Free Farewell to Reality by Jim Baggott

to be quantum theory’s fundamental flaws — its inconsistencies and incompleteness.
    Bohr stood firm. He resisted the challenges, each time ably defending the Copenhagen interpretation and in one instance using Einstein’s own general theory of relativity against him. But Bohr’s case for the defence relied increasingly on arguments based on clumsiness, an essential and unavoidable disturbance of the system caused by the act of measurement, of the kind that he had criticized Heisenberg for. Einstein realized that he needed to find a challenge that did not depend directly on the kind of disturbance characteristic of a measurement, thus undermining Bohr’s defence.
    In 1935, together with two young theorists, Boris Podolsky and Nathan Rosen, Einstein devised the ultimate challenge. Imagine a physical system that produces two photons. We assume that the physics of the system constrains the two photons such that they are both produced in identical states of linear polarization. * We have no idea what these orientations are until we impose a reference frame by performing a measurement. According to the Copenhagen interpretation, until the measurement, the actual orientations are ‘undetermined’ — all orientations are possible in the wavefunction, just as the single photon in the two-slit experiment can be found anywhere on the photographic film prior to the collapse.
    For the sake of clarity, we’ll call these photons A and B. Photon A shoots off to the left, photon B to the right. We set up our polarizing film over on the left. We make a measurement and determine that photon A has vertical polarization along our laboratory z axis.
    What does this mean for photon B? Obviously, we have not yet made any measurement on photon B, yet we can deduce that it, too, must have vertical polarization along this same axis. The physics of the process that produced the photons demands this. The polarization state of photon B appears to have suddenly changed, from ‘undetermined’ to vertical, even though we have made no measurement on it. And although we might in practice be constrained in terms of laboratory space, we could in principle wait for photon B to travel halfway across the universe before we make our measurement on A.
    All this talk of photons and their polarization states might seem rather esoteric, and it might be a little difficult to follow precisely what’s going on. But it’s important that we understand the nature of Einstein, Podolsky and Rosen’s challenge and so fully appreciate what’s at stake here.
    Let’s try the following (imperfect) analogy. Suppose I toss a coin. This happens to be a coin with some special properties. As it spins in the air, it splits into two coins. The operation of a ‘law of conservation of coin faces’ means that if the coin that lands over on the left (coin A) gives ‘heads’, then the coin that lands over on the right (coin B) gives ‘heads’ too. If coin A gives ‘tails’, then coin B also gives ‘tails’. There are no circumstances under which we would expect to observe the results ‘heads’—‘tails’ or ‘tails’—‘heads’.
    Now suppose I toss the coin and we look to see what result we got for coin A. We see that it lands ‘heads’. We know that coin B must also give the result ‘heads’ — the law of conservation of coin faces demands it.
    Our instinct is to assume that the properties of the two coins are established at the instant they split from the original coin. The coins split apart and separate in mid-air, in some way that ensures that we eventually get correlated results — ‘heads’—‘heads’ or ‘tails’—‘tails’. But assuming this implies that there are other variables involved of which we are ignorant, and that if this is all described by quantum theory, then the theory