I didn’t write about the Nobel Prize in physics on the day it was awarded, because it was awarded in an area of physics I’m not very familiar with, and I needed to educate myself.
The area concerned is ‘topological phases and phase transitions’ in matter. It’s not completely unfamiliar to me. Steve Bramwell, a leading expert in the area, is a member of my department at UCL, and I have been lucky enough to hear him talk about his work. He’s been pretty busy with interviews since Tuesday, and is quoted in the Guardian news article on the prize, here.
While trying to educate myself, I came across a great explanation of the general ideas behind the prize, by Brian Skinner at MIT, and I don’t think I can improve on it. If you want to understand the ideas and why they are important, I heartily recommend his article (written before the prize was awarded)¹.
Without rehashing that then, I just want to add something that really fascinates me about this kind of physics.
A key concept is the idea of topological defects, illustrated in Skinner’s article by a glitch in a zipper, or a vortex in a field of arrows. These defects exhibit many of the features we see in the so-called ‘fundamental’ particles of the Standard Model of physics. Some of the mysterious properties of these particles become a bit more intuitively obvious when the particles are pictured as vortices.
For example, the ‘Pauli exclusion principle’ is what stops electrons from overlapping each other, and constrains them to occupy different energy-levels around the atomic nucleus. These energy levels are responsible for all the chemical properties of the elements in the periodic table, so it’s an important principle to understand. I find it difficult to picture how this might work in the case of fundamental electrons. But if you look at the vortices in Skinner’s illustrations, it is easy to see why they can’t approach each other too closely - the arrows just point in the wrong way.
These topological ideas even give rise naturally to the idea of quantisation itself. You cannot move gradually between two topologically distinct forms – say a bagel and a Swedish pretzel (see video). You have to make a quantum leap. In topological terms, you have to make, or break, a connection.
So from one point of view this physics looks more fundamental than the search for the smallest constituents of matter. It is the physics of deep principles, which these constituents seem to obey.
On the other hand, all the discussion of these principles in terms of zips and arrows (or bagels) assumes that the zips and arrows and the forces between them exist. The experimental observations and applications are seen in solids, and the little arrows are magnetic atoms or molecules. All of them have to be made of something, and in the end what they are made of is the quarks and electrons of the Standard Model. Once you have got to that stage, you are reduced to saying that electrons behave like defects in a quantum field, you have no mechanistic model for that field, so you call it fundamental. And indeed it might be.
All I’m really saying is that there are two senses in which physics can be ‘fundamental’. There is the question of what everything is made of at its core – what are the smallest constituents of the universe? But there are also fundamental principles, which operate in many different systems, on many different scales.
These topological phases are a manifestation of some new principles, with far reaching consequences. That is why their discovery is so important.
¹ His swirling arrow diagrams are also lovely to look at. One of the key achievements behind the prize is the ‘Kosterlitz–Thouless phase transition’, corresponding to a temperature at which vortices and anti-vortices separate from each other, illustrated by the lovely animation half way down the article. Thanks to Stephen Curry and Aatish Bhatia for pointing me at this article.
Jon Butterworth’s book Smashing Physics is available as “Most Wanted Particle” in Canada & the US.
