[[Quantum Mechanics|Quantum mechanics]] is fundamentally different from many other theories in physics. Most theories are based on local interactions: one billiard ball pushes another one by touching it, an [[Electron|electron]] interacts locally with an [[Electric Field|electric field]], etc. It was only natural that interactions which do not follow this paradigm would be frowned upon. In 1935 Albert Einstein, Boris Podolsky and Nathan Rosen remarked exactly that: [[Quantum Mechanics|quantum mechanics]] allows interactions that they described as "spooky interactions at a distance", in other words, nonlocal interactions. The big question now: does Nature really allow these interactions or are we just missing information? Maybe, there is a common cause that controls these non-local interactions that we see. It took almost thirty years to settle this dispute. John Stuart Bell published an article which provided a recipe to test **experimentally** whether we were lacking information or whether Nature actually allows nonlocal interactions. He proposed to measure correlations and provided a maximal value for local theories in the form of a [[Bell Inequality|Bell inequality]]. Fast forward another thirty-five years: in 2015 the first fully convincing experiments were performed showing that Nature actually allows nonlocal interactions, a prediction which is compatible with [[Quantum Mechanics|quantum mechanics]]. Bell nonlocality is one of the beautiful examples how research progresses over almost 100 years to solve one of the most fundamental questions about the nature of Nature. >[!read]- Further Reading > - [[Bell Inequality]] > - [[Entanglement]] > - [[Nonlocal Game]] >[!ref]- References >- A. Einstein, B. Podolsky, and N. Rosen, Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?, Phys. Rev. **47**, 777 (1935). >- J. S. Bell, On the Einstein Podolsky Rosen paradox, Physics Physique Fizika **1**, 195 (1964). > - N. Brunner, D. Cavalcanti, S. Pironio, V. Scarani, and S. Wehner, Bell nonlocality, Rev. Mod. Phys. **86**, 419 (2014). > - S. Storz et al., Loophole-free Bell inequality violation with superconducting circuits, Nature **617**, 7960 (2023). >- L. K. Shalm et al., Strong Loophole-Free Test of Local Realism, Phys. Rev. Lett. **115**, 250402 (2015). >- M. Giustina et al., Significant-Loophole-Free Test of Bell’s Theorem with Entangled Photons, Phys. Rev. Lett. **115**, 250401 (2015). >- B. Hensen et al., Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres, Nature **526**, 7575 (2015). >- W. Rosenfeld, D. Burchardt, R. Garthoff, K. Redeker, N. Ortegel, M. Rau, and H. Weinfurter, Event-Ready Bell Test Using Entangled Atoms Simultaneously Closing Detection and Locality Loopholes, Phys. Rev. Lett. **119**, 010402 (2017).