THE UNIFICATION ARGUMENT
One way to construe the interaction claims in Smolin's quote is simply as suggesting a motivation for searching for a TOE: since everything interacts with everything else, it would be desirable if a TOE could be discovered to explain these interactions.
This claim, I will consider in section 7. In the present section I will construe the quote as containing an argument for the existence of a TOE (one that makes the mind “call out” for a TOE). The argument, whether construed as providing evidence sufficient to believe the conclusion (so that it is not a speculation), or just as providing some supporting reason for a speculative conclusion, makes two claims: (i) if entities subject to certain laws interact (e.g., electromagnetic and nuclear forces, the mind and the body), there must be, or there probably is, some more basic set of unifying entities and laws that govern such interactions; (ii) therefore, since everything interacts with everything else there cannot be separate, unrelated theories of nature governing all phenomena. There can only be one, a TOE, and it will unify all interactions.It is not completely clear how broadly or narrowly TOE theorists want to construe the idea of an “interaction,” and therefore whether the claim that everything interacts with everything else is true. Nor, even if everything does interact with everything else, does (ii) follow from (i)—that is, that there exists some fundamental TOE that unifies and explains all these interactions. Each interaction could be explained by some deeper unifying theory without its being the case that all interactions are unified and explained by the same theory. But putting these issues aside, I want to focus here on the weaker but important claim (i).
What sort of unification do TOE theorists have in mind here? In section 2, I distinguished three ways a theory might unify.
It might explain a range of different phenomena (Whewellian “consilience”); it might unify ontologically by reducing a more complex system to a simpler or more basic one; it might contain unified assumptions (Whewellian “coherence”). TOE theorists, I think, have in mind all three. But it is the second idea, ontological reduction, that is particularly important here in order to understand what TOE theorists are aiming for in presenting the “unification argument.”To see what such ontological unification by reduction might involve, let us contrast two cases in classical physics, both involving interacting forces. In the first case, consider the forces acting on each planet by the sun and by all the other planets. Newton's theory unified these forces ontologically by arguing that what may appear to be various forces of gravity (or “heaviness”) exerted on and by the planets are not many different types of force, but one, subject to one law—the universal law of gravity. Contrast this case with that of a parachutist falling who is subject to interacting forces of gravity and air resistance. These opposing forces combine to produce a much smaller acceleration (if any) than gravity alone. This type of combining of forces, even if we know how to compute it, is not what TOE theorists have in mind (or at least it is not all they have in mind). It is not ontological unification, since there are still distinct forces acting. Ontologically unifying these forces in such a case (if that were possible) would mean explaining both gravity and air resistance in terms of a single force (as in the case of Newton's universal gravity), or explaining these forces by invoking some more basic system, different states of which give rise to the two forces (as in the case of Maxwell's unification of electricity and magnetism by a set of equations governing the electromagnetic field).[168]
Suppose that the four fundamental forces admitted by current physics, subject to laws governing each, interact on certain occasions to produce a given result.
What string theorists have in mind is a TOE that unifies these forces ontologically. As I am construing claim (i) of the present interaction argument, from the fact that the forces interact, we may infer that there is a deeper law or theory unifying the forces by showing them to be manifestations of a single force or by showing that they are produced by some more basic system, different states of which give rise to these forces. Similarly, from the fact that physical and mental states interact to produce certain behavior, we may infer the existence of some more basic state that has or gives rise to both mental and physical properties. Why should this be so? What general reason might be offered for inferring such ontological unification from interaction?Perhaps an appeal can be made to the “common cause principle,” first introduced by Reichenbach,[169] according to which if two types of events or states of affairs are correlated, then, assuming they are not identical, either one causes the other or there is a common cause. So in physics, if two or more forces are found to operate together (say, electromagnetic and gravitational forces producing the motions of charged bodies with mass), and if one (say, the electromagnetic) does not cause the other (gravity), then there must be a common cause of both forces and hence of the effects produced. But the common cause principle is subject to counterexamples (in quantum mechanics, electromagnetic theory, and other cases).[170] There is no convincing empirical argument for the general validity of this principle. If there is a common cause in a given case, that can only be shown empirically by appeal to the facts of the case, not by appeal to the common cause principle.[171]
In the parachute example, there is no reason to infer that the forces of gravity and air resistance have a common cause or that they are identical from the fact that these forces interact to produce the effect.
Nor does one need to infer this in order to understand the effect in this case. To be sure, depending on the circumstances, one may be motivated by a desire for unification to try to understand certain interactions by proposing an ontological unification. But neither the fact of interaction nor the desire to find a unification provides a reason to infer that such a unification exists. Smolin himself seems to recognize this. A few pages after the earlier quoted passage (which strongly suggests an inferential argument), in commenting on the question of “whether all the forces we observe in naturemight be manifestations of a single, fundamental force,” he writes: “There seems, as far as I can tell, no logical argument that this should be true, but it is still something that might be true.”[172] Here, Smolin treats the “interaction” idea not as an evidential basis for a TOE but, rather, as a pragmatic basis for searching for one (a view I will discuss later).[173]
Finally, if: (a) interaction does require, or make probable, an ontological unification (of one of the two sorts I have mentioned); and (b) everything interacts with everything else, then there can be only one type of “fundamental” object. Suppose, in accordance with (b), two types of objects interact, and suppose both are claimed to be “fundamental.” If they do interact, then, in accordance with (a), there is, or is likely to be, an ontological unification. This means that either these objects are of the same type (e.g., both involve bodies exerting a gravitational force) or else there are more fundamental objects responsible for the interaction. In the first case, we have only one type of object, and in the second case, we do not have “fundamental” objects. To be sure, the existence of only one type of “fundamental” object is what string theorists and Nagel seem to want. But I am not making it a condition for a TOE that it postulate only one type of “fundamental” object. Nor, as I indicated earlier, do I accept assumptions (a) and (b), which necessitate that conclusion.
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