Quantum theory predicts that single photons can interfere with themselves, just like waves, when passing through a pair of slits. This may seem weird enough, but photons don’t always act like waves! In fact, we can change whether a photon acts like a particle or a wave merely by looking at it.
The image on the left was captured while sending a beam of single photons through a pair of slits. As discussed in my last post the photons are acting like waves, producing an interference pattern. In this experiment we know nothing about the individual photons passing through the apparatus. Most importantly, we don’t know which of the two slits an individual photon has passed through.
In the image on the right, I modified the experiment so that I could tell which slit each individual photon had passed through. Suddenly, the interference pattern disappears! By making the measurement I forced the photons to act like particles, which do not interfere.
Amazingly, the only change made between the experiments above was adding a filter between the slits and camera that detected the interference pattern. This filter changed how the photons interacted with the slits, even though they passed through the slits first! I think this is one of the coolest examples of the weirdness of quantum mechanics.