Free Farewell to Reality by Jim Baggott Page A

is in some sense incomplete.
    In the photon example, the two particles are said to be ‘entangled’. Because of the way they are produced, both photons are described by a single wavefunction. When we make a measurement on photon A, the wavefunction collapses and the polarization properties of both photons mysteriously become ‘real’. If photon B is halfway across the universe, then the collapse must reach out across this distance. Instantaneously.
    Likewise, quantum theory insists that our two coins don’t possess fixed properties the instant they’re created. It’s almost as though they each oscillate back and forth from ‘heads’ to ‘tails’ as they spin to the ground. But the moment we see that coin A has landed ‘heads’, then coin B must also land ‘heads’, even though the two coins may fall far apart from each other.
    Einstein, Podolsky and Rosen wrote: ‘No reasonable definition of reality could be expected to permit this.’ 12
    Despite what quantum theory says, Einstein, Podolsky and Rosen argued that it is surely reasonable to assume that when we make a measurement on photon A, this can in no way disturb photon B. What we choose to do with photon A cannot affect the properties and behaviour of B and hence the outcome of any subsequent measurement we might make on it. Under this assumption, we have no explanation for the sudden change in the polarization state of photon B, from ‘undetermined’ to vertical.
    We conclude that there is, in fact, no change at all. Photon B must have vertical polarization all along. As there is nothing in quantum theory that tells us how the polarization states of the photons are determined at the moment they are produced, Einstein, Podolsky and Rosen concluded that the theory is incomplete.
    â€˜This onslaught came down upon us as a bolt from the blue,’ wrote Belgian theorist Léon Rosenfeld, who was working with Bohr at his institute when news of this latest challenge reached Copenhagen. 13 The eminent English theorist Paul Dirac declared: ‘Now we have to start all over again, because Einstein proved that it does not work.’ 14
    Bohr’s response was simply to restate the Copenhagen interpretation. He argued that we simply cannot get past the wave shadows and the particle shadows. Irrespective of the apparent puzzles caused by the need to invoke a collapse of the wavefunction, we just have to accept that that’s the way it is. We have to deal with what we can measure and perceive. And these things are determined by the way we set up our experiment.
    Bohr argued that it does not matter that the polarization state of photon B can be inferred from measurements we make on photon A. By setting up an experiment to measure the polarization of photon A with certainty (along the z axis, say), we deny ourselves the possibility of measuring the polarization along any other axis (x or y ). And if we cannot exercise a choice, then the actual properties and behaviour of photon B are really rather moot. Even though there is no mechanical disturbance of photon B (no clumsiness), its properties and behaviour are nevertheless defined by the way we have set up the measurement on photon A.
    The Einstein—Podolsky—Rosen thought experiment pushed Bohr to drop the clumsiness defence, just as Einstein had intended. But this left him with no alternative but to argue for a position that may, if anything, seem even more ‘spooky’. The idea that the properties and behaviour of a quantum particle could somehow be influenced by how we choose to set up an apparatus an arbitrarily long distance away is very discomforting. Many years later the English physicist Anthony Leggett summarized this as follows:
    But in physics we are normally accustomed to require some positive reason before we accept a particular part of the environment as relevant to the outcome of an experiment. Now the [distant]