Why focus on the single slit experiment? It’s because this experiment, more than any other, highlights the crucial role of information transfer. This isn’t about the transfer of energy or force, but rather, information. For the purpose of this discussion, it’s important that we define ‘information’ as the universe does, not as humans typically understand it. The single slit experiment serves as a powerful demonstration of this concept.
Briefly, information is that which causes physical reality to assume the properties we observe. Particles, forces, and fields, all will have their own properties. One type of particle will be different from another type of particle, one kind of force different from another kind. It is this phenomenological information that gives observable reality the various identities we can see and measure.
But what is information in this context? Is it something we can actually measure? Is it something solid we can lay our hands on? No. Information of the basic, fundamental kind I am talking about is a 'quantum' property of the object we are studying. As previously mentioned, the concept of 'reality' doesn't apply in the context of quantum physics. The information we gather is as 'real' as the quantum object under observation, like a single electron. However, calling quantum objects 'real' is like calling a cat obedient. It feels good, but it's not quite right.
From it's very beginnings, the quantum theory and reality have been at odds with each other, and this continues even today. But, if there is a method to making some sense of the world we see around us, and the random, imaginary world quantum physics paints is correct, it must be found at the beginning of the quantum interpretation of things. We have to go back to its roots, and see what we can see. And, the single slit experiment, is just about the most basic, fundamental, and practical place to start.
When we look at the single slit experiment (I would recommend Feynman's Lectures for anyone really interested in a lucid description that makes sense of the results of experiment, as long as you don't ask why (If I'm feeling chatty, perhaps I'll tell you the reason physicists don't like to ask the 'why' questions too loudly in public, sometime). Feynman was wise enough to stick to results, not conjecture. But I should point out, even he starts with a 'double slit' experiment - you can always cover up one of the slits..., it shows us a glimpse of the shadowy world single particles of matter and energy live in. Common sense gives way to a realm of possibilities that make no sense. Yet, even in this topsy turvy world, there is one thing that we can understand and make use of. Probability. Quantum mechanics operates on statistical principles. For a single particle, its specifics are uncertain. However, when observing many particles, patterns start to appear. It is only when you begin to deal with large numbers of particles that order begins to emerge.
So, for a single atom of uranium, quantum physics is useless. Quantum physics gives almost no information about individual atoms. Yet they manage to do their thing anyway. And Quantum physics is one of our most powerful tools for obtaining information about the natural world.... yes it has its weaknesses. That is why we need to go all the way back to the very beginning, and that is the single slit experiment, and see if there is something there we have missed or misinterpreted.
Well, the single slit experiment is all about shooting individual particles through a slit cut in gold foil, or some other aperture. The particle will either hit the material that makes up the slit, or travel on through, and get detected as it hits the sensors aligned all around the slit for just such a joyous occasion. It can't get any simpler than that, for sure. Even the results are simple. The detector records an area where particles make it through. Everywhere else, except for the oddball particle that gets scattered and winds up hitting your mother-in-laws nose, there is a flat line. In other words, this is a simple experiment that has simple and explainable results -- it is well behaved, and that is *exactly* what we are looking for.
However you design it, the results of the experiment, whether the particle makes it through the slit or not, is governed by simple statistics. By putting the particle emitter on a track and moving it, the odds of the particle making it through the slit change. This is information, and we can control it. In essence, we can now control probability itself, at least within the confines of our experiment. This is why I brought up this ridiculously simple experiment and took so much time on it -- if nothing else, it is an example within quantum mechanics where we humans are able through simple means, to control the outcome of events by changing the nature of probability itself. This is very important - so much so that I am going to write a bit more...