Add some ice cubes to a pan on the stove. Within minutes, you will have observed the three most common phases of matter: solid, liquid, and gas. But at the extreme of nature’s limits, such as close to absolute zero (−273° C), the same matter enters strange and wonderful new phases.
The 2016 Nobel Prize in physics was awarded to three researchers whose work transformed our understanding of matter in such exotic states. David Thouless of the University of Washington, Duncan Haldane of Princeton University, and Michael Kosterlitz of Brown University took home the prize for, as the Nobel committee put it, “theoretical discoveries of topological phase transitions and topological phases of matter.”
I spoke to John Chalker, head of theoretical physics at the University of Oxford, and asked him to explain this year’s Nobel Prize in physics to my grandpa—who hasn’t studied anything beyond high-school science. Here is how he put it, edited and condensed for clarity.
There are two types of work that win Nobel Prizes. One involves specific achievements, such as the 2013 physics Nobel prize which was given for both the theoretical and experimental discovery of the Higgs boson. The other is for a set of new ideas that gradually become more important in the way we think. The 2016 prize belongs to the latter group.
Physics is used to understand everything from the very small, such as sub-atomic particles in quantum physics, to the very big, such as black holes in cosmology. This year’s prize deals with condensed-matter physics, which deals with things in the middle, including everyday materials such as semiconductors that are used to make computer chips.
When they began their work in the 1970s, this year’s winners were not thinking about applications that might emerge from their work. They were simply trying to solve scientific puzzles, and they used mathematical ideas from a field called topology, which is the study of geometrical properties of objects.
One such puzzle involved figuring out the weird behavior of superconductors, which are materials that under certain conditions, such as extremely low temperatures, offer no resistance to electrical current that flows through them. A normal superconductor wire would, for example, lose its abilities as it is warmed up to normal temperature. The phase transition would occur in a uniform manner, as in there would be no areas within the wire where superconductivity still occurred.
If instead of a wire the superconductor was made out of a thin sheet (“different geometry” as a topologist would say), then it would behave differently. Unlike the wire, it would not transition uniformly, which meant that in such thin sheets you had points that lost their superconductivity but others that didn’t. Thouless and Kosterlitz used topological concepts and developed the physics that explained this weird phase transition.
Such understanding can go a long way. For instance, in the 1940s, researchers spent a great amount of effort trying to understand semiconductors. These metals—which, as the name suggests, conduct only a little bit of electricity—were interesting materials but didn’t have much use. As scientists understood why they behave the way they do, they started developing applications for them. One such application led to the creation of the integrated circuit, which is today present in every computer and smartphone in the world.
Another puzzle involved understanding one of the most bizarre effects in physics: the quantum Hall effect. The mere discovery of the effect won the 1985 Nobel physics prize, but for many years nobody really understood why it happened.
Here’s the classical Hall effect, discovered in the 1870s: Take a sheet of metal and apply voltage to opposite edges, say the top and bottom. You’ll get an electrical current flowing across the sheet. Then apply a magnetic field that is perpendicular to the sheet. This will force electrons away from the top and bottom and towards the left and right edges, which creates a voltage drop across the latter two edges—this is called the Hall voltage. This phenomenon has practical applications: Without a magnetic field there is no Hall voltage, but with a magnetic field there is some; so, based on the magnitude of the Hall voltage, instruments can be used to measure the strength of a given magnetic field.
The quantum Hall effect takes this experiment a step further. Replace the metal sheet with a conductor that is only a single-atom thick and then cool the material down to near absolute zero. Now when you apply the perpendicular magnetic field a strange thing happens. The ratio of the current passing through the sheet and the Hall voltage created remains constant, no matter what material you use or what imperfections you add to it.
Thouless used topology to explain the quantum Hall effect. To a topologist, a bagel will be the exact same thing as donut, because both objects have a hole in it. In this case, for simplicity sake, the donut and bagel have the same “topological invariant” of one hole.
Thouless found that the quantum Hall effect in a thin sheet could be explained using the concept of topological invariants. The constant ratio observed was nothing but a topological invariant. In other words, every material used to test the experiment has the same current-to-voltage ratio.
In the 1980s, Haldane applied the principles of topology to predict that the quantum Hall effect should exist regardless of a magnetic field in certain types of material. In 2013, more than 25 years later, his predictions were shown to be correct when other scientists were able to make new materials to test the prediction.
The combination of all the three winners’ work has, through later discoveries, led to the development of materials called topological insulators. These materials are insulators on the inside but behave like normal conductors on the outside. Like the semiconductors of the 1940s, these topological insulators are bound to find some application that we can’t yet imagine.