Research (big picture)

"Today we cannot see whether Schroedinger's equation contains frogs, musical composers, or morality --- or whether it does not." --Richard Feynman

Studying "emergence" with physics

Many of the most important, interesting, and vexing problems we encounter are questions about the behavior of large collections of objects: How do atoms organize themselves to form diverse materials such as plastics, metals, fluids, magnets? How do connected neurons collectively result in intelligence? How do competing organisms give rise to ecosystems? How do humans organize themselves into groups, economies, and societies? Each of these questions shares the common thread of being a question about how entities come together to give rise to behavior not obviously connected to the underlying entities.

There are several reasons that condensed matter and ultracold atomic physics are wonderful areas in which to study this emergence. Firstly, unlike many other fields, we have a good understanding of the constituent pieces --- they obey Schroedinger's equation referenced above by Feynman --- and we can specifically focus on the means by which the individual pieces' behaviors turn into the collective behavior. Secondly, it is possible to do reproducible, tunable, quantitative experiments --- in contrast, you can't repeatedly create human societies in test tubes. I believe that remaining firmly grounded in experimental consequences is the only way to reliably develop theories, and physics offers us a grounded way to develop new methodologies for understanding emergent behavior.

Example of emergence in physics

For a concrete example, consider a gold atom, a tiny, basically spherical, transparent speck. None of metallic gold's properties are readily apparent --- its shine and color, its large thermal conductivity (think of a cold winter, when a metal doorknob feels much cooler than wood at the same temperature), and its low electrical resistance have no direct counterparts in the individual gold atom. Understanding how atoms collectively form metals was an early coup of solid state physics. This theory gave us, for example, an understanding of why copper is a metal while diamonds are insulators (not to mention explaining semiconductors and resulting in the transistor). This leads to another fascinating observation: although the emergent metal bears little resemblance to the individual atoms, vastly different atoms can give rise to similar behavior. For example, all metals --- be they aluminum, copper, silver, or gold atoms, or a combination --- share most of gold's characteristics listed above. This is not a coincidence: understanding this universality culminated in the development of the renormalization group in the 1970's. This idea of universality and the renormalization group has since permeated well outside of physics, for example to biology, chemistry, mathematics, and finance.

Applications

Finally, a little more practically, the fruits of studying emergent phenomena in physics have frequently led to technological advances: the transistor, hard drive, laser, and MRI are just a few examples which owe their existence to fundamental discoveries in condensed matter physics over the last century. As we are presently confronting many phases of matter which cannot be understood within our current framework of how properties emerge from their constituent particles, one imagines that these will have equally dramatic applications. One that is often talked about is the application of so-called topological states to quantum computing. Cold atomic systems also offer great promise for precision accelorometry, magnetometry, and emulation of real materials, for example high temperature superconductors. I have invested much of my attention in the latter role as analog systems to explore many body systems, and recently I have also been exploring quantum-enhanced metrology and other quantum-enhanced technologies.