The one slit optical interference experiment is described using the wave model, particle model and a prototype unified model of light called phots. The evidence is part of a project to investigate whether a unified model for light might be possible.
Introduction
Previous essays in this series challenged the assumption that wave-particle duality is just a fundamental mystery of the Universe and suggested that a rethink might be worth a try. They suggested putting old analogies aside, starting with a fresh sheet of paper and using a bottom up building process based solely on experimental evidence. To illustrate the idea they started a prototype called ‘phots’. This essay describes the classic one slit interference experiment using the wave model and also in terms of the emerging phot model.
Single Hole/Slit Experiment
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The single slit experiment can be done using a slit a few wavelengths wide or a small hole in a screen (as shown here). This illustration shows the experimental setup. The distance between laser and hole, and hole and screen is about a metre. The screen has been rotated to give a better view of the pattern observed.
Fresnel Diffraction from a Slit: The diagram shows the screen at increasing distance z from the slit and graph of the irradiance/brightness pattern on the screen. Drawing not to scale. The slit width is a small multiple of the wavelength. The intervals between the screen positions are about a meter. The diagram shows how the pattern changes with distance. Many illustrations show only the second last graph.
This diagram is a computer generated artist’s impression of what is said to be happening according to the wave model. Coherent plane waves enter from the left. The part of each wave that impinges on the hole (or use a slit if you like) can be modelled as a large number of tiny wavelets. These radiate towards the detector screen, but combine in phase (constructively) or out of phase (destructively) once they have travelled a reasonable distance away. The grey washed out areas radiating towards the screen become the dark zones. Note: Computer generated pictures and ‘artist’s impressions’ need to be treated with caution. They are useful to illustrate the explanation but may introduce features that do not correspond to real experimental outcomes.
One problem with the above computer generated ‘artist’s impression’ is that it shows wavelets expanding in directions up to 90 degrees on either side of the axis. In a real experiment, and as shown in the top diagram, the emergent light is confined within a narrow beam that gradually broadens with distance.
Conventional Explanations
Textbooks and on-line explanations invariably use the Huygens principle of imagining that light consists of wave fronts and that each wave front can be modeled as a superposition of innumerable little wavelets. These impinge on the hole in the screen and a subset of them get through. Every point of space in the hole (or slit) becomes a new source of hemispherical wavelets proceeding in the forward direction. No mention is made of the disruptive effects of all the wavelets that do not get through the hole. The wavelets spend all their travel time interfering with each other and then they interact with a screen. This reveals whether their interference is being constructive or destructive. The overall effect is a pattern of brightness and darkness, light in the middle and a fringe either side.
The same argument is also presented using the language of rays. Every point on the screen is illuminated by rays from the slit. The screen distance is very much larger than the slit width so these rays are essentially parallel. But some have to travel a little bit further than other to get to that point the screen. Divide the slit into a hundred divisions (say) and pair #1 with #51, #2 with #52 and so on all the way to #50 with #100. This accounts for all the light going from the slit to that point on the screen. If the path length of the second pair member is half a wavelength longer or shorter than that of the first pair member then those two rays will arrive fully out of phase and will combine destructively, effectively cancelling each other out. Same for all the other pairings and so that point on the screen will be dark.
This explains the last few graphs in the top diagram. The first few graphs are more complicated but the explanation is much the same.
A weakness in this explanation is the assumption that all the rays pairs are in an identical phase back at the slit. The effect works with ordinary monochromatic light and not only laser light, so this assumption needs some explanation.
It has to be recognised that the mathematics developed by Fresnel and others successfully describes the outcomes of numerous experiments involving interference effects. However, this does not mean that every aspect of the wave model is literally correct. It might be the case that the wave model gets the right answers for the wrong reasons. There are many aspects of the wave model that can be questioned. Here are two of them:
- The Huygens wavelet principle is mathematical sleigh of hand but hard to accept as literally true. Where are the generators of the wavelets in the hole? There is nothing there. The experiment still works in a vacuum. There are no electrons or dipoles or anything else to source the innumerable wavelets.
- How can the wavelets have the same wavelength as the source wave? They each carry a tiny amount of the energy in the incident wave. But energy in a light wave is proportional to its frequency (E=Planck’s constant x frequency). So the frequency of the wavelets should be very small. And since the wavelength is the speed of light c divided by the frequency, each wavelet should have an enormous wavelength. Which ruins the whole story.
The explanation in terms of rays is a closer to the modern understanding of light. Even so there is much to query. Why is there no discussion of polarisation? Why are some rays bent so far to the side?
Conjectural explanations of single slit diffraction in terms of phots
The ‘phot protocol’ recommends gathering as much evidence as possible before jumping to conclusions about what is going on. However it is human nature to compare, contrast and categorise new information as it arrives. This has something to do with how our brains work. The trouble is that light is not like anything else we know. Nothing else travels at the speed of light. Nothing else has a size range exceeding a billion orders of magnitude. It is as unique as it is fundamental. Hence it is dangerous to use analogies from everyday life because they will inevitably be grossly misleading or plain wrong. But we cannot help ourselves. The challenge therefore is to keep an open mind as long as possible and to keep all ideas, conjectures and pre-judgments on the table until they are conclusively ruled out by experiment.
By way of example, three conjectures are offered here for what is going on in a one slit interference experiment. For convenience of description, call the slit edges upper and lower. The diffraction pattern is a symmetric distribution of bands of light along an up and down line. If another set of slits is made in the screen at right angles to the first slits, forming a cross, then a two dimensional pattern appear on the screens – a bit like a waffle. Bit for now it suffices to worry about the simpler two slit experiment.
In the ‘phot protocol’ any explanation that fits the evidence is kept on the table until it is ruled out by further evidence, and new ideas are welcomed if there is a chance they might have merit. To illustrate this point here are four conjectured explanations for the one-slit diffraction experiment in terms of phots.
Conjecture 1 – Diffraction over two edges: The slit width is comparable to the effective width of the arriving phots. Many phots impinge on the upstream surface of the sheet material. Some are absorbed and re-emitted on the source side. Some are absorbed all together. Others discover the slit and ‘percolate’ through to the other side.
Some phots arriving with their path close to midway between the edges of the slit ‘squeeze through’ and proceed straight to the central bright band on the screen, especially if the slit is wide. However, many of the phots will have a path that is off centre relative to the axis of the experiment. Some will be closer to the upper edge of the slit and an equal number (on average over time) will be closer to the lower edge of the slit.
Provided the sheet is thin and the slit edges smooth, what happens then is basically diffraction over a knife edge at the bottom edge plus diffraction over a knife edge at the top edge.
Look at the brightness pattern for the screen closest to the slits in the middle diagram above. This looks like back-to-back graphs of the intensity patterns of diffraction over an edge. The suggestion here is that is almost exactly what they are. The top and bottom of the slit are effectively knife edges and pattern on the screen is a combination of back to back knife-edge diffraction patters.
This conjecture 1 explanation for the one slit diffraction pattern in terms of phots then goes back to the conjectured explanation for knife-edge diffraction in Essay 3.4 of (Van de Vusse, 2024).
That explanation had two components. The first was that some of the phots were ‘refracted’ around the edge because their finite width meant that part of them was affected by the matter in the screen. The same explanation was conjectured as an explanation for Arago’s spot.
The second component was a new phenomenon. The slit edge seemed to set up a set of barriers just outside the knife edge. Something a bit like a wire fence. The phots were forced to go either side of the ‘wires’ on their way to the screen. The ‘shadows of the wires’ became the dark part of the interference pattern on the screen. If the observer looks back towards the knife edge then the dark lines above the knife edge are very easy to see by eye (suitably protected if using a laser) or photograph.
Conjecture 2 – Double Phot Interference: This explanation is basically a version of the wave model explanation. As a phot makes its way through the slit it becomes trapped at the exit until a second phot comes along. The arrival of the second phot frees the first phot and the two end up going off together, more or less in the same direction. Their path is at an angle ‘theta’ up or down from the axis of the experiment. The two phots leave the slit at different places, separated by d/n where d is the slit width and n is an even integer. The latter determines which side band the phots will end up in. If the phots are very close together they end up in the central bright spot.
The two phots arrive at the screen at the same time and in the same place. What happens next depends on the phase difference between the two phots. If the phase difference is small then they both deliver their energy to the screen, more or less in the same place and at the same time. If they are out of phase then one phot delivers its energy a bit higher up the screen and the other phot delivers its energy a bit lower on the screen. The gap becomes the dark zone in the interference pattern.
Conjecture 3 – ‘Photlets’: Travelling through the slit causes a phot to disintegrate into two equal parts. Its two sine wave drivers separate. Each part keeps its orientation and phase, but gets only half the energy and half the spin of the parent phot. The new entities will be called photlets (a type of neutrino perhaps, or even a ‘ghost wave’.) They are quantum entangled.
The two photlets act almost exactly as described in the wave model explanation of one slit interference. They set off from the aperture in pairs. Their starting point is separated by the slit width divided by n, when n is a even integer. What happens when they approach the screen depends on their phase difference. If this is small they produce the effect of single phot at the point of impact. If the phase difference larger they still combine to recreate the original phot, but the impact site is higher or lower on the screen.
The pattern of bright and dark bands on the screen is explained by the same mathematics as used in the classic wave explanation, except now it applied to photlets and not wavelets.
The observer might use smoke to detect the path of the light between slit and screen. If an entangled pairs of photlets encounters a smoke particle it collapses into a phot and acts as if the smoke particle was part of a screen.
Conjecture 4 – Free space refractions: Phots whose path lies close to the mid point of the slit pass through without disturbance, especially if the slit is large. Off centre phots cause a ghost like disturbance in the dielectric properties of the medium in which the phot is traveling, even if this is a vacuum. This disturbance travels with the phot for a short distance. In that time it can causes the path of the phot to be deflected up or down. This forces the phots to travel along paths that ultimately show up as the diffraction pattern observed on the screen.
Discussion
The conjectural explanations of one-slit interference effects using the prototype phot model are used to illustrate the overall approach, as well as to show that explanations not reliant on an inadequate water wave analogy might be possible. The phot protocol recommends collecting as much evidence as possible before starting to form fixed ideas, and using a framework to assemble the evidence logically. It is natural to start collecting conjectures at the same time, but this should not prejudice the selection of evidence.
Summary
The usual wave based explanations of the one-slit interference patterns are often over-simplified and gloss over some substantial difficulties, even though the mathematics produces a good fit to the observations. Explanations that treat light as being made up of small particles do not work at all. The ‘phot protocol’ suggests abandoning inappropriate analogies bases on water waves and cannonballs, starting with an open mind and let the evidence from experiments slowly build up a holistic framework in which a complete self-consistent unitary understanding of light might start to emerge.
The prototype being presented as an example of this approach seems to be travelling reasonably well. The basic evidence suggests that light comes in discrete bundles of energy which have orientation, phase, spin and line of travel and which generate sinusoidal electro-magnetic effects during interactions. To avoid mental baggage these entities have been called phots. To accord with everyday evidence and special relativity they are considered to have effective width but no length and to do very little ‘in-flight’ except travel very fast. Whether or not they rotate in flight is an open question.
Phots are not waves and they are not particles. They are what they are. They do have characteristics of waves and of particles, so some might choose to think of them as a hybrid, but it is probably better not to do this and to stick to the phot protocol. There is a long way to go.
The one-slit optical interference experiment adds to the accumulation of evidence. In combination with earlier evidence, such as Arago’s spot and edge diffraction effects, it suggests the following. Phots interact instantaneously with the whole area of sheet around the slit. Some are absorbed and some of these are reflected. The rest come apart and percolate through the slit to the other side where they are reborn as “child-phots”. Those close to the centre line are essentially the same as their parent and form the middle of the pattern. Those off-centre are more complicated. They end up being sent off at some angles but not others. The interference pattern is essentially generated close to the slit and is then projected forwards. There is strong evidence that wave like interference is going on, but it occurs close to the slit.
The next step in the overall project is to tackle one of the most perplexing experiments in the history of physics – Thomas Young’s Double Split experiment dating back to 1801.
Reference
Van de Vusse, Sjoerd B.A., 2024, Some ideas and experiments for issues affecting modern physics, https://hereticalphysics.com.au
Author contact: SBAvan@utas.edu.au
Author’s location: Hobart, Australia
