# Heisenberg part 2: Leibniz, Camus, and Kuhn (oh my!)

Continuing my commentary on Heisenberg’s Physics and Philosophy (see my previous post for more), I have a few more scattered thoughts to get down. In no particular order:

## Heisenberg, Spekkens & Leibniz

In a recent talk, Rob Spekkens argues that a principle articulated by Leibniz, often called the identity of indiscernibles, provides justification for a number of desidirata (e.g. locality and noncontextuality) that have emerged over the past hundred years for interpretations of quantum mechanics. The principle can be stated as follows:

To suppose two things indiscernible is to suppose the same thing under two names.

This has been interpreted in many ways by many different people, but Spekkens rephrases it as

A symmetry observed empirically should be retained in any ontic theory intended to explain those observations.

This is quite a drastic step away from the previous statement of the principle. However, Spekkens notes that even if Leibniz wouldn’t have endorsed this statement, Einstein certainly used it to great effect, especially in the general theory of relativity: the inertial mass and gravitational mass have always been observed to have the same value, so Einstein created a theory that treats them identically. His endorsement of the principle of the identity of indescernibles is further evidenced in his writings and secondary sources.

In addition to this rather nice example of the rhetorical technique I like to call reductio ad Einsteinum, it turns out that Heisenberg himself actually articulated this principle in support of his interpretation over, for example, those of Bopp (whom I have never heard of) and Bohm1. He says:

it must be particularly mentioned here that Bohm’s language destroys the symmetry between position and velocity which is implicit in quantum theory…Since the symmetry properties always constitute the most essential features of a theory, it is difficult to see what would be gained by omitting them in the corresponding language…[Bopp’s interpretation] destroys the symmetry between particles and waves that otherwise is a characteristic feature of the mathematical scheme of quantum theory.

While Heisenberg uses the word ‘language’ rather than ‘ontology,’ he is clearly expressing an idea very near Spekken’s version of the principle of the identity of indescernibles.

## Heisenberg and Camus (??)

All right, this one’s a stretch. And while I haven’t seen or heard anything to suggest that Heisenberg had anything at all to do with existentialism (please drop me a line if you have), THE MYTH OF SISYPHUS WAS PUBLISHED THE SAME YEAR AS PHYSICS AND PHILOSOPHY so bear with me here.

Camus’ whole thing was l’absurd, which might be pithily described as the simultaneous truth of the following two (assumed to be obviously true) facts:

• We, as humans, have an innate drive to search for meaning in the world.
• There is no meaning in the world.

So what does this have to do with our present reading? Heisenberg says that

in order to understand [the world] we have to introduce some kind of order, and order means to recognize what is equal, it means some sort of unity. From this springs the belief that there is one fundamental principle, and at the same time the difficulty to derive from it the infinite variety of things.

If we take a physicist’s ‘meaning in the world’ to be ‘a theory of everything,’ and combine that with the very real doubt that there is a theory of everything, we have the physicist’s sense of the absurd (you read it here first, folks). Yes, this is almost definitely reading a little too strongly into Heisenberg, but since we’ve made it this far I’ll close with that famous quotation from Camus: “One must imagine [grad students] happy.”

## Heisenberg and Kuhn

Incommensurability is the idea that, to one degree or another, different scientific theories might be unable to compare results due to their speaking totally different languages. Again, it’s interesting to put Heisenberg’s book in the context of its time, as it preceded Thomas Kuhn‘s The Structure of Scientific Revolutions by only 7 years. While Wikipedia tells us that the idea of (in)commensurability dates back to the thirties, it was popularized by Kuhn’s book where Kuhn uses the example of Newtonian mechanics and special relativity: they cannot actually be directly compared because the very words time and space refer to entirely different concepts. Heisenberg has an interesting take on this notion, and says that

Above all, we see from these [various interpretations] how difficult it is when we try to push new ideas into an old system of concepts belonging to an earlier philosophy—or, to use an old metaphor, when we attempt to put new wine into old bottles. Such attempts are always distressing, for they mislead us into continually occupying ourselves with the inevitable cracks in the old bottles instead of rejoicing over the new wine, We cannot possibly expect those thinkers who a century ago introduced dialectic materialism to have foreseen the development of quantum theory. Their concepts of matter and reality could not possibly be adapted to the results of the refined experimental technique of our days.

The reason I find this so interesting is that Heisenberg uses this idea of incommensurability to support his own interpretation, which rests on ideas of language and the incompatibility of languages (specifically, our language and ‘the quantum language of the universe’). Kuhn’s version would rather have it that we can’t really compare any of the different interpretations, as, for example, particle doesn’t mean the same thing in any of them. Thus, Heisenberg’s conception of incommensurability is not so much between two theories, but between our theory (or language) and ‘the world’ itself.

On the other hand, Kuhn’s book is useful when we go ahead and compare interpretations anyway, since he argues that empirical validation/falsification is not the only way (and, in fact, not always a good way) to compare theories. This is handy when, in the wacky world of interpretations of quantum mechanics, all of the interpretations are usually considered to give the exact same experimental predictions. (An interesting series of talks that I saw recently claim some unique signatures of deBroglie’s interpretation in the CMB, which even distinguish it from Bohm’s interpretation).

## Heisenberg and World War II

Just in case anybody decides to read the book (please do), there are a couple of bits of information that I found in some background reading (read: a couple Wikipedia pages) that give some context to a couple of otherwise innocent remarks that Heisenberg makes.

The first and last chapters have some particularly juicy material. Interspersed with amazingly colonialist/Eurocentric discussions of how quantum theory will affect “the religious and philosophical foundations of the native culture” everywhere except Europe which will obviously be fine, Heisenberg grandstands about how important it is to think about nuclear weapons politically and culturally. Unfortunately, he sort of forgets to mention that he was the lead scientist on the Nazi project to build an atomic weapon during World War II, and that when questioned after the war he gave a generally-agreed-upon-to-be rather halfhearted apology. Oops!

Throughout the book, Heisenberg also makes some rather veiled comments about scientists resorting to ‘political methods’ to push their own theories. As far as I can tell, this is not some general comment about professional rivalry but rather a direct reference to Deutsche Physik, a group which emerged in Germany concurrently with the Nazi party’s rise to power. Literally “German Physics,” and sometimes called “Aryan physics,” these were a bunch of anti-Semites (including Max Planck) who rejected Einstein’s theory of special relativity because it was “Jewish Physics” (yes, you read that right). Since Heisenberg embraced special relativity, he was initially on bad terms with Deutsche Physik. But don’t worry! Heisenberg’s mom knew Himmler’s mom so Heisenberg’s mom called up Himmler’s mom and asked “Would you tell your son to stop bullying my son?” and thus Heisenberg was, as mentioned above, put in charge of the Nazi’s atomic bomb project. Glad they worked that out!
1Heisenberg also gives us this nice quote from Bohr, in response to Bohm’s hope that future experiments at a small enough scale will confirm his theory: “We may hope that it will later turn out that sometimes $2\times2=5$, for this would be of great advantage for our finances.”