"Although the other fundamental interactions-electromagnetism and the strong and weak forces-have been successfully married to quantum theory, the standard methods of quantization seem to fail for gravity9. This has motivated alternative approaches to the unification of gravity with quantum theory, including string theory, loop quantum gravity and proposals that gravity is not quantized at all but remains fundamentally classical10. A decisive factor in determining which route is correct has so far been lacking: experimental evidence."
"These theorems rest on the assumption that theories of classical gravity can involve only local operations and exchanges of classical information2,3,4,5,6,7,8. This is because non-local, action-at-a-distance processes are considered unphysical, and it seems natural that a classical gravitational interaction cannot transmit quantum information. Under this assumption, the interaction falls into a class of processes that, according to quantum information theory or generalizations7,8, cannot create entanglement, formalized as local operations and classical communication (LOCC) in quantum information theory3,5."
Standard quantization methods fail for gravity, motivating alternative approaches including string theory, loop quantum gravity, and proposals that gravity remains fundamentally classical. A decisive experimental test has been lacking. Feynman proposed a thought experiment placing an object of Planck mass (0.02 mg) in a quantum superposition of two locations before interacting gravitationally with another mass. Modern formulations treat the key signature as observation of entanglement between the massive objects. Several theorems show that physically realistic local classical-gravity theories limited to local operations and exchanges of classical information cannot create such entanglement. Under the LOCC formalism, creation of entanglement by gravity would therefore imply that gravity transmits quantum information.
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