In 1955, Werner Heisenberg traveled to Scotland to give a series of lectures which were later published as the book Physics and Philosophy (both originally in English). In it, Heisenberg lays out a (dare I say surprisingly?) cogent exposition and defense of The Copenhagen Interpretation (a term which he coined in this work, although it turns out it’s really just his interpretation and there are a number of different Copenhagen-type interpretations) and discusses a number of related issues. The book is interesting both as a historical artifact giving insight into the thoughts of one of the founders of quantum mechanics, and as a highly accessible (if biased) introduction to issues in the ontology/epistemology of modern physics. It made me feel things, so I’m going to write a second post with various scattered reactions; but first, here’s a brief summary/background of the work, along with one of those scatterred reactions. The teaser: according to Wikipedia, Heisenberg was a devout Lutheran; did he believe in the soul, and if so, is the existence/justification of a soul central to his interpretation?
The Copenhagen Heisenberg’s interpretation
Heisenberg’s interpretation of quantum mechanics rests on his observation of a number of dualisms: the most important are that between classical language and quantum ‘reality’ (in scare quotes because it’s unclear how deeply committed to reality Heisenberg is) and that between the physical system under study and the rest of the universe (the latter including the observer). These dualities are related to Bohr’s idea of complementarity, but it seems that Bohr takes complementarity as a principle to be applied to a number of phenomena, while Heisenberg observes a few dualities and builds his interpretation on those specific observations. Plus, complementarity is a super slippery idea that I don’t understand so we’re going to stick with Heisenberg.
First, classical language vs. quantum reality. In a perhaps Wittgensteinian move, Heisenberg makes the observation that human language is only suited to discuss and describe classical phenomena. While jargon like superposition and entanglement has been coined to describe quantum phenomena, we must by the virtue of our own classicality define, describe, and understand this jargon in terms of classical concepts and words. While we can describe the inputs and potential outputs of an experiment, and we have a mathematical theory (namely, quantum mechanics) that can predict a statistical relationship between these inputs and outputs, we fail to achieve a deep understanding of what happens ‘in between’ because we literally don’t have the words for it.
Second, the system under study vs. the universe and observer. We’ll discuss the division between these three concepts (system, observer, universe) in the next section, but for now I’d just like to note that Heisenberg seems hazy on quantum fundamentalism. Quantum fundamentalism, a brand of materialism/physicalism, is the idea that everything in the universe is, at bottom, quantum mechanical. When one applies this to the observer in Heisenberg’s interpretation, the measurement problem rears its ugly head. One might assume that most physicists at least tacitly believe in quantum fundamentalism—however, in a recent survey of a small sample of physicists, only 39% agreed with the statement “The observer is a complex (quantum) system.” (55% agreed with the statement “The observer…plays no distinguished physical role;” we might speculate that the difference is made up of materialists who are not quantum fundamentalists, or that the participants had some other notions in mind).
To return to Heisenberg (remember Heisenberg? This is a post about Heisenberg), while he never actually denies quantum fundamentalism or any related idea, his language suggests that he very seriously entertains the possibility that the observer cannot be reduced to a quantum system. This is demonstrated in the chapter “The Relation of Quantum Theory to Other Parts of Natural Science,” from which we can cherry-pick the following rather strong statements:
The only question…is whether the physicochemical concepts allow a complete description of the organisms…it will probably be necessary for an understanding of life to go beyond quantum theory and to construct a new coherent set of concepts, to which physics and chemistry may belong as ‘limiting cases’…If we go beyond biology and include psychology in the discussion, then there can scarcely be any doubt but that the concepts of physics, chemistry, and evolution together will not be sufficient to describe the facts…We would, in spite of the fact that the physical events in the brain belong to the psychic phenomena, not expect that these could be sufficient to explain them.
In particular, I’d note that Heisenberg doesn’t simply think there is a missing concept (for example, a theory of computation) that will show how psychic phenomena evolve/emerge from physics; rather, physics itself will be a “limiting case” of a larger theory encompassing both psychology and physics. Of course, this reading will get complicated when we turn to…
Heisenberg and Descartes
Heisenberg spends two chapters giving brief histories of atomism/monism and the role of Descartes’ dualism in natural science, respectively. He identifies the “Cartesian partition,” Descartes’ dividing of the world into ‘res cogitans‘ and ‘res extensa,’ as both a particularly productive frame of thought in the physics of the 19th century and as the leading reason why “scientists like Einstein” had trouble with Heisenberg’s interpretation. Rather than elaborating on this, Heisenberg spends some time talking about three different flavors of realism and then Kant, whom I would rather not touch with a ten foot pole in this context (note to self: actually read some Kant).
Instead of following this path I’d like to hang around with Descartes for a little longer, and divide Heisenberg’s ideas about the “Cartesian partition” into two separate notions: an ontic partition and an epistemic partition. I think that Heisenberg doesn’t sufficiently distinguish between the two, and this leads to some confusion about what he’s really trying to say about it. As noted above, Heisenberg says that the Cartesian partition led to great progress in the natural sciences by allowing investigators to talk about the external world (the ‘res extensa’) separately from both the ‘I’ (the ‘res cogitans’) and from God (both Descartes and Heisenberg were devout Christians). This is what I would like to term the epistemic partition—in essence, it’s just a mild statement of reductionism that says that we can gain knowledge of the external more or less without reference to the internal. The ontic partition, on the other hand, has more to do with the issue of quantum fundamentalism/materialism. This partition is a direct counterclaim to materialism: the res cogitans and the res extensa are made up of fundamentally different things; they may or may not interact, but an explanation of one does not lead to an explanation of the other.
When Heisenberg says that the Cartesian partition leads to the confusion of scientists who don’t accept his interpretation of quantum mechanics, it is unclear whether he’s talking about the epistemic or ontic partition; all we know is that he sees a conflict with his interpretation. This is certainly the case with the epistemic partition, as his interpretation requires the insertion of an observer into the calculations—although he doesn’t suggest “collapse-by-consciousness,” everything he says is in the context of an experiment, presumably conducted by humans. If he means that the ontic partition is in conflict with his interpretation, this would imply that he is in fact a materialist/monist; if he does not mean this, then it would imply the other. Given that he generally talks about the Cartesian position in epistemological terms (i.e. as leading to an increase in knowledge) and that this most obviously conflicts with his interpretation, I am inclined to say that he denies the epistemic partition rather than the ontic one even if Descartes really intended the latter/is usually discussed in those terms. I am also inclined to take this position because it supports my crackpot theory about…
So Heisenberg (a) was emphatic about the role of an observer in an experiment (b) was hazy on quantum fundamentalism and (c) maybe had funny ideas about Descartes’ philosophy. So what?
Well, while we like to think of scientists as purely objective automatons (in line with the epistemic Cartesian partition), the fact is we’re all human. And with being a human comes all of the messy feelings and taught beliefs that we gather through our lives. As such, the hypothesis “Heisenberg’s religion didn’t influence his science” seems far less likely than “Heisenberg’s religion did influence his science.” Many of the great physicists have acknowledged this explicitly, including Kepler looking for “divine harmony” and Einstein who famously remarked “Jedenfalls bin ich überzeugt, daß der nicht würfelt” (“I, for one, am convinced that He does not play dice”). Heisenberg himself discusses “religious or political creed” in the present work:
From what has been said one would be inclined to demand that the scientist should never rely on special doctrines, never confine his method of thinking to a special philosophy. He should always be prepared to have the foundations of his knowledge changed by new experience. But this demand would again be an oversimplification of our situation in life for two reasons. The first is that the structure of our thinking is determined in our youth by ideas which we meet at that time or by getting into contact with strong personalities from whom we learn…The second reason is that we belong to a community or a society. This community is kept together by common ideas, by a common scale of ethical values, or by a common language in which one speaks about the general problems of life.
What’s more, Physics and Philosophy was published as part of a series called “World Perspectives,” which includes titles such as Approaches to God; Recovery of Faith; The Art of Loving; or The Meaning of the Twentieth Century. Clearly it has a place not only in the physics canon, but also in the long tradition of work towards answering that famous question which Calvin found so trivial:
Again, so what? Well, anybody who has been on either side of a physicist-layperson discussion has probably participated in the following dialogue:
“So before measurement, it’s in a superposition, but once you look at it, it collapses into whatever you observe it to be.”
“Wait, once you look at it? Like a human has to be involved? Like you need consciousness or a soul or something for quantum mechanics to work?”
“Of course not, that’s ridiculous!”
The only claim I am trying to make is that this is not so ridiculous: regardless of what one believes about the existence or nonexistence of a soul, I think we have to entertain seriously the idea that one of the founders of quantum mechanics, who solidified and proselytized its most popular interpretation, was motivated at least partially by the idea that humans have a soul. I mean, that’s not a whole lot crazier than quantum mechanics, is it?