Is quantum entanglement real? When two photons are created at the same time, and then separated over some distance, are they still mysteriously in touch with each other, so that what one photon does affects what the other photon does — instantaneously?
The concept was first quantified by Bell in his 1964 paper, and known as Bell’s Theorem. It has since been demonstrated by many experiments, those of James Clauser and Alain Aspect. But many sceptics remain to be convinced, feeling that there must be some loophole. Something overlooked in the experimental setup may be “tipping off” one photon so that it knows ahead of time what the other photon is about to do. For example, hidden variables, such as suggested by David Bohm, could allow particles to communicate instantaneously with each other over large distances.
Why does this matter? Because at the heart of Bell’s theorem is the concept of super-determinism; the issue of whether free will exists or whether it is only an illusion. Perhaps the photon entanglement can be explained in a non-local way as a result of determinism. Bell stated the problem in a 1985 interview on BBC.
There is a way to escape the inference of superluminal speeds and spooky action at a distance. But it involves absolute determinism in the universe, the complete absence of free will. Suppose the world is super-deterministic, with not just inanimate nature running on behind-the-scenes clockwork, but with our behaviour, including our belief that we are free to choose to do one experiment rather than another, absolutely predetermined, including the ‘decision’ by the experimenter to carry out one set of measurements rather than another, the difficulty disappears. There is no need for a faster-than-light signal to tell particle A what measurement has been carried out on particle B, because the universe, including particle A, already ‘knows’ what that measurement, and its outcome, will be.
The idea of super-determinism was recently dealt a blow by an experiment performed by David Kaiser, Alan Guth and others from the University of Vienna, published in Physics Letters, Cosmic Bell Test: Measurement Settings from Milky Way Stars . The experiment set up involved shooting two photons in opposite directions, then measuring a polarization property of the photons simultaneously. In past, experiments measured a polarization direction selected by a random number generator. However there was always a possibility that somehow the random number generators could communicate with each other and allow the photons to “cheat”. To get around this problem, a telescope was installed at each measurement site, and trained on a star 600 light years away. The wavelength of the light entering the telescope was used to choose which polarization direction would be measured. If in some way, that measurement was pre-determined, the decision had to have been made 600 years ago when the starlight set out on its journey.
The experiment showed that the photon polarizations were correlated; the photons were entangled regardless of experimental setup. The proposed loophole, that the entanglement was due to a pre-determination was shown to be false..
One has to ask, why are such extraordinary efforts being made to establish the detailed nature of quantum entanglement? Haven’t enough experiments been done to show that it is real? Quantum entanglement is important because it may underlie what we see, and how we understand the nature of reality. The star experiment suggests that entanglement has a bearing on the existence of free will. Free will, choice may be a result of quantum entanglement. What can be more fundamental to our lives? Quantum entanglement may also, according to Roger Penrose, explain the nature of consciousness.