The double-slit experiment
The double slit experiment – in its simplest form passing light through two slits on a screen – shows that light has both wave and particle like properties, and that when both slits are open, there will be interference between those portions of the wave emerging from each slit, much the same as when water passes through slits in a board or screen:
So is it also with matter particles such as electrons. Imagine that electrons are fired from an electron gun at a barrier that has two narrow slits. When the left slit is open and the right slit closed, the electrons pass through one by one and appear thus on a detector screen; similarly so on the other side of the screen when the right slit is open and the left closed. But when both slits are open, what you see are alternating dark and light regions on the screen[1]. Regardless of which slit each individual electron passes through, it seems to “know” about the presence of both slits. The presence of the left slit thus changes the possible locations of electrons passing through the right slit and vice versa.
The reason behind these alternating dark and light regions is that they are caused by an interference pattern, similar to those caused by two waves on a pond when we drop in two pebbles. Where the peak of one wave crosses the peak of the other, the combined wave height is big; where the trough of one crosses the trough of another, the combined wave depression is deep; and where the peak of one crosses the trough of the other, the waves cancel each other out and the water remains level. This is referred to as an interference pattern. The resultant interference, depicted in the bright and dark regions on the screen in the diagram on the following page[2] can be explained in terms of an undulating probability wave. A depiction of the double-slit experiment can be found on the page entitled Feynman's sum over paths under the rubric "wave-particle duality", also reproduced on the page Energy, matter and light: waves and particles at the same time
[1] Greene (2011), 195.
[2] Greene (2011), 198.