Is Gravity a Mystery?

Is Gravity a Mystery?

Gravity was certainly viewed as a mystery upon its introduction into the learned world, on account of its action-at-a-distance powers. How can one body affect another across great distances without any connecting medium? But that is not the aspect of gravity I want to focus on; my concern is with the idea that gravity is an attractive force. It is commonly said that bodies exercise a gravitational pull on other bodies: they draw other bodies towards them by means of an emanating force. An object strays into the earth’s gravitational field and feels the gripping power of gravity pulling it downwards, and down it goes ever more rapidly. The earth actively attracts it while it is held in gravity’s grip. Gravity is thus an attractive not a repulsive force. But what is it we actually observe about such motion? We observe one object moving towards another; we observe no attractive force. This is an interpretation of what we observe. Similarly for tidal motion, the motions of the planets around the sun, and any other gravity-induced motion. Two or more objects are observed to be in relative motion and we hypothesize that there is an attractive force acting between them. But why don’t we interpret what we observe differently—why don’t we say that one object is surging towards another, actively moving in its direction under its own power? Why aren’t the waters of the world reaching out to the moon, surging in its direction? People can be irresistibly attracted to other people, powerless in their grip, but they can also just move towards them of their own volition: so why can’t we say that that’s what objects do when the force we call gravity is operating? It isn’t that the earth has pull; rather, meteors (and the like) have thrust. Isn’t this hypothesis equally compatible with the observed motions? The concept of attraction clearly derives from human psychological attraction, but inanimate bodies don’t attract in this sense; so what grounds the idea of gravitational attraction? Isn’t this gratuitous anthropomorphism with no objective basis? If we must have an invisible force, why can’t it be a propulsive force possessed by bodies in relation to each other? The sun isn’t pulling the planets in their orbits; the planets are propelling themselves around the sun—they have that active power. They propel themselves faster the closer they get to the more massive object, according to an inverse square law; but the object towards which they move exercises no pull on them—nothing like being yanked by a rope. Gravity is the power of objects to move towards other objects not the power to move other objects: the moving objects have the power, while the stationary object lacks any power to influence the movement of remote objects. Couldn’t there be a world in which this was the actual situation—no pull but all thrust? We have got into the habit of describing the motions we observe by using the concept of attraction, borrowed from human interactions, but aren’t the motions equally describable by using the concept of thrust? The sun isn’t coercing the planets to orbit it in the way they do; the planets do this of their own volition, so to speak—it is in their nature to move this way. They move according to Newton’s laws but not for Newton’s reasons: the explanation of their movements is quite different. Newton had no account of what confers the power of attraction—no physical basis for this power  [1]—but it is a question why we should talk this way to begin with, given that the observed motions are compatible with the rival hypothesis just sketched. The ability of an object to propel itself is admittedly puzzling, but on this score the two theories seem on a par. Why posit attraction?

            My point here is not that the thrust theory is true, or preferable to the pull theory; it is that we don’t knowwhich is true. Of course, it can also be said that neither theory is true: maybe there are no such forces in the universe but just the motions we observe. Such a view has been held by some physicists, and the General Theory of Relativity can be interpreted as replacing forces with the structure of space. We know that gravity exists in the sense that there are universal laws of motion of the kind adumbrated by Newton and Einstein, but we don’t know what kinds of force govern it. This is a mystery, an area of deep ignorance: we don’t know whether the earth pulls or whether objects seek it out or neither. Or both: maybe the earth pulls a bit and falling objects thrust a bit. We don’t know what forces bodies intrinsically have, only how they are disposed to move. We don’t know how forces act over empty space, but we also don’t know how the forces are distributed. I notice that some physicists eschew talk of attraction and pull, no doubt sensing their empirical vacuity; but then they are left with no causal explanation of the movement of bodies. Either movement is a mystery or gravity is a mystery or both. Not only do we not know what grounds the (alleged) gravitational pull of the earth (Newton’s lacuna); we don’t even know that it has any such pull. The whole thing starts to seem pretty damn mysterious, not to say spooky. We don’t know how gravity operates over the void; we don’t know what confers it; and we don’t know whether the force is attractive or propulsive. It is as if we observe a group of people moving around each other, some moving towards some central person, and we assume a force of attraction emanating from that central person—while it could be that they are all moving of their own accord with nothing coercive emanating from the center. The truth is that we are simply ignorant of the underlying causal structure of things.

            Now consider those billiard balls that so fascinated Hume. Here the causal nexus is one of propinquity—no action-at-a-distance (allegedly). We tend to assume that the cue ball carries the power to move the target ball: its motion is “transferred” to that ball. It has a pushing power, while the target ball just passively receives this power and is propelled in a certain direction with a determinate velocity. Hume was very concerned about this nexus, but he assumed that the cue ball has the coercive power. But is this the only possibility—what about the idea that the target ball has the power to move off when touched by the cue ball? It is the occasion of the movement but not the cause of it (Occasionalism without God): that is, the target ball propels itself away when triggered to do so by the cue ball. The case is like a person being given permission to do something and thereupon acts in a certain way: all the causal power comes from within the person not from the permission. Why isn’t this the way causality works? So-called causes don’t have causal power; so-called effects do. The triggering object has no power by itself to bring about a change in the affected object; rather, the latter object has the power to change when impinged upon by the former object. Contact causation is not a matter of a transfer of power (“energy”) from impinging object to impinged-upon object; it is a matter of the latter object having the power to move itself when certain conditions obtain involving the former object. This is an ignition-and-thrust model of causation (like rockets) not a transfer-of-power model. Active power is located in the effect object not the cause object (so-called). Again, the point is not that this model is the true theory of causation; it is that we don’t know it’s not true. We don’t know how causation works, even in this fundamental respect. Causation is thus a mystery. We talk in certain ways, probably deriving from human experience, but we have no justification for this way of talking over other ways—none that is warranted by the observable facts. Hume was right about how little observation of causal sequences tells us about the nature of causation; and it turns out that it doesn’t even tell us in which objects causal power is located. We are deeply ignorant of causation, even of the contact kind, i.e. causation is a mystery. We don’t know what is going onwith causation even in the simplest cases (which is why Occasionalism is even a theoretical possibility). The cause-effect nexus is like the mind-body nexus—an area of profound ignorance.

            Electromagnetic causation is also deeply mysterious. Here we are said to have attractive and repulsive forces (related to so-called positive and negative charge). But why employ the concepts of attraction and repulsion (pull and push)? All we observe are movements of particles towards or away from other particles; it is a further claim that attractive and repulsive forces govern these movements. Maybe particles propel themselves in the direction of other particles instead of being attracted by them (similarly for repulsion): there is no active pulling force, but rather a spontaneous tendency to move towards other particles. Or maybe there are no such forces but just brute motions, or maybe a bit of both. We don’t know. We just talk in a certain way because it makes intuitive sense to us given our psychology; there is no objective evidential basis for this mode of talk. So again, electromagnetic causation is mysterious—far from transparent. We try to summarize the motions we have observed with concepts drawn from common sense and originating in human agency, but really we are flailing in a sea of ignorance. Even so-called mechanistic causation is full of mystery, as Hume pointed out in the case of those careening billiard balls.

  [1] Note that mass could equally be the variable with respect to which a repulsive force is proportional. In a possible world in which massive objects repel other such objects, instead of attracting them, we could equally find a lawlike correlation with mass. There is no intrinsic necessary connection between mass and attraction—or none that we can discern.