Gregg Jaeger

Review:

"Work by John S. Bell and others since the 1960s has taught us that this world is also non-local, violating a fundamental principle of classical science. The failure of locality, the lack of any coherent (classical) particle concept and contravention of (classical) causality have often been taken to imply that the prospects are dim for a realist interpretation of quantum theory. Gregg Jaeger is more sanguine. The extraordinary empirical success of quantum theory means that we must take these radical concepts seriously. The premise of his new book is that we have not yet taken them seriously enough. Jaeger urges that the conceptual structure of quantum theory, combined with a realist orientation to science, requires a transformation of the basic metaphysics of physical objects and of causation. As he puts it, this is nothing less than a Copernican revolution in our understanding of the content and structure of the world. Jaeger's goal is to "pursue an interpretation of quantum mechanics that helps provide a world picture in which the most general and long-satisfactory categories such as individual object and cause continue to hold...

Jaeger stakes out a realist interpretive approach to quantum theory, in which quantities contained in the theory truly describe properties and objects of the quantum world. Jaeger begins by reviewing the formalism of non-relativistic quantum mechanics, with a particular emphasis on the concept of particle. He notes that particles have a very different nature in the quantum domain: they remain discrete entities, but they only sometimes have definite positions or trajectories, and crucially they lack both classical substantiality and primitive thisness. Jaeger also discusses non-local correlations, focusing on the pioneering work of Bell...

In response to these challenges, the realist ontology for quantum theory Jaeger develops includes three major elements: objectively real unsharp physical magnitudes, quantum individuals as basic objects, and quantum causation. Along the way, Jaeger offers somewhat technical accounts of a wide array of foundational issues in the interpretation of quantum theory... According to Jaeger, elements of reality may be "objectively indefinite" (p. 5)....Also needed on Jaeger's realist approach is a metaphysics for quantum causation. The failure of locality means that no classical account of probabilistic causality will be sufficient. According to Jaeger, quantum potentialities are the elements of reality that determine the future state of the system, so these are appropriate candidates for quantum causes... Finally, the most important component of Jaeger's realist metaphysics is his notion of a quantum object. He begins by assuming that any acceptable object ontology satisfies Leibniz's principle of identity of indiscernibles (PII)... Jaeger proposes as an alternative his quantum principle of individuation (QPI): "A system is an individual if and only if its state is entirely specifiable by a ray in the Hilbert space associated with it" (p. 154). This is a version of the "ray view" characterized by Tim Maudlin. These individuatable particles, and objects built up from particles, form Jaeger's ontology of the objects of the quantum domain.

Jaeger takes as a consequence of QPI that in the case of a system with entangled sub-systems, such as entangled photons in a Bell-type setup, these sub-systems are not individuals and hence not quantum entities. He notes that since the degree of entanglement of subsystems can depend on the choice of basis, QPI does not in general pick out a unique ontology of quantum individuals. What QPI does do, Jaeger emphasizes, is allow for the application of PII to the quantum domain... Jaeger's project is realist, and like most realists his motivations are explanatory. As he puts it, "only realism can explain the instrumental reliability of scientific methodology" (p. 126). He says that the particle concept continues to be valuable in quantum mechanics in order to provide "physical explanations" of quantum phenomena (p. 26). By contrast, an operational (anti-realist) particle concept should be rejected because it fails to explain. Similarly, "The potentiality interpretation enables the explanation of definite measurement outcomes" (p. 114), and this is a reason in its favour... Much work remains to be done to show how Jaeger's concepts of quantum cause, quantum potentiality and individuatable quantum objects explain the observable phenomena of the quantum domain."

Reviewed by Andrew Wayne, Notre Dame Philosophical Reviews (2014)