The Mystery of Motion

The Mystery of Motion

Consider a completely static universe—nothing moves in it. Within this universe there lies a sun and solar system just like ours except nothing is in motion, relatively or absolutely. Now ask yourself the question, “How would it move, if it were to move?” Can we infer from the properties it has how it would move? This seems difficult: the shape, color, chemical composition, volume, and mass of the objects don’t predict a unique course of movement. There is no entailment from these static properties to the dynamic properties of the objects.[1]That is, the dispositions to movement of our imagined solar system are not deducible from its other properties. They seem superadded not supervenient, certainly not reducible. So the basis of movement is not to be found in the intrinsic properties of objects. The question, then, is where it does come from—what is its origin? We might compare the question with a comparable question about consciousness: if we consider a universe just like ours physically, we find that consciousness is not entailed by the facts obtaining therein. God has more work to do in bringing consciousness into existence than just bringing physical organisms into existence. Similarly, God has more work to do in creating motion than just creating objects with static properties. The static world is like the zombie world—both are a step short of constituting motion or mind. A kind of dualism therefore seems indicated.

In Newton’s physics we find just such a dualism: what needs to be added is force. Gravitation causes planetary motion, and there exists a force of gravity. This force is proportional to mass, but it is not reducible to mass. There is a law linking mass and motion, but these are distinct attributes of objects. We only get motion when force is added to mass. Similarly, there may be laws linking brain states and consciousness, but there is no identity between the two. So Newton’s scheme is fundamentally dualist: force is to objects as the mind is to the body (under a dualist view of the latter). Force is an extra element or ingredient in the universe. Motion owes its existence to the existence of this force—itis the cause of motion.

But this motion-inducing force invites skepticism, often voiced. First, there is the question of its visibility: we don’t seem to see it—it is a theoretical postulate. Nor is it even likewhat is visible (as atoms are). It seems inherently unobservable (except by way of its effects). Second, we are told that it acts at a distance instantaneously; but that seems hard to credit, and it makes movement into an occult phenomenon. Third, how exactly is it related to other properties of objects? Are there possible worlds in which the same force operates but objects have very different intrinsic properties? Are there worlds in which feathers exert great gravitational force and suns very little? Fourth, and most concerning, the alleged force seems like an ad hocpostulation that simply summarizes observed motions: the force exists becauseof the observed motions—there is no more to its existence than the motions. There is certainly no evidence for its existence except the motions it allegedly produces. It looks suspiciously like a dormitive virtue brought in to explain soporific effects. Why do the planets move as they do? Because they are subject to a force that makes them move that way, i.e. they are disposed to move that way. The force doesn’t add anything; it just repeats what we already knew. It would be different if we could detect it independently of the motions it causes–but we can’t. How does the brain produce consciousness? By means of a force that works on (or in) the brain to generate conscious events—call it “psychical force”. That is hardly explanatory and smells like empty stipulation.

So should we get rid of this alleged force? General relativity is said to do just that: it replaces gravitational force with the structure of space. Massive bodies deform the geometry of space to cause the motions we observe. But how can spatial geometry cause motion? How can it trigger motion? Couldn’t there be a static universe consisting of curved space-time? Force at least offered to initiate motion—you just apply the force to an object. But how can curvy space initiate motion—where is the oomph? The theory tells us how an object will move through space when it moves, but it is silent on whythings move—what causes motion. The origin of motion is thus left obscure. It was just this problem that led to the doctrine of occasionalism: God is the origin of motion. Either God acts continuously to keep things moving or he starts the machine up and watches it play itself out. Proponents of occasionalism can’t see how motion comes about unless some sort of divine agency lies behind it—for what else could be the origin of motion? Similarly, God is supposed to create the mind and ensure its linkage to the body, since the body itself has no power to create minds. Thus motion argues the existence of God: no God, no motion; motion, therefore God. But theology aside, the doctrine is wheeled in to explain a physical fact, namely that things move. This seems unintelligible otherwise: for things need a reasonto move. Movement can’t be a brute fact.

Have we got over the problem of motion today? Does our current physics remove the puzzle? No, it ignores it. Sure, we have theories of motion that are predictive and mathematical—they characterize the laws that govern motion—but do they really explain why motion occurs? Do they explain what causes it? I don’t think they do–any more than current neuroscience explains what causes consciousness. The four forces recognized in contemporary physics are like Newton’s gravitational force in this regard: they are postulations made on the basis of nothing but observed motions. They are an I-know-not-what introduced to sum up observations. What isa force anyway? We might appeal to internal impressions of our acts of will, but that can hardly be an adequate basis for an ontology of physical forces. We are merely papering over our ignorance. Of course, positivistic philosophers and scientists might try to render the question meaningless, but I take it we need not travel down that dreary road again. There really is a genuine question about how motion comes to be—and we don’t seem to have much to say about it. It remains as mysterious as it ever was.

Three questions about motion may be distinguished, each calling for a theory, and each very difficult. First, what is the natureof motion—is it relative or absolute? Despite a preference for relative theories today, I don’t think this question has been resolved, or even properly posed—but I won’t pursue it now. Second, what are the lawsof motion? Here we have made impressive progress, beginning with Galileo and Newton, though quantum theory presents some substantial puzzles, especially in relation to microscopic motion. Third, what is the originof motion—what causes it? This is the question I am most concerned with here, and it is not answered by answers to the first two questions. It seems virtually invisible in today’s intellectual climate (as the problem of consciousness was until relatively recently). It is hard to get people to take the question seriously. There is a vague feeling that providing the laws of motion suffices to dispel all questions about motion: but that is surely wrong (as Newton himself fully recognized). How matter gives rise to motion is an unsolved mystery; as is the question of how it gives rise to just these motions and not those other ones. That is the point I tried to dramatize with the example of the static universe. Simply put, the question is why anything moves to start with. Was there motion before the big bang or did that event trigger motion? If so, how? Could there be an eternally static universe? Could matter, as we know it, stand stock-still? Could our universe ever cease to move? What would it take for motion to speed up? How exactly is the motion of an object related to its other properties? God was called “the prime mover”, but can he move? If not, how can he cause movement? I don’t think we have answers to these questions: we merely observe movements and then attempt to supply predictive laws governing them. But that project will only take you so far. We are very familiar with movement, both through our senses and in moving our own bodies; but what would we think if we had never experienced movement and then one day observed it to occur? Wouldn’t it strike us as miraculous? How could thisdo that? It would seem like an infusion from another realm not a predictable unfolding of what we already know. It would be like breathing life into a dead body. An infusion of electricity was necessary to do that in the case of Frankenstein’s monster, and a similar infusion would be needed to explain how immobile matter could become animated. The transition from immobility to movement through space is non-trivial, indeed remarkable. Earlier thinkers were impressed with this phenomenon, as contemporary thinkers appear not to be—I think we should return to earlier wonderment. How doesthe universe contrive to contain movement? We might label this the “motion-body problem”.[2]

Colin McGinn

[1]This basic point is found in Hume’s celebrated discussion of causation in An Enquiry Concerning Human Understanding. It is no accident that he uses the example of the motion of a billiard ball to illustrate his thesis that it is impossible to deduce causal powers from intrinsic properties of the cause. There is no reason, independently of “custom” (experience), to expect any specific motion upon impact given knowledge of the known properties of objects, or any motion at all.

[2]Similar themes have been voiced by Noam Chomsky, who observes that motion used to be regarded as a “hard problem”—and should still be so regarded.