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July 8, 2006

How many Circles are there in the World?

Posted by Urs Schreiber

On the right notion of torsor for suspended U(1).

Consider the 2-group Σ(U(1)): a single object, U(1) worth of morphisms.

Try to build a bundle of categories whose typical fiber is a torsor for that.

Since Σ(U(1)) is strict, you’d guess strict 2-torsors would do. But they don’t.

So weaken things. As a category in sets, Σ(U(1) is equivalent to the 2-group Tor(U(1)), whose objects are U(1)-torsors and whose morphisms are torsor morphisms (HDA V, p. 51).

A U(1)-torsor is a circle S 1. We need to take care not to identitfy different circles S 1, S˜ 1, etc.

Hence Tor(U(1)) contains a lot of objects. How many circles are there, in the world?

But what we want is a topological 2-torsor. So we want not just all circles, but all continuous families of them.

In other words, what we are really looking for is not the collection of all circles, but the moduli space of circles.

Pick any topological space X and consider the collection of continuous families of circles parameterized by X. This collection is the same as maps from X into the moduli space of circles, otherwise known as BU(1), the classifying space for circle bundles.

Hence we want our Σ(U(1))-torsor to have a space of objects which looks like BU(1).


In fact, each point in this space corresponds to a circle, in the sense that we have the canonical circle bundle on BU(1), known as the universal princiapl U(1)-bundle EU(1).

Given two circles S 1 and S˜ 1, the space of torsor morphisms between them is

(2)Mor(S 1,S˜ 1)=S 1×S˜ 1U(1).

Similarly for any continuous family of circles.

Hence we find that the space of morphisms of our Σ(U(1))-torsor is

(3)Mor T=EU(1)×EU(1)U(1),

with the obvious source and target maps

(4)EU(1)×EU(1)U(1) t BU(1) s BU(1).

In other words, T is nothing but the transport groupoid of the universal U(1)-bundle


Let’s try to build a bundle of categories with typical fiber T.

Take any BU(1)-bundle EX, classified by an element in H 3(X,). Assume we can find a continuous map from the total space E to BU(1) which restricts on each fiber to a representative of the canonical circle bundle.

We get a category internal to the category of bundles

(6)L× XL L L× XLU(1) t E s L E X

whose bundle of objects is E and whose bundle of morphisms is L× XLU(1), where L is, on each fiber of E, the canonical circle bundle.

The same game can be played for more general 2-groups. Let (HG) be any crossed module, regarded as a strict 2-group.

Again, regarded in sets this is equivalent to Bitor G(H), which are H bitorsors whose left and right action are related by an element in G (just ordinary bitorsors if G=Aut(H)).

Again, this is huge. How many H bitorsors are there in the world?

What we want is continuous families of them. As before, this are nothing but H-bitorsor bundles whose left and right actions are related by elements in G.

The moduli space of all (GH)-bitorsors is known. It’s the classifying space for (GH)-bibundles, known as the geometric realization of the nerve (HG).

As before, we would want to build bundles of categories with typical fiber the transport groupoid of the universal H-bibundle over (HG).

Say (HG) is the string 2-group (), such that (HG) is String itself. We’d start with a String-bundle and re-interpret each fiber as a category, namely as the transport groupoid of the universal (GH)-bibundle.

Up to the fact that I don’t know if this the construction I am describing always exists as a continuous thing.

Posted at July 8, 2006 12:17 PM UTC

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Re: How many Circles are there in the World?

Hi! I’m in Shanghai! It’s a lot of fun. There’s an email of yours I need to answer. Luckily, I just got broadband in my apartment yesterday, so it should be much easier now.

I’ve thought a lot about this “moduli space of all circles” business. I gave a course on it in a summer school on homotopy theory in Calgary. You can see the notes here:

Higher Gauge Theory,
Homotopy Theory
and n-Categories

Some of the pictures are missing - even in the file with scanned-in pictures - but you can probably figure out how they should go. If anyone has questions, they should ask.

Among other things, I describe how and why one should go from the category of all U(1)-torsors to the space BU(1), the classifying space for U(1)-bundles. This makes crucial use of the fact that the category of U(1)-torsors is a topological category. The topology on the space of objects is discrete, but the category on the space of morphisms is not.

I forget if I explained this fact in these notes, but actually the category of U(1)-torsors is a smooth category, and this makes BU(1) into a smooth space in the sense of Chen. We need this when doing differential geometry with U(1)-bundles.

In my notes I also explain how to iterate this business indefinitely, and turn the topological category of U(1)-n-torsors into the space Bn(U(1)), which is the classifying space for U(1) n-bundles.

While first thinking about these issues, I was very confused about how a U(1)-bundle over a smooth space X was a smooth map from X into the category of all circles. Clearly such a bundle assigns a circle (a U(1)-torsor) to each point of X, namely the fiber over that point. But how does one make the concept of a smoothly varying choice of circle precise?

I discuss this question and give my best answer so far in these course notes.

Posted by: John Baez on July 9, 2006 3:02 AM | Permalink | Reply to this

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