I examine some problems standing in the way of a successful `field interpretation' of quantum field theory. The most popular extant proposal depends on the Hilbert space of `wavefunctionals.' But since wavefunctional space is unitarily equivalent to many-particle Fock space, two of the most powerful arguments against particle interpretations also undermine this form of field interpretation. 1. Introduction 2. Field Interpretations and Field Operators 3. The Wavefunctional Interpretation 4. Fields and Inequivalent Representations 4.1. The Rindler representation 4.2. Spontaneous symmetry breaking 4.3. Coherent representations 5. The Fate of Fields in Interacting QFT 6. Conclusions
I examine some problems standing in the way of a successful field interpretation' of quantum field theory. The most popular extant proposal depends on the Hilbert space of wavefunctionals.' But since wavefunctional space is unitarily equivalent to many-particle Fock space, two of the most powerful arguments against particle interpretations also undermine this form of field interpretation. 1. Introduction 2. Field Interpretations and Field Operators 3. The Wavefunctional Interpretation 4. Fields and Inequivalent Representations 4.1. The Rindler representation 4.2. Spontaneous symmetry breaking 4.3. Coherent representations 5. The Fate of Fields in Interacting QFT 6. Conclusions
There are significant problems involved in determining the ontology of quantum field theory (QFT). An ontology involving particles seems to be ruled out due to the problem of defining localized position operators, issues involving interactions in QFT, and, perhaps, the appearance of unitarily inequivalent representations. While this might imply that fields are the most natural ontology for QFT, the wavefunctional interpretation of QFT has significant drawbacks. A modified field ontology is examined where determinables are assigned to open bounded regions of spacetime instead of spacetime points.
This is an opinionated survey of some interpretive puzzles in quantum field theory. The problem of inequivalent representations is sketched, including its connections with competing accounts of physical equivalence. The controversy between variant formulations of the theory, algebraic versus Lagrangian, is given a conciliatory resolution. Arguments against particles are addressed, demarcating clearly between different forms of particle interpretation. Field interpretations are then considered, including wavefunctional, spacetime state realist and Heisenberg operator realist interpretations. Ruetsche’s coalesced structure interpretation is presented and juxtaposed with an alternative, more traditional view of the theory’s laws and state space. Finally, the CPT theorem is discussed, together with its implications about the nature of spacetime.
The availability of unitarily inequivalent representations of the canonical commutation relations constituting a quantization of a classical field theory raises questions about how to formulate and pursue quantum field theory. In a minimally technical way, I explain how these questions arise and how advocates of the Hilbert space and of the algebraic approaches to quantum theory might answer them. Where these answers differ, I sketch considerations for and against each approach, as well as considerations which might temper their apparent rivalry.
Philosophical reflection on quantum field theory has tended to focus on how it revises our conception of what a particle is. However, there has been relatively little discussion of the threat to the "reality" of particles posed by the possibility of inequivalent quantizations of a classical field theory, i.e., inequivalent representations of the algebra of observables of the field in terms of operators on a Hilbert space. The threat is that each representation embodies its own distinctive conception of what a particle is, and how a "particle" will respond to a suitably operated detector. Our main goal is to clarify the subtle relationship between inequivalent representations of a field theory and their associated particle concepts. We also have a particular interest in the Minkowski versus Rindler quantizations of a free Boson field, because they respectively entail two radically different descriptions of the particle content of the field in the very same region of spacetime. We shall defend the idea that these representations provide complementary descriptions of the same state of the field against the claim that they embody completely incommensurable theories of the field.
Philosophical reflection on quantum field theory has tended to focus on how it revises our conception of what a particle is. However, there has been relatively little discussion of the threat to the ‘reality’ of particles posed by the possibility of inequivalent quantizations of a classical field theory, i.e. inequivalent representations of the algebra of observables of the field in terms of operators on a Hilbert space. The threat is that each representation embodies its own distinctive conception of what a particle is, and how a ‘particle’ will respond to a suitably operated detector. Our main goal is to clarify the subtle relationship between inequivalent representations of a field theory and their associated particle concepts. We also have a particular interest in the Minkowski versus Rindler quantizations of a free Boson field, because they respectively entail two radically different descriptions of the particle content of the field in the very same region of spacetime. We shall defend the idea that these representations provide complementary descriptions of the same state of the field against the claim that they embody completely incommensurable theories of the field.
It is very likely that most readers and authors of articles appearing in Classical and Quantum Gravity are interested in what might loosely be termed the philosophy of science. It is equally likely, with perhaps a few exceptions, that the research of these same readers and authors has not been influenced in the least by the work of professional philosophers of science. This is certainly the case for those of us who work in quantum field theory. One must therefore ask who the intended audience is for a book which purports to discuss the philosophical implications of quantum field theory. In the preface, the author states that there are two intended audiences for his book. The first is `other interested interpreters of physics' who wish to learn something about quantum field theory in as painless a way as possible. The second is for students of physics who are bogged down in the details of learning quantum field theory and cannot see the overall picture. I am not convinced that either audience will find this book to be of much use. There are two basic problems with this book. The first is that the quantum field theory in it is of the old-fashioned kind, and is not really representative of the way the subject is either taught or thought of today. The second is that there is not sufficient background material present to make this book suitable for a novice reader. For example, I wonder how much a reader who has not studied quantum mechanics in any depth will get out of a treatment which launches into Hilbert space and a discussion of states without much in the way of proper introduction. Any reader who is familiar with the workings of quantum field theory will find much to disagree with in this book. For example, the author tries to make a case that a Fock space description is better than talking about particle labels and restricting to symmetric or antisymmetric states. The reason claimed for this is that there are states in Fock space which are neither symmetric nor antisymmetric under particle interchange which must be discarded. (In the author's words, this introduces `surplus formal structure'.) However, in Fock space an equally arbitrary choice is made by focusing on relations among the creation and annihilation operators which involve only commutation or anticommutation relations. Although this is acknowledged by Teller, no satisfactory explanation is given as to why one procedure is any less arbitrary than the other. The book is incomplete in many ways, with many interesting points mentioned just in passing. For example, the discussion around the Newton--Wigner position operator would have been greatly enhanced by including more details, such as an explanation as to why quanta described by Newton--Wigner states seem to propagate at superluminal speeds. It is hard to believe that a reader who has not met the Rindler vacuum before would find Teller's discussion to be very informative. After being so critical of this book, I should mention a part which I did enjoy and would recommend to students of physics. This is the last chapter of the book which contains a discussion of renormalization theory. It is my experience that students often find renormalization theory confusing, and may find Teller's semiqualitative discussion illuminating.