Henry Margenau (1901-1997) was, like Eddington, a physicist with interests in the philosophy of science. He had written on the topics he addresses here already 15 years earlier, and he was at this point on the verge of publishing a book, The Nature of Physical Reality (McGraw-Hill, 1950), that combined accounts of the content of physics with a philosophical interpretation of it.
In this article, Margenau approaches quantum mechanics slowly, and you will find him doing little more than alluding to physics before the last few paragraphs of the first assignment, with his actual discussion of quantum theory not beginning until the second half of the second assignment. However, although it may not be initially apparent, the earlier parts of the article lay the groundwork for what he intends to say about quantum mechanics later.
Assignment for Fri. 11/11: §§1-2 (pp. 287-295)
• Margenau’s discussion of sense experience in §1 is introductory. Much of his discussion will concern the relation of physical theory to such data. (His comment about data and habita on p. 289 concerns the Latin etymology of the words: data is related to terms for giving and gifts while habita is related to terms for having or possessing.)
• As you read Margenau’s discussion on pp. 290f of the continuity between the sensory and rational, you might compare it to Hanson’s idea of theory-laden observation. Margenau’s discussion of the transition from immediate perception to physical objects by integration and reification (pp. 292f) leads him to the idea of “constructs” and the question of their validity. This whole discussion is very close the initial part of Eddington’s ch. XII, and the view of Russell to which Margenau compares his own on p. 294 was quite close to the one Eddington adopts. Margenau talk of “rules of correspondence” (p. 294) might remind you of Carnap’s “correspondence rules” (KHR, pp. 320ff), and Margenau does have in mind something like Carnap’s idea.
Assignment for Mon. 11/14: §§3-5 (pp. 295-302)
• The ideas of “states” and “observables” that Margenau introduces in his discussion of the flower at the beginning of §3 are central terms in quantum mechanics. The distinction between “latent” observables and “properties” or “possessed” observables (p. 297) is his own, and it is central to his understanding of that theory. Notice also his description at the end of the section of what he calls the “postulate of causality.” It’s status in quantum theory is, for him, the key philosophical issue concerning that theory.
• The distinction between “physical” and “historical” reality at the beginning of §4 is used to describe the difference between quantum and classical mechanics. Bohr introduced the idea of “complementarity” (which Margenau associates with this distinction) initially to speak of the wave-particle duality and related features of quantum theory. In his book, Margenau offers the following gloss of one of Bohr’s statements of the principle:
Physics has a choice, Bohr says, between describing nature in terms of classical observables and in terms of abstract states.… The first choice permits visualization but requires that causality be renounced; the second forbids visualization but allows causality to be retained. And these alternatives can never reconciled.
The Nature of Physical Reality, 421.
Margenau goes on to say that, while Bohr has asked science “to resign itself to an eternal dilemma” (ibid., p. 422), he thinks science has chosen the second alternative.
The “wave” function Margenau mentions on p. 299 is the same as the ψ-function he speaks of on p. 300. On the one hand, the development of this wave over time is determined by the Schrödinger equation he mentions. On the other hand, the rule of “correspondence” tying this state to observations of position identifies the amplitude of the wave at a given point with the probability of the particle being observed at that point. Margenau’s way of understanding this is that a causal tie between states at different times is “restored” (in the face of observables that had “refused to give consistent values in repeated observations”) by giving the correspondence rules a “statistical character” (see p. 295) that connects a single state with the frequency of occurrence of the values of an observable in an “aggregate of data” (i.e., the probabilities of particular observations is manifested by their relative frequency in an aggregate of observations).
The term “hidden parameters” (p. 300) refers to an alternative approach to handling the variability of observables: this variability might be traced to a variability in as yet unrecognized features of the state (in the way adding charge to mass, position, and velocity makes possible an account of variations in color). Although initially attractive to many (including Einstein), the scope for such a modification of quantum theory was gradually narrowed (both before and after Margenau wrote) until the “hidden variable theories” still consistent with observations were stranger than quantum theory itself.
• Margenau’s analogy with genetics may not be very helpful if you think in terms of current molecular biology. To get his point, you need to imagine a time when the only access to the genetic constitution of an organism was by way of the observable characteristics of it and its descendants.