No-communication theorem

In physics, the no-communication theorem (also referred to as the no-signaling principle) is a no-go theorem in quantum information theory. It asserts that during the measurement of an entangled quantum state, it is impossible for one observer to transmit information to another observer, regardless of their spatial separation. This conclusion preserves the principle of causality in quantum mechanics and ensures that information transfer does not violate special relativity by exceeding the speed of light.

The theorem is significant because quantum entanglement creates correlations between distant events that might initially appear to enable faster-than-light communication. The no-communication theorem demonstrates that the failure of local causality does not imply that "spooky action at a distance," a phrase originally coined by Einstein, can be used to communicate faster than light.