ERWIN SCHRÖDINGER called it the "defining trait" of quantum theory. Einstein could not bring himself to believe in it at all, thinking it proof that quantum theory was seriously buggy. It is entanglement: the idea that particles can be linked in such a way that changing the quantum state of one instantaneously affects the other, even if they are light years apart.

This "spooky action at a distance", in Einstein's words, is a serious blow to our conception of how the world works. In 1964, physicist John Bell of the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, showed just how serious. He calculated a math inequality  that encapsulated the maximum correlation between the states of remote particles in experiments in which three "reasonable" conditions hold: that experimenters have free will in setting things up as they want; that the particle properties being measured are real and pre-existing, not just popping up at the time of measurement; and that no influence travels faster than the speed of light, the cosmic speed limit.

As many experiments since have shown, quantum mechanics regularly violates Bell's inequality, yielding levels of correlation way above those possible if his conditions hold. That pitches us into a philosophical dilemma. Do we not have free will, meaning something, somehow predetermines what measurements we take? That is not anyone's first choice. Are the properties of quantum particles not real - implying that nothing is real at all, but exists merely as a result of our perception? That's a more popular position, but it hardly leaves us any the wiser.

Or is there really an influence that travels faster than light? Cementing the Swiss reputation for precision timing, in 2008 physicist Nicolas Gisin and his colleagues at the University of Geneva showed that, if reality and free will hold, the speed of transfer of quantum states between entangled photons held in two villages 18 kilometres apart was somewhere above 10 million times the speed of light
Whatever the true answer is, it will be weird. Welcome to quantum reality.