Here is my four-year-old animation of the entanglement experiment …
Now consider this possible animation of the experiment, which illustrates the assumption that each electron is in a linear combination of up and down spin. It imitates the superposition (or linear combination) with up and down arrows on each electron oscillating quickly.
Notice that if you mouse click on any frame in the timeline, you will see that total spin = 0 is conserved. When one electron is spin up the other is always spin down. But what justifies the assumption that the spin of each electron is in a superposition of up and down? For the Copenhagen Interpretation, it is simply that we do not know which it is.
Since quantum mechanics says we cannot know the spin until it is measured, our best estimate is a 50/50 probability between up and down.
This is the same as assuming Schrödinger’s Cat is 50/50 alive and dead. But what this means of course is simply that if we do a large number of identical experiments, the statistics for live and dead cats will be approximately 50/50%. We never observe/measure a cat that is both dead and alive!
As Einstein noted, QM tells us nothing about individual cats. Quantum mechanics is incomplete in this respect. He is correct, although Bohr and Heisenberg insisted QM is complete, because we cannot know more before we measure, and reality is created (they say) when we do measure.
Despite accepting that a particular value of an “observable” can only be known by a measurement (knowledge is an epistemological problem, Einstein asked whether the particle actually (really, ontologically) has a path and position before we measure it? His answer was yes.
Einstein believed strongly in conservation principles – conservation of energy, momentum, angular momentum, and spin. Once a particle is on a path, with a spin, what could possibly change its spin mid-flight to its detection?
Here is an animation that assumes the two electrons are randomly produced in a spin-up and a spin-down state. Einstein’s objective reality suggest that they remain in those states no matter how far they separate, provided neither interacts (is decohered) until the measurement. Too simple to be true?
Almost every presentation of the EPR paradox begins with something like “Alice observes one particle…” and concludes with the question “How does the second particle get the information needed so that Bob’s measurements correlate perfectly with Alice?”
There is a fundamental asymmetry in this framing of the EPR experiment. It is a surprise that Einstein, who was so good at seeing deep symmetries, did not consider how to remove the asymmetry.
Consider this reframing: Alice’s measurement collapses the two-particle wave function. The two indistinguishable particles simultaneously appear at locations in a space-like separation. The frame of reference in which the source of the two entangled particles and the two experimenters are at rest is a special frame in the following sense.
As Einstein knew very well, there are frames of reference moving with respect to the laboratory frame of the two observers in which the time order of the events can be reversed. In some moving frames Alice measures first, but in others Bob measures first.
If there is a special frame of reference (not a preferred frame in the relativistic sense), surely it is the one in which the origin of the two entangled particles is at rest. Assuming that Alice and Bob are also at rest in this special frame and equidistant from the origin, we arrive at the simple picture in which any measurement that causes the two-particle wave function to collapse makes both particles appear simultaneously at determinate places with fully correlated properties (just those that are needed to conserve energy, momentum, angular momentum, and spin).