Author’s Introduction (Extract)
- The chapter is a long one; so a little guidance may be helpful.
- I spend the remainder of this first section describing the kind of presentism I prefer, and contrasting it with other A-theories. It turns out that there is considerable pressure on presentists to accept the existence of certain things that, intuitively, are “in the past”. (A plausible response to the pressure, introduced in section I, will prove relevant to the question of section III: what theory of the manifold should presentists adopt in the face of Theodore Sider’s worries about cross-temporal states of motion?)
- Section II introduces SR and GR as theories about the structure of a four-dimensional manifold; I claim it is “safe” to assume substantivalism, and I focus on the Minkowskian manifold structure of SR.
- In section III, I argue that a presentist who accepted SR should have to suppose that the present slices the Minkowskian manifold in a certain way; and that its past and future locations would constitute a foliation of the manifold. I offer presentists two ways of thinking about the metaphysics of this ostensibly four-dimensional entity, including one that manages to reject “past” (i.e., formerly occupied) and “future” (i.e., soon to be occupied) space-time points. I show that the presentist who takes my approach to the manifold can deal with the kinds of fundamental cross-temporal relations needed by post-Newtonian theories of motion and gravitation.
- In section IV, I argue that the conflict with SR is not as deep as has been suggested. In particular, the oft-heard charge of “inconsistency” is not so straightforward as it is made to seem. Whatever disharmony remains between the A-theory and SR is of dubious significance. SR is of interest mainly as an approximation to GR, and it is even less clear whether presentism is inconsistent with GR. Furthermore, quantum theory may well call for radical changes in our conception of space-time; and some of the proposed changes promise to reinstate a way of slicing the manifold that would have to coincide with the A-theorist’s division of the manifold into a series of successive presents. It is unclear whether these versions of quantum theory will win out over competitors that leave space-time looking more as Relativity sees it. But SR is false1, and GR faces challenges from an even more impressively confirmed physical theory2. These facts can hardly be irrelevant to the significance of arguments that assume their truth.
- Summing Up
- The main problems for presentism discussed in this paper were:
- Sider’s argument that presentists lack adequate grounds for physically important cross-temporal relations involving motion; and
- Objections based on inconsistency with Relativity, especially those based on alleged inconsistency with SR.
- My main conclusions can be summed up as follows:
- Sider’s problems about cross-temporal relations require that presentists take manifold structure seriously. In a Galilean or Relativistic universe, physically fundamental cross-temporal relations force the presentist to admit the existence of formerly occupied points, or find some kinds of surrogates for them in the present. The simplest strategy would be to adopt a growing-manifold presentism; but I suggested a way to maintain a one-slice presentism with persisting trajectories that passed through no-longer-existing points.
- I tried to show that the conflict with Relativity is not so deep as one might think, while also calling into question the relevance of conflict with SR or GR. If we inhabited a manifold that appeared to have the metrical properties of Minkowski space-time or of some foliable manifold satisfying GR, those of us who are presentists would not automatically be forced to reject SR or GR, because we would not automatically have to posit laws involving additional intrinsic space-time structure. The present might march in step with some physical phenomenon, and so obey laws that do not directly appeal to manifold structure going beyond the metric. Even if the presentist were forced to posit such additional structure, she would not be radically scientifically revisionist, so long as she accepts the existence of the manifold and recognizes the fundamentality of its structure in scientific explanations. Furthermore, the relevance of whatever conflict there might be between presentism and either form of Relativity remains uncertain. SR is false3=1, and GR is challenged by quantum theory. Although most physicists who are looking for a theory of quantum gravity are trying to get by without imposing a preferred foliation upon space-time, some think we need to do so. I lack the expertise to hazard an informed guess about the relative chances of these two kinds of theories. But some of the reasons proposed for introducing a physically distinguished foliation would play right into the presentists’ hands. So long as we are allowed to appeal to principles like (94) or (9*5), we would have good reason to suppose that physics had come around at last, and found a use for genuine simultaneity after all.
- Pushing Back
- Metaphysicians — including the numerous theoretical physicists and philosophers of physics who moonlight as metaphysicians — should naturally like to be able to invoke the prestige of physics in settling disputes. After all, scientific questions actually do get settled occasionally, unlike so many of the larger questions of metaphysics; it would be nice if stable results in physics could provide some leverage on the slippery problems of metaphysics. And this metaphysician, at any rate, agrees that physics simply must be relevant to many of the metaphysician’s central concerns. What part of metaphysics is more exciting than the attempt to locate ourselves — thinking and feeling human agents — in the physical world? Since physics represents our best efforts to describe the fundamental nature of that world, metaphysicians cannot ignore advances in physics if we are serious about this project.
- However, when appealing to findings from empirically well-grounded disciplines, philosophers face a strong temptation to overstate their case — especially if their philosophical opponents can be relied on to be relatively innocent of new developments in the relevant science. I fear that some B-theorists have succumbed to the temptation, judging by the relish with which they often pronounce a verdict based on Relativity. They can practically hear the crunch of the lowly metaphysician’s armor giving way, as they bring the full force of incontrovertible physical fact down upon our A-theoretically-addled heads. But what actually hits us, and how hard is the blow? SR is false6=1; GR’s future is highly uncertain; and the presentist’s conflict with either version of Relativity is shallow, since the presentist’s manifold can satisfy the same geometrical description as a B-theorist’s manifold, and afford explanations of all the same phenomena in precisely the same style. In these circumstances, how could appeal to SR or GR justify the frequent announcements that the A-theory–B-theory dispute has been “settled by physics, not philosophy”?
For the full text, see Zimmerman - Presentism and the Space-Time Manifold.
Footnotes 1, 3, 6:
- This is a rather bald and contentious claim, presumably justified at length in the text.
- Is this on account of the EPR Paradox?
- Ie. Quantum Mechanics, it later turns out.
- For any events e1 and e2, e2 is causally dependent upon e1 only if, when e2 was happening, e1 had already happened.
- For any events e1 and e2, e2 is causally dependent upon e1 only if, when e2 was happening, e1 was happening or had already happened.
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2019
- Mauve: Text by correspondent(s) or other author(s); © the author(s)