As I mentioned in my previous post, I’ve recently started out a new postdoc at IST – the Instituto Superior Tecnico in Lisbon, Portugal. Making the move from North America to Europe with my family was a lot of work – both before and after the move – involving lots of paperwork and shifting of heavy objects. But Lisbon is a good city, with lots of interesting things to do, and the maths department at IST is very large, with about a hundred faculty. Among those are quite a few people doing things that interest me.
The group that I am actually part of is coordinated by Roger Picken, and has a focus on things related to Topological Quantum Field Theory. There are a couple of postdocs and some graduate students here associated in some degree with the group, and elsewhere than IST Aleksandar Mikovic and Joao Faria Martins. In the coming months there should be some activity going on in this group which I will get to talk about here, including a workshop which is still in development, so I’ll hold off on that until there’s an official announcement.
I’ve also had a chance to talk a bit with Pedro Resende, mostly on the subject of quantales. This is something that I got interested in while at UWO, where there is a large contingent of people interested in category theory (mostly from the point of view of homotopy theory) as well as a good group in noncommutative geometry. Quantales were originally introduced by Chris Mulvey – I’ve been looking recently at a few papers in which he gives a nice account of the subject – here, here, and here.
The idea emerged, in part, as a way of combining two different approaches to generalising the idea of a space. One is the approach from topos theory, and more specifically, the generalisation of topological spaces to locales. This direction also has connections to logic – a topos is a good setting for intuitionistic, but nevertheless classical, logic, whereas quantales give an approach to quantum logics in a similar spirit.
The other direction in which they generalize space is the -algebra approach used in noncommutative geometry. One motivation of quantales is to say that they simultaneously incorporate the generalizations made in both of these directions – so that both locales and -algebras will give examples. In particular, a quantale is a kind of lattice, intended to have the same sort of relation to a noncommutative space as a locale has to an ordinary topological space. So to begin, I’ll look at locales.
A locale is a lattice which formally resembles the lattice of open sets for such a space. A lattice is a partial order with operations (“meet”) and (“join”). These operations take the role of the intersection and union of open sets. So to say it formally resembles a lattice of open sets means that the lattice is closed under arbitrary joins, and finite meets, and satisfies the distributive law:
Lattices like this can be called either “Frames” or “Locales” – the only difference between these two categories is the direction of the arrows. A map of lattices is a function that preserves all the structure – order, meet, and join. This is a frame morphism, but it’s also a morphism of locales in the opposite direction. That is, .
Another name for this sort of object is a “Heyting algebra”. One of the great things about topos theory (of which this is a tiny starting point) is that it unifies topology and logic. So, the “internal logic” of a topos has a Heyting algebra (i.e. a locale) of truth values, where the meet and join take the place of logical operators “and” and “or”. The usual two-valued logic is the initial object in , so while it is special, it isn’t unique. One vital fact here is that any topological space (via the lattice of open sets) produces a locale, and the locale is enough to identify the space – so is an embedding. (For convenience, I’m eliding over the fact that the spaces have to be “sober” – for example, Hausdorff.) In terms of logic, we could imagine that the space is a “state space”, and the truth values in the logic identify for which states a given proposition is true. There’s nothing particularly exotic about this: “it is raining” is a statement whose truth is local, in that it depends on where and when you happen to look.
To see locales as a generalisation of spaces, it helps to note that the embedding above is full – if and are locales that come from topological spaces, there are no extra morphisms in that don’t come from continuous maps in . So the category of locales makes the category of topological spaces bigger only by adding more objects – not inventing new morphisms. The analogous noncommutative statement turns out not to be true for quantales, which is a little red-flag warning which Pedro Resende pointed out to me.
What would this statement be? Well, the noncommutative analogue of the idea of a topological space comes from another embedding of categories. To start with, there is an equivalence : the category of locally compact, Hausdorff, topological spaces is (up to equivalence) the opposite of the category of commutative -algebras. So one simply takes the larger category of all -algebras (or rather, its opposite) as the category of “noncommutative spaces”, which includes the commutative ones – the original locally compact Hausdorff spaces. The correspondence between an algebra and a space is given by taking the algebra of functions on the space.
So what is a quantale? It’s a lattice which is formally similar to the lattice of subspaces in some -algebra. Special elements – “right”, “left,” or “two-sided” elements – then resemble those subspaces that happen to be ideals. Some intuition comes from thinking about where the two generalizations coincide – a (locally compact) topological space. There is a lattice of open sets, of course. In the algebra of continuous functions, each open set determines an ideal – namely, the subspace of functions which vanish on . When such an ideal is norm-closed, it will correspond to an open set (it’s easy to see that continuous functions which can be approximated by those vanishing on an open set will also do so – if the set is not open, this isn’t the case).
So the definition of a quantale looks much like that for a locale, except that the meet operation is replaced by an associative product, usually called . Note that unlike the meet, this isn’t assumed to be commutative – this is the point where the generalization happens. So in particular, any locate gives a quantale with . So does any -algebra, in the form of its lattice of ideals. But there are others which don’t show up in either of these two ways, so one might hope to say this is a nice all-encompassing generalisation of the idea of space.
Now, as I said, there was a bit of a warning that comes attached to this hope. This is that, although there is an embedding of the category of -algebras into the category of quantales, it isn’t full. That is, not only does one get new objects, one gets new morphisms between old objects. So, given algebras and , which we think of as noncommutative spaces, and a map of algebras between them, we get a morphism between the associated quantales – lattice maps that preserve the operations. However, unlike what happened with locales, there are quantale morphisms that don’t correspond to algebra maps. Even worse, this is still true even in the case where the algebras are commutative, and just come from locally compact Hausdorff spaces: the associated quantales still may have extra morphisms that don’t come from continuous functions.
There seem to be three possible attitudes to this situation. First, maybe this is just the wrong approach to generalising spaces altogether, and the hints in its favour are simply misleading. Second, maybe quantales are absolutely the right generalisation of space, and these new morphisms are telling us something profound and interesting. The third attitude, which Pedro mentioned when pointing out this problem to me, seems most likely, and goes as follows. There is something special that happens with -algebras, where the analytic structure of the norm makes the algebras more rigid than one might expect. In algebraic geometry, one can take a space (algebraic variety or scheme) and consider its algebra of global functions. To make sure that an algebra map corresponds to a map of schemes, though, one really needs to make sure that it actually respects the whole structure sheaf for the space – which describe local functions. When passing from a topological space to a -algebra, there is a norm structure that comes into play, which is rigid enough that all algebra morphisms will automatically do this – as I said above, the structure of ideals of the algebra tells you all about the open sets. So the third option is to say that a quantale in itself doesn’t quite have enough information, and one needs some extra data something like the structure sheaf for a scheme. This would then pick out which are the “good” morphisms between two quantales – namely, the ones that preserve this extra data. What, precisely, this data ought to be isn’t so clear, though, at least to me.
So there are some complications to treating a quantale as a space. One further point, which may or may not go anywhere, is that this type of lattice doesn’t quite get along with quantum logic in quite the same way that locales get along with (intuitionistic) classical logic (though it does have connections to linear logic).
In particular, a quantale is a distributive lattice (though taking the product, rather than , as the thing which distributes over ), whereas the “propositional lattice” in quantum logic need not be distributive. One can understand the failure of distributivity in terms of the uncertainty principle. Take a statement such as “particle has momentum and is either on the left or right of this barrier”. Since position and momentum are conjugate variables, and momentum has been determined completely, the position is completely uncertain, so we can’t truthfully say either “particle has momentum and is on the left or “particle has momentum and is on the right”. Thus, the combined statement that either one or the other isn’t true, even though that’s exactly what the distributive law says: “P and (Q or S) = (P and Q) or (P and S)”.
The lack of distributivity shows up in a standard example of a quantum logic. This is one where the (truth values of) propositions denote subspaces of a vector space . “And” (the meet operation ) denotes the intersection of subspaces, while “or” (the join operation ) is the direct sum . Consider two distinct lines through the origin of – any other line in the plane they span has trivial intersection with either one, but lies entirely in the direct sum. So the lattice of subspaces is non-distributive. What the lattice for a quantum logic should be is orthocomplemented, which happens when has an inner product – so for any subspace , there is an orthogonal complement .
Quantum logics are not very good from a logician’s point of view, though – lacking distributivity, they also lack a sensible notion of implication, and hence there’s no good idea of a proof system. Non-distributive lattices are fine (I just gave an example), and very much in keeping with the quantum-theoretic strategy of replacing configuration spaces with Hilbert spaces, and subsets with subspaces… but viewing them as logics is troublesome, so maybe that’s the source of the problem.
Now, in a quantale, there may be a “meet” operation, separate from the product, which is non-distributive, but if the product is taken to be the analog of “and”, then the corresponding logic is something different. In fact, the natural form of logic related to quantales is linear logic. This is also considered relevant to quantum mechanics and quantum computation, and as a logic is much more tractable. The internal semantics of certain monoidal categories – namely, star-autonomous ones (which have a nice notion of dual) – can be described in terms of linear logic (a fairly extensive explanation is found in this paper by Paul-André Melliès).
Part of the point in the connection seems to be resource-limitedness: in linear logic, one can only use a “resource” (which, in standard logic, might be a truth value, but in computation could be the state of some memory register) a limited number of times – often just once. This seems to be related to the noncommutativity of in a quantale. The way Pedro Resende described this to me is in terms of observations of a system. In the ordinary (commutative) logic of a locale, you can form statements such as “A is true, AND B is true, AND C is true” – whose truth value is locally defined. In a quantale, the product operation allows you to say something like “I observed A, AND THEN observed B, AND THEN observed C”. Even leaving aside quantum physics, it’s not hard to imagine that in a system which you observe by interacting with it, statements like this will be order-dependent. I still don’t quite see exactly how these two frameworks are related, though.
On the other hand, the kind of orthocomplemented lattice that is formed by the subspaces of a Hilbert space CAN be recovered in (at least some) quantale settings. Pedro gave me a nice example: take a Hilbert space , and the collection of all projection operators on it, . This is one of those orthocomplemented lattices again, since projections and subspaces are closely related. There’s a quantale that can be formed out of its endomorphisms, , where the product is composition. In any quantale, one can talk about the “right” elements (and the “left” elements, and “two sided” elements), by analogy with right/left/two-sided ideals – these are elements which, if you take the product with the maximal element, , the result is less than or equal to what you started with: means is a right element. The right elements of the quantale I just mentioned happen to form a lattice which is just isomorphic to .
So in this case, the quantale, with its connections to linear logic, also has a sublattice which can be described in terms of quantum logic. This is a more complicated situation than the relation between locales and intuitionistic logic, but maybe this is the best sort of connection one can expect here.
In short, both in terms of logic and spaces, hoping quantales will be “just” a noncommutative variation on locales seems to set one up to be disappointed as things turn out to be more complex. On the other hand, this complexity may be revealing something interesting.
Coming soon: summaries of some talks I’ve attended here recently, including Ivan Smith on 3-manifolds, symplectic geometry, and Floer cohomology.