In physics, the principle of locality states that an object is only directly influenced by its immediate surroundings.
The de Broglie–Bohm theory, also known as the pilot-wave theory, Bohmian mechanics, the Bohm or Bohm’s interpretation, and the causal interpretation, is an interpretation of quantum theory. In addition to a wavefunction on the space of all possible configurations, it also postulates an actual configuration that exists even when unobserved. The evolution over time of the configuration (that is, of the positions of all particles or the configuration of all fields) is defined by the wave function via a guiding equation. The evolution of the wave function over time is given by Schrödinger’s equation. The theory is named after Louis de Broglie (1892–1987) and David Bohm (1917–1992).
No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.
In a well-known 1935 paper, Einstein and co-authors Boris Podolsky and Nathan Rosen (collectively “EPR”) sought to demonstrate by a paradox that QM was incomplete. This provided hope that a more-complete (and less-troubling) theory might one day be discovered. But that conclusion rested on the seemingly reasonable assumptions of locality and realism (together called “local realism” or “local hidden variables“, often interchangeably).
realism meant the moon is there even when not being observed.
These assumptions were hotly debated within the physics community, notably between Nobel laureates Einstein and Niels Bohr.
In his groundbreaking 1964 paper, “On the Einstein Podolsky Rosen paradox”, physicist John Stewart Bell presented an analogy (based on spin measurements on pairs of entangled electrons) to EPR’s hypothetical paradox.
Based on EPR a choice of measurement setting here should not affect the outcome of a measurement there (and vice versa). After providing a mathematical formulation of locality and realism based on this, he showed specific cases where this would be inconsistent with the predictions of QM theory.
locality and realism=> QM theory false
Using quantum entanglement of photons instead of electrons, John Clauser and Stuart Freedman (1972) and Alain Aspect et al. (1981) convincingly demonstrated that the predictions of QM are correct in this regard.
QM is right => locality or realism are false
Concept of realism is now somewhat different from what it was in discussions in the 1930s. It is more precisely called counterfactual definiteness; it means that we may think of outcomes of measurements that were not actually performed as being just as much part of reality as those that were made.
Locality is short for local relativistic causality.
Freedom refers to the physical possibility to determine settings on measurement devices independently of the internal state of the physical system being measured.
Conclusive experimental evidence of the violation of Bell’s inequality would drastically reduce the class of acceptable deterministic theories but would not falsify absolute determinism, which was described by Bell himself as ‘…not just inanimate nature running on behind-the-scenes clockwork, but with our behaviour, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined’.