The Double Slit experiment

It's Sunday, and while others may today visit a temple or a church or a mosque, I will be reading one of those books on quantum physics that are meant for lay people like myself ("In Search of Schroedinger's Cat", by John Gribbin). It is a thrilling read. Quantum mechanics results are so whacky and difficult to wrap one's brain around that they're almost mystical.

Let me tell you about one of the simplest experiments that shows the weirdness of the world we live in - the double slit experiment. Perhaps you performed this experiment in school - we did. All you need to perform it is a small light source (a bulb, let us say), an opaque sheet with two fine slits in it, and another sheet to act as a screen. If you put the bulb on one side, the slits in the middle and the screen on the other side, what will you get?

You might think that you will get two bands of light on the screen, one for each slit from the light passing through. But reality is different. Let me turn to Wikipedia to describe it accurately:

The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright and dark bands on the screen. However, at the screen, the light is always found to be absorbed as though it were made of discrete particles, called photons.

So you have individual photons not going in a straight line to the screen but instead hitting the screen in certain areas but not others. When you look at the total picture created by thousands of photos, you see these alternating bright and dark bands.

Yet if one of the slits is closed, the interference collapses and all you see is one single line on the screen, not alternating bright and dark bands. The photon acts like a particle moving in a straight line. Hmm...

But it gets weirder...

The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present.

So:
One photon, one slit = Particle moving in a straight line
One photon, two slits = Something going through both slits and ending up as a particle on the screen (not necessarily in a straight line with either slit)? Yet if you send one photon after another, you will see each crash onto the screen at what seems like a random point, but gradually a pattern will emerge - the same dark and light bands that can be statistically predicted.

Something very interesting is going on here. Feynman once said that the double-slit experiment "contains the only mystery".