Motion Again


Causation and Motion



Consider a universe containing just two objects, A and B (suppose they are tennis balls). They are moving relatively to each other at 30mph and are a million miles apart. Conventional opinion has it that it is arbitrary to declare one at rest and the other in motion; we can only regardone as at rest while the other moves in relation to it. We cannot suppose that one might be genuinely (objectively, absolutely) at rest and the other genuinely (objectively, absolutely) moving—that is, moving relative to space itself. Motion is coordinate-dependent, and we get to choose what is to be the coordinate. But suppose we apply a force to one of these objects, say A—we throw the ball in a certain direction. It moves relative to B; but B also moves relative to it, with no force applied. That is, we causeA to move, but not B. It would be bizarre to suggest that we also applied a force to B making it move relative to A. Isn’t this a non-arbitrary reason to suppose that A is in motion and not B? We can’t tell just by looking which is really at rest and which really moving, but once we know the causal history we can make such a judgment. The object B did not have its state of motion changed by the act of throwing it, but the object A did. Thus causation and motion are linked, both ontologically and epistemologically: causation gives rise to motion and we use this fact to determine what is moving (objectively, absolutely). This is how we ordinarily think of actual motions in our universe: when things are observed to move relatively we use causal facts to decide which is moving absolutely (i.e. relative to space). That is the epistemic basis for judgments of non-relative motion.

Generally speaking, causation produces non-relative changes in an object, as in changing the color or shape of an object. We don’t suppose that painting an object red, say, could effect a color change in some other remote object—the analogue of causing motion in ball B by throwing ball A. Causation brings about local changes in the object acted on. Nor do we suppose that it is somehow arbitrary which object changes color or shape—as if we are free to say that some object a million miles distant changes color or shape when we apply a paintbrush to an object in front of us or hit it with a hammer. So if motion is anything like these properties it is a local property of the object acted upon, objectively and absolutely, not a property that can be decreed to hold of a remote object in relative motion with respect to the given object. Of course, relativemotion is relative, but causation gives us reason to suppose that there is also non-relative motion—motion that reflects the causal order of things. If I order you fetch me an apple, thus causing your body to move in certain ways, it would be bizarre to suggest that I have thereby caused the whole material universe to move relatively to your body, which I have taken to be at rest for the nonce; though it is quite true that your body and the universe have been in a state of relative motion, and that I couldhave chosen to make your body my frame of reference for the purpose of describing this relative motion. Intuitively, your body was caused to move by my command, not the rest of the universe, even though there was a state of relative motion between them. In a universe equipped with causation, then, there is absolute motion (as well as relative).

The difference between motion and color or shape is just that we can seechanges in the latter but not in the former just by looking at the object in question. But this is not a deep fact about motion: absolute motion could be imperceptible yet real. And not seeing is not the same as not knowing: we can know that absolute motion has occurred by knowing what the operative causes are. Movement through absolute space is not perceptible because of the featureless nature of space, but it can be inferred by knowing the causal history of the object (e.g. whether it was recently thrown). In a sense, then, absolute motion is not an empirical property in the way that color and shape are: but it can be real nonetheless. Only a form of idealism would deny this possibility.

The relativity of motion is a central tenet of Einstein’s relativity theory, both special and general. But it is noteworthy that no other science treats its central properties as similarly relative. Einstein’s (supposed) revolution in physics has not been mirrored in geology, biology, psychology, economics, etc. There has been no replacement of absolute notions by relative ones—as if an animal could only be a mongoose in relation to another animal, or a belief something you can have only relative to someone else. Objects simply have intrinsic non-relative attributes; they don’t have to be regarded as elements of systems that confer on them whatever properties they possess. This makes Einstein’s mechanics anomalous among the sciences: only it deals in properties that (allegedly) consist in relations to a coordinate system. We don’t, for example, think that Darwin’s theory of natural selection makes evolution relative to a choice of coordinates. Animals are not said to evolve with respect to one reference frame but not with respect to another. Why should the motion of bodies be an exception to this rule? It would be different if Einstein’s “revolution” had carried over to the other sciences, but it hasn’t. Only in certain (dubious) branches of anthropology could anyone say that we have discovered that certain apparently absolute properties are really relative—viz. ethical properties—but that would be a wholly superficial analogy. So mechanics alone traffics in the kind of relativity proposed by Einstein. No one ever says that an animal could be a mongoose relative to one set of animals but not relative to a different set, or that individuals have beliefs relative to one set of people but not relative to another set. There is nothing arbitraryin the claim that a certain animal is a mongoose or that a given person believes that the sky is blue—as if we could with equal right describe things differently by simply changing our frame of reference. Recognizing the existence of absolute motion, because of causation, thus brings physics into line with the other sciences. In trying to free physics from supposed “metaphysical elements” by banishing the idea of absolute motion (and absolute space and time) relativity theory makes physics exceptional among the sciences.[1]



[1]This is, of course, quite contrary to the erroneous lay idea that Einstein transferred the general relativity (subjectivity) of all our supposed knowledge into that bastion of objectivity known as physics. In fact, he claimed a relativity in physics notfound elsewhere.

2 replies
  1. Giulio Katis
    Giulio Katis says:

    I am not sure why you discuss Einstein. Your analysis of motion, and the questions you ask, can be accommodated entirely within classical physics – specifically Galilean relativity/invariance and Newtonian mechanics. As far as I can tell, you nowhere engage the specific insights of Special or General Relativity (e.g. the invariance of the speed of light, curvature of space-time, time and space dilation).

    Putting that question aside, when you discuss motion you do not make the critical distinction made in physics between uniform motion (motion in a straight line with constant speed) and non-uniform motion (motion that is not in a straight line or which changes speed). The relativity you speak of in physics only applies to objects A and B that are moving uniformly relative to one another. (This is a direct consequences of the laws of classical mechanics, i.e. pertaining to gravity, being second order differential equations. This appears to reflect a deep feature about the universe.)

    But the examples you give regarding ‘change’ all appear to involve non-uniform motion. If A is moving non-uniformly to B, physics would not claim their reference frames are indistinguishable.

    In fact, I’d hazard a guess that you might consider uniform motion not really being a type of motion that involves much, or qualitative, change at all.

    Either I’ve missed a key point in your argument (perhaps relating to a distinction between Einstein’s theories and Newton’s), or you’ve overstated the nature of relativity in physics.

    The insight of Galileo and codified by Newton, which I believe most people still have not internalised, could be described as very much a Taoist insight: from the perspective of mechanics, to describe the “state” of something you need to describe its position as well as its velocity; and therefore the laws of mechanics, which describe how state changes, must be in terms of the change of uniform change (and boundary or initial conditions). Of course, Newton and others developed the Calculus, which has had a much bigger impact on the human world than the I Ching…

    • Colin McGinn
      Colin McGinn says:

      But the general theory is officially opposed to absolute motion too, unless we regard it as going back on the special theory. My point is that accelerated motion has to be viewed as absolute, but then that will carry over to uniform motion. It’s quite true, however, that you can believe in only relative motion and not be an Einsteinian. I think that all the startling results of STR spring from an assumption of relative motion (combined with other factors like the constant speed of light).


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