This essay gives three examples as to how the results of Young’s Double Slit Experiments might be explainable in ways that are consistent with the experimental fact that light comes in discrete parcels. This is significant because Young’s Double Slit experiments are generally held to be impossible to describe in terms of photons and hence they contributed to the awkward compromise called wave-particle duality.
This essay is part of a series arguing that a unified model for light might be possible. It recommends an approach that avoids misleading analogies, systematically builds up evidence from experiments, keeps options open as long as possible and uses deductive logic to see what emerges. As an example the essays have followed a line of evidence that has led to a prototype model called phots.
Keywords:
Some Background to the Double Slit Experiment
Thomas Young (1773-1829) was an English polymath who made contributions to many fields including physics, physiology and Egyptology. In his double slit experiment of 1801 Young used a pinhole in a curtain as the source of light, passed the light either side of a playing card and managed to get an interference pattern on the far wall. In later versions he used a candle as the source of light, a prism to get monochromatic light, and a small hole in a screen to make the light spatially coherent. He then passed the light through two parallel slits and was able to observe pattern of light and dark bands on a screen.
Young’s own drawing is shown above. A and B are the slits and C, D E, F are places on a screen where the wavelets are out of phase with each other.
Young presented the results of his experiment to the Royal Society in 1803 and described it in a book published in 1807. His explanation of the results incorporates the ideas of Christiaan Huygens over a hundred year earlier. Young is credited with starting the revival of the wave model of light.
In the modern demonstrations of this experiment, a coherent light source such as a laser beam illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The multiple dark stripes would not be expected if light consisted of classical particles forced to travel in straight lines through the slits.
The experiment works even with incident light that is so dim that it has to be considered to be mostly single photons. A low-intensity double-slit experiment was first performed by Geoffrey Taylor in 1909 (Taylor, G. I. 1909).
The fact that the experiment works with ‘feeble’ light has led most physicists to agree with Paul Dirac that it shows that photons can and do interfere with themselves, thus forcing science to adopt the principle that a photon is both a particle and a wave.
This photo of a single slit pattern and a double slit pattern comes from Wikipedia. The slits were 0.7mm apart. In the top image one of the slits has been closed and faint bands can be seen either side of the central patch of red light. The bright middle section is twice as wide as the side bands. If the screen is bent slightly to focus on the slits, the side bands are equally spaced. In the double slit pattern underneath, dark lines appear where there was brightness before when just one slit or the other was open. The dark lines are equally spaced. The number of dark lines depends on the distance between the two slits.
Quote from Wikipedia entry on the double slit experiment: “In quantum mechanics, this experiment is considered to demonstrate the inseparability of the wave and particle natures of light and other quantum particles (wave–particle duality). Richard Feynman was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.”
The Experiment
A typical experimental setup for Young’s double slit experiment is shown below.
The diagram refers to a typical Young’s double slit experiment. Laser light illuminates two slits of width s spaced apart by distance d. Call the slits A and B. The middle diagram (b) shows light intensity curves a and b and these represent the patterns observable if just the A slit is used, or just the B slit is used, with the other slit covered over. Patterns a or b are observed in such one slit experiments. Curve c is a diagrammatic graph obtained by simply adding curves a and b. The solid wavy line in the lower diagram shows the light intensity pattern observable if both slits are open. It has the same overall pattern as curve c (redrawn to scale) but the observed intensities are dramatically rearranged into bright bands separated by dark stripes.
The double slit experiment works best with monochromatic light. Note that the experiment can also be done with electrons, or even atoms and larger particles.
General Observations/Evidence
In general terms the two slit interference pattern on a screen looks like the additive combination of two single slit experiments with a rake dragged through the whole pattern.
If the experiment is enveloped in fine smoke the paths taken by the light can be seen. They seem to emanate from a region between the two slits, close to the plate. In textbooks and online demonstrations of the experiment this aspect of the experiment is rarely discussed, and if it is discussed the schematic drawings often contradict each other. Light is sometimes shown as if it emanates from a point on the sheet/plate midway between the two slits, or from the inner edges of the slits, or from the middle of each slit, or the rays are shown as being parallel and just appear in free space somewhere between the slits and the screen.
Using smoke or other means to trace the rays back from the screen suggests that the gross pattern forms quite close to the slits and is then projected forward. The pattern changes and evolves somewhat as the screen distance is increased and this also needs to be explained.
The experiment has been done using light that is so dim that it has to be considered as a stream of single phots, possibly with some turning up in twos and threes close together, but definitely not a whole swarm at the same time. This evidence strongly suggests that that photons can “interfere with themselves”. However, it does not rule out the possibility that photons can also interfere with each other in suitable circumstances. Furthermore, it might be possible that the self-interference is actually not quite what it seems.
Conjectural phot based explanations about the famous double slit experiment need to take into account the evidence from other optical experiments, especially diffraction experiments. In turn the evidence from double slit experiments can inform, inspire and constrain the interpretations of other relevant experiments.
If a credible, comprehensive, consistent model of light can be developed the next step would be to develop new experiments and predictions to test the model still further.
Preliminary discussion
If slit B is closed then the light through slit A creates a one slit interference pattern on the screen. Opening the B slit as well then gives a pattern which basically looks like the pattern from the A slit combined with the pattern from the B slit – but with one drastic change. The pattern is a lot more wavy. It looks like a fine toothed comb has raked through everything. Dark stripes appear in the pattern and the missing light ends up in the bright patches. Opening the B split has affected the results from the A slit.
This is the challenge to explain – how can the light through B affect the light through A and vice versa? The classic wave explanation involves constructive and destructive wave interference from B and that putting the screen in the way of the light shows the results of all this interference at that particular distance from the screen as measured along the axis of the experiment. (If the experimenter curves the screen to focus on the slits it gives a slightly sharper picture).
Preceding essays have discussed a protocol for incorporating the evidence from optical experiments into a holistic logical framework. While illustrating this approach a prototype model began to emerge and was given a working title – phots.
The prototype phots do not have a wavelength. In fact they have no length at all, in keeping with Special Relativity and the fact that they travel at the speed of light. They do however have an effective width that is more than enough to encompass both slits at once. They also have inherent energy, orientation, phase and spin and they deliver their energy in the form of sinusoidal electro-magnetic disturbances when they interact.
The protocol suggested for trying to come up with a unified model for light is to put aside misleading analogies, accumulate evidence from experiments, develop a logical framework, keep an open mind and let the evidence do most of the talking. In that spirit here are three conjectural explanations for the double slit experimental results that do not use the wave model, but use the prototype phot model instead.
Double Slit Conjecture #1: A Dielectric Fence
The idea here was suggested by observations of diffraction over an edge and through a single slit. Dark lines could be seen hovering near the edges and between the edges and these cast ‘shadows’ all the way to the observer’s eye. This gave rise to the idea that some effect or other was actually causing an electro-magnetic barrier to the progression of light. A diagram will illustrate the idea.
Double slit experiment explained without waves? Conjecture #1 The drawing shows the type of explanation that might be consistent with a unified model for light. Here the combined effect of light from the A and B slits is being streamed through some sort of ‘dielectric fence’ close to the sheet with the slits. Furthermore it is conjectured that the fence can be seen with the naked eye or camera and looks similar to the dark lines that appear above the blade in knife edge diffraction.
Phots encountering the barrier are forced to do a deflection or sidestep to get around them. If their path encounters a barrier head-on the sidestep is up to a quarter of a ‘Cperiod’ sideways and if the encounter is less severe the sidestep is smaller. A Cperiod is here defined as the distance light travels in one period of the disturbances that the phot creates on impact. (Readers will recognise this as having the same dimension as the wavelength used in the wave model, but ‘wavelength’ is a term being avoided here because of all the mental baggage it brings).
The conjecture is of course incomplete in that it does not explain exactly how the dielectric fence is created or how it works exactly. One suggestion notes that a lot of electro-magnetic disturbances are being delivered to the plate and these might influence the dielectric medium surrounding it. Perhaps they create a standing wave in the vicinity of the slits and this acts to add the fine detail fringe to the underlying one-slit phenomena.
A good thing about the suggestion is that it lends itself to experimental investigation. Here are some ideas:
- Record the screen patterns on film at set distances and calculate exactly where the dark lines appear to be emanating from. Use smoke to give a visual impression.
- Check where the fringe effect first starts to appear.
- Tilt and bend the plate containing the slits in various ways.
- Thin and thicken the plate to see if this has any effect.
- Investigate the effects (if any) of placing prisms of various materials between the slits on the immediate downstream side of the plate.
- Heat the various parts of the plate in different ways, while keeping the slit widths and separation constant, to check for any effects e.g. color changes.
- Note that in the intersecting edges experiment described in Essay 3.4 (van de Vusse, 2024) it was noticed that no visible light came through the area closest to the intersection of the blades. Does this dark patch have any relevance to the formation of the dark areas in the double slit experiment?
- In the double holes version of this experiment, is it possible to use side by side optical fibres instead of the two holes?
- Check what happens if a third slit is introduced.
Double Slit Conjecture #2: Pairs of Phots
When a phot encounters the slits its progression is slowed down drastically and it envelops the material between the slits in electro-magnetic energy. If another phot arrives within a few periods of this disturbance a child phot is created in both the top slit and the bottom slit. They are a pair. The two phots set off in the same direction at the same time and with the same phase.
When the pair of phots arrives at the screen they each start to interact with it. They deliver their energy in the form of sinusoidal electro-magnetic disturbances. This is where wavelike constructive and destructive interference occurs. If the packets of energy are separated in space and time by one period they do not interfere with each other. Both phots impact the screen. If separated in space and time by half a period they push each other aside and interact with the screen at locations further apart. This creates the dark stripes in the overall pattern. It is basically the same story as the wave model but occurs on a pair by pair basis.
Double slit experiment explained without waves? Conjecture #2.
Sequential images
1. Mono-chromatic phots approach double slits.
2. The first phot creates an electromagnetic disturbance between the slits.
3. The second phot triggers the creation of a pair of phots.
4. The pair of phots approaches the screen and each starts to interact with it.
5. If the space-time separation between the interactions is an integral multiple of the period, each phot interacts as normal.
6. If the space-time separation between the interactions is not an integral multiple of the period, each phot interaction is pushed aside until it is.
A good thing about the suggestion is that it lends itself to experimental investigation. Here are some ideas:
- Replace the screen with a very sensitive photo multiplier, use very dim light and see if the arriving light tends to occur in pairs.
- Try to disrupt the final pattern by interfering with pair formation at the slits. For example, use silvered glass for the plate and parallel scratches instead of slits.
Similar ideas have been proposed before. See for instance a paper put forward by Shan-Liang Liu in 2017 (Shang-liang Liu, 2017). The basic idea there is that in feeble light experiments the pattern is created in the detector screen itself. One photon energises the atoms in the screen (or photo-multiplier etc.) and before this turns into a detection event a photon arrives from the other slit and modifies the location of the overall combined event.
Double Slit Conjecture #3: Interfering Photlets
When a phot impacts on the divider between the two slits it disintegrates into two equal parts. Its double sine wave drivers become two single sine wave dividers. Each part keeps its phase. Each part keeps half the energy and half the spin of the parent phot. The new entities will be called photlets. (Perhaps we can think of them as entangled neutrinos?) They operate in tandem with their sibling photlet.
The photlets behave in a similar way to the wavelets described in the wave model explanation. One photlet radiates from the top slit towards the screen, the other from the bottom slit. The paths are parallel, or at least very similar.
The direction they both take together is influenced by where their parent phot intersected the slit divider. If it was off centre their path will be away from the axis of the experiment.
The first photlet to reach the screen starts the impact process. (In quantum terms it collapses the probability wave). The second photlet instantly joins in the impact effects from the first. The combined effect delivers the same energy as if the parent phot was impacting at that point.
Double slit experiment explained without waves? Conjecture #3
An incident phot divides into two ‘photlets’. The photlets travel a similar but not identical path to the screen. Path differences lead to phase differences. If the photlets arrive with an integral number of periods difference in phase they produced the effects of a single photon. If the photlets arrive with a fractional number of periods difference in phase they still produce the effects of a photon, but the location of the impact moves to one side or the other. This creates a dark band.
The location of the impact depends on the phase difference between the photlets. If the phase difference is small they combine to produce the effect of single phot at the point of impact. If the phase difference is larger they still combine to recreate the original phot, but the impact site is higher or lower on the screen (i.e. towards the centre or the outside of the pattern).
The explanation is as yet undecided about whether the photlets leave the slits simultaneously but arrive at the screen a short time apart, or alternatively leave the slits in their own good time but arrive simultaneously. Either way they have a slightly different world line and this accounts for the variabiliy of the location of their combined impact.
A variation on this conjecture theme is that the photlets recombine in flight but with a residual uncertainty about their exact line of travel. If they are in phase the path is straightforward. If they are out of phase their path flutters to one side or the other.
The above conjecture #3 is similar to the wave explanation, except it uses phots and photlets instead of waves. The photlets do not act singly but always together. (They do not have enough spin to interact with an electron by themselves.)
Most of the novelty in the conjecture comes from the idea that the incident phot can dissociate into two parts. This gives some detail to Dirac’s conclusion that two slit experiment shows that a photon can interfere with itself. It also explains why rapidly covering up one slit or the other can upset the results. Furthermore it resolves the complexities around experiments that try to work out which slit a particular photon has gone through. It suggests that if two slits are available then a phot can effectively go through both of them.
Discussion
The conjectures above are intended to provoke ideas outside the usual wave and particle paradigms. We know that light is just one thing, so one explanation should be able to explain all relevant experiments. Three suggestions have been provided above to show that it might be possible. Of course, at least two of the suggestions must be wrong. This is where the ‘phot protocol’ comes to the fore.
The phot protocol recommends using experimental evidence rather than prejudicial presumptions and inappropriate analogies. Keep an open mind about the possibilities and let Nature do the talking.
The next steps would be to keep on adding experimental evidence. Apart from the experiments suggested above, a good area to consider might be the class of experiments associated with the Mach-Zender interferometers.
The conjectures have been influenced by experiments using very low levels of light. However, the suggestion that photons can interfere with themselves does not exclude the possibility that they might also be able to interfere with each other. In fact, the experiments considered so far in this series of essays are starting to suggest some additional possible properties for phots. For example (on a tentative basis) …
- Monochromatic phots travelling close together in the same direction at the same time tend to link up into pairs. Their widths overlap and they adopt the same phase.
- Monochromatic child phots created close together in space and time tend to form a pair that sets off in the same direction.
- When monochromatic phots travelling close together in space and time encounter a screen they each create the usual effects but the exact location of these effects is affected by the phase difference between them. If the space-time difference is less than one period the disturbances push each other aside, thus creating the dark bands seen in the interference pattern.
If (1) is valid and pairs of phots can link up with other phots then over vast reaches of space it is conceivable that light from a small intense source of light could form long lines of phots linked side by side together. Similar to one dimensional ‘wavefronts’.
Summary
This essay is part of a series of essays suggesting it is time for a fresh attempt to develop a unified model of light, and recommending an approach based solely on experimental evidence. To illustrate the approach, the evidence from basic optical experiments was considered and a prototype model slowly began to take shape.
Young’s Double Slit interference experiment is so difficult to explain using a particle model for light that it was a key reason for adopting the wave-particle duality compromise over a century ago. A unified model for light will inevitably have to prove that it can satisfactorily explain the double slit results and others like them.
To illustrate the ‘phot protocol’ approach to developing a unified model, three conjectures were presented to suggest that it might be possible to develop an explanation of the double slit results without using wave or wavelets. Several experiments are suggested to test these conjectures further.
Although created for heuristic purposes the prototype phot model seems to be travelling reasonably well. Whether or not it is on the right track remains to be seen. The hope is that it provokes good questions, fresh thinking and new ideas.
References
Shan-Liang Liu, 2017, arXiv:1709.10344v3 [physics.gen-ph] 10 Nov 2017
Taylor, Geoffrey, I. 1909, Interference Fringes with Feeble Light, Prof. Cam. Phil. Soc. 15: 114.
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
