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by Katie Howard
- Outcome independence: the outcome of the experiment at site A does not depend on the outcome of the experiment at site B.
- Parameter independence: the outcome of the experiment at site A does not depend on the choice of detector setting at site B.
The concept of locality prohibits any influences between events in space-like separated regions. Think of it in terms of Maxwell’s equations, where the electric and magnetic fields are plane waves travelling at a constant speed, which is the speed of light. If there is causality between two non-local events, the time delay must be larger than the time light takes to travel from the first to the second event.Leggett has proposed to consider theories that maintain the assumption of outcome independence, but drop the assumption of parameter independence. It is worth remarking at this point that the attribution of fundamental importance to this factorization of the locality assumption can easily be criticized. Whilst it is usual to describe the outcome at each site by ±1 this is an oversimplification. For example, if we are doing Stern-Gerlach measurements on electron spins then the actual outcome is a deflection of the path of the electron either up or down with respect to the orientation of the magnet. Thus, the outcome cannot be so easily separated from the orientation of the detector, as its full description depends on the orientation.Nevertheless, whatever one makes of the factorization, it is the case that one can construct toy models that reproduce the quantum predictions in
They are able to falsify this combined assumption because the (wrong) assumption implies, through
The sane conclusion is, of course, that we must finally do what all fully sane friends of Max Born did in 1926, take quantum mechanics seriously, and abandon “realism” (I don’t mean political realism which is good but quantum realism which is bad!): only probabilities may be predicted and it makes no sense to talk about the “real” values of observables of a quantum system before these values are measured. Only results of experiments have a physical meaning.
Nevertheless, some people still insist that it is plausible that “realism” holds and locality is what is violated. Relativity requires that the fundamental degrees of freedom - such as quantum fields - must evolve according to local and causal laws. This statement must be true with accuracy: it’s not only beautiful but it has been experimentally validated.
On the other hand, Zeilinger now argue that they have falsified a large class of “nonlocal realist” theories, too, because the measured correlations are higher even than what “nonlocal realist” theories allow. I don’t quite know how they can achieve such a goal. It is clearly a theoretical goal.
I think that every sane quantum physicist can predict the result of all these experiments and there can’t be any new surprises here: quantum mechanics works and physics behind all these experiments is controlled by the same simple laws that give clear predictions to every setup. On the other hand, they must be using some “improved” version of
Nevertheless, I endorse their position that the results of all these experiments make any attempt to preserve “realism” - i.e. to deny the probabilistic nature of quantum mechanics - highly contrived. The more you understand how these experiments work, the more you agree with us.Such models of physical realism, suggesting that the results of observations are consequence of the properties carried by physical systems, are called hidden-variable theories. The idea is that all measurement outcomes depend on pre-existing properties of objects that are independent of the measurement. The limitation of quantum theory then would be that we do not know all variables, they are hidden from us.