APA 2014-2: Against causal reductionism

by Massimo Pigliucci

Second report from this year’s meeting of the Eastern Division of the American Philosophical Association. This session, under the general heading of philosophy of science, was actually constituted of just one talk, entitled “Against causal reductionism” and delivered by Chris Weaver (Rutgers University) (the session was chaired by Michael Hicks, Rutgers University).

As you’ll see from my interspersed and final comments, I actually disagree with the author’s main thesis, and even the way he goes about defending it. But the talk did stimulate my neuronal firing, and I hope it will generate some thoughtful discussion here, as well as provide another example of what professional philosophers do at their daily job.

Weaver began by observing that the consensus view in physics is that the universe began in a very unusual state of very low entropy, but maintains that the characterization of such initial state as “unusual” is based on the highly questionable principle of indifference [1] and is otherwise difficult to defend. I don’t really have a bone to pick in that particular fight, and it honestly wasn’t clear to me how exactly this related to the main topic of the talk, causal reductionism, but that’s where we are going next.

Weaver thinks that causal reductionism is based on an unsound argument from physics. Causal reductionism – in this context – is the view that causal interactions reduce to non-causal facts, that they are nothing above and beyond lawfully related events, where natural laws themselves are not “causal.” (Of course, there is a huge literature on the very concept of causation itself, but we’ll leave that for another day. [2])

Causal reductionism, according to Weaver, is not implied by empirical analyses of causation but depends instead on the following “argument from physics”:

P1) Physical science only requires natural law and physical history (no causation)
P2) If 1, then causal reductionism holds
C) Causal reductionism holds

Therefore, according to Jonathan Schaffer, among other authors, “causation disappears from sophisticated physics.” Weaver doesn’t buy it, and proposes to attack the first premise above, by arguing that there is a distinction between the formalism and the interpretation of any given theory T, and that while some particular physical theory T lacks the formalism of causation, any sensible interpretation of T will have to include causal talk.

Take the idea of a gravitational field in the general theory of relativity (henceforth, GTR). While the formalism of the theory (i.e., the equations) doesn’t include any talk of causes, the field itself has to be interpreted – according to Weaver – as having causal properties: if gravitation is the conformation of space-time itself, then obviously it has causal properties.

GTR is based on four principles: relativity, general covariance, finitude of the speed of light, and equivalence [3]. Weaver maintains that the principle of equivalence (between gravitational and inertial mass) includes language concerned with the effects of gravitation, i.e., it is framed by using talk of causality. Even when it is not expressed using causal language per se, the principle is usually accompanied by language that implies causal notions (for instance in Sean Carroll’s 2003 textbook [4]). Indeed, Einstein himself understood the principle of equivalence in causal terms, which Weaver demonstrated by using a number of direct quotations.

So, the idea is that although the field equations of GTR do not explicitly include any causal talk, one cannot avoid but interpret them as telling us about the behavior of the gravitational field, including, therefore, its (causal) effects on the motion of objects and free particles.

Weaver appropriately quoted a number of experts on GTR confirming his interpretation that Einstein did talk of the theory in interpretive causal terms, whenever he was discussing how the theory accounts for the inertial motion of bodies.

(At this point in the talk I had the perhaps obvious thought that while all of the above is fine, GTR is still a classical theory, and that causal talk – and hence Weaver’s approach – still break down when we get to quantum mechanics, or to a future quantum theory of gravity. Turns out the author himself dealt with this objection toward the end of his talk, not at all in a satisfactory manner, I think.)

Yet another example of causation within GTR is the theory’s prediction of gravitational waves, which do causally affect both fields and matter. The recent (alleged) discovery of gravitational waves [5] was the result of observation of effects of such waves on the cosmic background radiation, and moreover, gravitational waves are certainly emitted by causal interactions.

Weaver maintains therefore that causation enters sound physical science by way of a proper understanding of general cosmological theories such as GTR. After all, spacetime points, according to GTR, are associated with “domains of influence” that are almost universally interpreted causally.

Crucially, GTR does not reduce the above causal influence talk to anything more fundamental or primitive, regardless of mistaken (according to Weaver) identification by some authors of causal influence structure with light cone structure [6].

Finally, as it is customary structure in many philosophy talks, the author considered (and attempted to rebut) a number of possible objections to his thesis.

The first such objection notes that the dynamical laws of GTR are time reversal invariant, which means that any causal reading of those equations implies a denial that causes necessarily precede their effects.

Here Weaver surprised me by simply shrugging the objection away! The principle that causes must precede their effects, he maintained is false. What he was referring to was the possibility of simultaneity of cause and effect, which has been proposed by others. The problem is that this would require a radical principle of instantaneous action at a distance, of the kind that Newton was worried about, as Weaver himself was keenly aware. The best he had to offer was the claim that instantaneous action at a distance may not actually be the case, but that the mere possibility is not incoherent. Okay then, but I thought we were talking physics, not logic.

Worse, the point about the time reversal invariance of GTR equations would also imply backwards causation, which Weaver accepts (well, he has to!) on the grounds that GTR does not preclude closed timelike curves [7], which in turn would make time travel possible [8].

Weaver also nodded toward the problem with quantum mechanics that I raised above, but said, and I quote: “well, you know, quantum gravity is a mess…”

A second objection points out that GTR is not itself a fundamental physical theory, so the causal reductionist should not be worried, at the least not yet. GTR will eventually have to yield to quantum mechanics in ways that would rub out any attempt to understand the causal activity of the gravitational field as fundamental physical activity.

To this Weaver, rather astoundingly, simply bit the bullet and acknowledged that okay, so maybe fundamental physics can do away with causality, but not all physical theories can. But this is worse than a Pyrrhic victory, I think. To begin with, because it is well established in philosophy of science that “special sciences” (i.e., everything but fundamental physics, including non-fundamental physics) do effectively deploy causal talk and cannot, apparently, do without it [9]. The puzzle of causal vs non-causal talk in science has always been at the fundamental level. If it turns out that the only reason GTR has to engage in causal talk is because it isn’t a fundamental theory (and we know it isn’t!), and that once we move to more fundamental levels of description and explanation causal talk yields to nomological talk (i.e., talk in terms of laws of nature) then the game is up for the critic of causal reductionism. Boy would I love to hear about this from physicists and philosophers of physics!
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Massimo Pigliucci is a biologist and philosopher at the City University of New York. His main interests are in the philosophy of science and pseudoscience. He is the editor-in-chief of Scientia Salon, and his latest book (co-edited with Maarten Boudry) is Philosophy of Pseudoscience: Reconsidering the Demarcation Problem (Chicago Press).

[1] Principle of indifference, Wiki entry.

[2] See: The Metaphysics of Causation, by Jonathan Schaffer, Stanford Encyclopedia of Philosophy.

[3] Principle of relativity; General covariance; Equivalence principle.

[4] Spacetime and Geometry: An Introduction to General Relativity, by S. Carroll, Addison-Wesley, 2003.

[5] Gravitational wave discovery looks doubtful in new analysis, by Clara Moskowitz, Scientif American, 22 September 2014.

[6] Light cone, Wiki entry.

[7] Closed timelike curve, Wiki entry.

[8] Unwinding time, by S. Carroll, The Wall Street Journal, 17 December 2011.

[9] See, for instance: Every Thing Must Go: Metaphysics Naturalized, by J. Ladyman and D. Ross, Oxford University Press, 2007.