Schrödinger’s equation is at the heart of physics, John Barrow tells us in his introduction to Cosmic Imagery: Key Images in the History of Science, then adds that Schrödinger’s Cat is an “apparent paradox”.
But his section on “that darned cat” gives no reason for the word “apparent”. Despite Niels Bohr answering that the only reality is our knowledge of the cat, most of us admit Schrödinger was right to see as absurd the quantum mixture of dead and alive cat-states.
Bohr couldn’t answer why he required a human observer – can the cat itself (or living substitute) not function as observer? The second famous paradox, EPR, shows that measuring one particle of a quantum-entangled pair in effect measures its distant partner without disturbing it directly, so beating Heisenberg indeterminacy and speed-of-light signals. Barrow gives a picture of light-rings of photons entangled as output of a UV-pumped non-linear crystal, but a pair of down-converted photons generally have unequal frequencies, not equal as he shows, and the picture doesn’t represent ‘entanglement’. John
Wheeler’s famous depiction of the quantum mechanical world as ‘a great smokey dragon’ showed the difficulty of visualization – instead of clear boundaries and contours, reality seems to dissolve into a hazy fuzz.
From the Review in Contemporary Phys. 51(4), 1-2 (2010): Cosmic Imagery: Key Images in the History of Science by John D. Barrow