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One of the greatest challenges in fundamental physics is to reconcile quantum mechanics and general relativity in a theory of quantum gravity. A successful theory would have profound consequences for our understanding of space, time, and matter. This collection of essays written by eminent physicists and philosophers discusses these consequences and examines the most important conceptual questions among philosophers and physicists in their search for a quantum theory of gravity. Comprising three parts, the book explores the emergence of classical spacetime, the nature of time, and important questions of the interpretation, metaphysics, and epistemology of quantum gravity. These essays will appeal to both physicists and philosophers of science working on problems in foundational physics, specifically that of quantum gravity.
In sections 6 and 7 of their paper in this volume, French and Rickles raise the question of the logical relations between the indistinguishability postulate (IP) and the various senses in which particles might fail to be individuals. In section 6 they refer to the convincing arguments of French and Redhead (1988) and of Butterfield (1993) that IP does not logically entail non-individuality, understood several ways – even though, as all seem to concede, there is something perverse about taking bosons and fermions to be individuals. Going the other way, the possibility of IP violating ‘quons’ (Greenberg, 1991) shows that if non-individuality is taken to mean the absence of continuous distinguishing trajectories, characteristic of standard quantum mechanics (QM), then non-individuality does not entail IP. Nor, as French and Rickles point out, do substance or haecceity views of individuality.
But what if we conceive of individuality in terms of the Principle of the Identity of Indiscernibles (PII)? First, French and Redhead (1988) and Butterfield (1993) have given theorems showing that bosons and fermions violate PII, while the former have also demonstrated violations of PII in the case of a certain paraparticle state. But these cases, as I will explain (and as French and Rickles point out), cover just a very few of the possible kinds of quantum particles, and so for each kind the question arises as to whether it violates PII.
I want to take issue with the definition of enantiomorphy that Pooley gives in his paper in this volume. His account goes something like this:
(a) Suppose that the relationist has an account of the dimensionality of space, according to which space is n-dimensional.
(b) The relations – especially the multiple relations – between the parts of a body determine whether it is geometrically embeddable in n-dimensional spaces that are either (only) orientable or (only) non-orientable.
(c) Then ‘an object is an enantiomorph iff, withrespect to every possible abstract [n]-dimensional embedding space, each reflective mapping of the object differs in its outcome from every rigid motion of it.’
This account depends on the truth of (b). Suppose that a body were embeddable in both orientable and non-orientable spaces of n dimensions. Then it might fail to be an enantiomorph, not because any of its possible reflections in physical space was identical to a rigid motion of the body, but because in some abstract space a reflection and a rigid motion of its image are identical. Pooley (in note 14) makes this point, but claims that the burden of proof falls on the opponent of his account to show that (b) is false.
The greatest challenge in fundamental physics is how quantum mechanics and general relativity can be reconciled in a theory of 'quantum gravity'. The project suggests a profound revision of our notions of space, time and matter, and so has become a key topic of debate and collaboration between physicists and philosophers. This volume collects classic and original contributions from leading experts in both fields for a provocative discussion of all the issues. This volume contains accessible introductions to the main and less well known approaches to quantum gravity. It includes exciting topics such as the fate of spacetime in various theories, the so-called 'problem of time' in canonical quantum gravity, black hole thermodynamics, and the relationship between the interpretation of quantum theory and quantum gravity. This book will be essential reading for anyone interested in the profound implications of trying to marry the two most important theories in physics.
In recent years it has sometimes been difficult to distinguish between articles in quantum gravity journals and articles in philosophy journals. It is not uncommon for physics journals such as Physical Review D, General Relativity and Gravitation and others to contain discussion of philosophers such as Parmenides, Aristotle, Leibniz, and Reichenbach; meanwhile, Philosophy of Science, British Journal for the Philosophy of Science and others now contain papers on the emergence of spacetime, the problem of time in quantum gravity, the meaning of general covariance, etc. At various academic conferences on quantum gravity one often finds philosophers at physicists' gatherings and physicists at philosophers' gatherings. While we exaggerate a little, there is in recent years a definite trend of increased communication (even collaboration) between physicists working in quantum gravity and philosophers of science. What explains this trend?
Part of the reason for the connection between these two fields is no doubt negative: to date, there is no recognized experimental evidence of characteristically quantum gravitational effects. As a consequence, physicists building a theory of quantum gravity are left without direct guidance from empirical findings. In attempting to build such a theory almost from first principles it is not surprising that physicists should turn to theoretical issues overlapping those studied by philosophers.
But there is also a more positive reason for the connection between quantum gravity and philosophy: many of the issues arising in quantum gravity are genuinely philosophical in nature.