March Meeting day 2

Today was probably the maximum crowd at the APS meeting. Saw a number of talks and had lots of conversations. One particularly interesting talk was about variations on this result. In condensed matter we've become used to the idea of "photonic band gap" or "photonic crystal" materials, systems where a spatially periodic pattern of dielectric contrast (e.g., glass vs. air) results in optical properties that mimic the electronic properties of crystals: bands (energy ranges) where light can propagate freely, and (photonic) band gaps, where light is reflected and can't propagate. This talk was about weird "hyperuniform" disordered dielectric structures that nonetheless have a complete photonic band gap (in all directions) in 2d, and the fact that by introducing voids and defects in these systems, it's possible to make very selective and directional waveguides. I need to read more about the math behind this. The experiment uses microwaves rather than visible light, meaning that it's possible to build such structures by hand using sapphire plates and rods on the centimeter scale.

The Buckley Prize session was also very good, though I missed talks in the middle. Very crowded, particularly for Charlie Kane's talk. That one would have been fun if it'd been a full hour - it felt like he had to abbreviate some of the discussion to fit into the 30+6 minute slot.

Another highlight this afternoon was the big Kavli session about the mesoscale. The lead talk was from Bob Laughlin, who is always entertaining. He focused on the big open question of whether there are laws that emerge in biology. By his definition, a law is a quantitative relationship between measured parameters that always holds. Some laws are (apparently) fundamental, like the force between two point charges in vacuum. Others are emergent, like the relationship between stress and strain in elastic media, or the Navier-Stokes equations that govern hydrodynamics. In the emergent situation, emergent laws hold when the system is sufficiently macroscopic. Laughlin's big question is, are there universal quantitative relationships that emerge in biological systems (beyond the trivial ones already mentioned, like elasticity being useful for describing cell membranes)? He says that there are hints all over, but it's very hard to do the definitive experiments because biology is just so complicated and our experimental tools are comparatively invasive and crude. When asked to describe biologists in one word, he said "frustrated".

The talk also put forward two definitions of what condensed matter physicists do, both of which are very good. Via Laughlin, Michael Fisher says: "Our job [as condensed matter physicists] is to discover and understand the phases of matter and the transitions between them." Laughlin himself says: "Our job [as condensed matter physicists] is to discover the emergent laws of nature, and hand them to engineers so that they can be put to use." Good stuff. On a lighter note, I realized partway through the talk that every now and then it's entertaining to imagine Laughlin's words as if said by William Shatner. "We trust the emergent laws of rigidity and hydrodynamics...to make an airplane...that can go up to 35000 ft...and not...explode!"