In this experiment, tiny bits of matter (photons, electrons, or any atomic-sized object) are shot towards a screen that has two slits in it. On the other side of the screen, a high tech video camera records where each photon lands. When scientists close one slit, the camera will show us an expected pattern, as seen in the video below. But when both slits are opened, an “interference pattern” emerges – they begin to act like waves. This doesn’t mean that atomic objects are observed as a wave (even though it recently has been ovserved as a wave), they just act that way. It means that each photon individually goes through both slits at the same time and interferes with itself, but it also goes through one slit, and it goes through the other. Furthermore, it goes through neither of them. The single piece of matter becomes a “wave” of potentials, expressing itself in the form of multiple possibilities, and this is why we get the interference pattern.
How can a single piece of matter exist and express itself in multiple states, without any physical properties, until it is “measured” or “observed?” Furthermore, how does it choose which path, out of multiple possibilities, it will take?
Then, when an “observer” decides to measure and look at which slit the piece of matter goes through, the “wave” of potential paths collapses into one single path. The particle goes from becoming, again, a “wave” of potentials into one particle taking a single route. It’s as if the particle knows it’s being watched. The observer has some sort of effect on the behavior of the particle.