The Double-Slit Experiment Of Quantum Theory Through Systemic Functional Linguistics [19]

Hawking (1988: 58-9):

Interference can also occur for particles, because of the duality introduced by quantum mechanics. A famous example is the so-called two-slit experiment (Fig. 4.2). Consider a partition with two narrow parallel slits in it. On one side of the partition one places a source of light of a particular colour (that is, of a particular wavelength). Most of the light will hit the partition, but a small amount will go through the slits. Now suppose one places a screen on the far side of the partition from the light. Any point on the screen will receive waves from the two slits. However, in general, the distance the light has to travel from the source to the screen via the two slits will be different. This will mean that the waves from the slits will not be in phase with each other when they arrive at the screen: in some places the waves will cancel each other out, and in others they will reinforce each other. The result is a characteristic pattern of light and dark fringes.

 

The remarkable thing is that one gets exactly the same kind of fringes if one replaces the source of light by a source of particles such as electrons with a definite speed (this means that the corresponding waves have a definite length). It seems the more peculiar because if one only has one slit, one does not get any fringes, just a uniform distribution of electrons across the screen. One might therefore think that opening another slit would just increase the number of electrons hitting each point of the screen, but, because of interference, it actually decreases it in some places. If electrons are sent through the slits one at a time, one would expect each to pass through one slit or the other, and so behave just as if the slit it passed through were the only one there—giving a uniform distribution on the screen. In reality, however, even when the electrons are sent one at a time, the fringes still appear. Each electron, therefore, must be passing through both slits at the same time!

Blogger Comments:

From the perspective of Systemic Functional Linguistic Theory, wave-particle duality is the complementarity of potential and instance.  On this view, it is particles, not waves, that pass through the slits.

Importantly, if only one particle is emitted in the double-slit experiment, there is no interference pattern recorded on the detector screen. The interference patterns only begin to appear as more and more particles are detected. This means that the interference cannot be a property of each single instance. And this means that each detected particle only goes through one slit or the other, not both.

Instead, the "interference" is a property of the quantum system as potential, as described by the wave function, with the different frequencies of particle impacts instantiating the different probabilities of the quantum system potential.