Vol. 1, No. 3, pp. 195—200, 1964
Did Feynman think that entanglement is just another "situation in quantum mechanics [which] can...be explained by saying, 'You remember the case of the experiment with the two holes? It's the same thing'"? Not entirely. In his very brief comments on Bell's Theorem, Feynman said it was "no big deal," just another way of presenting what quantum mechanics already knows. See
YouTube.com. But entanglement involves something more than the mysterious wave function.
A scientific theory can only be judged by its agreement with experiments. Although the experimental results are only statistical, quantum theory predictions are more accurate by several orders of magnitude than classical physics!
And entanglement is no exception. The two-particle wave function Ψ
12 predicts the outcomes perfectly and completely. First, it predicts that the sequence of spins for each particle will be completely random! Secondly (and counterintuitively) it predicts that the total spin will be the same as it was at initial entanglement, conserving total spin angular momentum.
Quantum mechanics predicts that although the first measurement is perfectly random, the second measurement, no matter how far away when measured, will be exactly what is needed to conserve total spin, leading so many thinkers for nearly a century to believe that the first measurement must have "influenced" the second, that the particles are "communicating" at faster than light speed!
And thousands of Bell experiments have confirmed these theoretical predictions perfectly!
Is the "
weirdness" of entanglement a part of the "
only mystery" of quantum mechanics? It is, but not the only part, because quantum mechanics tells us precisely what
causes the two particles to always appear
perfectly correlated. The total spin of the particles is a
constant of the motion that is
conserved at all times, constraining the spins to always agree. We can view this constraint as a
common cause in the past light cone of the two particles, coming from the apparatus that initially entangled them, located centrally between the two measurements.
The great exaggeration of entanglement capabilities comes from thinking that Alice and Bob are
communicating at faster than light speed with one another. They are not. Alice and Bob's measurements are creating bits of information when they collapse the two-particle wave function coming from the entanglement apparatus located centrally between them. But since the bits are randomly generated, no meaningful information is being communicated from the causal center to Alice and Bob s well.
A ← CC → B
Starting most prominently with Einstein himself, critics of the
Copenhagen Interpretation say that it denies a "reality" independent of
(conscious) "observers."
This is extreme and seriously flawed, especially the absurd claim that particles "do not exist" when they are not being observed. What does not exist is the particular value of properties which quantum mechanics says has different
possible values when measured.
The conservation of matter and energy means that aA material particle cannot go in and out of "existence."
"Do you really believe the moon is not there when you are not looking at it?," Einstein famously asked his colleague and later biographer, Abraham Pais (
Rev. Mod. Phys. 51, 863–914 (1979), p. 907).
N David Mermin
What "doesn't exist" when no one is observing is up-to-date
information (
knowledge) about a particle's properties, where is it exactly, how is it moving, what is its internal state, etc.
And when an observer does make a measurement, the so-called
collapse of the wave function does not mean that anything physical is moving! It is just information changing!
The foolish idea that a particle goes out of existence and returns when observed contradicts a basic principle of physics, more fundamental then the mechanical laws of motion, namely the
conservation of mass and energy, as well as conservation of properties like angular momentum and electron spin, which are critical to explaining
entanglement.