A number of philosophers of science and science writers have written books and articles on quantum entanglement describing it as weird, impossible, illogical, and unreal.

Physicists too have described quantum phenomena as unreal, as defining a new "quantum reality," and as simply impossible to understand in terms of pre-quantum classical physics.

We review ten recent books by philosophers of science and prominent science writers along with key arguments from physicists back in the 1950's and 1960's. We discuss a few insights into quantum phenomena that may be gained by examining the information that is available about quantum states before and after a quantum measurement, especially when total momentum is conserved.

The founders of quantum mechanics themselves held some extreme views that quantum phenomena might be beyond human understanding.

Niels Bohr, "There is no quantum world."

Werner Heisenberg, "We cannot find how nature really is, only what we can know about it."

Albert Einstein, "Spooky action-at-a-distance."

Richard Feynman, "Nobody understands quantum mechanics." "The two-slit experiment is the [central] mystery of quantum mechanics." Feynman called it the one and only mystery, and we now know it explains entanglement as well.

Only one person explicitly says that conservation of angular momentum (spin) can explain the perfect correlations between entangled distant particles. It was Eugene Wigner in 1963, after the work of David Bohm (1952) but just before John Bell (1964). Bell, and Bohm, and Einstein before them, all used conservation of momentum implicitly to know about properties of the widely separated particle.

The criticism is correct that the particular z-spin (or x-spin or y-spin) that we measure did not exist before we measure, because we know that definite spins in all directions cannot exist simultaneously, and we can choose to measure in a random direction. But it is also correct that if we measure either particle, and find ("create the reality") that z-spin is up, then because the total z-spin must remain zero by conservation of angular (spin) momentum, the other article's z-spin will be found to be down, as long as the measurement is made in the (previously agreed upon) same direction.

Before measurement the spin state of the two particles (total zero spin with no preferred direction is called a singlet state) is rotationally symmetric, the same in all directions. Whichever direction we choose to measure, the two spins we find in that direction will necessarily be opposite to satisfy the conservation of angular momentum. It is not because we measure one or the other particle first, then that particle causes the other to line up. It is because the collapsing two-particle wave function Ψ₁₂ is a shared property of both particles, as Erwin Schrödinger showed in 1935 when he defined entanglement.

The two-particle wave function Ψ₁₂ collapses to the product of disentangled single-particle wave functions Ψ₁ Ψ₂. Together they still satisfy the conservation of angular momentum. If Ψ₁ is spin up, Ψ₂ will be spin down. This is not because one acts on the other.

It is a measurement that disentangles (or decoheres) the two-particle wave function, causing the "collapse of the wave function" that separates the two particles. Einstein hoped the entangled particles could separate simply by moving far enough apart, but Schrödinger explained to Einstein that is not the case, and Schrödinger has been shown correct by entanglement experiments over vast distances. Only a measurement that collapses the wave function can disentangle the particles.

Note that Niels Bohr's Copenhagen Interpretation that the value of spin measured "did not exist" before the measurement is correct. The measurement direction was chosen by the experimenter. Werner Heisenberg called this the "free choice? of the experimenter. Whether spin is found to be up or down was not determined, but random. Paul Dirac called this "Nature's choice."

The spin and its direction certainly were not determined back at the time of the initial entanglement as some have suggested. Nor were they predetermined at the beginning of the universe as determinist mathematical physicists sometimes claim. These properties both come into existence (emerge) perfectly correlated at the first measurement, caused by (Heisenberg and Bohr might say "created" by) the measurement.

Ananthaswamy, A. (2019). Through two doors at once: The elegant experiment that captures the enigma of our quantum reality. Dutton.

Ball, P. (2020). Beyond weird. University of Chicago Press.

Becker, A. (2018). What is Real? New York: Basic Books.

Brody, J. (2020). Quantum entanglement. MIT Press.

Herbert, N. (2011). Quantum reality: Beyond the new physics. Anchor.

Kaiser, D. (2011). How the Hippies Saved Physics, Counterculture, and the Quantum Revival. Norton.

Kumar, M. (2008). Quantum: Einstein, Bohr and the great debate about the nature of reality. Icon Books Ltd.

Lindley, D. (2008). Where does the weirdness go?: why quantum mechanics is strange, but not as strange as you think. Basic books.

Maudlin, T. (2011). Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics. Wiley-Blackwell; 3rd edition

Musser, G. (2015). Spooky Action at a Distance: The Phenomenon that Reimagines Space and Time--and what it Means for Black Holes, the Big Bang, and Theories of Everything. Macmillan.