The Cat and EPR Paradoxes

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


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3 Responses to The Cat and EPR Paradoxes

  1. Andy Howe says:

    Never mind the wretched cat – according to Niels Bohr’s Cogenhagen interpretation of QM, the whole universe only exists because humans observe it. This is deifying humanity.
    I believe Bohr was satsified that any “unconcious” measurement by some form of detector would be sufficient, and the cat would certainly be a sufficient detector long before the box was opened.
    …. Andy Howe, Sheffield, UK [New Scientist letter, p.30, 28 May 2011]

    • iconoclastcymru says:

      What is your evidence for Bohr ‘believing’ any form of detector would suffice? We now now know of atomic-sized detectors as well as the common microchip 0/1 states.
      What counts as a detector is an essential problem shown up by Schrodinger’s ‘Cat’. Remember the “mind boggling” interpretation of photon cascades by the Zeilinger school, that what counts is whether ‘welcher weg’ for a photon could be determined, not whether there was a detector in place to determine it (see Marshall).
      * Wang, Zou, Mandel, Phys. Rev. A, 44, 4614 (1991)

  2. Andy Howe says:

    Cf. wikipedia on what Bohr believed:
    ” Niels Bohr, never had in mind the observer-induced collapse of the wave function, so that Schrödinger’s Cat did not pose any riddle to him. The cat would be either dead or alive long before the box is opened by a conscious observer.[5] ”
    …. indeed the cited reference did say what the not-always-reliable Wikipedia claims it says [Ed: as likely written by the same J Faye]!
    Faye, J (2008-01-24). “Copenhagen Interpretation of Quantum Mechanics”. Stanford Encyclopedia of Philosophy. The Metaphysics Research Lab Center for the Study of Language and Information, Stanford University. Retrieved 2010-09-19.)

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