Photon Physics and the Classical Ontology Paul A. Klevgard, Ph.D. pklevgard@gmail.com Sandia National Laboratory, Ret. 1.0 The Classical Ontology – We live in a material world and physicists, like all of us, model reality accordingly. A particle is a material model as is a wave hosted (or not) by a medium. Both have been used as models for radiation. The problem is that together they are incompatible and apart each explains some features of radiation but not others. Niels Bohr tried to bridge the compatibility gap – discrete particle vs. continuous wave – by accepting both as consequences of an unknowable reality based on opposites. Clearly this was to make a defect into a virtue or in software engineering parlance, declaring a bug to be a feature. Material models are based upon the classical ontology founded on existence, mass and particle space motion. Trying to model radiation and the photon based upon these has been a failure.1 Existence: Radiation is not something existing (persisting) over time; rather it is occurring in time; radiation's essence is oscillation frequency in time. Matter: In-flight radiation is completely independent of anything material. Photon kinetic energy (KE), not rest mass, explains photon momentum; termination at a point can be explained without invoking the particle impact analogy. Space motion: Radiation's utilization of space is unique; it differs from how matter-inmotion utilizes space. We know this because a photon has no defined trajectory; it takes all available paths to a target. The classical ontology of entities based on existence, matter (particles) and space motion (trajectory) is still very useful to us, but not in a naïve way of porting its constituents directly into radiation. A better approach is to take quantized entities that exist (mass/particle) 1 Existence and occurrence are used in these pages strictly within the confines of physics. They encompass physical reality and are the two co-equal divisions within traditional ontology as expanded for physics. Entities either exist or occur and the more familiar one, of course, is matter existing in space and persisting over time. Might there be an occurrence counterpart we have missed? 2 plus entities that occur (radiation/photon) and compare them at a higher (ontological) level for their commonalities. Radiation is the transfer of kinetic energy (KE) from one location (photon origin) to another (photon reception). It is generally assumed that the photon is an entity as is the material particle. Thinking of "entity" as a generic concept, the difference between quantized mass and energy entities is essentially a consequence of inversion or switching of: 1) categories (existence vs. occurrence) and 2) dimensions (space vs. time). Among the common features between existing and occurring entities are: quantization in one dimension or the other; and storage by E = mc2. 2.0 Time, the Photon and Kinetic Energy – The photon has kinetic energy (KE) with no rest mass; its logical compare is (inertial) rest mass with no KE. The justification here is that this makes both entities stationary in a dimension. Inertial rest mass is stationary in space by definition. And the photon is stationary in time because anything at the speed of light incurs infinite time dilation. A requirement that entities be stationary in one dimension excludes moving electrons from consideration here. Their duality differs from that of photons in that it involves a real, rest mass particle which is not the case for the photon. Both mass and energy have their conservation laws. A rest mass particle obeys the law of the conservation of mass as it progresses in time. Successive observers in time experience the same particle with the same mass; they have the existing particle and its mass in common. We explain this by saying that time progression does not affect existing particles; that time progression is orthogonal to existing objects in space. Photon kinetic energy (KE) too has its conservation law. Photon energy must be constant as it progresses in space within an inertial system. Assume we have a single photon from a laser moving on a line (ideal case) through space. This photon presents the exact same KE to all space observers along its path were they to measure it. Now imagine this linear-path photon encountering and passing through a double slit. The photon interferes with itself and now has multiple paths toward termination on some target screen. Observers situated on these multiple, diffracted paths have the same (possible) experience as the observers on the linear path: if the photon terminates on their path it will have the undiminished KE from photon 3 origin. All of these paths have the photon KE in common although the diffracted paths differ in probability of photon reception. This behavior of the single photon tells us that space-residing, material devices (slits, pinholes) cannot affect (fractionate) photon KE but they can fractionate probability of photon reception. This means that the photon has two identities or facets: 1. A potential (probabilistic) identity that has a presence in space and so will fractionate there. 2. A KE identity not divisible in space and common to (available for) all possible space path observers. Neither simple progressing in space (linear path in a vacuum or in a medium) nor progressing-diffracting in space (multi-path) has any effect upon photon KE. Photon KE is therefore orthogonal to space and what happens there, namely diffraction and velocity diminution (e.g., in glass). And something is orthogonal to a dimension when it does not reside in that dimension. We conclude that photon KE does not reside in space where it might dissipate or rarefy. Photon KE resides in time! 2 3.0 Entity Identities – Any entity must: 1) be quantized; 2) reside in a dimension; and 3) store its opposite via E = mc2. We take these requirements in order. First, quantization for an entity is a given; entities are discrete, not continuous. Second, to reside in a dimension means to occupy an interval (or volume) in that dimension; this ensures that the entity is "physically real" (a dimensional presence). Third, an entity storing its opposite via E = mc2 limits our real entities to mass and energy. The rest mass particle is an entity: it is quantized (space discrete); it resides in (occupies) space; and it stores its opposite, namely potential energy (e.g., thermal, intrinsic). By these criteria, photon KE is also an entity. It is quantized (time discrete oscillation cycles); it resides in time as we have seen; and it stores its opposite, namely potential (relativistic) mass. 2 Locating photon KE in time allows it to occur/oscillate as we shall see. Photon KE as a mere quantity with no hosting dimension doesn't allow that. 4 Since entities store their opposite, they have two facets/identities, namely a kinetic (unstored) identity and a potential (stored) identity. The kinetic identity always resides in one dimension: matter occupies space and photon KE occupies time. What each of them stores will progress in the alternate dimension toward release. Thermal energy stored by matter will, during time progression, be released at some point as radiation (emission). Similarly, photon KE as a radiation entity stores potential (relativistic) mass; at some point during space progression that stored mass is released to a target at photon reception. Nature is even-handed between existence and occurrence, matter and radiation.  Quantized matter is an entity: its kinetic identity is mass; its potential identity is stored (thermal, intrinsic) energy.  Quantized radiation is an entity: its kinetic identity is KE; its potential identity is stored (relativistic) mass. While entities have two identities, their kinetic identity is primary. Hence it is convenient to refer to the whole entity just by its kinetic identity. So "rest mass" is shorthand for the material entity and "photon KE" for the radiation entity. Physicists mostly study rest mass in motion where they can ignore the (potential) energy the mass stores. But if physicists only studied transitory (inertial) particles stationary in space they would not ignore what those particles store and release over time. So it is with photons; we only study them (can only study them) as stationary in time so we cannot ignore what they store and release over space. 3 What is stored by an entity has its own identity. A particle's rest mass is not the same as the particle's intrinsic energy even though the two are related by E = mc2. A photon's KE is not the same as that photon's stored, relativistic mass. An entity's kinetic (unstored) identity resides (occupies an interval) in a dimension. That entity's potential (stored) identity progresses toward release in the opposite dimension. Stationary entities (inertial rest mass, the photon) make use of space and time in a way that differs for matter-in-motion. The photon can have incompatible attributes because it has two identities operating in different dimensions. The photon's KE identity resides in time and its oscillation cycles occupy time intervals there. The photon's alternate (potential, probabilistic) identity progresses in 3 Release of photon stored mass eliminates the photon. For a transient particle, release of its stored, intrinsic energy eliminates (annihilates) the particle. 5 space as we shall see. The photon is not a simple object; dualism of photon attributes relies upon a dualism of photon identities.4  The photon entity has it kinetic (unstored) identity, namely KE, residingoccurring in time; its alternate (latent-stored) identity progresses/occurs in space. 4.0 First Identity: Photon Kinetic Energy in Time – When a charge is accelerated the photon KE created must reside somewhere so it can be available at photon termination. Attaching the KE as quantity to a massless, multi-path photon is to fall back into modeling based on existence, matter and space. The photon is an (energy) occurrence; it is not an "object" with a defined space presence. Work done on a charge creates something without rest mass; it creates photon KE as an entity in time whose oscillation does not involve anything material residing in space. Photon KE then is pure oscillation residing in time; such pure occurrence is not unknown in physics.5 Because photon KE resides in time, its cycles cannot be interrupted by any spacedependent intervention or instrument we devise. Material objects will only receive whole action quanta although the time rate of delivery (energy) varies with cycle frequency. Hence photon KE is always quantized for amount received and you can never receive half a cycle. The classical ontology (existence, matter, space) imagines that some object (particle, virtual or real) carries KE as a payload along space paths. This is the only mode of transport the classical ontology permits: some entity/object residing in one dimension, namely space, is transported in the same dimension. Thus you may transport your space-residing chair from its current space position (your den) to its new space position (your living room), a transport with a defined space trajectory. This model works fine for space-residing chairs moving in space; it does not work for the photon KE which does not reside in space and hence can have no defined trajectory there as we shall see. The photon two identities function in different dimensions. The photon's first (kinetic) identity resides in, is hosted by, time; its second, conjoined (potential) identity progresses in 4 Our instruments respond to photon KE and we can infer photon probabilistic space paths. These two sides or facets of the photon are – for lack of a better word – hereafter called "identities." 5 Physicists have embraced the concept of oscillation of "nothing" or oscillation from "nothing"; vacuum state fluctuations in QFT are just that. 6 space. As a pure oscillatory occurrence, photon KE is orthogonal to space where its potential identity progresses on paths. Think of it in a Cartesian analogy; pure oscillation on the vertical (time) axis extends upward without limit. On the horizontal (space) axis there is a dependent probability wave whose frequency happens to match the oscillation on the vertical axis. The two axes are orthogonal and cannot intersect since one is behind the other. Being orthogonal makes photon KE common to (shared by) all possible observers on said diverging space paths. Photon KE is not transported through space from one location (origin) to another (termination) because it does not reside there. By residing in time, photon KE is not involved in space progression at the speed of light; that is left to the photon's potential identity (next section). Conceptualizing: Dimensions having orthogonal functionally (residing vs. progressing) and entities being common to observers separated in a dimension are difficult concepts. If they were easy the photon would have been deciphered long ago. These concepts are a little less difficult for stationary rest mass objects. If we reflect upon the stationary objects around us we realize that their persistence (progression) in time obviates their being transported in time. The chair you are sitting in is simply present as the same chair to all subsequent observers who see it or use it; it is common to you and to them. An existing chair in space does not require relocation (time transport) for time-separated observers. This key concept of no transport and being common applies to photon KE with space and time, mass and KE reversed. For the single photon there may be multiple observers (detectors) at different space locations on the many paths that the photon potential identity follows. Each observer hopes to receive the photon's kinetic identity (time-residing KE). Each space-separated observer has the possibility of receiving the same (shared, common) occurring photon KE residing in time. Because of this sharing, the occurring photon KE in time does not require relocation (space transport) for space-separated observers.6 6 A note on orthogonal dimensions: Physics in general, classical and modern, focuses on mechanics and so is even-handed regarding space and time: the moving particle changes positions in both. But if we remove the particle's space progression (make it stationary) and look at its persistence in time, then things appear different. Space and time are now orthogonal since space contains (hosts) the particle while time renders that particle shared and always available to observers at different time locations. This change from moving to being stationary removes KE but it alters neither space nor particle rest mass. It does eliminate one progression (in space) and brings to the fore a persistence and availability in time that was always present. This change yields a different physics (different from mechanics) because the two dimensions are now orthogonal, having dissimilar functionality: entity hosting vs. entity sharing-commonality. This physics also applies to radiation but that has been obscured by our attempts to apply the classical ontology to radiation. When a photon is created its KE resides in time where it is stationary due to infinite time dilation; meanwhile different observers (on available paths) in the orthogonal (space) dimension all share the very same photon KE although only one can actually receive it. 7 Summing up: Matter resides in space; radiation (photon KE) resides in time. Each has a presence/availability in the alternate dimension: time for matter (where we call that availability "persistence"); space for radiation (no word for "space availability"). This alternate dimension is where an entity will be shared by (common to) observers in that alternate dimension: photon KE shared by all observers on available space paths; rest mass shared by all observers on time paths. It is in that alternate dimension where an entity's potential identity will progress to determine the probabilistic release of what is stored (next section). Photon KE is but one identity of what we call the photon. The second identity must provide for the: 1) probabilistic nature of photon reception; 2) spreading and progressing (rapidly!) on all available space paths; and 3) collapse of what is on those space paths. 5.0 Second Identity: Photon Potential (Relativistic) Mass – If a single photon of known wavelength enters a double slit one can calculate probable reception locations on a target screen. The mathematics tells us that something travels those photon space paths between double slit and target; something immaterial that is latent and hence probabilistic. And what is on those widespread photon paths must disappear without a trace when photon KE is received at a point. This dependency of probabilistic, latent space path content upon time-residing photon KE tells us that E = mc2 storage must be involved. We conclude that the photon's second identity must be its stored, potential (relativistic) mass.7  Potential (relativistic) mass is the photon's second identity; it progresses along all available space paths as an occurring waveform.  Like the at-rest particle, the photon's identities reside in one dimension and progress in the alternate dimension. 7 Potential (relativistic) mass is a bit controversial these days; even Einstein spoke against it. The dispute is less about physics and more about semantics and pedagogy ("don't confuse students!"). Some argue that relativistic mass is the same as (kinetic) energy and so we should employ the latter and discard the former. What they mean is that relativistic mass depends upon KE; however, that does not make them the same. The photon has both and they may cause different effects: in the Compton Effect the deflected (re-emitted?) photon has reduced frequency (energy change); it also has a new direction (momentum change). Stored mass, potential mass and relativistic mass all refer to the same thing: the mass a photon has. Photon potential/relativistic mass is measurable as a quantity; what is new here is its progressing, waveform space presence. It occurs as a consequence of photon KE. It does not exist! Only rest mass exists. 8 So the photon is a single entity but with two identities, kinetic and potential. Photon KE is the kinetic (unstored), time-residing identity. It is common for observers on space paths. A photon's stored (relativistic) mass is the potential, space-progressing identity. It has the waveform, will diffract and therefore vary in intensity for individual space paths. By residing in time, photon KE has no role to play in space transport. Photon KE employs its potential identity to execute speed-of-light, probabilistic space progression. When our Sun sends us a photon, it is photon potential (relativistic) mass – not photon KE – that space progresses for eight minutes to reach us. Stripping photon KE of its space transport function is very (extremely!) difficult for us; we are so immersed in the classical ontology (existence, matter, space) where KE is the payload of a space-navigating particle. But that view just leads to paradoxes. Entities either exist or occur as a whole: the host (kinetic) identity and the stored (potential) identity both either exist (particle) or occur (photon KE). This means the occurrenceoscillation of photon KE is shared with the mass it stores. Hence photon potential (relativistic) mass progresses and oscillates giving it the waveform. Its space progression and wave nature means it diffracts into multiple paths at pinholes or slits. Photon potential (relativistic) mass has a space presence but only as something latent (stored) that occurs. It only interacts with itself yielding all the usual wave behavior: interference, superposition, reinforcement and diffraction. But our knowledge of its behavior is indirect since our physical instruments only receive photon energy and momentum. Because of this, various features of photon reception are not well understood. They include: 1) the objective reality of probability; 2) randomness; and 3) instantaneous collapse. 5.1 Objective Probability in Space – We can model photon wave progression mathematically and we recognize that our computed waveforms only govern probable (potential) photon reception. Nevertheless, we don't make the connection between photon probability and photon stored (relativistic) mass. This despite the fact that release of something stored is generally probabilistic (e.g., radioactive energy release). For many, a more attractive explanation for photon probability is to invoke 9 something abstract and mathematical that was designed for the electron: the wave function in multi-dimensional configuration space.8 Potential (relativistic) mass, when not discounted (i.e., rejected), is only seen as a mere quantity. It is never granted a space presence such that it can interact with slits and pinholes. We have noted that kinetic energy was identified in the mid-nineteenth century and made into a quantity possessed by matter-in-motion. At the turn of the twentieth century physicists were grappling with another problem: the increase of effective mass with velocity. Once more they opted for a quantitative formulation. So both KE and relativistic mass got incorporated into physics as simple energy/mass quantities and denied a dimensional presence. All of this is understandable given the knowledge of the day, but we have paid a price for those choices; and we keep paying. Photon potential (relativistic) mass is the E = mc2 consequence of photon KE. They are both mass-free, oscillatory occurrences, but photon KE is stationary in the time dimension where it resides and oscillates. In contrast, orthogonal photon relativistic mass must progress in the space dimension. With no rest mass involved and all the photon's KE residing in time, photon potential (relativistic) mass is a collapsible wave of probable photon reception (of photon KE transfer). Nature gives us only two types of entities: quantized matter (particle) and quantized radiation (photon KE). Each store their opposite by virtue of E = mc2. That which is stored (thermal energy or relativistic mass): 1) depends upon, is hosted by, the entity's kinetic identity; 2) is probabilistic for release in its progression dimension which alters (or eliminates) its host; The potential (relativistic) mass wave of a single photon interferes with itself when passing through a pinhole or double slit. The result on a target screen is regions whose intensity may be high (wave crest reinforcement) or low (wave crest cancellation). The intensity distribution on the screen determines the probability of photon KE reception, but only in the aggregate. Individual reception is random. 5.2 Photon KE Reception as Random – Photon KE can only terminate (impinge) on matter and at a point in space and time (absorption). Photon KE resides in time and to terminate on space-residing matter it must bring 8 Applying the wave function to radiation is not correct. You can't write a Schrödinger equation for a massless "particle." 10 about a cross (an orthogonal intersection) of occurring KE in time with existing rest mass in space. In a word it becomes an event: 1) something involving both KE and rest mass; 2) something that is discrete in two dimensions because the two entities involved reside in opposing dimensions. Photon reception on matter is the intersection of ontologically different entities (existing vs. occurring). But we don't see that and insist that KE is a quantity carried by a particle. Alas, Nature's ontological subtlety eludes us and we keep trying to make photon KE into the payload of a particle that "impacts" matter; this only leads to paradoxes. That which controls the tendency (probability) of photon KE termination on matter is photon potential (relativistic) mass which is continuous as a waveform progressing in space. Meanwhile, the actual transfer of quantized time-residing KE to quantized space-residing matter is discrete and event based; it cannot be continuous. So it is that photon termination is individually random but in aggregate it reflects the distributed intensities of photon potential (relativistic) mass. Whenever something is stored – potential energy (thermal, radioactive) in matter, or potential (relativistic) mass in radiation – there is always an associated probabilistic rate for aggregate release. But this aggregate release rate is combined with randomness for individual release: random in time for thermal emission; random in space for photon absorption. Photon emission from a warm body is discrete/random but it is correlated to that body's (bulk, continuous) property, temperature. Photon absorption is discrete/random but it is correlated to the local (continuous) wave intensity of photon potential (relativistic) mass.9 5.3 Collapse – Collapse of radiation's probable space paths at photon reception was something that bothered Einstein deeply; he regarded it as what we now call nonlocality. Ironically, what collapses is something he disowned: relativistic mass. Total Collapse due to Photon Termination: Photon potential (relativistic) mass waves progressing over space paths have but a single (common) source of occurrence and the latter is orthogonal to said waves. That is, the photon's potential (space wave) identity is an expression of the photon's orthogonal kinetic (time oscillation) identity. Photon potential (relativistic) mass waves progressing in space depend upon this single time source for their entire occurrence, whether that occurrence is for oscillation/progression or for cessation of oscillation. When a 9 The general argument in these pages is that entities of matter (mass) and radiation (KE) will be parallel. Photon stored mass is to photon absorption (reception) as a body's stored energy (heat) is to photon emission. 11 photon terminates, photon KE in time ends that oscillation upon which all space-progressing potential (relativistic) mass depends. The cessation and disappearance of dispersed spaceprogressing waves is instantaneous and nonlocal because: 1) these waves have a single, orthogonal point of failure; and 2) these are waves of occurrence and have neither rest mass nor energy. Partial Collapse due to Path Blockage: A single photon may have multiple paths some of which are blocked by a detector preventing potential (relativistic) mass wave transmission. If termination occurs on that detector, then a general path collapse is triggered as described above. If termination does not occur on that detector, the potential (relativistic) mass waves the detector blocks cannot progress in space and they collapse without a trace. Any unblocked waves continue in space with enhanced probability of release. Photon potential (relativistic) mass waves occur and progress; they will cease to occur (collapse) when denied space progression. Similarly, rest mass particles will cease to exist if denied time progression. Photon is not Unitary: A typical analysis in texts, journal papers and all over the web is some version of this: "whether photons polarized in one orientation will pass through a second (or third) polarizer at a different orientation is predictable in the aggregate but not in the instance which shows how QM is: 1) based on chance; 2) beyond our understanding [Bohr]." This is to regard the photon as an entity with no features/identities. What actually happens is both more complicated and less mystical; it is explained by a photon's two identities. Photon potential (relativistic) mass as pure waveform occurrence passes through all polarizer filters with greater or lesser (or zero) transmission depending upon the filter angle. Where this wave is partially (totally) blocked: 1) it may (will) initiate photon KE reception on the filter; or 2) the blocked waves may themselves collapse with no energy being transferred and the unblocked waves continue. At some point, early or later, the photon's KE will terminate either on a filter or on a target screen. A mythical, undifferentiated photon-as-unit does NOT pass through one or many polarizer filers. A real photon entity has its two identities playing separate roles when encountering polarizing filters. Photon KE in time does not navigate space-residing filters; it is unaffected by them but will at some point terminate undiminished as governed by the photon's potential identity. It is this space-progressing identity, photon potential (relativistic) mass, which navigates filters and suffers rarefaction in the process. Termination is random for the individual but in the aggregate conforms to potential mass wave intensity. 6.0 Photon as Particle – 12 The debate over radiation as a particle (Newton) or as a wave (Huygens) got reopened early in the twentieth century. Those quantum pioneers born before 1900 (Einstein, Bohr, Planck) grew up with Maxwell's wave theory of radiation and found it difficult adjusting to radiation quanta and the dualism it implied.[1, p.231,233] But succeeding generations of physicists grew accustomed to advancing science by studying particle properties; the conflict with the wave behavior of radiation got pushed to one side. Richard Feynman [2] exemplifies this shift to particle physics; he argues that radiation is composed of particles [p. 14] and dismisses wave behavior as a paradox not worth pursuing. [p. 24, 81] But the photon does not behave like a material particle: it has no rest mass, no defined location or trajectory and it leaves no tracks in a cloud chamber. In spite of all this, Feynman's idea of photon-as-particle is still widely embraced. There are two reasons for this. First, when explaining something unknown everyone's first impulse is to employ familiar concepts. Photons deliver energy and momentum to a point in space and this looks very much like particle impact. This (lazy) idea that the photon is a particle traversing space and impacting on a material target: 1) depends upon a bad analogy; 2) applies the matter-based classical ontology to radiation. It is doubly wrong. Second, it is assumed that energy cannot be real on its own;10 it must be a property of something and that something is a particle (or field), even if the particle is Imaginary (virtual). In addition, photon wave behavior does not lend itself to KE interactions. Photon waves, as we have seen, are probabilistic; they can collapse without a trace and they don't carry KE. So the wave side of photon behavior – something necessary for explaining optical phenomena – is of little use to physicists writing energy exchange equations. It is the termination (and emission) of photons where KE is involved; this energy resides in time but physicists with their existencemass-space worldview can imagine that said energy is a quantity belonging to a particle moving in space. This permits equations to be written based entirely on the classical ontology. In the words of Abraham Pais, physicists "...call a photon a particle because, just like massive particles, it obeys the laws of conservation of energy and momentum in collisions, with an electron say (Compton Effect)."[1, p. 350-1] 10 The reader will note that this sentence is not phrased as: "...energy cannot exist on its own..." Radiation kinetic energy does not exist, it occurs. We use the phrase "to exist" to mean "to be real." Our obsession with existence, matter and space carries over to our language: something is real only if it exists (and resides in space); this is very parochial. Actually, something is real if it involves mass or energy and resides in a dimension, space or time. In light of E = mc 2 and relativity (where space and time are equal), why should it be otherwise? 13 So it is easy to see why photon-as-particle is a popular analogy and why it is mathematically useful. But neither of these rationales survives serious scrutiny and photon-asparticle still leads to paradoxes. Photons terminate at a space and time point because that is how two orthogonal entities intersect. That is, photon KE residing in time can only meet (impinge upon) target matter residing in space at a point (an event) in space and time that both share. Our limited view of what is real (i.e., particles, impact, photon as simple object) leads us to underestimate the subtlety of nature. 7.0 The Photon in the Double Slit – A photon's two identities are interdependent yet are very different. One is kinetic (unstored), the other potential (stored); one resides in time and registers on our instruments and the other progresses in space and must be inferred because it involves probability. This is what has made the modeling and interpretation of radiation so difficult. Identity #1: Photon oscillation KE residing in the time dimension naturally passes through the space-residing slits unaffected and terminates (transfers its energy) on a material target at a point in space and time. Identity #2: Photon potential (relativistic) mass is a space-progressing waveform. When it divides to go through both slits its two paths mutually interfere. This creates multiple space paths of differing intensity governing probable release (termination). The identification of this as wave behavior is correct but with a caveat: these are retractable, collapsible waves of probability (stored mass); these waves do not carry energy as sound waves and water waves do. When diffracted, water waves dissipate their energy; diffracted light waves do not since they have no energy. Experimenters have tried to resolve the wave versus particle identity of the photon by modifying the double slit. By placing a photon detector behind one or both of the slits the hope is to determine "which way" (particle) information. One Slit Blocked: When waveform potential (relativistic) mass enters a slit and impinges on a detector just behind that slit, one of two things will happen. Photon reception may occur and the waves in the other, no-detector, slit collapse instantly. Alternately, no photon termination results for this slit's detector and the waves it blocks collapse and disappear (Section 5.3) while 14 waves in the other slit continue on in space. This latter case leads people who believe in the classical ontology to assert that "the particle [and its KE payload] chose the no-detector slit." Two Slits Blocked: We assume the detector at each slit has an equal chance of termination (reception). If termination occurs at detector A, then photon potential mass waves at detector B simply disappear since A has triggered a global wave collapse. It is a mistake to assume that nothing ventured into the slit where termination did not take place; potential (relativistic) mass traverses both slits. All slits of the right size can diffract a photon. Hence wave effects occur whether one slit or both slits are open. Identifying one-slit passage as "particle behavior" and two slit passage as "wave behavior" is incorrect. Both constitute (probabilistic) wave behavior since that is how radiation functions in space. We use the wrong (classical) ontology when we regard photon KE as a "particle" travelling space paths. Because it resides in time, photon KE is simply common to all available space paths its waveform potential identity traverses. Interaction-free measurement: The photon's two identities are unitary but function in two dimensions. The potential (relativistic) mass of a single photon progresses in space and so can be split into two paths, say by the first beam splitter of an interferometer; each path/arm then has a 50% probability of terminating the entire photon. The photon KE of course does not split. If you block one of the two arms and termination does not occur, then the other arm immediately converts to 100% probability of termination. But you have thereby altered the photon; one of its paths is "live" (potential/relativistic mass progressing) and the other path is "dead" (no potential/relativistic mass). This eliminates any possibility of interference when reuniting the two paths of the single photon at the second beam splitter, one path active, the other not. One photon identity (namely occurring potential/relativistic mass) has "touched" the obstacle and undergone a change without the other radiation identity (photon KE) "registering" (terminating) on the obstacle. Put another way, the potential (space wave) identity has been altered but not the kinetic (KE) identity; our instruments can measure the latter but not the former. Commentators call this "interaction-free measurement."[3] But in fact interaction of one photon identity has taken place, namely the blockage and collapse of potential (relativistic) mass on one path; this constitutes a change of the photon. The mystery of interaction-free (counterfactual) measurement disappears once you understand the role space-progressing photon potential (relativistic) mass plays. That is, once you realize the photon has two identities and you stop applying the classical ontology (existence, mass, space) to the realm of radiation (occurrence, energy, time). 15 8.0 Conclusion – Previous attempts at understanding the photon over the decades have failed because they have been based upon a classical ontology. The photon is not a particle following a defined trajectory in space; it can only be understood based upon an ontology of occurrence, KE and time. The function of radiation is the transmission of work done (KE) from one space location (photon origin) to another space location (photon termination). Work done on a charge creates an occurring entity: photon KE as pure oscillation residing in time. This time-residing KE entity has (via E = mc2) stored (relativistic) mass that progresses (and oscillates) in space (as a wave) toward release of this mass. Thus the photon has two identities: 1) a kinetic identity as KE residing and oscillating in time; and 2) a potential identity as relativistic mass progressing in space as a wave of probable release/termination. Innumerable observers can share the same entity (photon KE or inertial rest mass) if they do not reside in the entity's dimension. For example, your desk residing in space is common to (shared by) observers at different time locations because your desk and those observations of it made at different time locations are orthogonal; desk and observations are in opposing dimensions. Similarly a photon's KE residing in time is common to (shared by) observations of it made at different space locations; photon KE and observers (detectors) are in opposing dimensions. This means that photon KE does not have to "travel" (particle-like) to distant space locations because being in time it is already common to those locations. It is the photon's potential (relativistic) mass that does the speed-of-light travelling while also determining probable photon reception (KE transfer) to a material target (termination).  The photon is a quantum whole (an entity) with two identities.  Photon oscillatory KE resides in time and so does not diffract (fractionate) in space.  Photon waveform potential (relativistic) mass progresses on all available space paths, can diffract and determines probable photon reception (termination).  These two identities permit the photon to display opposing attributes: 1) a discrete, particle-imitating nature since time-residing photon KE can only intersect 16 (impinge upon) space-residing matter at a point; and 2) a probabilistic wave nature due to photon potential (relativistic) mass oscillating and progressing in space (these are Einstein's "ghost" waves [4, p.2-3]). Reception: Particle physicists exploring the Standard Model have little concern for waveparticle duality; their focus is on radiation as mediating energy (and force) between spaceresiding particles. They can do their physics quite well using the classical ontology. It is hard to see them changing their view from photon KE in space to photon KE in time; perhaps in a generation or two down the road. Earlier converts might be philosophers of physics although they too travel a road with deep ruts. Gifted amateurs and popularizers still care about waveparticle duality and its paradoxes. They are free of institutional inertia and don't have to worry that pursuing duality is a career dead end; they may be first to convert. Unfinished: The preceding has granted photon KE entity status and equality with rest mass with both residing in one dimension while progressing in the opposite dimension. But if we locate radiation KE in time does that give us two conflicting varieties of KE, one for radiation and one for matter-in-motion? And what about the instantaneous collapse of a photon's potential (relativistic) mass? This suggests a type of nonlocality as Einstein noted long ago.[5, p.51-2] These are open questions for another time and place. * * * * * * * * * * * * * * * * Sources [1] Pais, Abraham. Niels Bohr's Times, In Physics, Philosophy, and Polity, Clarendon Press, Oxford, 1991. [2] Feynman, Richard. QED: The Strange Theory of Light and Matter, Princeton Univ. Press, 1985. [3] Wikipedia, The Free Encyclopedia, s.v. "Elitzur–Vaidman bomb tester," (accessed February, 2020), https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_tester 17 [4] Dongen, Jeroen van. "The interpretation of the Einstein-Rupp experiments and their influence on the history of quantum mechanics," https://arxiv.org/abs/0709.3226 (accessed Nov. 2019). [5] Becker, Adam. What Is Real? Basic Books, New York, 2018.