Quantum mechanics is a phrase that often evokes a feeling of mystery and esoteric power. It was developed at the beginning of the twentieth century to explain the results of experiments that were not consistent with classical Newtonian mechanics. The revolutionary insights attributed to quantum mechanics, though, are not actually a product of its equations and theories. Instead, they arise directly from the seemingly bizarre behavior of particles and forces demonstrated in laboratory experiments. We can bypass all the complex math and theories and go right to the easily visualized experiments to discover what they have to offer.
In this article I will be describing the behavior photons of light as they pass through openings in an opaque surface and then accumulate on a target surface. I will be describing what is called a double slit experiment. I chose to present the experiment involving light since it is the easiest to do, but similar experiments can be carried out using electrons or even larger particles, with the same results.
As you would expect, light going through a single slit in a solid surface forms an image of the slit on a target surface beyond it. In contrast, if you shine light through two closely spaced slits in the solid surface, you get what is called an interference pattern. Instead of the simple image of two slits next to each other on the target surface, you see a larger number of alternating light and dark bands. The diagram that follows was copied from http://grad.physics.sunysb.edu/~amarch/. It depicts light coming from a source to the left, through the slits and falling on a target surface. The pattern on the target is an example of an interference pattern.
The interference pattern of this experiment was initially thought by physicists to be the result of the wave nature of light as it passes through the two slits. This kind of interference can be visualized by imagining throwing two small stones simultaneously into a calm pool of water. As the waves from the stones spread out and run into each other, the crests and troughs of the waves add or subtract from each other as they overlap. Recent experimental results, though, raise questions about this interpretation.
Instead of using a bright beam of light to perform this experiment, it is possible to use photons that are emitted so slowly that only one photon at time goes through the entire apparatus and is recorded at the target surface before the next photon starts out. Since there is only one photon in the device at any one time, you would probably expect that there is no way for the photons to interfere with each other. That would mean that if you recorded the point of impact of enough individual photons to build up a clear pattern on the target surface, you would just get two bright spots, one for each slit. This is not what happens! When the single photons accumulate, they show an “interference” pattern, just like the high intensity beams of light do. Think about what this means. Even though the light photon is an incredibly small particle that travels in a straight line through air, it somehow can determine whether the surface it passes through has a single slit or multiple slits. The individual photons change their paths depending on the number of slits without any known physical means to respond to the configuration of the molecules that make up the surface with the slits.
This experiment violates our common sense assumption that all physical objects are separate and autonomous if they are not physically connected in some fashion. The behavior we see here is also automatic and lawful; every time we run this experiment we get exactly the same result. It is how the world works. In some mysterious fashion, the light photons always adjust their paths according to the configuration of the slits in surface through which they pass. There is some manner of nonphysical wholeness of the entire experimental set-up.
This experimental result also contradicts several centuries of scientific theories starting with Isaac Newton. Most scientists have envisioned the entire universe as unfolding according to the purely physical interactions of particles and forces over time. In keeping with this view, some have even likened the universe to an enormously large mechanical clockwork. In contrast, experiments like this not only show us that some sort of mysterious, nonphysical connection exists between physically separate particles, through this connection they always take each other into account and act accordingly. While scientists have discounted even the possibility that any sort of nonphysical interactions might exist, we see now that such interactions absolutely do exist. Furthermore, this conclusion implies that the universe unfolds in accord with at least two completely different systems of interaction, one physical and another nonphysical. Both must be understood to provide the most complete and accurate description of our world.
That some sort of nonphysical connection exists between physically separate particles is just the first of a number of very surprising conclusions about how our world works. As this series progresses the conclusions will be like pieces of a puzzle that begin to interlock into each other. They will suggest a much larger, very elegant picture that we are just beginning to understand. In the next article, I will present another variation of this two slit experiment that will also provide fascinating insights.