"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.