## Colloquium

### The physics of strong correlations: an enduring puzzle

**Speaker: Chris Hooley (University of St. Andrews)**

**Date: Wednesday 9 March 2022**

**Time: 15:00**

**Venue: N/4.07**

In the famous words of Sir Arthur Eddington, “We used to think that if we knew one, we knew two, because one and one are two. We are finding that we must learn a great deal more about ‘and’.” This pithy quotation captures the essence of the problem of complexity in physics. In many cases, even when we know the microscopic ingredients of a system, predictions of the behaviour of the entire system do not seem to follow easily from this knowledge.

In this talk, I shall try to explain why not. I shall draw on various examples from physics where strong correlations produce surprising effects, including traffic flow and traffic jams, the interiors of neutron stars, and electrical transport in nearly magnetic metals. I shall discuss Phil Anderson’s famous 1972 article “More Is Different” – which does not say what many people say it says – and whether the advent of increasingly high-performance computing should change our view on some of these questions of emergence and explanation.

**Short bio:** Chris Hooley is a Senior Lecturer in theoretical condensed matter physics at St Andrews, and also Operations Director of the Scottish Doctoral Training Centre in Condensed Matter Physics. His research interests centre on the quantum many-body problem and its applications in the physics of strongly correlated electrons in crystalline solids, cold atomic gases, and nanoscale devices. His active research topics at the moment are: the use of matrix product states to describe strongly correlated electron systems; vortex-mediated melting in layered systems with competing orders; the effect of the trapping potential on the many-body physics of ultracold atomic gases; the interpretation of partition function zeros at complex temperature; Majorana-paired mean-field states in magnetic systems; relativistic corrections to the electron-electron interaction in metals at low temperatures (with his student Sam Ridgway); and soliton solutions of the Einstein-Maxwell-Dirac equations (with his student Alasdair Leggat, co-supervised with Professor Keith Horne). He also has research interests in non-equilibrium physics, particularly the out-of-equilibrium Kondo model, and in magnetostrictive effects, including the Invar phenomenon.