Physics Pieces

Quite a few remarkable talks recently that I thought I'd write briefly about, especially the last one.

Jim Peebles gave a lecture last week about "finding" the big bang (cosmic microwave background and that kind of thing). An interesting account of the historical developments. As physics improves by successive approximations, it always leaves something unknown for the next generation to find out.

Rodney Baxter gave a talk on his work on hard hexagons, and had lessons about publishing conjectures even if you don't fully understand them, so others can work on them too.

Slava Rychkov gave a talk about ideas that might be pretty exciting for future work. Essentially, they use conformal field theory techniques ("conformal bootstrap") with some success in greater than two space dimensions (e.g. on the 3D Ising model), complementing the use of renormalisation group techniques.

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.

Physics Pieces (Part 4)

Talks by two Princeton professors who visited Oxford last week. Dan Marlow gave a talk about luminosity measurements at the Large Hadron Collider, discussing the various technical issues involved when pushing equipment to its limits to test the limits of science. Duncan Haldane spoke about geometry and topological phases of matter, starting from the Gauss-Bonnet theorem and the generalisation by Chern. Interesting history about how Karplus and Luttinger's paper in 1954 (Phys. Rev. 95, 1154–1160) on the anomalous Hall effect was discredited and finally vindicated in 2005.

Fellow graduate student Curt von Keyserlingk gave a talk on the Toric Code too, a toy model involving plaquette and vertex terms in the hamiltonian for the spins. There's a nice pictorial representation of the states of the system, and similar models can lead to what are known as "anyon statistics" for quasiparticles that are neither fermions nor bosons.

Roser Valenti from Goethe University in Frankfurt gave a talk on frustration, using density functional theory to come up with effective models and studying them with quantum Monte Carlo and other methods. Interesting attempt to understand caesium copper chloride and caesium copper bromide by substituting the chlorine atoms with bromine atoms and vice versa.

Martin Rees from Cambridge delivered a public lecture on "The Limits of Science" last week too, and I took the kids to it. He had some encouragement for beginning scientists, who might feel that so many major advances have been made that it's hard to push further forward. His idea was that each advance leads to new questions, and the computational power we have at our disposal is a powerful tool that previous generations did not have. Yet there are likely to be things beyond our ability to comprehend, which require posthuman intelligence. There is a huge gulf between what we can do today and what we should do, whether in research or in public policy, and we need to choose wisely with a longer term view of leaving a fair inheritance for future generations.

Physics Pieces (Part 3)

It's a new month!

This is going to be a short piece since I've already reported on some of the talks from last week. There was a talk by Jacques Laskar from IMCCE in Paris. Unfortunately I missed this one but there is a video that he showed at the end of the talk.

Joanna Haigh from Imperial College London gave a talk about how changes in the sun affect climate. The prevalence of sunspots has a 11-year cycle of fluctuations, and recent data has suggested a spectral distribution quite different from what is usually assumed.

Physics Pieces (Part 2)

Another exciting week has gone by, and the current week is unrelentless in seeking to outdo the last.

It's just mid-week and already David Weiss from Penn State gave us a summary of the various cold atom experiments going on in his lab - ranging from quantum computing on a 3-d lattice to a quantum version of "Newton's cradle" and a search for the electron dipole moment.

Fiona Burnell, a post-doc in our group, gave an informal presentation on a toy model for fractional topological insulators. Useful ideas to help me navigate the zoo of condensed matter systems out there. She did her PhD at Princeton under Shivaji Sondhi and had started her studies there while I was still there as an undergraduate.

But the real focus of this post was last week.

Alessandro de Silva from ICTP Trieste spoke about quantum quenches, studying the behaviour of systems quickly thrown from an equilibrium state to another via a non-equilibrium process.

Chris Hooley from St Andrews gave an entertaining lecture, based on some initial research into the use of complex temperatures for the thermodynamic partition function. It was useful for me as he spent a lot of time motivating the discussion and talking about quantum critical points.


David Nelson from Harvard gave a very accessible presentation on the packing problem on a curved surface. Very nice simulations and visuals for a practical and mathematical topic, and actually had everyone in the audience looking at a golf ball (the distribution of the dimples on the surface is related to how viruses arrange their capsomeres).

Lev Ioffe from Rutgers talked about quantum coherence, and what I really enjoyed were the introductory graphics with spherical cows and the philosophical idea that if the environment itself is quantum, then how does quantum decoherence set in? So it's like Schrödinger's cats looking at Schrödinger's cats.

With the complement of courses and problem sets, I am slowly but surely progressing on the path of understanding nature better!

Physics Pieces (Part 1)

A quick review of the week gone by...

A talk given by Olga Sikora (who worked in Nic Shannon's group in Bristol) was on spin ice and mentioned that frustration can occur as "charge frustration" as well. Professor Shannon is based in Oxford this year, and is subsequently headed to the new Okinawa Institute of Science and Technology next year.

A talk by Shivaji Sondhi presented a very interesting description of quantum liquids by considering the vortex structure induced in superfluids when coupled with real-space rotation in a bucket. Professor Sondhi was my advisor for my very first research project on the Ising model during Junior Year at Princeton.

A talk by Mark Newman on networks. Excellent for showing the application of physical insight (mainly statistical physics) into problems such as social networks, such as a simple proof of why it usually is that your friends on average have more friends than you (Ref). Professor Newman is a prolific author and did his DPhil at Oxford, co-supervised by Robin Stinchcombe and David Sherrington.