Davies & Gribbin (1992: 203-5):
A classic example [of wave vs particle detection] is provided by a famous experiment first performed by Thomas Young in England in the early nineteenth century. Young carried out his experiment with light, but an exactly equivalent experiment has now been performed using electrons. In the original experiment, a point source of light illuminates two narrow adjacent slits in a screen, and the image of the light that passes through the slits is observed on a second screen (Figure 33).
You might guess that the image would consist of two overlapping patches of light; in fact, it is made up of a series of bright and dark stripes, known as interference fringes.
The appearance of interference fringes in Young's experiment is a clear demonstration of the wave nature of light. Wave interference occurs in any wave system when two (or more) waves come together and overlap. Where the waves arrive in step they reinforce each other; where they are out of step they cancel each other. In Young's experiment the light wave from one slit intersects the light wave from the other slit to produce the bright and dark stripes, as the two waves alternately add together and cancel each other out. And it is important to appreciate that if either one of the slits is covered, the striped pattern disappears.
Paradoxical overtones emerge if one now regards the light as composed of particles — photons. It is possible to weaken the light source until only one photon at a time passes through the slit system, and to record the cumulative effect of many photons arriving one after the other at the second screen over a long period of time. Each photon arrives at the image screen and makes a spot on a photographic plate. In the equivalent electron experiment, single electrons are fired through a double-slit system, and the "image screen" is a sensitive surface like that of a television screen. The arrival of each electron makes a spot of light on the screen, and a video of the buildup of the spots of light shows how a pattern emerges as more and more electrons pass through the system.
Recall that one cannot know in advance, because of the inherent uncertainty of the system, precisely where any given photon or electron will end up. But the cumulative effect of many "throws of the quantum dice" will average out the distribution into a well-defined pattern. Moreover, this pattern shows the same series of interference bands as obtained with a strong source. The puzzle is this. Each particle, be it photon or electron, can clearly pass through one slit alone. And each particle, as the buildup of spots on the image screen indicates, behaves like a particle when it arrives, striking the screen in just one place.
Blogger Comments:
As this description makes clear, it is particles (photons, electrons) that are fired through the double-slit system and it is particles that are detected on the screen as a pattern of dots. From the perspective of Systemic Functional Linguistic Theory, the instantial frequency pattern that accumulates on the screen manifests the probabilities of the system potential.
The interference pattern that appears is thus a manifestation of the overlap of two probability waves, since each slit provides its own range of probable trajectories. The most frequent detections occur where the probability waves reinforce each other, and the least frequent detections occur where the probability waves cancel each other out.
The reason why the interference pattern disappears when there is only one possible slit for the particle to pass through is that, in this instance, there is only one wave of probability, and so no overlap of different waves.
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