The 2016 Nobel prize in physics has been awarded, and it wasn’t for gravitational waves. This was a huge surprise, since the direct detection of gravitational waves is one of the all-time biggest breakthroughs in astronomy. It’s not only confirmed a prediction of general relativity, and verified the existence of black holes, it’s also opened up an entirely new field of observational astronomy. Gravitational waves was considered such a shoe in for the Nobel that it’s lack of an award has sent science writers scrambling. So what did win? Topological phase transitions. The official notice awarded David J. Thouless, F. Duncan M. Haldane, and J. Michael Kosterlitz “for theoretical discoveries of topological phase transitions and topological phases of matter.”
Topology is an area of mathematics that looks at the geometry of different things, and how they are related. Everything from the curvature of space and time to the social networks of Facebook are related to topology. In the case of this year’s prize, it involved the application of topology to things like superconductivity and superfluids. For example, a typical fluid such as water has a certain amount of viscosity. This is why when you stir your coffee the swirl eventually dies down. But some things like liquid helium can be cooled to the point where it becomes a superfluid with no viscosity. If you were to stir a superfluid, it would keep swirling indefinitely. Such swirls are known as vortices. Because these vortices last indefinitely, they can be considered part of the topology of the superfluid.
Which brings us to this work. One of the interesting questions in condensed matter is how and why materials transitions from regular to super. What’s really going on in the structure of a material that makes a material superfluid or superconductive? To try to figure this out, the laureates focused on thin layers of material. So thin that it can basically be considered a two-dimensional surface. For a long time it was thought that superfluidity, for example, couldn’t exist in such a constrained state, but it turns out that they can, and when they do, things like vortices start behaving in interesting ways. When vortices in a fluid form, they’ll tend to be distributed in a random way. Some might interact with each other, but it’s all rather chaotic. However when a fluid cools below a certain point, the vortices pair up, and move together in pairs. This shift in the topological behavior occurs at a critical temperature, and thus is a topological phase change.
It’s actually a surprising result, because it means not only can materials have phase changes where the macroscopic behavior of a material shifts from solid to liquid to gas, but there are also more subtle phase transitions where the topology of a material changes in discrete ways. As we’ve begun to learn over time, these subtle topological shifts can bring exotic changes to a material, and we’re still learning about their applications.
So while it’s not gravity waves, this award-winning work has made waves of its own, and that makes it a worthy choice for this year’s Nobel prize.