Department of Physics and Astronomy Colloquia
Fall 2004
(1997/98,
1998/99,
1999/00,
2000/01,
2001/02,
2002/03,
2003/04)
Colloquia are Wednesdays 4:00pm in David Rittenhouse
Laboratory (209 South 33d Street), in room A8, unless otherwise noted. Colloquia are preceded by a department tea at
3:30 in room 2E17.
September 15:
The seventh Walter Selove Lecture
Sidney Nagel
University of Chicago
Physics at the Breakfast Table
Many complex phenomena are so familiar that we forget
to ask whether or not they are understood. In this lecture, I will
discuss several familiar cases of effects that are so ubiquitous that
we hardly realize that they defy our normal intuition about why they
occur. The examples of poorly understood classical physics that I
will choose can all be viewed at a breakfast table: the anomalous flow
of granular material, the long messy tendrils left by honey spooned
from one dish to another and the pesky rings deposited by spilled
coffee on a table after the liquid evaporates. These are all
non-linear hydrodynamic phenomena which not only are of technological
importance but can also lead the inquisitive into new realms of
physics.
Additional lecture, Tuesday September 14, 12:00pm:
Jamming and Failure: Solids on the Edge
This talk will address two questions: (a) how does a particulate
system jam and develop a yield stress in a disordered state and (b)
are there similarities between the variety of ways in which such a
jammed particulate solid can fail and return to the fluid state. I
will describe our search for similarities between failure caused by
changing two different control parameters: decreasing the density at
zero applied shear stress and increasing the shear stress at constant
density. There is a clean signature of an excess low-frequency
contribution to the density of vibrational states as failure is
approached along these different pathways. This suggests a natural
explanation for the excess specific heat and the Boson peak seen in
glasses at low temperatures. A measurement of the density of states
may also be used as a measurable precursor to failure in strained
systems on the verge of rupture. The work I describe was done in
collaboration with Andrea Liu, Leo Silbert and Corey O'Hern.
Host: Nelson
October 13:
Sean Carroll
University of Chicago
Why is the Universe Accelerating?
A variety of observations have led cosmologists to conclude
that the
universe is dominated by a mysterious form of "dark
energy" (in
addition to the well-established "dark matter",
which now seems
prosaic by comparison). This dark energy could
be vacuum energy (a
cosmological constant), or something dynamical
and slowly
evolving. All of the possibilities are very exciting,
and future
observations have promise for distinguishing between
them. I will
give an overview of the theoretical proposals for dark
energy and
the observational constraints which any model must satisfy.
Host: Balasubramanian
November 10:
Sajeev John
University of Toronto
Photonic Band Gap Materials: Engineering the Fundamental Properties
of Light
Photonic Band Gap (PBG) materials are artificial, periodic, dielectrics that enable engineering of the most fundamental properties of electromagnetic waves. These properties include the laws of refraction, diffraction, and spontaneous emission of light. Unlike traditional semiconductors that rely on the propagation of electrons through an atomic lattice, PBG materials execute their novel functions through selective trapping or "localization of light" using engineered defects within the dielectric lattice. This is of great practical importance for all-optical communications and information processing. Three dimensional (3D) PBG materials offer a unique opportunity for simultaneously (i) synthesizing micron-scale 3D optical circuits that do not suffer from diffractive losses and (ii) engineering the electromagnetic vacuum density of states in this 3D optical micro-chip. This combined capability opens a new frontier in integrated optics as well as the basic science of radiation-matter interactions.
We review recent approaches to micro-fabrication of photonic crystals with a large 3D PBG centered near 1.5 microns. These include direct laser-writing techniques and holographic lithography. We review the concept of a hybrid 2D-3D PBG hetero-structure in which a 2D photonic crystal micro-chip layer is suitably lattice matched and embedded within a 3D PBG material. This microchip layer contains optical wave-guides and optical micro-cavities that enable frequency selective control of spontaneous emission of light from atoms. Unlike traditional wave-guides that confine light in a high refractive index medium using total internal reflection, these air-wave-guides operate using the principle of light localization for confinement of light along a low refractive index path.
We demonstrate a nearly universal approach to ultra-dense, three-dimensional, integrated optics in general 3D PBG architectures. These 3D optical circuit paths are constructed using broadband, loss-less, chip-to-chip interconnects between 2D micro-chip layers, intercalated within the 3D PBG host material. Unlike electronic micro-circuitry, each air-wave-guide path can simultaneously conduct hundreds of wavelength channels of information, throughout the 3D micro-chip.
In addition to exhibiting diffraction-less flow of light through micron-scale bends, this optical micro-chip allows the engineering of very large and abrupt changes in the local electromagnetic density of states as a function of frequency. This leads to unprecedented frequency selective control of spontaneous emission, modification of the blackbody radiation spectrum, and some fundamentally new optical functions unattainable in conventional photonics.
Host: TBA
Tuesday, November 30 in DRL room A2
Robert Brandenberger
McGill University
Challenges for String Cosmology
The inflationary scenario provides the current paradigm of
early Universe cosmology. Although this scenario has been
very successful phenomenologically, it is plagued by
serious conceptual problems. I will discuss some of
these problems and explain why superstring theory might
provide a good framework in which to address these
issues. I will give an overview of some of the key challenges
which a new paradigm of the early Universe based on
string theory faces, and will discuss one approach to
addressing these questions, "string gas cosmology".Host: Jimenez
Tuesday, January 11 at 3:00pm in DRL room
A4 (note unusual day and time):
Vijay Balasubramanian
University of Pennsylvania
The Puzzle of Cosmic Acceleration
Astrophysical measurements suggest that the universe is undergoing a
period of accelerating expansion driven by a cosmological constant.
A previous episode of acceleration, i.e. inflation, is invoked to
solve numerous problems of Big Bang cosmology and to explain scale
invariance of the CMBR fluctuation spectrum. However, cosmic
acceleration poses many fundamental challenges for theoretical physics
ranging from the basic ("How is mass measured in an accelerating
universe?") to the almost philosophical ("How did time begin?"). I
will describe these puzzles and assess progress towards resolving
them.
Host: Nelson
January 12:
Nigel Goldenfeld
University of Illinois
Biocomplexity in Action: Pattern Formation and Microbial Ecology at Yellowstone's Hot Springs
Biocomplexity is the term that is becoming used to describe efforts to understand strongly-interacting dynamical systems with a biological, ecological or even social component. I provide a brief overview of why this field is not only interesting for physicists, but can benefit substantially from their participation.
Microbes are the "dark matter" of biology. Modern microbiology represents a fascinating opportunity for physicists to contribute to biology, because microbes are simple forms of life, but not too simple. Moreover, the techniques of statistical mechanics are ideally suited to exploring problems with biocomplexity, such as the ecology of microbial communities and the evolutionary dynamics of microbial genomes.
The ability to distinguish both ancient and modern geological features that are biologically influenced from those that are purely abiotic in origin can potentially advance our understanding of the timing and pattern of evolution, and may even provide a tool with which to identify evidence for life on other planets.
Host: Nelson
February 9:
March 16:
Paul Selvin
University of Illinois at Urbana--Champaign
Fluorescence Imaging with One Nanometer Accuracy (FIONA): Application to Nanoscale Motors
We have achieved 1.5 nm resolution using fluorescence imaging, approximately 300 times better than the diffraction limit of conventional light. Recently we have been able to increase the time resolution to 1 msec, from a previous value of 500 msec. Using this increased time-resolution, we have looked at molecular motors inside living cells. We have been able to see individual cargos being moved by individual kinesin and dynein, two important motors. We find that both kinesin and dynein move cargo 8 nm per ATP (the universal food of the cell), in opposite directions in a cell. Amazingly, these two molecular motors do not engage in a tug-of-war, but appear to be cooperative, giving the particle extra speed.
Host: Nelson
April 13:
Jean Carlson
University of California, Santa Barbara
Complexity and Robustness: Understanding Dynamics and Feedback in Nature
Host: TBA