Double Slit Weirdness
When a proper light source (coherent – light from a single source all at the same frequency) is placed in front of a screen with a narrow slit, the light is diffracted (spread out) as it goes through the slit and appears as a shaded band centered on a screen or photographic film. The light is scattered by the edges of the slit as shown in the first figure.
Single Slit Diffraction.
Double Slit Setup
If we add two more slits located side by side between the first slit and the screen, the light passing through each of the new slits is diffracted again such that the photons from each slit are bent across each path and combine to reinforce or cancel each other where they strike the screen. The result is an interference pattern (light and dark bands) on the screen.
If you block either of the two middle slits, the interference pattern disappears. The diffracted light from the forward slit is diffracted again by the open slit in the second screen, but no interference pattern emerges because the other slit is blocked.
It can be shown that when both slits are open, the interference pattern seen can be duplicated by drawing the photon as a wave at the frequency of the source at each of the second screen slits and then combining the resulting waves by addition and subtraction of the waves where they mix behind the slits. This addition and subtraction of the waves into the interference pattern seems to prove that the light is a wave. Indeed, if the light is a particle, it would imply that their interaction cancels particles in the dark areas or at the very least, they bump each other into specific areas on the screen. We know that does not happen.
However, if you replace the screen by a photographic film and reduce the intensity so that only a few hundred photons are sent through before the film is developed, you will find that you can see the individual places the photons hit as dots on the screen. You can also see that the individual dots are organized so that they fall into the bands of the interference pattern and duplicate it. Keep the film in place long enough and the patterns become more complete. Put a cover over one of the slits before you start and you will find that the film still shows dots, but no interference pattern, only the diffraction band.
The dots seem to prove that the light is indeed a particle, not a wave, but yet they seem to interfere with each other like a wave when both slits are open. A mystery, but not necessarily weird.
Quantum Weirdness at Work
Now if the light source is reduced in intensity enough to send only one photon at a time, a weird result can be seen if the photographic film is left long enough (days or even months in a very dark box) where both slits are left open. The interference pattern continues to develop on the film, even though there is no possibility of interference (or even photon bumping) unless the individual photons go through both slits somehow.
A test for this is to remove the film and put a detector behind the slits. But then we find that the detectors only detect one photon at a time and only through one slit or the other, never both. The interference pattern never develops if either slit is covered. Quantum Weirdness at work.
There have been some very inventive tests such as using super-fast mirrors behind the double slits that switch in and out of the path between the time the photon leaves the slit and before it arrives at the screen. The results are the same. When either slit is covered, the interference pattern disappears but when both are open it reappears, even if the photons arrive only one at a time.
Since the photographic plates seem to prove that the light is a photon and never goes through both slits, the quantum weirdness problem arises and part of the current explanation is that the measurement (putting a detector in the path) always disturbs the measurement. In fact a whole class of quantum theory has developed around the inability to make precise measurements due to the measurement disturbance problem.
Quantum Electrodynamics (QED) explains this behavior using simple diagrams to show the probabilities of where the photons will land. It cannot predict where any one photon will land, but given enough photons, it can predict the pattern very accurately. QED does little to explain the weirdness of it all.
I have a simple theory on how all this works that I will get to eventually. Next I want to talk about Polarized Light Weirdness.