# Young’s double slit experiment revolutionized physics

One of the strangest experimental results ever observed is that of the single particle double slit experiment. It is one of the most stunning illustrations of how the quantum world is fundamentally different from the large scale world of our physical intuition. In fact, the double slit experiment suggests that the fundamental nature of reality may not be physical at all, at least not in any way that we perceive.

Let’s start with the simple. Imagine a single periodic wave,suchasripplesonalake. Some distance away, those waves encounter a barrier with two gaps cut in it. Most of the wave is blocked, but some ripples still pass through the gaps. When the ripples start to overlap each other on the other side, they produce a really interesting pattern known as an interference pattern that is observed on a screen. This happens because at some points, the peaks from the two ripples overlap to create higher peaks, and at other points the troughs of the ripples overlap to create even deeper troughs.

What makes this pattern so incredible is its universality — every type of wave, from water waves, to sound, to light waves, displays this pattern. The double slit interference of light was rst observed by Thomas Young in 1801. He found that a source of light passing through two very thin slits produces an interference pattern in the bands of dark and light stripes on a screen behind.

The complication is this — we also know that light comes in indivisible bundles of electromagnetic energy called photons, as demonstrated by Einstein through the photoelectric effect. So light is composed of discreet packets, but also displays wave phenomena! In the double slit experiment, each photon must decide which of the two slits to go through. It can’t split in half and recombine on the other side, because it’s discreet. What one should see, with the photon model of light, is two bright spots representing the photons that got through the slits, and total darkness otherwise. How does one reconcile what one sees, an interference pattern, with what is expected, two bright spots.

It turns out that if you re photons, one at a time, at a screen with two slits such that each photon has equal probability of passing through either of the two slits, then withtime,onascreenbehind, the re ection points of the individual photons aggregate together, into denser and less dense areas, exactly like an interference pattern. The interference pattern has nothing to do with how each single photon’s energy is spread out, but how each photon dumps all of its energy at a single point. The pattern essentially emerges in the distribution of the photons themselves.

Many completely unrelated photons must essentially know the nal positions of each other. Each photon has no idea where a previous photon landed or where any future photon will land, and must have a nal landing point that forms a pattern with all the others. How can that be?

It turns out that some even weirder things than photons have been shown to produce this interference pattern. Shoot electrons through a single pair of slits, and you’ll get the same result. This effect has even been observed with whole atoms and whole molecules. Buckminster Fullerine (Buckyballs), are gigantic molecules of 60 carbon atoms, and have been observed to produce double slit interference.

We have to assume that the photon, electron, or Buckyball passes through both slits as a wave. That wave then interacts with itself to create an interference pattern.

Except here, the peaks and troughs of that pattern are the regions where there is more or less a chance the particle will nd itself on the screen. It seems like a wave of unde ned positions, that at some point, for some reason, resolves itself down to a single certain position. This distribution of possibilities is at the heart of quantum mechanics, and is governed by what is known as the probability wave function. So what is a probability wave really made of? Well, we know where our wave particle is at the beginning and end of its journey. We know our particle’s starting point — a light source, electron-gun, or Buckyball launcher. And we know its end point — the screen, or rather, wherever it releases its energy. The particle seems more particle-like at the beginning and end, but more wave-like in between. That wave holds the informa- tion about all the nal pos- sible positions of the particle, and every possible position at every stage in the journey. In fact, the wave must map out all the possible paths that the particle could take. In essence, we have this in nite family of ‘could be’ trajectories, as opposed to a single known trajectory. For some reason, near either end, this family of possible outcomes resolves into a single trajectory.

Within the mysterious span between the creation and the detection, is the particle anything more than a space of possibility?