"The very notion that particles like protons or electrons are rotating solid objects of size and shape doesn't fit the quantum worldview. And when scientists try to measure a particle's spin, they get one of two answers: up or down. There are no in-betweens in quantum mechanics."
True as far as it goes, but atoms are more complex systems. An atomic electron wavefunction - orbital - has a shape related to its orbital angular momentum [
https://en.wikipedia.org/wiki/Azimuthal_quantum_number ]. But of course the electron itself is point like as a particle.
Can anyone explain why this isn't an article about conservation of total spin of all the particles in the container? Isn't that a better way to explain it as opposed to quantum entanglement?
I think the article author labored more to explain entanglement, which is what the experiment observes, than to explain the total entangled spin of the singlet states, which is what the experiment measures. They use the strength of the applied magnetic field to see the entanglement:
"Fig. 3 Evidence for long-range entanglement. Points show the KF-obtained variances (ΔF^2_z) of the rotating-frame spin component F_z as a function of delay since Fz was last aligned along the laboratory z axis, and thus subject to measurement by Faraday rotation. ... As seen in these data, increased gradient B0 causes a faster relaxation toward the thermal spin state (TSS) value, as is expected for a gas of singlets ..."