Guest Editorial: Truth, Beauty and Supergravity

I begin with a warning: theoretical physics is an edifice built over the centuries by some of mankind's greatest minds, using ever more complicated and sophisticated concepts and mathematics to cover phenomena on scales billions of times removed—in directions both bigger and smaller—from our human dimensions, where our simple intuition or primitive language cannot pretend to have any validity. So any popular discussion is necessarily impressionistic, being couched in terms of classical analogies that do not really apply. This warning label should be attached to all such accounts, the present one included. However, I have tried to focus here on an aspect that involves more human attributes of our subject

Radiative Effects in Meson-Nucleon Scattering

An integral equation is derived which sums the contributions of a certain (infinite) set of radiative corrections to lowest-order meson-nucleon scattering. This equation for the S matrix is examined and an approximate treatment given. The resulting scattering amplitude is exhibited in the Thomson and high-energy limits

Electromagnetic Effects in Two-Nucleon Systems

Corrections to the electromagnetic properties of two-nucleon systems are examined by means of the relativistic two-body equation. An effective addition to the potential is derived, which describes the properties of the system in an external electromagnetic field. In particular, the correction to the deuteron's quadrupole moment is evaluated to lowest order in the meson-nucleon coupling. The result, for pseudoscalar (pseudoscalar) symmetric theory, is ΔQ=−1.3 × 10^(−28)(g^2/4π) cm^2

Inequivalence of first and second order formulations in D=2 gravity models

The usual equivalence between the Palatini and metric formulations of Einstein theory fails in two spacetime dimensions for both of its standard and ``Kaluza--Klein" reduced versions. In first order, there is a Weyl invariance that prevents full specification of the affinity and hence of metricity of the manifold

Duality transformations of Abelian and non-Abelian gauge fields

Duality transformations, i.e., rotations of electric and magnetic fields into each other, are implementable by a time-local generator for source-free Maxwell theory in an arbitrary spacetime metric. The Maxwell action ∫(E^2 − B^2) as well as the stress tensor components, e.g., E^2 + B^2, are duality invariant and the generator is conserved. In the formally analogous Yang-Mills case, duality transformations cannot even be consistently implemented

Electromagnetic Mass Shifts, Equal-Time Commutators, and Jost-Lehmann Representation

Electromagnetic mass shifts, equal-time commutators (ETC), and their relation are analyzed, using the Jost-Lehmann representation for the forward Compton amplitude. The divergences in δ m are determined by Lorentz scalars associated with the ETC (when these exist). These results are illustrated by models with currents bilinear and linear in canonical fields. Existence of the ETC implies that the Jost-Lehmann representation is unsubtracted

Gravity from self-interaction redux

I correct some recent misunderstandings about, and amplify some details of, an old explicit non-geometrical derivation of GR.Comment: Final, amplified, published version; GRG (2009