In

physics, the

**no-communication theorem** or

**no-signaling principle** is a

no-go theorem from

quantum information theory which states that, during measurement of an

entangled quantum state, it is not possible for one observer, by making a measurement of a subsystem of the total state, to communicate information to another observer. The theorem is important because, in

quantum mechanics,

quantum entanglement is an effect by which certain widely separated events can be correlated in ways that suggest the possibility of communication

faster-than-light. The no-communication theorem gives conditions under which such transfer of information between two observers is impossible. These results can be applied to understand the so-called paradoxes in

quantum mechanics, such as the

EPR paradox, or violations of

local realism obtained in tests of

Bell's theorem. In these experiments, the no-communication theorem shows that failure of local realism does not lead to what could be referred to as "spooky communication at a distance" (in analogy with Einstein's labeling of

quantum entanglement as requiring "spooky action at a distance" on the assumption of QM's completeness).