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Conference on the Foundations of Quantum Mechanics: Historical Analysis and Open Questions

THE FIRST STEPS OF QUANTUM ELECTRODYNAMICS: WHAT IS IT THAT'S BEING QUANTIZED?

Silvio Bergia

Published Jun 1, 2006• 0 citations
Publisher

Abstract

Quantum field theory, and in particular quantum electrodynamics (QED), are milestones in the history of twentieth century physics. Not all interpretative problems presented by them, however, can be said to have been settled once and for all. In this paper the first papers on QED by P. Jordan and P.A.M. Dirac are briefly analysed, with the intent to show that the authors confronted themselves from the beginning with the essential of those problems, that their answers were not unambiguous, and that there are still some open questions concerning the subject matter. The potentiality and up-to-dateness of Dirac's work are particularly emphasised.

DOI
10.1142/9789812773258_0006

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Quantum field theory, and in particular quantum electrodynamics (QED), are milestones in the history of twentieth century physics. Not all interpretative problems presented by them, however, can be said to have been settled once and for all. In this paper the first papers on QED by P. Jordan and P.A.M. Dirac are briefly analysed, with the intent to show that the authors confronted themselves from the beginning with the essential of those problems, that their answers were not unambiguous, and that there are still some open questions concerning the subject matter. The potentiality and up-to-dateness of Dirac's work are particularly emphasised.
This paper summarizes our papers over the past years which – taken together – effectively amount to a classical interpretation of QED. Our very first paper started exploring a basic intuition: if QED is the theory of electrons and photons, and their interactions, then why is there no good model of what electrons and photons actually are? We have tried to address this perceived gap in the theory – further building on the Zitterbewegung model of an electron – ever since. We thought we should write one final paper to provide some history – acknowledgements, basically – and summarize the key principles of the interpretation.
The study of light through the centuries has produced a number of theories and experiments. Newton advanced the corpuscular theory, building on the early Greek view of matter composed of atoms. Then Young essentially proved the wave nature of light through the interference experiments. Maxwell provided the firm theoretical basis for the wave nature. During the late 1800s and early 1900s, further experiments showed predictions based on a continuous wave led to the well-known ultraviolet catastrophe. Plank and Einstein originated and developed the notion of the photon as an elementary quantum of propagating electromagnetic (EM) energy. The first half of the 20th century saw the development of the field quantization and the quantum electrodynamics (QED). Feynmann labeled QED as ‘‘the best theory we have’’ to describe the matter-light interaction. The second half of the 20th century witnessed the development of the coherent and squeezed optical states attributable to Glauber and Yuen. The study of light and electromagnetic fields has a long history. This chapter explores the nature of light in terms of quantum optics.
After recalling episodes from Pascual Jordan's biography including his pivotal role in the shaping of quantum field theory and his much criticized conduct during the NS regime, I draw attention to his presentation of the first phase of development of quantum field theory in a talk presented at the 1929 Kharkov conference. He starts by giving a comprehensive account of the beginnings of quantum theory, emphasising that particle-like properties arise as a consequence of treating wave-motions quantum-mechanically. He then goes on to his recent discovery of quantization of ``wave fields'' and problems of gauge invariance. The most surprising aspect of Jordan's presentation is however his strong belief that his field quantization is a transitory not yet optimal formulation of the principles underlying causal, local quantum physics. The expectation of a future more radical change coming from the main architect of field quantization already shortly after his discovery is certainly quite startling. I try to answer the question to what extent Jordan's 1929 expectations have been vindicated. The larger part of the present essay consists in arguing that Jordan's plea for a formulation without ``classical correspondence crutches'', i.e. for an intrinsic approach (which avoids classical fields altogether), is successfully addressed in past and recent publications on local quantum physics.
Abstract We continue the discussion of the abstract transformation theory which led to the development of second quantization. Jordan had already suggested (in the foundational papers on matrix mechanics by Heisenberg, Born, and himself) that the electromagnetic field should be represented by a matrix, i.e., quantized. Dirac eventually took up this idea and proposed quantizing the field by treating the field as an operator. Jordan (with others) showed how the same technique could be applied to Bosons and then Fermions. This established the roots of quantum field theory. It also cleared up some of the interpretational issues of the time.
Abstract This book is unique in covering phenomena in photon- matter interactions in a unified way over a range of many orders in energy. The quantum field theoretic approach to the fully relativistic theory of quantum electrodynamics (QED) is presented together with the non- relativistic theory in both confined and unconfined geometries. The predictions of QED have been verified to a greater accuracy than any other physical theory. Moreover QED is a paradigm for other gauge theories and is presented in such a way that the generalisation to other gauge theories is natural. Gauge and Poincare symmetry properties and the non-existence of a photon wave function are thoroughly discussed. Starting from the Dirac equation the non-relativistic interaction of the electron with the electromagnetic field is derived as an effective Hamiltonian of multipole expansions. Much of quantum optics is based on the lowest order dipole approximation. From this point on the treatment of fully relativistic QED and quantum optics is done in parallel. Applications of perturbation theory such as Compton and Moller scattering and the theory photdetection are given. After the impressive successes of QED, the limitation of the theory and the necessity of electroweak theory and quantumchromodynamics are discussed. The remaining chapters are devoted to quantum optics inside cavaties. Various approaches to open systems such as master equations are discussed within the context of active systems (e.g. the laser) and passive systems. Semi- classical approximarions are shown to imply a rich non- linear dynamics including chaos for certain parameter regimes. The effect of fluctuations on such non-linear dynamics is also studied. The final chapter is devoted to highly non- classical states of the light field such as photon number, squeezed and two photon entangled states. The latter are studied for the important system of parametric down conversion and the localisation properties of photons are characterised in terms of asympotic tails in photodetection probabilities as a function of time delay. The range of the book has wider benefits. Workers in quantum optics will gain a deeper understanding of the foundations of their subject and field theorists will see concrete examples of open systems, which are beginning to impinge on fundamental theories.