## The Photon as a Relativistically Atomic Interaction: An Epistemic Reinterpretation of Quantum Phenomena

**Abstract:**
The nature of the photon and the interpretation of quantum mechanics have been subjects of debate for a century. Current interpretations grapple with wave-particle duality, the measurement problem, and the non-locality of entangled states. We propose a novel interpretation wherein the photon is not an entity (particle or wave) traversing spacetime, but rather represents a discrete, atomic act of interaction between two charged particles. From the perspective of a hypothetical frame co-moving with the photon (i.e., along a null geodesic), the emission and absorption events are co-local and simultaneous due to relativistic effects (zero proper time interval). For an observer in a subluminal frame, these events appear separated in space and time. We argue that the quantum wave function associated with a photon does not describe the state of a traveling entity, but rather represents the observer's epistemic uncertainty regarding the future absorption event, conditioned on the known emission event. This framework offers a parsimonious explanation for wave function collapse and the non-locality of entanglement as updates of knowledge concerning these atomic interaction events.

**Keywords:** Quantum Interpretation, Photon, Relativity, Epistemic Wave Function, Entanglement, Causality, Measurement Problem.

**1. Introduction**

Quantum mechanics (QM) is an extraordinarily successful theory, yet its foundational interpretation remains a subject of ongoing discussion. Central to quantum electrodynamics (QED), the photon concept exemplifies many interpretational challenges. While quantum field theory (QFT) provides a unified framework where entities like photons are excitations of quantum fields, capable of exhibiting both wave-like propagation patterns and particle-like discrete interactions, the task of interpretations often involves elucidating how these diverse manifestations arise from a single underlying reality and how to conceptualize measurement outcomes. Current interpretations, such as the Copenhagen interpretation, introduce observer-dependent collapse, while others like Many-Worlds [1] or Bohmian Mechanics [2] offer different ontological pictures, each with its own set of conceptual intricacies. The Wheeler-Feynman absorber theory [3, 4] previously explored interactions as fundamental but focused on time-symmetric electrodynamics.

This paper advocates for a distinct interpretation: the "photon" is not an independent entity (neither a classical particle nor a classical wave) propagating through spacetime. Instead, it *is* the indivisible, atomic act of energy-momentum exchange between two charged material systems (hereafter referred to as "emitter" and "absorber"). We posit that the perceived separation of emission and absorption is an artifact of the observer's subluminal reference frame, and that from a light-like perspective (along a null geodesic), these events are fundamentally unified.

**2. The Photon as a Spacetime Interaction Event**

Our central thesis is that a photon represents a single, irreducible interaction event linking an emitter and an absorber.

**2.1. Relativistic Co-locality of Emission and Absorption**
In Special Relativity, the spacetime interval $ds^2$ along the path of light is null:
$ds^2 = (c dt)^2 - dx^2 - dy^2 - dz^2 = 0$
This implies that the proper time $\tau = \int ds/c$ experienced along a light-like trajectory is zero. While a "photon's frame of reference" is ill-defined in standard SR for massive objects, the concept of a null interval between emission (E) at $(t_E, \mathbf{x}_E)$ and absorption (A) at $(t_A, \mathbf{x}_A)$ is robust. If we consider the interaction itself as the fundamental "event," then from the perspective of this null interval, E and A are not separated by any proper time. They constitute a single, indivisible spacetime event. The appearance of spatial and temporal separation is a consequence of projecting this null interval onto an observer's subluminal inertial frame.

**2.2. The "Shortest Path" and Unobservability of the Intermediate "Photon"**
The principle of least action, manifesting as Fermat's principle for light (shortest optical path), dictates the trajectory of this interaction. Crucially, if the photon *is* the interaction, then there is no intermediate "photon-particle" or "photon-wave" to be observed *en route*. Any attempt to "observe" the photon between E and A would necessitate an interaction, thereby defining a new absorption event and, consequently, a new, shorter E-A interaction.

This perspective offers a straightforward resolution to the issue of causality. An observer O cannot observe the emitter *after* emission and subsequently observe the absorber *before* absorption in such a way as to disprove the atomic nature of the E-A event. The causal structure of spacetime for subluminal observers ensures that if E is in O's past light cone, A can be in O's past light cone, future light cone, or elsewhere (spacelike separated). The ordering of E and A is preserved for all observers if they are timelike or null separated.

**3. Observer Perspectives and the Nature of the Wave Function**

For an observer O moving at $v < c$, the relationship between the emission (E) and absorption (A) events can be categorized based on O's light cone structure:

1. **E and A in O's Past Light Cone:** Both emission and absorption have been observed. The interaction is a known, completed event. This is the classical observation of a light signal received.
2. **E and A in O's Future Light Cone:** Both emission and absorption are predicted to occur in the future. This is classical prediction based on known laws and initial conditions.
3. **E in O's Past Light Cone, A outside O's Past Light Cone (i.e., in O's future or spacelike separated):** The emission has occurred (or is known to have occurred). The absorption event, however, has not yet entered O's causally knowable past. This scenario, we propose, corresponds to the quantum mechanical description.
4. **E outside O's Past Light Cone, A in O's Future Light Cone (and potentially in O's past relative to some *later* observation time):** The emission is not yet known, but the absorption is predicted for the future. This is symmetric to case 3 but less commonly considered from a single observer's evolving perspective.

**3.1. The Wave Function as Epistemic Uncertainty**
In scenario 3, the observer knows the circumstances of emission (e.g., an excited atom at $\mathbf{x}_E$ at $t_E$). The photon, as an interaction, is "destined" for a specific absorption event at $(\mathbf{x}_A, t_A)$. However, for observer O, $(\mathbf{x}_A, t_A)$ is not yet determined *from their perspective*. The wave function $\Psi(\mathbf{x}, t)$ in this interpretation does not describe an objectively existing wave propagating through space. Instead, it represents the observer's (or the community's) epistemically limited knowledge – a probabilistic estimation of where and when the absorption event *will* occur, given the information about the emission.

The wave function evolves according to Schrödinger's equation (or its relativistic counterpart) because this evolution correctly describes how the probability distribution of potential absorption sites changes over time and space, constrained by the properties of the emitting system and the environment of potential absorbers.

**3.2. Wave Function Collapse as Knowledge Update**
When the absorption event A enters the observer O's past light cone (i.e., $t_A$ becomes less than O's current time $t_O$, and the light signal from A reaches O), the observer gains direct information about A. At this moment, the epistemic description – the wave function – "collapses." This is not a physical process affecting a real wave, but an update of the observer's knowledge. The probabilistic estimation becomes a certainty regarding the completed interaction. This naturally resolves the measurement problem: measurement is simply the acquisition of information about the absorption end of the interaction.

**4. Reinterpreting Quantum Phenomena**

**4.1. Wave-Particle Duality**
The "wave-like" properties (e.g., interference in a double-slit experiment) arise from the probabilistic nature of where the interaction will complete. The wave function describes the probability amplitudes for the E-A interaction to terminate at different points on a detection screen. The "particle-like" properties (e.g., discrete detection in a photomultiplier) arise because the interaction itself is atomic and indivisible – it occurs at one point or not at all. There is no object that is sometimes a wave and sometimes a particle; there is only an interaction whose completion point is described probabilistically until observed. For interference, the wave function would describe the sum over all possible E-A interaction pathways, with the final distribution of interaction endpoints (absorptions) forming the interference pattern. The choice of "which path" is only meaningful if an interaction occurs along that path, which would destroy the interference pattern by defining a new intermediate absorption.

**4.2. Entanglement and Non-Locality: An Epistemic-Correlational View**

Consider two entangled photons, $\gamma_1$ and $\gamma_2$, produced from a common source S. In our interpretation, this implies two potential atomic interaction pathways are initiated: $S \leadsto A_1$ (for $\gamma_1$) and $S \leadsto A_2$ (for $\gamma_2$). Crucially, these are not independent. The conservation laws at S establish profound correlations between the *eventual* outcomes of these interactions, should they occur.

The joint wave function, $\Psi(\gamma_1, \gamma_2)$, describes our epistemic state regarding the correlated probabilities of how these interactions $S \leadsto A_1$ and $S \leadsto A_2$ will complete. It does *not* imply that each interaction pathway carries a set of locally predetermined values for all possible measurements, as Bell's theorem rigorously rules out such local hidden variable theories [Ref Bell].

Instead, the "determination" lies in the *structure of the correlations themselves*, imposed at S. When an observer becomes aware of the completion of the $S \leadsto A_1$ interaction at a specific absorber $A_1$ (with a specific measurement setting and outcome), their knowledge regarding the *potential* completion of the $S \leadsto A_2$ interaction is instantaneously updated. This update reflects the inherent, non-classical correlations established at the source. No physical influence travels superluminally between $A_1$ and $A_2$. The "spooky action" is an update of probabilistic knowledge about a system whose parts were always interconnected in a way that transcends classical locality.

The challenge for this interpretation, as for any seeking to uphold locality in the signaling sense, is to articulate how these strong, Bell-violating correlations arise without resorting to pre-set local hidden variables. One avenue within this framework is to consider the entangled "photons" not as two separate null-interval interactions, but as aspects of a single, more complex interaction structure $S \leadsto (A_1, A_2)$, where the "atomic act" might span all involved particles. The apparent separate paths in spacetime are projections of this underlying correlated structure. The "photon's own frame of reference" argument, where emission and absorption are co-local and simultaneous, would need to be extended to this joint interaction, suggesting a deep interconnectedness that is not fully captured by viewing them as two independent paths whose properties just happen to be correlated.

**5. Discussion**

This interpretation offers several advantages:
* **Ontological Parsimony:** It eliminates the need for a wave/particle object traversing spacetime, replacing it with the concept of an interaction.
* **Resolution of Measurement Problem:** Collapse is an epistemic update, not a physical process.
* **Intuitive Explanation of Non-Locality:** Entanglement's non-locality becomes a correlation of knowledge about predetermined (but unknown) properties of interaction endpoints.
* **Relativistic Consistency:** The core idea is rooted in the null spacetime interval characteristic of light-like phenomena.

Challenges and Future Directions:
* **Formalism:** While conceptually appealing, fully translating this interpretation into the mathematical formalism of QFT, particularly for virtual photons and complex field interactions, requires further development. The "interaction" itself would need to be the quantized entity.
* **Interference Details:** A more detailed model of how the probabilistic choice of absorption sites leads to specific interference patterns (e.g., considering the density and properties of all potential absorbers contributing to the wave function) is needed. This could potentially connect to a "sum over histories" approach where histories are defined by E-A pairs.
* **Delayed Choice Experiments:** This interpretation seems to handle delayed choice experiments naturally. The future "choice" of measurement setup simply determines which set of potential absorption events (and thus which interaction probabilities) are actualized. The photon "interaction" itself is not "deciding" anything retrocausally; our knowledge of the boundary conditions (including the detector setup) shapes our predictions.

This interpretation shares some common ground with transactional interpretations [5] but differs in its emphasis on the singular, atomic nature of the interaction and the purely epistemic role of the wave function for subluminal observers.

**6. Conclusion**

We propose that the photon is not an entity that travels, but rather *is* a discrete, atomic act of interaction between two charged particles. Relativistically, this interaction, spanning from emission to absorption, can be viewed as a single event with no proper time duration. For subluminal observers, the emission and absorption appear separated, and the intervening period is characterized by quantum uncertainty. This uncertainty is epistemic, described by a wave function that quantifies the observer's knowledge about the eventual absorption event, given the emission. Wave function collapse is the update of this knowledge upon observation of the absorption. Entanglement reflects correlated knowledge about multiple such interaction events.

This "Atomic Interaction Interpretation" offers a potentially unifying and ontologically simpler framework for understanding quantum phenomena, grounding them in relativistic principles and an epistemic view of quantum states. Further work is required to fully flesh out its implications and formal mathematical structure within QFT.

**7. References**
[1] Everett, H. (1957). "Relative State" Formulation of Quantum Mechanics. *Reviews of Modern Physics*, 29(3), 454–462.
[2] Bohm, D. (1952). A Suggested Interpretation of the Quantum Theory in Terms of "Hidden" Variables. I & II. *Physical Review*, 85(2), 166–193.
[3] Wheeler, J. A., & Feynman, R. P. (1945). Interaction with the Absorber as the Mechanism of Radiation. *Reviews of Modern Physics*, 17(2-3), 157–181.
[4] Wheeler, J. A., & Feynman, R. P. (1949). Classical Electrodynamics in Terms of Direct Interparticle Action. *Reviews of Modern Physics*, 21(3), 425–433.
[5] Cramer, J. G. (1986). The Transactional Interpretation of Quantum Mechanics. *Reviews of Modern Physics*, 58(3), 647–687.