And what’s the business model? How do they expect to make money?

Let K be a knot in S^3. We denote its zero-framed surgery by N_K.
Then we conjecture that K is topologically slice if and only if there exists a an epimorphism from pi_1(N_K) onto a ribbon group,
(i.e. a group with a Wirtinger presentation of deficiency one and with abelianization Z) such that
Ext_Z[G]^1(H_1(N_K,Z[G]),Z[G])=0.

The condition looks funny, but for G=Z this is just the condition that the Alexander polynomial vanishes.
The `if’ direction of the conjecture was shown for G=Z by Freedman and for G=Z \ltimes Z[1/2]
in our paper. It is difficult to make much progress on the conjecture as long as we don’t know whether surgery works for all groups.
The `if’ direction has been shown precisely for the two ribbon groups which are solvable, i.e. for which surgery is known to work.

Topological concordance is a 4-dimensional surgery problem. One let’s surgery on the knot bound a 4-manifold, and tries to measure the obstruction to changing this to a homology circle. Roughly, one expects these obstructions will lie in 1-1 correspondence to concordance classes, as they do in high dimensions.

One would like to define “homology surgery obstruction groups” for 4-manifolds, which simplifies in high dimensions to the Cappell-Shaneson surgery theory used to classify homology cobordism of high dimensional manifolds. But things seem much more complicated. In particular, in dimension 4, Freedman tells us we can iterate Whitney disks, and again roughly speaking, if we do so with sufficient control we can find embedded disks in a compactified regular neighborhood of the tower of Whitney disks. The Whitney trick allows us to embed spheres in homology classes, and excise 2-dimensional homology classes through surgery.

The filtrations arise from this perspective. One builds a Whitney tower one stage at a time, sometimes unbuilding and rebuilding to make the tower higher. This recursive process is reflected through the filtrations on the concordance group.

The tricky part arises because the group of the 4-manifold and the three manifold boundary have closely related fundamental groups, a connection captured through various versions of Poincare duality (linking and intersection theory.) This is strikingly different than in high dimensions, where all slice knots are slice with complements that have fundamental group the integers. Understanding this connection deeply lies at the heart of building and computing these filtrations. This connection of the fundamental groups of the 3 and 4 manifolds began with Casson and Gordon, and their seminal concordance invariants. Work of others since has elaborated that model, a non-trivial task.

One might hope to come from the other end, and build a surgery group globally instead of recursively, that is, instead of building approximations of surgery groups via invariants indexed on filtrations. Mark Powell and I have strong ideas of how to do this, and hope to publish something eventually. But this, like the filtrations approach, still must detect the way the fundamental group can change under concordance, and the resulting theory will be quite abstract. Only time will tell how computable it may be.

As for smooth concordance, as in high dimensions, one ideally wishes to build a “smoothing theory” which determines concordance classes of knots in a given topological concordance class. By additivity, it would suffice to compute smooth concordance of topologically slice knots. But modern 4-dimensional smoothing theory has resisted this approach, and one can’t know if this sort of 4-dimensional smoothing theory exists. We’re still trying to understand any good global picture of smoothing theory.

The Cochran-Harvey-Horn bi-polar filtration is an admirable step in the direction of conceptually merging topological and smooth concordance theory.