On Dualities and Equivalences Between Physical Theories | 22nd of June 2018
Jeremy Butterfield, Cambridge University | See video here
The main aim of this paper is to make a remark about the relation between (i) dualities between theories, as `duality' is understood in physics and (ii) equivalence of theories, as `equivalence' is understood in logic and philosophy. The remark is that in physics, two theories can be dual, and accordingly get called `the same theory', though we interpret them as disagreeing---so that they are certainly not equivalent, as `equivalent' is normally understood. So the remark is simple: but, I shall argue, worth stressing---since often neglected. My argument for this is based on the account of duality developed with De Haro: which is illustrated here with several examples, from both elementary physics and string theory. Thus I argue that in some examples, including in string theory, two dual theories disagree in their claims about the world. I also spell out how this remark implies a limitation of proposals (both traditional and recent) to understand theoretical equivalence as either logical equivalence or a weakening of it.
Topological order in quantum and classical physics | 21st of January 2021
Tom Lancaster, Durham University | See video here
The fractional quantum Hall (FQH) fluid has been advanced by several authors as a candidate for demonstrating strong emergence, most notably through its hosting particles with fractional electronic charges. The properties of the FHQ fluid can be traced back to topological order: a non-symmetry breaking ordering of electrons that relies on whole-system interactions. Although this exotic state of affairs might seem a quirk of quantum mechanics, this is not the case. In this talk I shall discuss the possibility of emergence in both the FQH fluid and, in an example drawn from the classical world of soft-matter physics, in a system of entangled polymer rings.
Network structure and the spread of epidemics | 11th of February 2020
Clara Stegehuis, University of Twente | See video here
Many real-world networks contain groups of densely connected nodes, also called communities. We use random graph models to show that these community structures strongly influence the behavior of epidemic processes on networks: community structures can both enforce as well as inhibit epidemic processes. Our models further show that the exact internal structures of communities barely influence the behavior of percolation processes across networks. We then investigate how the final size of an epidemic is influenced by contact tracing and quarantining. We show that the effectiveness of such tracing processes strongly depends on the network structure. In contrast to previous findings, the tracing procedure is not necessarily more effective on networks with heterogeneous degrees. We also show that network clustering influences the effectiveness of the tracing process in a non-trivial way: depending on the infectiousness parameter, contact tracing on clustered networks may either be more, or less efficient than on networks without clustering.
Hydrodynamic limits and duality | 4th of March 2021
I will introduce some basic models of interacting particle systems and introduce the concept of hydrodynamic limit, fluctuations around the hydrodynamic limit and large deviations from the hydrodynamic limit.
Large-scale dynamics of biomembranes and membrane-associated proteins | 18th of March 2021
Lipid bilayer membranes are self-assembled structures with the thickness of a few nanometers that can form an assortment of geometries of several micrometers in size, vital to the function of living cells. The so-called peripheral proteins, that can bind to the surface of membranes, are responsible for shaping and remodeling biomembranes. The necessary cooperative action of a multitude of curvature-inducing proteins leads to the emergence of the rich membrane geometries observed in living cells. In order to model this highly dynamic system close to its native spatiotemporal scales, a mesoscopic model that can accurately mimic membrane mechanics and solvent hydrodynamics is needed. In this talk, I present our coarse-grained dynamic membrane model , and the corresponding approach to hydrodynamics that leads to realistic membrane kinetics . I will talk about the entropic membrane-mediated interactions and investigate the kinetics, stationary distributions, and the free energy landscape governing the formation and break-up of protein clusters on the surface of the membrane.
 M. Sadeghi, T. R. Weikl, and F. Noé. Particle-based membrane model for mesoscopic simulation of cellular dynamics. J. Chem. Phys., 148(4):044901, 2018.
 Mohsen Sadeghi and Frank Noé. Large-scale simulation of biomembranes incorporating realistic kinetics into coarse-grained models. Nat. Commun., 11(1):2951, 2020.
How good risk management can cause financial crises | 1st of April 2021
Florian Wagener, UvA
Risk management is as old as finance. It is based on the observation that total risk can be reduced by spreading it out over market participants. However, new financial instruments are regularly at the root of global financial crises. We propose a mechanism how the availability of more financial instruments may destabilise markets when traders have heterogeneous expectations and adapt their behaviour according to performance-based reinforcement learning.