A preceding essay argued that it might be possible to build a single, simple, self-consistent model for light and it would be timely to do so. As a provocation it makes a start on constructing a bottom up approach it calls ‘phots’. This essay continues this work by collecting more key experimental evidence and using it to build a framework for a unified model of light. A start is made on checking whether such a model might be able to explain the major types of optical experiments involving diffraction and interference effects.
Introduction
Using two mutually inconsistent models for light is unsatisfactory. Wave-particle duality was a compromise over a hundred years ago, but it was not meant to be permanent. An earlier essay in this series argued that a single consistent model for light might be possible. It recommended a bottom up approach that avoids inappropriate analogies and just follows the evidence from experiments until a new model starts to take shape. To illustrate the idea it made a start on developing a simple prototype and began incorporating evidence from basic but fundamental optical experiments. This essay continues that process, focusing on some basic but fundamental experiments involving diffraction and interference.
Diffraction
Diffraction is the name given to a variety of path bending and interference pattern effects when light passes close to the edge of a solid object, or through a screen with one or more holes or slits, or is reflected from a solid with many small lines, ledges or pits. Diffraction effects can resemble more general interference effects so categorising the effects can be a bit confused, but this does not matter. The point is that any self consistent understanding of light has to come to terms with a whole set of refraction, diffraction and interference effects.
There are countless experiments and variations on these experiments involving diffraction and interference effects using various parts of the electromagnetic spectrum, and there are equivalent experiments using electrons and other small particles instead of light. This essay will add flesh to the phot framework using the results from experiments involving diffraction over an edge and through a slit.
Arago’s Spot (also known as a Poisson spot)
Isaac Newton (1642-1727) favored a corpuscular concept for light, partly because a ray of light does not spread out much as it travels. Christiaan Huygens (1629-1695) was able to explain some key optical phenomena using the idea that light had a wavefront and developed a whole set of secondary wavelets during interactions. However, the corpuscular model predominated for about a hundred years. In 1803 Thomas Young presented his work on diffraction through parallel slits to the Royal Society and this brought the wave model back into contention.
Pierre Laplace (1749-1827) and Siméon Poisson (1781-1840) were both supporters of the particle model. Poisson argued that if light were a wave it would bend around solid objects, like the sound of a bell around an intervening building, and this was not observed.
Poisson and Francois Arago (1786-1853) were both members of the French Academy of Science. Arago decided to check Poisson’s argument experimentally. In 1818 Arago used a small intense source of light and a small round smooth solid object and examined the object’s shadow. To general surprise it was observed that there was a bright spot in the middle of the shadow. This gave a boost to the wave model.
Arago influenced Augustin-Jean Fresnel (1788-1827) who conducted numerous experiments and developed an advanced mathematical description based on the Huygens wave model. This was successful in predicting and explaining many of the effects observed. The wave model and mathematics continued to reign supreme, especially when overlaid by the electro-magnetism relationships identified by James Clerk Maxwell (1831-1879).
Poisson spot (aka Arago Spot). Modern version. A point like source of monochromatic light (e.g. red light from a Neon-Helium laser) spreads out enough to shine on and around a small solid object such as a coin or ball bearing and casts a shadow on a screen several metres further away. In the right conditions a small bright spot appears in the middle of the shadow.
Note that if you place your eye at the location of Arago’s spot and look back along the experiment, you would not expect to see a spot of light on its way to meet you. What you should see is a ring of light coming from the periphery of the round object placed in the path of the incident light. (Do not do this if using a laser! It could damage your eyes. Use a camera instead).
Wave Model Explanation: The source presents a spherical wavefront to the object and at that point can be considered to be made up of innumerable small wavelets with the same wavelength. These superimpose as they proceed with constructive and destructive interference between them. The light intensity at any point on the screen comes from the summation of all the wavelets arriving at that point at the same time. They all interact constructively right in the middle and that produces the bright spot.
Note that the explanation does not consider polarisation, let alone spin, and does not explain why exactly the wavelets take the form required to produce the answer.
Particle Model Explanation: The 19th century particle model found it difficult to explain this effect. The modern photon model and quantum mechanics resorts to modelling the experiment as a superposition of innumerable possible outcomes which resolves itself on a photon by photon basis when each photon interacts with the screen.
Phot model interim suggestion: There are two effects to explain – light bending and light producing a bright ‘splash’ in some places but not others. The bending downwards is reminiscent of refraction and could simply be that be the side of the phots closest to the object slows down, thus slewing the path of the phot. Note that the edges used are invariably thin and smooth. Hence the bottom of the phot has a good chance of re-emerging. The symmetry of the setup suggests a natural focus point for such phots, somewhere along the axis of the experiment. In keeping with the protocol of the phot approach more evidence will be gathered before indulging in speculation.
Oil Film Interference Pattern
The diagrams above show the wave model explanation for light and dark bands seen when monochromatic light reflects from the top and bottom surface of a thin film, such as oil on water. On the left two waves are combining constructively to produce a bright band. On the right they are combining destructively, producing a dark band. It is fair to ask – if the combined wave on the left has higher energy, why does it still have the same frequency? And on the right hand side – where did the energy go?
Suggested Explanation in terms of Phots: Experiments of the sort described above usually still display the same effects when the intensity of the incident light is turned lower and lower. So low that the light can be thought of as a stream of single photons. Hence the phot model is looking for an explanation that works phot by phot. (In general, wave model explanations struggle at very low intensities.)
This thin film interference pattern only involves two surfaces. Phots arrive with a small but significant range of angles of incidence. The observer is on the right. Here the observer is conceived as a photon multiplier/counter that can be moved up and down to scan what is happening. It observes a pattern of light and dark fringes as it moves higher, corresponding to steeper angles at which the reflected phots reach it.
Thin Film Interference: The phot on the left is absorbed or transmitted. The phot on the right is incident at a slightly smaller/steeper angle and reaches the observer. ∆d is the difference in path length. W is the effective width of the phot. The diagram is just a sample of the overall pattern.
For the experiment to work the thickness of the film has to be comparable to W, the ‘effective width’ of the phot. As a phot comes close to the oil film it engages the atoms in the oil film in a zone several W wide around ‘ground zero’. The depth of this impact zone extends to the lower surface of the oil film. The parent phot delivers its composite package of sinusoidal electro-magnetic energy, momentum and spin. What happens next is that either the phot is absorbed, or a child phot is born and reflected upwards. The outcomes are about equally likely. If a child phot is born it will be a reflected phot with the same energy level as its parent.
The difference in this situation from a normal refraction or absorption event is the role played by the second surface. It sets up the other end of an ‘interference path’. If the length of the interference path is approximately equal to W (or a multiple of W) then the incident phot will be absorbed. If the length of the interference path is approximately equal to an odd multiple of W/2 a child phot is created at the surface and reflected outwards. (Actually, this also depends on what the thin film is resting on. If it resting on a medium with a lower refractive index then the pattern of light and dark is reversed. As discussed previously (for total internal reflections) ‘reflections’ from interfaces between a medium and another medium with a lower refractive index are not mirror images and are better thought of as severe path bending.)
The diagram shows a sample of two phot paths. The phot on the left is being absorbed, the phot on the right is reaching the observer. But what is going on where the diagram shows the dotted lines? What is actually causing the interference?
If there are innumerable phots in the incident beam, we might imagine that they are interfering with each other. We might even imagine a pattern of electromagnetic resonances being set up in the denser medium that then affects the fate of incoming phots. But we want an explanation that can work at the level of a single phot. We’ve ruled out splitting up incoming phots. The evidence from any phots scattered from within the medium is that they are the same colour/frequency/energy level as the incoming phots. So we have a bit of a mystery.
It is a major piece of evidence that we need to interpret correctly. Furthermore the same issue crops up in numerous diffraction and interference experiments and phenomena. So it is a key to unlocking the nature of light without resorting to wave-particle duality.
The following answer may have merit or it might not. Its purpose is to give an example of the sort of ideas that need to be considered.
Ghost Waves (see also Pilot Waves): The evidence is calling for a new feature to be recognised. There seems to be a candidate that has been around almost as long as wave-particle duality has existed. Something called a de Broglie pilot wave.
Rather than adopt de Broglie’s ideas in full at this stage, this essay will give the concept a working tile – ‘ghostwaves’ (Einstein used the same term at the 1927 Solvay conference, but what he meant by this not clear to me). A ghostwave may turn out to be a de Broglie wave. But I am going to call it a ghostwave for now to avoid prejudging the outcome.
Quotes from Wikipedia: “De Broglie presented the pilot wave theory at the 1927 Solvay Conference. However, Wolfgang Pauli raised an objection to it at the conference, saying that it did not deal properly with the case of inelastic scattering. De Broglie was not able to find a response to this objection, and he and Bohm abandoned the pilot-wave approach.”
“In 1952, David Bohm, dissatisfied with the prevailing orthodoxy, rediscovered de Broglie’s pilot wave theory. Bohm developed pilot wave theory into what is now called the de Broglie–Bohm theory. The de Broglie–Bohm theory itself might have gone unnoticed by most physicists if it had not been championed by John Bell, who also countered the objections to it. In 1987, John Bell rediscovered Grete Hermann’s work, and thus showed the physics community that Pauli’s and von Neumann’s objections “only” showed that the pilot wave theory did not have locality.”
“Lucien Hardy and John Stewart Bell have emphasized that in the de Broglie–Bohm picture of quantum mechanics there can exist empty waves, represented by wave functions propagating in space and time but not carrying energy or momentum, and not associated with a particle. The same concept was called ghost waves (or “Gespensterfelder”, ghost fields) by Albert Einstein. The empty wave function notion has been discussed controversially.”
“An alternative to the standard understanding of quantum mechanics, the De Broglie–Bohm theory states that particles have precise locations at all times, and that their velocities are influenced by the wave-function. So while a single particle will travel through one particular slit in the double-slit experiment, the so-called “pilot wave” that influences it will travel through both. … The de Broglie-Bohm theory produces the same statistical results as standard quantum mechanics, but dispenses with many of its conceptual difficulties.”
Note the word ‘locality’ in the quote above. Locality is a principle in physics that there is no action at a distance, and that objects can only be influenced by their immediate surroundings and things within their immediate surroundings. For an action at one point to have an influence at another point, something has to convey the effects. That ‘something’ could be an impulse or wave in some sort of medium in which they are both immersed, or it could be some sort of messenger or particle that carries the influence from one object to the other. Locality evolved out of the field theories of classical physics.
While open to the idea of pilot waves the phot model will start by assuming locality is valid (because it is common sense). Another essay in this series suggests that optical Bell experiments have not demonstrated faster than light action-at-a-distance is real. Action at the speed of light is however another matter entirely and is entirely plausible.
In the thin film interference effect described above, a potential plain-English explanation goes as follows. When a phot strikes the top surface it undergoes a bit of trauma. It suddenly slows down. Its package of energy and spin unravels. This process sends a short ghostwave into the medium. It is a wavelike ripple in three dimensions and it spreads out as it propagates, which means it dilutes and diminishes quite quickly. It is not a ripple in the atoms and electrons of the medium – that is a job for the electromagnetic disturbances carried in the phot’s package. Rather it is a ripple in the medium between the atoms and electrons. A bit like a gravitational wave perhaps. More about this later.
The ghostwave travels into the medium at the speed of light and is reflected back from the bottom interface. But so does the child phot. They travel together. Same speed, location and direction. The ghostwave carries no energy. It is an ‘empty wave.’ It is just a pattern in the aether. But it does have the ability to influence the future manifestation of the child phot.
As for Pauli’s objection to de Broglie’s idea, it is possible that this fits into the same category as an objection raised against Galileo’s ideas by the wise men of the day. They told him the Earth could not possible be spinning on a daily basis because “the birds would not be able to keep up”. It was a good point, but capable of resolution. Perhaps Pauli and de Broglie are both right. If a phot interacts in one way it can produce ghostwaves. If it interacts in another way it can undergo Compton scattering.
Newton’s rings
The experimental setup: a convex lens is placed on top of a flat surface and is illuminated from above by 650 nm red laser light pointing straight downwards. Here the observation is made from above using a low light microscope. The small amounts of light reflected from the top and bottom surfaces just add to the general glare and are not important. The centre is dark because the light reflected from the curved interface does not have a phase change, and it combines destructively with light reflected from the bottom surface, which does have has a 1800 phase change.
Quote from Wikipedia: “The pattern is created by placing a very slightly convex curved glass on an optical flat glass. The two pieces of glass make contact only at the center, at other points there is a slight air gap between the two surfaces, increasing with radial distance from the center to the microscope” … “Light from a monochromatic (single color) source shines through the top piece and reflects from both the bottom surface of the top piece and the top surface of the optical flat, and the two reflected rays combine and superpose. However the ray reflecting off the bottom surface travels a longer path. The additional path length is equal to twice the gap between the surfaces. In addition the ray reflecting off the bottom piece of glass undergoes a 180° phase reversal, while the internal reflection of the other ray from the underside of the top glass causes no phase reversal. The brightness of the reflected light depends on the difference in the path length of the two rays
Constructive interference (a): In areas where the path length difference between the two rays is equal to an odd multiple of half a wavelength (λ/2) of the light waves, the reflected waves will be in phase, so the “troughs” and “peaks” of the waves coincide. Therefore, the waves will reinforce (add) and the resulting reflected light intensity will be greater. As a result, a bright area will be observed there.
Destructive interference (b): At other locations, where the path length difference is equal to an even multiple of a half-wavelength, the reflected waves will be 180° out of phase, so a “trough” of one wave coincides with a “peak” of the other wave. Therefore, the waves will cancel (subtract) and the resulting light intensity will be weaker or zero. As a result, a dark area will be observed there. Because of the 180° phase reversal due to reflection of the bottom ray, the centre where the two pieces touch is dark.”
This explanation is straightforward and in accord with standard textbooks. But one can still ask questions. The explanation assumes that the light arrives as convenient pairs of rays with exactly the same path and the same initial phase as each other. How likely is that? Furthermore, in all the constructive and destructive interference, what is happening to the energy? If waves combine constructively as described above, does this not imply a higher level of energy and thus higher frequency and hence a colour change? And does not the wave model require the wavelength within the glass to be shorter by a factor n, where n is the refractive index of glass. So what does that do to the explanation? And why so much reflection from surface 3? If 96% of the incident light penetrates surface 1, why expect anything less from surface 3? Second last question. What is going on in regards to polarisation? Constructive and destructive interference between two waves as described only happens when they have the same orientation. Last question – what is the wave model explanation if the incident light intensity is so low that it can be thought of as a stream of individual photons?
The conventional answer to the question about where does the energy goes seems to be that the light intensity in the bright bands is the same as that of the incident beam and that the energy in the dark bands goes right through the apparatus and can be observed from the opposite side as a complementary set of rings. In practice a bit less energy to allow for some absorption and scattering. But this is a bit puzzling. If a ray is used to cancel another ray on the top side of the apparatus, how can it be discovered to be leaving the scene via the underside of the experiment?
Suggested Explanation of Newton’s Rings in terms of Phots: In the phot model, numerous phots descend vertically upon the surface 1 and most are reborn as child phots travelling down to surface 2. The rest are scattered or absorbed.
The process creating Newton’s rings occurs at surface 2 (the curved surface in the setup shown in the diagram). This is where the dense medium meets the less dense medium. Child phots are created which either proceed into the gap and on to surface 3, or they go back the way their parents came from and end up as Newtons’ rings. The phenomenon is driven by the closeness of surface 3. Take surface 3 away and the whole effect vanishes.
Something strange happens in the zone around surfaces 2 and 3. Something makes some phots go back the way they came from to form the bright pattern of rings. The remaining phots pass through the apparatus and form a ‘negative image’ of Newton’s rings when observed from the opposite side. In other words, if you photograph the underside of the experiment you can see the bright rings replaced by dark rings and the dark rings replaced by bright rings.
What seems to be happening is total internal reflection of some phots at surface 2, in a pattern dictated by the closeness of surface 3. Denote the gap distance by D. In the wave model explanation a bright ring is said to appear whenever 2D is an odd multiple of L/2, where L is the ‘wavelength’ of the light. (But is this the wavelength of light in the glass or the wavelength of light in the gap?).
Phot model possible explanation (i) is that at certain distances D, numerous phots are trapped into resonances between surfaces 2 and 3. The geometry of the experiment means that this occurs in rings around the point of contact between the two glass plates. The resonances change the interface and block the onward progress of bout half of the phots following behind. Such phots are reflected upwards and become the bright rings that give the experiment its name.
Phot model possible explanation (ii) is the formation of phots at surface 2 is affected by ghostwaves reflected from surface 3. As with thin film interference, the ghostwaves determine whether the phot will go back the way it came from or proceed into the air gap. It is an interference effect with a key difference – it occurs between a phot and its own ghostwave. Whenever the back and forth distance between surface 2 and surface 3 is odd multiple of W/2 the phots will be influenced to go back the way they came. The overall process is similar to thin film interference, except now the ghostwave travels back and forth in the thin air gap.
Both phot explanations have some elementary plausibility. The suggested protocol in such cases is to avoid jumping to conclusions and to wait for a fuller picture to emerge from other experiments.
Summary
This essay is part of a set of essays that challenge the attitude that the inherently inconsistent wave-particle model for light is just a fundamental mystery that we have to put up with. It suggests abandoning inadequate, old fashioned and misleading analogies and using a fresh, open-minded approach that creates a framework for accumulating experimental evidence until the overall description of light becomes clear. As a provocative straw man it makes a start on developing such an approach and gave it the working title – Phots.
This essay looks at some key experiments involving interference effects. The conventional explanations are found wanting, quite apart from the fact that they contradict each other. The naive straw man fares rather better, at least intuitively.
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
