Wave interference. When two stones are dropped close together in a pond, they create two circles of ripples. As the ripples spread, the two circles intersect. Where a wave peak from one circle hits a wave-peak from another, the two peaks produce a higher peak. But where a peak hits a trough, their interference creates a flat plane. Wave interference and 2 slits In the example in the circle on the right, the effect of wave interference can be seen as concentric ripples pass through a barrier containing two slits. This effect was studied by Young who used light beams, in his experiments below...
Young's 2-slit experiments A light-beam shining through a slit in a card projects itself on the wall as a single bar of light. This pattern indicates that light is ``corpuscular,'' travelling through the slit and hitting the wall in tiny pellet-like packets. But when a second slit is cut next to the first, the result is a set of parallel stripes. This pattern indicates that the two slits of light run into each other and set up an interference pattern. Wave peaks reinforce each other into bright stripes, others canceling out, produce the dark bands in between. With 2 slits, light appears to be wave-like.
2 slits + 1 photon = strange results Now reduce the intensity of the light source so that only one photon at a time is released. Aim the photon through the slit in any direction, and it will cast a bar of light on the wall. But if both slits are open, no matter where you point the photon, it will always land on a bright stripe, and not where a dark band was.
The strange quantum phenomenon of this experiment is that when both slits are open, a single photon encounters wave interference. It's as if it were interfering with another photon - although there is no other photon. But if you close the second slit, the interference pattern vanishes and the photon stops behaving like a wave. It resumes behaving like a pellet.
The polarized film experiments In these experiments, an ``interconnectedness'' appears to exist between photons. To discover how, we first set up a detector made of polarized film. Aim a photon at the detector. If that photon has the same polarization as the detector (A), the photon always passes right through it. Rotate the polarization of the detector to be at right angles to the photon (B), and the photon is always blocked But if you rotate the detector to some angle in between (C) the photon will sometimes pass through and sometimes be blocked. Bell theorized the effect of matching-polarity photons being emitted by a light source. Two detectors are placed in the path of the photons. When the detectors are aligned identically, the photons behave identically. But when one detector is rotated out of line with the other, one photon may pass while the other is blocked. Using math to determine the amount of mismatching, Bell predicted 50% mismatching. In fact, the mismatching was closer to 75%. This indicated that the photons were not acting independently of one another but within a single quantum system, or `interconnectedness.''