The quantum framework is in some respects deeper than spacetime -
the information flows which are deployed by the
framework are difficult to characterize in spacetime terms.
"Before the measurement there is no particle pair; there is only a
gigantic atom. This atom pervades all space. The experiment dematerializes the atom, and in its place two particles appear. Each materializes, as it must in the universe, so as to preserve the laws
of nature."
Marvin Chester,
Primer of Quantum Mechanics
Generally, physical systems have
global properties which continue to evolve globally even as the system becomes
spatially separated - when the "parts" of the system are measured then they will
manifest correlations which embody (or "cash in") the previous global state.
Might these correlations be the result of local processes, perhaps involving other,
unknown or "hidden" facts about nature? No. (This is precisely what John Bell proved, in 1964.)
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The process generating the correlations is "nonlocal".
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Can it be used for signalling then? No. Given quantum theory
signalling via the quantum process is not in the cards...
- Yet quantum theory is
an idealization, and some have shown how relaxing aspects of the idealization
uncovers nonlocal possibilities.
- More fundamentally, we are nonlocal by virtue of simply being. What happens in the
universe, and in our selves, is very different than would happen in a locally bound,
classicalesque world.
Einstein, Podolsky and Rosen first focused attention on correlated, spatially
extended quantum systems in 1935, though in their argument they assumed
'locality' in order to find fault with quantum theory. 25 years later
Bell's result showed that EPR's assumption was mistaken.
In 1989, Greenberger, Horne and Zeilinger sharpened Bell's results further
by considering correlated states with 3 or more entangled particles.
The full extent of nonlocality as a physical fact is not well understood - for example,
does a superfluid exhibit nonlocality? Generally, almost any "collapse of a
wavefunction" appears to be nonlocal: is this an artifact of our description?
Since the 1964 analysis of John Bell (Bell's theorem) it is widely recognized that in
some sense the nonlocality is real - quantum mechanics is a much different theory
than one could assemble with local parts. In the words of Henry Stapp
(1977),
"The present formulation asserts that a theory entails a nonlocal connection
if there is no conceivable way for the results in each region to be
independent of the choice made in the other region. Quantum theory has
such a nonlocal connection: That is what Bell actually discovered."
It is probably not possible to signal using quantum nonlocality, though in my
view it is a complete mistake to conclude from this that all of the nonlocality
"washes out" in practice. (In fact it does not wash out!)
One could turn the question around by viewing the quantum description as
completely global, and ask what is the root of the apparent, provisional locality.
