Relativistic quantum theory poses a number of conceptual problems over and above the non-relativistic mechanics. From a purely mathematical point of view it is also much more sophisticated. Even the kinematic theory poses considerable difficulties: locality, antimatter, negative energy, and charge are already systematically linked in ways that cannot simply be paraphrased. A further complication is that in the midst of this structure we have what is usually called the Newton-Wigner representation (or, in the case of the Dirac theory, the Foldy-Wouthuysen representation), in which fields and states are no longer covariantly described and where the “local” self-adjoint quantities (NW-local observables) do not obey microcausality, but only satisfy equal-time commutators with respect to a particular inertial frame. If we pass to this represenation, we obtain at a stroke the basic structure of non-relativistic quantum field theory; it is not too hard to descend from that to a many-particle mechanics, and to recover the usual definitions of localization.
Relativistic quantum theory poses a number of conceptual problems over and above the non-relativistic mechanics. From a purely mathematical point of view it is also much more sophisticated. Even the kinematic theory poses considerable difficulties: locality, antimatter, negative energy, and charge are already systematically linked in ways that cannot simply be paraphrased. A further complication is that in the midst of this structure we have what is usually called the Newton-Wigner representation (or, in the case of the Dirac theory, the Foldy-Wouthuysen representation), in which fields and states are no longer covariantly described and where the “local” self-adjoint quantities (NW-local observables) do not obey microcausality, but only satisfy equal-time commutators with respect to a particular inertial frame. If we pass to this represenation, we obtain at a stroke the basic structure of non-relativistic quantum field theory; it is not too hard to descend from that to a many-particle mechanics, and to recover the usual definitions of localization.
The aim of relativistic quantum mechanics is to describe the finer details of the structure of atoms and molecules, where relativistic effects become nonnegligible. It is a sort of intermediate realm, between the familiar nonrelativistic quantum mechanics and fully relativistic quantum field theory, and thus it lacks the simplicity and elegance of both. Yet it is a necessary tool, mostly for quantum chemists. Pilkuhn's book offers to this audience an up-to-date survey of these methods, which is quite welcome since most previous textbooks are at least ten years old. The point of view of the author is to start immediately in the relativistic domain, following the lead of Maxwell's equations rather than classical mechanics, and thus to treat the nonrelativistic version as an approximation. Thus Chapter 1 takes off from Maxwell's equations (in the noncovariant Coulomb gauge) and gradually derives the basic aspects of Quantum Mechanics in a rather pedestrian way (states and observables, Hilbert space, operators, quantum measurement, scattering,. Chapter 2 starts with the Lorentz transformations, then continues with the Pauli spin equation and the Dirac equation and some of their applications (notably the hydrogen atom). Chapter 3 is entitled `Quantum fields and particles', but falls short of treating quantum field theory properly: only creation/annihilation operators are considered, for a particle in a box. The emphasis is on two-electron states (the Pauli principle, the Foldy--Wouthuysen elimination of small components of Dirac spinors, Breit projection operators. Chapter 4 is devoted to scattering theory and the description of relativistic bound states. Chapter 5, finally, covers hyperfine interactions and radiative corrections. As we said above, relativistic quantum mechanics is by nature limited in scope and rather inelegant and Pilkuhn's book is no exception. The notation is often heavy (mostly noncovariant) and the mathematical level rather low. The central topic is the description of atoms and molecules, including relativistic effects. The author fulfils this program in a reasonable way and offers a valuable tool to the targeted audience. I am not overly enthusiastic about the end result, but I might be prejudiced. Clearly, going further would require the full power of quantum field theory, but this is clearly beyond the scope of the book.
The rampant success of quantum theory is the result of applications of the 'new' quantum \nmechanics of Schrödinger and Heisenberg (1926-7), the Feynman-Schwinger-Tomonaga Quantum \nElectrodynamics (1946-51), the electro-weak theory of Salaam, Weinberg, and Glashow (1967-9), \nand Quantum Chromodynamics (1973-); in fact, this success of `the' quantum theory has depended \non a continuous stream of brilliant and quite disparate mathematical formulations. In this carefully \nconcealed ferment there lie plenty of unresolved difficulties, simply because in churning out fabulously \naccurate calculational tools there has been no sensible explanation of all that is going on. It is even \nargued that such an understanding is nothing to do with physics. A long-standing and famous \nillustration of this is the paradoxical thought-experiment of Einstein, Podolsky and Rosen (1935). \nFundamental to all quantum theories, and also their paradoxes, is the location of sub-microscopic \nobjects; or, rather, that the specification of such a location is fraught with mathematical inconsistency. \nThis project encompasses a detailed, critical survey of the tangled history of Position \nwithin quantum theories. The first step is to show that, contrary to appearances, canonical quantum \nmechanics has only a vague notion of locality. After analysing a number of previous attempts at a \n`relativistic quantum mechanics', two lines of thought are considered in detail. The first is the work \nof Wan and students, which is shown to be no real improvement on the usual `nonrelativistic' theory. \nThe second is based on an idea of Dirac's - using backwards-in-time light-cones as the hypersurface \nin space-time. There remain considerable difficulties in the way of producing a consistent scheme here. \nTo keep things nicely stirred up, the author then proposes his own approach - an adaptation \nof Feynman's QED propagators. \nThis new approach is distinguished from Feynman's since the propagator or Green's function \nis not obtained by Feynman's rule. The type of equation solved is also different: instead of an \ninitial-value problem, a solution that obeys a time-symmetric causality criterion is found for an \ninhomogeneous partial differential equation with homogeneous boundary conditions. \nTo make the consideration of locality more precise, some results of Fourier transform theory are \npresented in a form that is directly applicable. \nSomewhat away from the main thrust of the thesis, there is also an attempt to explain the \nmanner in which quantum effects disappear as the number of particles increases in such things as \nexperimental realisations of the EPR and de Broglie thought experiments.
The Dirac and Foldy-Wouthuysen (FW) representations of relativistic quantum mechanics produce conflicting predictions for fundamental observables like location, spin, and probability density, creating a fundamental stalemate. This representation crisis undermines quantitative results across the field, yet researchers continue to work within restrictive methodologies—low-dimensional models, paraxial approximations, or classical circuit simulations—that mask rather than resolve the underlying mathematical tension. We propose the Relational Event-Coordination (REC) framework, which redefines position as a relational property emerging from a Hilbert space of spacetime events rather than an intrinsic particle operator. Within this framework, Lorentz symmetry acts as a geometric unitary, and observables arise from covariant clock POVMs conditioned against spacetime foliation. The REC framework recovers the Dirac density where covariance and continuity requirements dominate, while mapping to FW variables when classical alignment becomes necessary. Neither representation holds privileged status; both describe the same underlying events under different conditioning regimes. We identify testable predictions involving twisted electron beams in non-inertial frames and high-energy particles in intense laser fields, offering pathways toward empirical resolution of the representation paradox. Keywords: Dirac representation, Foldy–Wouthuysen transformation, relational quantum mechanics, relativistic observables, Event-Hilbert Space, Newton–Wigner localization, POVM, representation crisis
Up to this point, our study of quantum mechanics has concerned itself with the behaviour of particles that inhabit a Galilean spacetime. For many purposes, in atomic, molecular and condensed matter physics, this theory is quite adequate. We saw in earlier chapters, however, that our actual spacetime has a structure which is much closer to that of the Minkowski spacetime of special relativity and that more general structures must be considered when gravitational phenomena are signicant. From a purely theoretical point of view, it is therefore important to formulate quantum theory in a way which is consistent with these more general spacetimes. The benets of constructing a relativistic quantum theory actually go far beyond the aesthetic satisfaction of making our geometrical and quantum-mechanical reasoning compatible. For one thing, we shall discover that the relativistic theory provides a deeper understanding of spin and the distinction between fermions and bosons, which in the non-relativistic theory appear simply as facts of life that we must strive to accommodate. Also, of course, there are many situations in which relativistic eects become observable, for which non-relativistic theory provides no explanation. The most obvious are high-energy scattering experiments, in which particles acquire kinetic energies comparable with or greater than their rest energies mc2, and the correct 4-momentum (3.33) must be used. There are, however, more subtle eects, such as the spin-orbit coupling that is essential for interpreting atomic spectra, which are also of relativistic origin.
Relativistic quantum theory supports two competing notions of particle locality. Newton--Wigner localization provides a sharp position operator on the one-particle Hilbert space but is not Lorentz covariant and leads to instantaneous spreading for positive-energy states. Foldy--Wouthuysen localization, widely used in precision experiments, yields a robust quasi-classical particle description but is representation-dependent and does not coincide with strict field-theoretic locality. We show that this tension is not a paradox in UV-complete nonlocal quantum field theories whose ultraviolet physics is encoded by an entire-function regulator. In such theories the physically accessible observables form a quasi-local algebra generated by regulated smearings with an intrinsic nonlocality length. Sharp Newton--Wigner projectors are not operationally realistic, and Newton--Wigner and Foldy--Wouthuysen localization become equivalent non-singular notions of locality in the same measurement algebra.
Given an isolated system of either free or interacting particles and the associated realization of the ten conserved Poincaré generators its total conserved time-like 4-momentum defines its inertial rest-frame as the 3+1 splitting whose space-like 3-spaces (named Wigner 3-spaces) are orthogonal to it and whose inertial observer is the Fokker–Pryce 4-center of inertia. There is a discussion of the problem of the relativistic center of mass based on the fact that the 4-center functions “only” of the Poincaré generators of the isolated system are the following three non-local quantities: the non-canonical covariant Fokker–Pryce 4-center of inertia, the canonical non-covariant Newton–Wigner 4-center of mass and the non-canonical non-covariant Mőller 4-center of energy. At the Hamiltonian level one is able to express the canonical world-lines of the particles and their momenta in terms of the Jacobi variables of the external Newton–Wigner center of mass (a non-local non-covariant non-measurable quantity) and of Wigner-covariant relative 3-coordinates and 3-momenta inside the Wigner 3-spaces. This solves the problem of the elimination of relative times in relativistic bound states and to formulate a consistent Wigner-covariant relativistic quantum mechanics of point particles. The non-relativistic limit gives the Hamilton–Jacobi description of the system after the separation of Newtonian center of mass. Finally there is the definition of the non-inertial rest-frames whose 3-spaces are orthogonal to the total 4-momentum of the isolated system at spatial infinity.