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That means sending a photon through the slits and only then making the decision to record its arrival observed or unobserved. How would the photon “know” what you’ve decided?
Now Alain “Hidden” Aspect at the Laboratoire Charles Fabry de l’Institut d’Optique in France and a few pals have performed the trick for the first time. They say that photons are not so easily tricked: switching the method of observation actually changes the outcome of the experiment, which means the photon somehow knows what you’ve done.