Physics and the Physical

Physics and the Physical

It sounds reasonable—indeed tautological–to say that physics is about the physical, as psychology is about the psychological. But that is not clearly true. Consider Newton’s physics: it includes not only physical things in the ordinary sense but also space and time—as well as gravitational force. That last item raised eyebrows at the time owing to its “occult” nature (it wasn’t “mechanical”), but the first two items also raise questions. Are space and time physical? Intuitively they are not, but the question is clouded by lack of clarity about the meaning of the term “physical”. They certainly contrast with chunks of matter in a number of respects, according to Newton: they lack mass, they are not solid and impenetrable, they don’t move around, they can’t be sensed, they are not made of atoms, they have no shape, and they are infinite and eternal.  Space and time contrast rather sharply with matter—they are, if anything, immaterial. Yet they are essential to the way physics understands the world; in particular, they are how motion is defined, i.e. translation of place over time. Newton was not himself a materialist, believing both in the soul and in God, so he had no materialist scruples about accepting this capacious ontology: he had no wish to keep physics physical. He was not a physicalist about physics. Someone claiming to reduce the mind to physics, say, would not be a physicalist under this conception of physics, i.e. someone who believes only in physical things. In physics we have material bodies as well as space and time (and force), the latter not being physical in the sense applicable to material bodies (mass, solidity, motion, etc.) We could say that, for Newton, the world of physical things exists within a larger world of non-physical things. And these things are not just trivially non-physical (as radiation may be said to be), but fundamentally different in nature from what is physical. The physical thus exists against a background of completely non-physical things. If anything, space and time belong on the side of God, not on the side of matter (consider their infinity and eternality)—at least as Newton sees things.

            It is understandable that a physicist with empiricist and physicalist tendencies will balk at Newton’s ontology, because he includes realities that are non-physical and imperceptible. A positivist will be suspicious of such things (see Mach), especially one who wants physics not to stray from the physical. Einstein was just such a physicist: he had decided positivist sympathies and he wanted to find a “physical meaning” (his phrase) for such terms as “time” and “space”. Accordingly, in the special theory of relativity he replaced talk of time with talk of clocks—physical objects in space. No more superordinate time dimension marching on in splendid isolation from the physical world; instead there are just physical clocks and their readings. He physicalizes time; or he eliminates it in favor of clocks. As a result we get the familiar but still startling “discoveries” of special relativity, such as the relativity of simultaneity and “time dilation”. These claims are all really about the behavior of clocks in various conditions of motion. Clocks are finite, perishable, mutable, physical things, unlike the Newtonian time dimension. Motion is understood as change of position correlated with different clock readings, with each object assigned its own clock in Einstein’s thought experiment. There are thus as many “times” as there are assigned clocks, and hence “time” has not the absoluteness we might expect—or better, there are just clocks in this model with time itself eliminated from consideration.

            What about space? What “physical meaning” can we give to space? The first thing is to do is make motion relative: objects only move relative to other objects not in relation to absolute space. Thus we replace the impalpable Newtonian spatial dimension with perceptible physical objects—nothing non-physical in the picture. Movement through space becomes change of position relative to a chosen physical object, as time becomes the changing behavior of clocks. Time and space, as Newton understands them, drop out, to be replaced by relations among physical objects. Hence we have physicalism about physics. Moreover, in general relativity space comes to have some of the characteristics of matter: instead of being fixed and unchangeable, it acquires the ability to bend, as steel rods may bend. Note that space doesn’t bend in space, as if there is some extra spatial dimension behind the curvature of space; rather, space simply is the collection of all such curves. Just as what we call “time” is regarded as a collection of clocks, so space is regarded as a collection of (gravity-induced) bends—which brings space much closer to matter than it was under the Newtonian conception. Only something physical in nature can literally bend, so we have brought space into the physical arena. The old Newtonian dualism of space and matter has been replaced by a quasi-monism of bendable being. Indeed, one might wonder how space can be curved, with corresponding causal powers, unless it is a form of matter (of an etiolated kind, to be sure). Thus Einstein physicalizes space in the general theory as he physicalized time in the special theory. Now physics has become comprehensively physical under the new dispensation. All this might sound counterintuitive and confused, but it is the result of ruthlessly physicalizing the Newtonian system. To put it differently, this is what you get if you insist on finding “physical meaning” for the terms of standard pre-Einsteinian physics–you distort their meaning beyond recognition. What implications there might be, or not be, for the nature of matter, motion, space, and time, as they exist in nature, is very much left an open question; one might suppose, none. We have simply stopped talking about one thing (motion through space over time) and started talking about something quite different (clock readings of events in varying relative positions). Likewise, instead of referring to gravitational effects in space, we describe space as itself curved: light rays bend in space near massive bodies because space itself curves like a physical thing; it isn’t that space remains unchanged while things move differently through it. These are all physicalizing tendencies designed to free physics from the ontological heterogeneity of classical Newtonian physics. They result from adopting physicalism about physics. No doubt this tendency reflects empiricist assumptions, given that the physical is deemed perceptible; so Einstein’s style of physics results from Newton’s physics put through the sieve of empiricism. Otherwise put, it changes the subject.  [1]   

            Can we conclude from this that physics would look very different if it was never subjected to the physicalizing tendencies in question, themselves an offshoot of empiricist epistemology? It appears that we can. Let us imagine a world like the world Newton describes except that we stipulate that there are no physical objects in this world, i.e. no objects with mass, solidity, or perceptibility. There are, however, things that move through space over time—a bit like neutrinos, perhaps. Compared to our weighty solidities these hypothetical things are not material at all—they are wispy penetrable things. They exist in a universe of absolute space and time, which themselves are not physical. There are no sentient beings in this universe, and there are no clocks or observers of any kind. Nevertheless, there are laws of motion, mathematically expressed—let’s say Newton’s laws. Then there is in principle a physics of this world, with forces and equations governing these forces. In this world physics is entirely, not merely partially, non-physical (whatever quite the notion of the physical comes to—I have stipulated possessing mass and solidity for present purposes). That is, there is a science of motion for this world, tailored to the entities it contains; whether there is anything physical is beside the point. So physics is not essentially about the physical as such; it is about motion in space over time.  [2] The label “physics” is therefore misleading in that it suggests that the science in question deals essentially with what is physical (compare “psychology”). We might want to rename it “motion science” or some such. Indeed, our actual physics contains entities often deemed non-physical (in some sense) such as fields of force and certain massless particles. Newton’s physics dealt with the motions of material bodies, but his general framework is not necessarily tied to that ontology. By contrast, Einstein’s conception of physics ties it firmly to the physical, even to the point of physicalizing time and space—or, more accurately, replacing them with surrogates deemed more “meaningful”. I think this was a mistake, but I haven’t attempted to argue that here; my aim has been rather to set out the underlying methodological and metaphysical issues more perspicuously than is usual. The relationship between physics and the physical is actually quite contentious; certainly, we must not assume that what physics deals with is ipso facto physical in any well-defined sense.  [3]  

Colin McGinn

  [1] Here I am influenced by some unpublished work of Randolph Lundberg, though I don’t attribute my conclusions to him.

  [2] If we borrowed from Descartes the notion of an immaterial substance, we could specify a world that contains such entities in a state of motion governed by laws. The science of these motions would still be physics.

  [3] We may note that, according to the OED, the word “physic” was used in the Middle Ages and later to refer to medicine, and that it comes from the Greek phusis meaning “nature”. Thus it did not originally connote the idea of corporeal matter. It is certainly not analytic that physics is the study of corporeal matter, which is why it can include what is not material. It was Einstein who edged physics towards the physical, in our modern sense, with his insistence that we find “physical meaning” for terms like “space” and “time”. Under “physical” in the OED we find “relating to things perceived through the senses as opposed to the mind; tangible or concrete”. This is the sense that Einstein surely intended, though he says little to clarify his meaning. Very likely he was just taking over Mach’s positivist critique of Newton.