EPR paradox, 2

Two assumptions drove the desire to find a local realist theory:

1. Objects have a definite state that determines the values of all other measurable properties, such as position and momentum.

2. Effects of local actions, such as measurements, cannot travel faster than the speed of light (in consequence of special relativity). Thus if observers are sufficiently far apart, a measurement made by one can have no effect on a measurement made by the other.
   
— Wikipedia on Bell’s theorem

It is no longer possible to adhere to both the principle of locality (that distant objects cannot affect local objects), and counterfactual definiteness, a form of ontological realism implicit in classical physics. Some interpretations of quantum mechanics hold that a system lacks an actualized property until it is measured, which implies that quantum systems exhibit a non-local behaviour. Bell’s theorem proved that every quantum theory must either violate local realism or counterfactual definiteness.

— Wikipedia on Naive realism

1. The principle of locality:

There are two possible meanings of “locality” here.

1.1 The principle is correct in a sense that no causal influence can be faster than light.

1.2 The principle is incorrect in a sense that distant particles can be entangled. Correlation without causation can be instantaneous.

Assume that a pair of particles are entangled. Measuring one particle will collapse the wave function, which governs both particles, instantaneously.

2. Counterfactual definiteness:

2.1 It is correct in a sense that an object has a definite quantum state.

2.2 It is incorrect in a sense that, most often than not, the definite quantum state is not corresponding to a definite classical state (aka eigenstate). Instead, that quantum state is a superposition of different eigenstates. 

— Me@2012-04-07 11:36:01 AM

2012.04.07 Saturday (c) All rights reserved by ACHK