Thursday, November 10, 2011

Physics Pieces (Part 5)

Interesting set of seminars last week.

Claudio Castelnovo from Royal Holloway, who was a post-doc in Oxford, gave a talk about magnetic monopoles in spin ice, which was featured on the cover of Nature in 2008. He gave a brief theoretical introduction and described results by Morris et al. from neutron scattering experiments with an applied magnetic field to compare against this model of emergent electrodynamics in a 3D condensed matter system.

Daniel Khomskii from Koeln University, Germany spoke on frustrated Mott insulators. The virtual hopping of Hubbard spins around a triangle in a triangular lattice gives an orbital current in third order perturbation theory, which was a somewhat analogous result to magnetic monopoles in spin ice (electric charge instead of magnetic monopoles).

Dieter Jaksch, associated with the atomic and laser physics sub-department of Oxford, and with the Centre for Quantum Technologies in Singapore, gave a talk about strongly driven quantum systems. He described the application of tensor network and path sum approaches to their areas of research, on impurities in optical lattices, Rydberg lattices and tetrahertz pump-probes.

Martin Rees gave another talk, this time on real and counterfactual universes. This was the 8th Dennis Sciama memorial lecture, and Rees was Sciama's graduate student. Stephen Hawking had been Sciama's graduate student for about 2 years when Rees joined the research group. Interestingly, Roger Penrose, who made contributions to astrophysics during the 1960s, was at the talk as well.

Rees gave some of the history of the steady-state theory versus big bang theory in the 1940s and 50s, and spoke about the "tuning" problem of the cosmological constant and the amplitude of fluctuations during the big bang. To produce a universe like ours, there seem to be strong constraints on the allowed value of the cosmological constant (cosmic expansion rate), and some sensitivity also to the fluctuations - too much fluctuations and there are too many black holes, too little and there are too few stars. And gravity needs to be weak so the universe has enough time to become interesting.

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