opposite spin. Therefore, particle production alone does not determine measured spin, and realism does not happen. Measuring system and particle together, as a new system, determine measured spin.
spin detection
If two spin-1/2 particles are in singlet state, three detectors oriented at -120, 0, and +120 degree angles perpendicular to moving-particle path can measure one particle's spin. Probability that both spins have opposite values is cos^2(A/2), where A is angle.
Physical Sciences>Physics>Quantum Mechanics>Wavefunction>Collapse>Non-Local
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Date Modified: 2022.0224