پارادخش ِ EPR pârâdaxš-e EPR
*Fr.: paradoxe EPR*
A thought experiment developed in 1935 by A. Einstein (1879-1955),
Boris Podolsky (1896-1966), and Nathan Rosen (1909-1995)
to demonstrate that there is a fundamental inconsistency in
→ *quantum mechanics*.
They imagined two physical systems that are allowed to interact
initially so that they will subsequently be defined by a single
quantum mechanical state. For example, a neutral → *pion*
at rest which decays into a pair of → *photon*s. The pair
of photons is described by a single two-particle → *wave function*.
Once separated, the two photons are still described by the same wave
function, and a measurement of one → *observable*
of the first system will determine the measurement of the corresponding
observable of the second system. For example, if photon 1 is
found to have → *spin*
up along the *x*-axis, then photon 2 must have spin
down along the x-axis, since the final total → *angular momentum*
of the two-photon system must be the same as the angular
momentum of the initial state. This means that we know the
spin of photon 2 even without measuring it. Likewise, the
measurement of another observable of the first system will determine
the measurement of the corresponding observable of the second system,
even though the systems are no longer physically linked in the
traditional sense of local coupling (→ *quantum entanglement*).
So, EPR argued that quantum mechanics was not a complete theory, but it could be
corrected by postulating the existence of → *hidden variable*s
that furthermore would be "local". According to EPR, the specification of these
local hidden parameters would predetermine the result of measuring any observable of
the physical system. However, in 1964 John S. Bell developed a theorem,
→ *Bell's inequality*,
to test for the existence of these hidden variables.
He showed that if the inequality was satisfied,
then no local hidden variable theory can reproduce the predictions of
quantum mechanics. → *Aspect experiment*. A. Einstein, B. Podolsky, N. Rosen: "Can quantum-mechanical
description of physical reality be considered complete?"
Phys. Rev. 41, 777 (15 May 1935); → *paradox*. |