Antiparticles and the "Backward-in-Time" Representation

Antiparticles and the “Backward-in-Time” Representation: A Structure-First Reading of the Feynman–Stueckelberg Picture examines the familiar statement that an antiparticle may be represented as a particle travelling “backward in time,” separating its legitimate quantum-field-theoretic content from stronger ontological interpretations. The paper distinguishes positive- and negative-frequency field modes, particle and antiparticle operators, charge conjugation, time reversal, reversed fermion-line orientation, crossing relations, and CPT symmetry. Antiparticles are treated in accordance with modern quantum field theory as positive-energy excitations of the conjugate sector of a charged field. The backward-in-time picture is retained as a representation of amplitudes and field structure rather than as evidence for macroscopic retrocausality. A cautious Phase Compatibility Theory interpretation is then developed. The antiparticle sector may provisionally be described as a conjugate registration sector, but this terminology is not identified with charge conjugation, complex conjugation, time reversal, or a simple opposite phase sign. Version 2.2 further clarifies that conjugacy does not imply mutual exclusion. Particle and antiparticle sectors may coexist as separately resolved states, while an allowed interaction may change the jointly resolvable organisation. Annihilation is therefore not treated as destruction into nothing, but as transformation into other permitted field excitations while conserved physical quantities remain accounted for. The schematic relation A + A^C -> R is introduced only as an organisational placeholder for this proposed re-resolution and not as a completed physical map. The paper explicitly does not identify matter with positive Phase, antimatter with negative Phase, or particle-antiparticle conjugacy with a universal 180-degree phase relation. Version 2.2 also sharpens the formal validation programme. Any future Phase-to-QFT construction must define an explicit registration map, recover the particle-antiparticle mode structure and operators, distinguish that map from charge conjugation and time reversal, reproduce propagators and crossing relations, preserve Lorentz covariance, locality, unitarity, conservation and CPT structure, and recover pair production and annihilation through one consistent formalism. A genuinely Phase-specific empirical discriminator must follow from that construction rather than be selected retrospectively. The thermodynamic arrow remains explicitly separate. Antiparticle physics, negative-frequency modes, annihilation, and time-reversal-related amplitudes are not presented as evidence for an entropy mechanism or macroscopic time reversal. Version 2.2 status: unrestricted pre-audit conceptual and formal-development paper. It provides a bounded interpretation and prospective validation programme. It does not claim a new QED derivation, a completed Phase-to-QFT operator mapping, or empirical evidence selecting Phase Compatibility Theory over standard quantum field theory. Version 2.2 revisions include strengthened primary-source treatment of the Feynman-Stueckelberg history, clearer separation of hole-theory history from modern QFT, tightened CPT and observer-language boundaries, explicit coexistence and re-resolution treatment of conjugate sectors, pair-production and annihilation closure as a mandatory formal gate, and a sharper distinction between interpretive Phase vocabulary and completed operator-level physics.