Forthcoming in Studies in History and Philosophy of Modern Physics, 2003

Review of
William Lane Craig
Time and the Metaphysics of Relativity
Kluwer Academic Publisher, 2001
Mauro Dorato
Department of Philosophy
University of Rome Three
Via Ostiense 234, 00146, Rome, Italy
           e-mail:dorato@uniroma3.it

In his recent book Time and the Metaphysics of Relativity, William Lane Craig uses the concept of time to try to reconstruct strong conceptual links between theology, metaphysics and physics, three vertices of a triangle that until the 17th century were much less separated than they are today. According to Craig, the task of the philosopher of time is not just to use the prestige of contemporary physical theories to support certain metaphysical views (the A versus the B theory), or, more questionably, certain views of God (his timelessness versus his omni temporality). He also claims that one should be guided by certain a priori theological and metaphysical hypotheses in order to interpret and even evaluate physical theories. In this respect, the main aim of the book is to rehabilitate a neo-Lorentzian interpretation of relativity and to show its connection with cosmic time, which Craig questionably regards as a direct measure of God’s, tensed, A-theoretical Time. Not by chance, the present volume is part of a remarkable tetralogy, the other three dealing with God, Time and Eternity, The Tensed Theory of Time and The Tenseless Theory of Time. 
As “an accessible, largely non-mathematical, philosophically informed introduction to relativity theory and the concept of time contained thereof” (p. ix), the book would do its job quite well if it were not so biased toward certain preconceived metaphysical and theological views. The trouble is not that introductions should be philosophically neutral, but rather that the evaluation of some arguments should be more careful, as shown by his one-shot rejection of the metaphysical significance of Gödel’s rotating solution to Einstein’s field equations (p.157). On other occasions, large batteries of quotations from the primary and secondary literature are good instances of arguments ex auctoritate and thereby fail to advance a substantially new claim. Nevertheless, Craig’s attempt of rehabilitating a neo-Lorentzian interpretation of relativity is challenging: not only does the author possess an encyclopedic knowledge of the historico-philosophical literature on time and relativity, but he also shows a good command of the technical material, two virtues that among theologians are quite uncommon and deserve attention.
After a brief historical background of the special theory, serving as an introduction to Lorentz’s compensatory hypotheses (ch.1), he presents Einstein’s special theory (ch. 2), its conceptual novelty (ch. 3), and its empirical basis (ch.4). I don’t have particular objections to this initial part, except that I don’t agree with the author’s remarks about the impossibility of deciding in all cases when an hypothesis is ad-hoc. If an hypothesis H explains experimental results purely post-factum, has no independent support, and does not entail new testable results, I would say that H is ad hoc. I take it that Lorentz’s Molecular Force hypothesis met all these requirements, something that, pace Craig, does not seem a matter of “philosophical taste” (p. 16). 
The fifth chapter presents what the author (unfortunately) calls two “interpretations” of relativity theory. The first is a relativity interpretation, the “3+1 reading” of the theory that Einstein originally proposed, based on the centrality of the notion of inertial frames, light signals, and relative three-dimensional spaces and global time coordinates. According to Craig, the ontology presupposed by such an interpretation is one of things or substances, or alternatively, in Lewis’ terminology, of entities that persist in time by enduring. Prima facie at least, this interpretation regards Minkowski spacetime merely as a mathematical instrument, and seems more in accord with a tensed view of time. On the other hand, within the spacetime interpretation, Minkowski spacetime is understood realistically, objectivity is identified with invariance and frame-independence, while the world is regarded as composed of tenseless, four-dimensional, perduring entities having spatiotemporal parts, or simply of pointlike events, sewn together in a block universe. The corresponding metaphysics of time is provided by the B theory, characterized by a becomingless world. 
Surprisingly, Craig goes so far as claiming that what I would have called two formulations of the same theory entail “strikingly different metaphysical visions of reality.”(p. 81). The word “interpretation” seems inappropriate for relativity precisely because of its ontological implications: while in quantum theory we have different interpretations because of the radically different ontologies implied, say, by Bohmian mechanics or by the many mind interpretations, I would claim that, despite Craig’s arguments, nothing of the kind applies to special relativity. 
Terminological questions aside, I don’t see why adopting the spacetime “formulation” of special relativity (perhaps within a model-theoretic view of scientific theories) should force one to be a spacetime realist: later in the book Craig himself preaches an instrumentalist approach to the spacetime interpretation (p. 192-3). Conversely, using the 3+1 formulation for the purposes of describing a particular physical system is not incompatible with Minkowski-spacetime-realism, i.e., with the belief that certain physical systems exemplify the spatio-temporal structure given by Minkowski spacetime. 
Furthermore, Craig claims that (i) both interpretations are inimical to becoming and that (ii) the relativity interpretation is “explanatorily deficient” vis à vis the spacetime interpretation. As to (i), I will just note that the spacetime formulation is not necessarily wed to the block-view of the universe, as Craig assumes almost without giving arguments (p. 104). Agreed: since in the spacetime formulation a becoming relation can be objective if and only if it is invariant, such a relation must be local, meaning that the “physical present” reduces to a point-like event. The friend of becoming’s first task is then to explain the apparent conflict with our experience, undoubtedly making us believe that “the present” extends to the boundaries of space (p.102, p. 152). As noted by Stein (1991), however, if we consider (i) the finite temporal interval which is psychologically necessary for judgments of temporal succession (“this is before that”) and (ii) the enormous speed of light, we have the elements to explain the phenomenological experience of the spatial extendedness of the present. Since experiments on human beings report that below a threshold t of approximately 50 ms we judge two temporally separated light signals as simultaneous, within a sphere of 15.000 km (= ct) everything in our visual field looks “psychologically simultaneous” (see Dorato 2002a). To this important psycho-physical fact, that explains away an intuition that presentists regard as charged with ontological significance, I would add that becoming should be explicated in a slightly deflated but philosophically satisfactory way, namely by identifying the successive coming into being of events at (proper) times with their mind-independent occurring at those times (Dorato 2002b). If these necessarily sketchy remarks are correct, the non-physical, main metaphysical motivation adduced by Craig to introduce a third, neo-Lorentzian interpretation of relativity – namely, the rescue of a mind-independent becoming – is lost. 
As to the charge of “the explanatory deficiency of the relativity interpretation” vis à vis the spacetime interpretation (the second claim above), consider that talking of the “reality” of the relativistic effects – and, consequently, of our need of explaining them – is ambiguous. Craig says that time dilations and lengths contractions are “real”, and if by “real” he means “measurable”, his claim is correct. If instead “real” means “due to a causal mechanism” or “due to a force”, his claim is wrong, because in this second sense the relativistic effects are not real at all, so that there is nothing to explain. In the “real = measurable” case, the relativity of simultaneity on the one hand, and the light-clock hypothesis on the other do suffice to explain the relativistic effects, in such a way that the “spacetime explanation” – given in terms of “different perspectives on a unique four-dimensional reality” – simply completes and does not compete with the “relativity explanation”. However, since in “the real = caused” case Einstein’s theory tells us that there is simply nothing to explain, Grünbaum’s analogy with the modern principle of inertia vis à vis Aristotelian physics is completely appropriate, despite Craig’s attempt at rebutting it (p. 99). Such an analogy points to the fact that according to Aristotle any motion, even the rectilinear, uniform motion that we call inertial, requires a cause and therefore an explanation in terms of a mover, while in classical physics, causes (i.e., forces) need to be introduced only to explain departures from inertial motion. Grünbaum argues that an analogous relationship holds for Lorentz contractions and time dilations before and after Einstein’s special relativity. In conclusion, the question of which of the two formulations of relativity we should prefer is merely pragmatic, and as such it does not have the metaphysical consequences hoped for by Craig.
In the second part of the book Craig argues that relativity theory, in both “interpretations,” is based on an outdated positivist epistemology (ch. 7, 8). Granting that Einstein’s criterion for synchronizing two clocks at a distance is verificationist in essence, however, it does not follow that just because the positivist theory of meaning has been abandoned, we should feel justified to reintroduce in science wild metaphysical hypotheses with no independent support from science. 
Nevertheless, it would be unfair to blame Craig for his uncontrolled taste for metaphysical speculations, given that the final part of the book seems to oscillate between two contrasting philosophical positions. According to the first, “relativity physics…is not necessarily saying anything that is relevant for the metaphysician” (p. 152), a claim that tends to be advanced whenever evidence coming from physics is against his metaphysical views. The second position is that physics “confirms” certain metaphysical and theological views over others, a claim that is put forth whenever evidence for the existence of a privileged frame (coming for instance from cosmic time or quantum non-locality) seems more reassuring. If this impression is well-founded, Craig’s book is essentially guided by an apologetic attempt and opportunistically uses physics and metaphysics for his purpose.
As evidence for the oscillation toward the first of the two positions above, consider that in dealing with Gödel’s closed-time universe or (p.157), or with Barbour’s “end of time” in quantum gravity (p. 160) – which are “bad news” for his metaphysical inclinations – he writes that physical time and metaphysical time are to be regarded as distinct or independent of each other, as measures and thing measured are (the reference to Newton, the other hero of the book beside Lorentz, is obvious). Since Craig assumes that only an outmoded verificationism could identify physical time with metaphysical time (p. 160-162, p. 170), for him the abandonment of positivism automatically entails the independence of metaphysics from physics. Given this independence, however, why worry about physics at all and write a book on the metaphysics of relativity? Furthermore, Craig seems to tacitly assume that the domain of metaphysics is shut for the physicist or the philosopher of physics, as only theologians can tread it. However, an overcoming of positivism is compatible with a physicalist metaphysics, or with the claim that there’s nothing but physical systems out there.
Coming now to the oscillation toward the second position, the last chapter is meant to support the A theory of time and the existence of a privileged frame with empirical arguments, essentially given by the contingent existence of cosmic time in certain symmetrical cosmological models of the universe and by highly controversial issues coming from quantum non-locality, stressed by Maudlin and others. The evidence for a connection between a neo-Lorentzian interpretation of special relativity and the existence of cosmic time, however, is very thin, except for the fact that they both would make a more hospitable environment for global becoming, by him questionably regarded as evidence for the temporality of God. To theological speculations of this sort, we could reply by recalling Laplace’s answer to Napoleon, asking him about the role of God in physics: “Sire, je n’ai pas eu besoin de cette hypothese!”. 
In conclusion, by focusing on my criticism, I feel I have not done justice to the scope of this book and to the interesting proposal of reconsidering, in the wake of Bell, a neo-Lorentzian interpretation of relativity as a possible path that future physics could take. At the moment, an addition to the standard structure of Minkowski spacetime a privileged but undetectable frame of absolute rest per se would not contradict Einstein’s theory, but should be regarded as superfluous for the physical description of the world. Is evidence coming from quantum physics sufficient to reverse this judgment? We should be thankful to the author both for having stressed this important, still unanswered question and for having reminded us that trying to establish the compatibility of the scientific image of time with its manifest image is one of the most important tasks of contemporary philosophy of physics.

References

Balashov, Y. and Janssen M. (2002), Presentism and Relativity, http:/philsci-archive.pitt.edu, forthcoming in British Journal for the Philosophy of Science.
Dorato, M. (2002a), Kant, Gödel and Relativity, in P. Gardenfors, K. Kijania-Placek and J. Wolenski (eds.), Proceedings of the Invited Papers of the 11th International Congress of the Logic Methodology and Philosophy of Science, Synthese Library, Kluwer, Dordrecht, pp.329-346. 
_____(2002b), On Becoming, Cosmic Time and Rotating Universes, in C. Callender (ed.) Time, Reality and Experience, Proceedings of the Royal Institute of Philosophy Conference for 2000, Cambridge University Press, Cambridge, pp. 253-276.
Stein H. (1991): “On Relativity Theory and Openness of the Future”, Philosophy of Science 58, pp. 147–167