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The idea that correlation does not imply causation is well known to scientists and statisticians, but now physicists in Canada have shown that it is not always the case in the weird world of quantum mechanics.

Research in medicine, economics and many other disciplines often relies on showing a statistical correlation between two variables. It is often not clear, however, whether a change in one variable actually causes a shift in the other or whether the two variables are related via a third unmeasured factor. In a drug trial, for example, a higher recovery rate among those who take a certain drug compared with those who choose not to take the drug could be related to a third factor that is linked causally to both – perhaps those who choose not to take the drug are less ill than the others. The answer is to carry out randomized drug trials, in which drugs and placebos are distributed randomly. This means that one variable – whether or not a patient chooses to take the drug – is controlled, rather than being left alone.

In the latest work, a team led by Kevin Resch of the University of Waterloo in Canada and Robert Spekkens of the Perimeter Institute for Theoretical Physics, also in Waterloo, has discovered that in quantum mechanics it is possible to find out whether or not two variables are linked causally without having to control one of the variables. Both variables can in fact be left free, with causation established purely by studying the pattern of correlations that emerge from repeated trials of the quantum system.

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