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Joshua G. Fenwick

Publications and source records attributed to Joshua G. Fenwick.

5 recordsLinked to original sources
arXiv2026

Photon localization: a comparative study

We compare different measures for photon localization in terms of two-dimensional Gaussian wave packets. We find that all measures start to coalesce if the wave packet has evolved for times which are larger than a few times the inverse momentum-space width of the package. However, the Landau-Peierls wave function yields the largest positive offset -ct>0 while the scalar Fourier transform of the wave packet yields the smallest offset and converges towards -ct=0 fastest. We also discuss local detection of photons through a model detector consisting of ions in ion traps. The position-dependent detection probabilities are inferred from the scattering matrix. We find that the local detection probability for photons can be expressed in terms of three of the proposed localization measures, viz. the Landau-Peierls wave function, the energy wave function, and the Hawton density. Those three porposals also remain close throughout the time evolution of the single-photon wave packet.

quant-ph↗
arXiv2026

Consistency Conditions and the Derivation of Harmonic Structure in Einstein-Maxwell-dilaton Theory

Many exact solutions of Einstein-Maxwell and Einstein-Maxwell-dilaton theory share a common structural pattern in which the metric functions are built from harmonic functions on a specified spatial base, often taken to be flat. We investigate this pattern by considering the Einstein-Maxwell-dilaton theory in arbitrary dimensions, where the dilaton field is non-trivially coupled to the Maxwell field and to a Liouville-type potential proportional to the cosmological parameter. Without imposing either the base geometry or the harmonic form of the metric function in advance, we show that for the generic branch, the field equations force the dilaton couplings to be equal, restrict the spatial base geometry to be Ricci flat, and make the metric function harmonic on this base. The resulting spacetime can then be written in terms of a conformal potential. We also consider a purely spatial branch, which instead leads to a distinct constraint on the coupling constants. These results provide a unified field-equation derivation of the harmonic behavior and conformal structure that appear in several classes of solutions, including multi-center geometries, cosmological solutions, and dynamical black holes.

gr-qc↗
arXiv2025

Antisymmetric tensor portals to dark matter

Both freeze-in of very weakly coupled dark matter and freeze-out of initially thermalized dark matter from the primordial heat bath provide interesting possibilities for dark matter creation in the early universe. Both scenarios allow for a calculation of baryon-dark matter coupling constants as a function of dark matter mass due to the constraint that freeze-in or freeze-out produce the observed dark matter abundance. Here we compare the resulting coupling constants in the two scenarios if dark matter couples to baryons through an antisymmetric tensor portal. The freeze-in scenario predicts much smaller coupling in agreement with the nonthermalization postulate. We find that the couplings as a function of mass behave very differently in the two scenarios.

hep-ph↗
arXiv2023

Relativistic probability densities for location

Imposing the Born rule as a fundamental principle of quantum mechanics would require the existence of normalizable wave functions also for relativistic particles. Indeed, the Fourier transforms of normalized k-space amplitudes yield normalized x-space wave packets which reproduce the standard k-space expectation values for energy and momentum from local momentum pseudo-densities. However, in the case of bosonic fields, the wave packets are nonlocally related to the corresponding relativistic quantum fields, and therefore the canonical local energy-momentum densities differ from the pseudo-densities and appear nonlocal in terms of the wave packets. We examine the relation between the canonical energy density, the canonical charge density, the energy pseudo-density, and the Born density for the massless free Klein-Gordon field. We find that those four proxies for particle location are tantalizingly close even in this extremely relativistic case: In spite of their nonlocal mathematical relations, they are mutually local in the sense that their maxima do not deviate beyond a common position uncertainty $Δx$. Indeed, they are practically indistinguishable in cases where we would expect a normalized quantum state to produce particle-like position signals, viz. if we are observing quanta with momenta $p\ggΔp\ge\hbar/2Δx$. We also translate our results to massless Dirac fields. Our results confirm and illustrate that the normalized energy density provides a suitable measure for positions of bosons, whereas normalized charge density provides a suitable measure for fermions.

quant-ph↗