This is the series I wish would have existed a month ago when I first started learning about ultrafilters and ultraproducts. First of all, what’s the motivation for learning about these ultra-objects? I imagine that if you’re here, you probably already have some degree of interest in ultrafilters, ultraproducts, or nonstandard models of arithmetic. But I’ll see if I can bolster that interest a little bit.

The most exciting application to me personally is that ultraproducts give you a recipe for constructing new mathematical structures out of familiar ones. Out of some model M one can construct a a new model *M, which typically has a much richer and stranger structure, but is nonetheless elementarily equivalent to M (this is called Łoś’s theorem). Elementary equivalence means that all the expressive resources of first-order logic are insufficient to tell M and *M apart: they agree on all first-order sentences.

For example, the ultraproduct of ℝ (the real numbers) is *ℝ, the hyperreal numbers. The hyperreals contain an enormous supply of infinitesimal quantities clustered around every real, as well as infinitely large quantities. And all the usual operations on ℝ, like addition and multiplication, are already nicely defined in *ℝ, meaning that these infinitesimals and infinities have a well-defined algebraic structure. And Łoś’s theorem tells us that ℝ and *ℝ are elementary equivalent: any first-order sentence true of the reals is also true of the hyperreals.

Another example: the ultraproduct of ℕ (the natural numbers) is *ℕ, the hypernaturals. The hypernaturals don’t contain any infinitesimals, but they do contain an uncountable infinity of infinite numbers. And since *N and N are elementarily equivalent, *ℕ is a model of true arithmetic! This is super exciting to me. True arithmetic is this unimaginably complicated set of sentences; there’s no Turing machine that decides which sentences reside in true arithmetic, nor is there a Turing machine with a halting oracle that does the job, nor even a Turing machine with a halting oracle for Turing machines with halting oracles! The computational complexity of true arithmetic is the limit of this sequence of progressively harder halting problems: you can only decide the set if you have an oracle for the halting problem, plus an oracle for the halting problem for TMs with oracles for the halting problem, plus an oracle for the halting problem for TMs with oracles for the halting problem for TMs with oracles for the halting problem, and so on. This level of complexity is known as **0**^{(ω)}.

So true arithmetic is this impossibly uncomputable set of sentences. We know trivially that ℕ is a model of true arithmetic, because that’s how true arithmetic is defined { φ | ℕ ⊨ φ }. And we also know that true arithmetic has nonstandard models, because it’s a first-order theory and no first order theory can categorically define ℕ (I think the easiest way to see why this is true is to apply the Löwenheim-Skolem theorem, but – self-promotion alert – there’s also this very nice and simple proof from first principles). And ultraproducts allow us to construct an explicit example of one of these nonstandard models! You can actually write down some of these nonstandard numbers (they look like infinite sequences of natural numbers) and discover their strange properties. For example, (2, 3, 5, 7, 11, …) is a nonstandard prime number, (1, 2, 4, 8, 16, …) is infinitely even, and (0!, 1!, 2!, 3!, 4!, …) is a multiple of every standard natural number. We’ll dive into all of this soon enough.

Ultrafilters and ultraproducts also have applications outside of logic. A fairly basic result about ultrafilters (that every ultrafilter over a finite set contains a singleton) is equivalent to Arrow’s impossibility theorem in voting theory (that any voting system with unanimity, independence of irrelevant alternatives (IIR), and finitely many voters contains a dictator). And the existence of a singleton-free ultrafilter over an infinite set (a **free*** *ultrafilter) shows that with infinitely many voters, there’s a non-dictatorial voting system with unanimity and IIR. There’s a pretty good description of those results here, but finish reading my post first!

Nick Bostrom in an old paper titled *Infinite Ethics* describes how to apply ultrafilters to resolve some of the issues that arise when trying to apply utilitarian ethics in a universe with infinitely many experiencers. Suppose that the current universe contains an infinite number of experiencers, each of whom is having an experience with a valence of +1. By pressing a button, you can immediately transition to a universe where everybody is now experiencing a valence of +2. Clearly pressing the button is a utilitarian good. But 1+1+1+… = ∞ = 2+2+2+…. So it looks like a standard account of utility as real numbers says that the utility of the +2 world is the same as the utility of the +1 world (both just ∞). But if we treat utilities as hyperreal numbers instead of ordinary real numbers, then we can do the comparison and get the right answer. It’s worth noting that this approach doesn’t fix all the problems raised by an infinite universe; for instance, if pressing the button only increases a finite number of experiencers from valence +1 to valence +2, then the net utility of the universe is unchanged, even with hyperreal utilities. (This corresponds to a property that we’ll see soon, which is that a free ultrafilter contains no finite sets.)

Okay, introduction over. Hopefully you’re now excitedly asking yourself the question “So what are these ultrafilters and ultraproducts anyway??” The story begins in the next post!

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