Colloquia

Department of Physics Colloquia Schedule

SPRING 2008

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February 7, 2008
LISA RANDALL
Harvard University

"Warped Geometry: Consequences and LHC Signatures"

Extra dimensions, and warped geometry in particular, might hold the key to the solution to the hierarchy problem of particle physics. We discuss the underlying mechanism and how the LHC will test this idea.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (Pappalardo Community Room)

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February 14, 2008
EDWARD FARHI
MIT

"A quantum computer can determine who wins a game faster than a classical computer"

Imagine a game where two players go back and forth making moves and at the end of a fixed number of moves each position is either a win or a loss for the first player.  In this case, if both players play best possible, it is determined at the first move who wins or loses.  

To figure out who will be the winner you need not look at all of the final positions but only at N.⁷⁵³ where N is the number of final positions.  I will show that with a quantum computer the exponent can be reduced to 1/2.  The technique involves quantum scattering theory.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (Pappalardo Community Room)

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February 21 , 2008
ADAM COHEN
Harvard University

"Electro-optical traps for single molecules in water at room temperature"

Just as traps for single ions in vacuum enabled long observation times and precision measurements, traps for single biological molecules in solution would enable extended observation and precision measurements.  I will describe progress toward that goal.    

The Anti-Brownian Electrokinetic trap (ABEL trap) uses fluorescence particle tracking and real-time feedback to induce an electrokinetic drift in a particle that cancels its Brownian motion.  I will describe trapping of single molecules of DNA, protein, viruses, and quantum dots, and discuss what can be learned from a trapped molecule.

Time: 4:15pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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February 28, 2008
PHUAN ONG
Princeton University

"The Role of Vortices in Limiting Tc in Cuprate Superconductors"

Superconductivity in the copper oxides occurs at temperatures much higher than in all other metals.  There is growing evidence that the Cooper pairs actually survive to even higher temperatures. 

I will discuss Nernst and torque magnetometry experiments which suggest the scenario that, above Tc in the cuprates, long-range phase stiffness is destroyed, rather than the gap order parameter.  In the Nernst experiment, the vortex current produced by a temperature gradient generates a Josephson E-field perpendicular to the applied field H.  A large Nernst signal eN persisting to a high onset temperature ~130 K is observed in nearly all cuprate families.  Extensive Nernst experiments in the cuprates LSCO, Bi 2201, and 2212 yield a 3D phase diagram (x,T,H) in fields up to 45 T. 

This picture has been confirmed by recent torque magnetometry experiments.  In a tilted H, local planar supercurrents associated with vortices above Tc produce a torque that deflects a cantilever.  The inferred diamagnetism provides thermodynamic evidence for the vortex liquid picture suggested by the Nernst effect.  Recent high-temperature STM experiments providing direct test of these ideas will also be discussed. 

Research supported by NSF.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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March 6 , 2008
CHRISTOPHER STUBBS
Harvard University

"Testing Gravity with Telescopes"

The discovery of the accelerating expansion of the Universe demands that we test our understanding of the foundations of gravity on all accessible scales.

I will describe 3 examples of how optical wavelength telescopes can be brought to bear on this challenge:

1) Testing the self-consistency of the MOND scenario, an alternative to the dark matter hypothesis, by measuring the 3-d kinematics of galaxies

2) Testing the principle of equivalence with lunar laser ranging

3) Linking optical variability to sources of gravitational radiation

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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March 13 , 2008
DISTINGUISHED DAVID AND EDITH HARRIS LECTURE
BORIS KAYSER
Fermilab

"Exploring the Neutrino Questions"

Neutrinos are among the most abundant particles in the universe. The discovery that they have nonzero masses has raised a number of very interesting questions about them, and about their connections to other areas of physics and astrophysics. After briefly reviewing what has been learned about the neutrinos so far, we will identify the open questions, explain why they are interesting, and discuss ideas and plans for answering them through future experiments.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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March 20, 2008
GRADUATE OPEN HOUSE
SARA SEAGER
MIT

"Extrasolar Planets: Interiors, Atmospheres, and the Search for Habitable Worlds"

For thousands of years people have wondered, “Are we alone?” With over 250 exoplanets known to orbit nearby stars, this question has moved from science fiction to  mainstream study. Now that the existence of exoplanets is firmly established, a new era of “exoplanet characterization” has begun. A subset of exoplanets—called transiting planets—pass in front of their stars as seen from Earth. Transiting planets have opened a whole new opportunity for exoplanets, because their physical properties, including average density and basic atmospheric properties, can now be routinely measured. The race to find habitable exoplanets has accelerated with the realization that big Earths orbiting small stars can be both discovered and characterized with current technology. These ideas will lead us down a path to the ultimate goal of space-based discovery and characterization of Earth analogs.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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March 27 , 2008
No colloquium talk – Spring Break

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April 3, 2008
GEOFFREY WEST
Santa Fe Institute

"Universal Scaling Laws in Living Systems from Cells to Cities; Towards a Unifying Theory of Biological and Social Structure and Organization"

Life is very likely the most complex phenomenon in the Universe manifesting an extraordinary diversity of form and function over an enormous range. Yet, many of its most fundamental and complex phenomena scale with size in a surprisingly simple fashion. For example, metabolic rate scales as the 3/4-power of mass over 27 orders of magnitude from complex molecules up to the largest multicellular organisms. Similarly, time-scales, such as lifespans and growth-rates, increase with exponents which are typically simple powers of 1/4. It will be shown how these "universal" 1/4 power scaling laws follow from fundamental properties of the networks that sustain life, leading to a general quantitative, predictive theory that captures the essential features of many diverse biological systems. Examples will include animal and plant vascular systems, growth, cancer, aging and mortality, sleep, DNA nucleotide substitution rates. These ideas will be extended to social organisations: to what extent are these an extension of biology? Is a city, for example, "just" a very large organism? Analogous scaling laws reflecting underlying social network structure point to general principles of organization common to all cities, but, counter to biological systems, the pace of social life systematically increases with size. This has dramatic implications for growth, development and sustainability: innovation and wealth creation that fuel social systems, if left unchecked, potentially sow the seeds for their inevitable collapse.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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April 10, 2008
DAVID WINELAND
National Institute of Standards and Technology

"Quantum Information Processing and Quantum Metrology with Trapped Ions"

Over the last decade or so, there has been an explosion of activity in quantum information processing (QIP).  Arguably, Shor’s algorithm for factoring large numbers stimulated much of the interest; however experimentally, this application remains a distant goal.  Nearer-term possibilities include practical quantum communication and quantum simulation.  Also, we can apply some simple QIP procedures to metrology; examples where entanglement can lead to improved detection sensitivity in interferometry and spectroscopy are discussed in the context of trapped ions.  If we can eventually apply some of these techniques to large numbers of ions, we might realize a direct analog of Schroedinger’s cat. It is also interesting to consider extending some of these methods to other quantum systems.

*Supported by IARPA, ONR, and NIST

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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April 17, 2008
PETER BORDEN
Solar Business Group, Applied Materials, Inc.

"Making a Better Solar Cell"

We start with an overview the market drivers for photovoltaics (PV) – the direct conversion of sunlight to electricity.  In general, these center on the reduction of cost per generated kWH, although efficiency provides a significant premium in many markets.  Today, there are two dominant approaches to PV:  wafer-based silicon and thin film.  The former has a decided efficiency advantage, while the latter has a cost advantage.  For both approaches we review of the methods used to achieve the best efficiencies, largely through maximizing collection of generated carriers, light trapping, physical design to minimize contact obscuration and series resistance, and bandgap engineering to optimize the match to the solar spectrum.   This leads to an understanding of performance gaps and opportunities for improvement.  The talk concludes with an overview of how technologies from related fields, such as flat panel displays, IC fabrication, and laser processing are being exploited to fill both fill gaps and enable scaling of production volume to drive down costs.  

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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April 24, 2008
STEVEN KIVELSON
Stanford University

"Electronic Liquid Crystals"

Highly correlated quantum fluids are fluids in which the typical interaction strength between neighboring particles is large compared to the zero-point kinetic energy. It is somewhat miraculous that such systems exist at all, since in the limit of large enough interactions, one generally expects frozen (probably crystalline) states to be ubiquitous. However, as is the case in classical complex fluids, for quantum systems, as well, there can exist a host of phases intermediate between the quantum gas, in which interactions are weak, and the quantum crystal, in which the zero-point kinetic energy is small. Classical liquid crystals are frequently viewed as phases that occur due to the complicated shape of the constituent molecules, such as the highly elongated "nematogens" that give rise to most nematic liquid crystals with which we are familiar; this might make the occurrence of such phases in electron fluids seem unlikely. However, liquid crystals can also be viewed as partially melted solids, and from this perspective they can (and do) occur in correlated electron systems, as well.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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May 1 , 2008
ANGELA OLINTO
University of Chicago

"The Highest Energy Particles"

After almost a century of observations, the ultra-high energy sky has finally displayed an anisotropic distribution. A significant correlation between the arrival directions of ultra-high cosmic rays measured by the Pierre Auger Observatory and the distribution of nearby active galactic nuclei signals the dawn of particle astronomy. These historic results have important implications to both astrophysics and particle physics.

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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May 8, 2008
STEPHEN WOLFRAM
Wolfram Research

"Hunting for Our Universe and Other Adventures on the Frontiers of Computation"

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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May 15 , 2008
NAT FISCH
Princeton University

"Manipulating Phase Space in Plasma with Waves"

Time: 4:15 pm
Place: Room 34-101 / MIT

Refreshments @ 3:45 pm in 4-349 (The Pappalardo Community Room)

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