Send single photons through a double slit and they will somehow interfere with themselves to produce an interference pattern, as if they were waves. That’s quantum mechanics for ya. ‘Cept it don’t work if the photons are being watched, in which case each photon appears to pass through one slit or the other, as if it were a particle.

Somehow the photon knows it is being watched, which sends physicists and philosophers a-shiverin and and a-tremblin back to their textbooks.

In 1984, the American physicist John Wheeler dreamt up a way of tricking the photons. He suggested switching the method of observation after the photon had passed through the slit.

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.

So how to explain this. Either some unknown laws of physics are telling the photons that a switch has taken place during the experiment (and do so at several times the speed of light) or the wave-particle nature of quantum mechanics is correct.

Take yer pick.

Ref: arxiv.org/abs/0710.2597: Wheeler’s Delayed-choice Thought Experiment: Experimental Realization and Theoretical Analysis