Free Quantum Theory Cannot Hurt You by Marcus Chown

spin, the second with anticlockwise spin—their spins cancel. Physicists say their total spin is zero. Of course, the pair of electrons can also have a total spin of zero if the first electron has an anticlockwise spin and the second a clockwise spin.
    Now, there is a law of nature that says the total spin of such a system can never change. (It’s actually called the law of conservation of angular momentum.) So once the pair of electrons has been created with a total spin of zero, the pair’s spin must remain zero as long as the pair remains in existence.
    Nothing out of the ordinary here. However, there is another way to create two electrons with a total spin of zero. Recall that, if two states of a microscopic system are possible, then a superposition of the two is also possible. This means it is possible to create a pair of electrons that are simultaneously clockwise-anticlockwise and anticlockwise-clockwise.
    So what? Well, remember that such a superposition can exist only as long as the pair of electrons is isolated from its environment. The moment the outside world interacts with it—and that interaction could be someone checking to see what the electrons are doing—the superposition undergoes decoherence and is destroyed. Unable to exist any longer in their schizophrenic state, the electrons plump for being either clockwise-anticlockwise or anticlockwise-clockwise.
    Still nothing out of the ordinary (at least for the microscopic world!). However, imagine that, after the electrons are created in their schizophrenic state, they remain isolated and nobody looks at them. Instead, one electron is taken away in a box to a faraway place. Only then does someone finally open the box and observe the spin of the electron.
    If the electron at the faraway place turns out to have a clockwise spin, then instantaneously the other electron must stop being in itsschizophrenic state and assume an anticlockwise spin. The total spin, after all, must always remain zero. If, on the other hand, the electron turns out to be spinning anticlockwise, its cousin must instantaneously assume a clockwise spin.
    It does not matter if one electron is in a steel box half-buried on the seafloor and the other is in a box on the far side of the Universe. One electron will respond instantaneously to the other’s state. This is not merely some esoteric theory. Instantaneous influence has actually been observed in the laboratory.
    In 1982, Alain Aspect and his colleagues at the University of Paris South created pairs of photons and sent members of each pair to detectors separated by a distance of 13 metres. The detectors measured the polarisation of the photons, a property related to their spin. Aspect’s team showed that measuring the polarisation of photons at one detector affected the polarisation measured at the other detector. The influence that travelled between the detectors did so in less than 10 nanoseconds. Crucially, this was a quarter of the time a light beam would have taken to bridge the 13-metre gap.
    At the bare minimum, some kind of influence travelled between the detectors at four times the speed of light. If the technology had made it possible to measure an even smaller time interval, Aspect could have shown the ghostly influence to be even faster. Quantum theory was right. And Einstein—bless him—was wrong.
    Nonlocality could never happen in the ordinary, nonquantum world. An air mass might split into two tornadoes, one spinning clockwise and the other anticlockwise. But that’s the way they would stay—spinning in opposite directions—until finally they both ran out of steam. The crucial difference in the microscopic, quantum world is that the spins of particles are undetermined until the instant they are observed. And, before the spin of one electron in the pair is observed, it is totally unpredictable. It has a 50 per cent chance of being clockwise and a 50 per cent chance of being anticlockwise (once again we come up