Here's a thought experiment #1

Light and shadow

What makes quantum mechanics so confounding? Consider the following iconic, oft-repeated experiment: A beam of light shines through two parallel slits cut into a barrier and falls on a strip of photographic film beyond the barrier. Since light itself consists of a stream of particles —photons — it seems reasonable to assume that the photons pass through one slit or the other en route to the film.

And if physicists set up the experiment with a photon detector at each slit, that is indeed what they see: Photons hurtle randomly through either the first slit or the second, which results in two separate clumps of dots forming on the film.

A slight adjustment, however, profoundly alters the results. If physicists remove the photon detectors, the pattern created on the film changes completely. Instead of two clusters of dots, alternating light and dark bands appear across the film, what physicists call an interference pattern. 

That pattern could form only if each individual photon somehow spread out like a wave and went through both slits simultaneously. Bright bands develop on the film where two wave crests coincide; overlapping crests and troughs create the dark bands. In other words, photons behave like particles with detectors present and like waves without detectors. 

- Discover USA May 2017 "The war over reality" by tim folger-