Applied Physics / Physics Colloquium

Polymer self-assembly plays a critical role in a range of soft-material applications and in the organization of chromosomal DNA in living cells. In many cases, the polymer chains are composed of incompatible monomers that are not regularly arranged along the chains. The resulting phase segregation exhibits considerable heterogeneity in the microstructures, and the size scale of these morphologies can be comparable to the statistical correlation that arises from the molecular rigidity of the polymer chains. To establish a predictive understanding of these effects, molecular models must retain sufficient detail to capture molecular elasticity and sequence heterogeneity. This talk highlights efforts to capture these effects using analytical theory and computational modeling. First, we demonstrate the impact of structural rigidity on the phase segregation of copolymer chain in the melt phase, resulting in non-universal phase phenomena due to the interplay of concentration fluctuations and structural correlation. We then demonstrate how these effects impact the phase behavior in statistical random copolymers and in heterogeneous copolymers based on chromosomal DNA properties. With these results, we demonstrate that the spatial segregation of DNA in living cells can be predicted using a heterogeneous copolymer model of microphase segregation.

Transport in strongly quantum systems is challenging to understand. I will describe a recently obtained bound on transport in terms of a characteristic quantum velocity (the Lieb-Robinson velocity) and the local thermalization time. This bound sheds some light on experiments in both condensed matter systems and ultracold atomic gases. At finite temperatures, a more powerful velocity is the so-called butterfly velocity, that is intimately related to quantum chaos. This velocity is still poorly understood; I will present some forthcoming results that constrain the temperature dependence of the butterfly velocity in terms of the underlying quantum scrambling of the system.

d = 4 N = 2 Field Theory and Physical Mathematics

I will explain the meaning of the two phrases in the title. Much of the talk will be a review of the renowned Seiberg-Witten formulation of the low-energy physics of certain four dimensional supersymmetric interacting quantum field theories. In the latter part of the talk I will briefly describe some of the significant progress that has been made in solving for the so-called BPS sector of the Hilbert space of these theories. Investigations into these physical questions have had a nontrivial impact on mathematics.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

Dark matter is as mysterious as it is ubiquitous. Cosmological evidence raises more questions than it answers about the origin and nature of the most abundant kind of matter in the Universe. Terrestrial experiments searching for answers have focused mainly on the possibility that the constituent of dark matter is a new particle near the Higgs boson mass scale - at the upper limit of the energy ranges ever explored in the laboratory. But recent years have seen a growing interest in the possibility that dark matter is made of particles in a far more pedestrian mass range, comparable to protons or electrons or somewhere in between. Such light dark matter particles could be hiding under our noses, kinematically easy to produce in the laboratory but difficult to detect because they are only produced rarely, through feeble interactions. I will discuss the theoretical underpinnings of sub-GeV dark matter, and the intriguing possibility that dark matter could be our first window into a "dark sector" with new particles and interactions. I will also discuss prospects for new small-scale experiments to explore these ideas, and the exciting prospect that the most strongly motivated parameter space is within reach of next-generation experiments.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

High energy density physics may be loosely defined as the study and application of matter and energy above one megabar in pressure – roughly 1 eV/atomic ion at solid density. This regime is characterized by strong ionization, the ubiquity of shocks, fast hydrodynamic instabilities, and the importance of radiation transport in the energy balance of the medium. The microphysics of this regime necessarily deals with the combinatorial complexity of multiply excited atomic ions interacting with radiation. Beyond normal terrestrial experience until recently, the high energy density regime is now the subject of concerted laser and pulsed power experimentation. Examples of applications include stellar astrophysics and inertial confinement fusion. In this colloquium, I will discuss recent theoretical and experimental developments in three significant areas that exemplify the challenges and impact of this physical regime: radiation transfer in local thermal equilibrium and more generally non-local equilibrium, and dynamical viscosity.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

New three-dimensional measurements of the positions and velocities of stars, in particular from the Gaia observatory, have provided unprecedented information on the dynamics of the Milky Way and nearby galaxies. Stellar streams and phase-space structures have been characterized, pointing towards an active recent accretion history by the Milky Way. In this talk, I will discuss how these observations inform hierarchical structure formation, the Milky Way, the Local Group of galaxies, and the nature of dark matter. I will discuss what we can expect from future Gaia data and future astrometric observations.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

Newton's Principia ignited the Scientific Revolution, but the work-sheets and sketches showing how he composed his masterpiece have been lost. Fortunately, he left behind enough clues to make it possible to give a plausible reconstruction of how he did it. Surprisingly, such a reconstruction has not been attempted before. In the winter of 1679, Robert Hooke initiated a correspondence with Newton outlining the physics of planetary motion. But Hooke was unable to formulate his concepts in mathematical form, and afterward, Newton accomplished this formulation, which allowed him to give a geometrical expression for the passage of time, thus laying the foundations for the Principia. On Dec.10, 1684, four months after a visit of Edmond Halley, Newton sent the first manuscripts for the Principia to the London Royal Society, which he had made "designedly abstruse to be understood only by able Mathematicians". This lack of clarity remains up to the present time. In his talk, I will show, however, that with simply a pencil and a ruler, and without any calculus, good approximations of orbits for central forces can be calculated graphically that also clarify the content of the Principia.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

Ranging across Botswana, Bolivia, Nepal, Denmark, The Navajo Nation, and small-town New Jersey, the intricacies of being an international science educator are explored. Does everyone think and communicate as "we" do? How can we maintain our sanity in an increasingly insane world? What are ways that we can best communicate scientific knowledge? These topics are explored in a web of poignant and often humorous anecdotes. The speaker, award-winning educator Phil Deutschle, holds degrees in Physics and Astronomy, is the author of, "The Two-Year Mountain: A Nepal Journey" and "Across African Sand: Journeys of a Witch-Doctor's Son-in-Law," and is the producer of the feature-length documentary, "Searching for Nepal."

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

In the past 10 years, high energy gamma-ray astrophysics has undergone a renaissance. Dramatically improved capabilities from both ground based and space based observatories have combined to unveil dozens of new classes of gamma-ray emitters among the thousands of new sources, and studied each one with unprecedented spatial and spectral capabilities. Continuous monitoring of the high-energy gamma-ray sky has uncovered numerous outbursts from active galaxies, gamma-ray bursts and the discovery of transient sources in our galaxy – some with surprising counterparts. In this talk I will review some of the science highlights from the past decade with an emphasis on the surprises and remaining open questions.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore
Location: Hewlett Teaching Center, Rm. 200

Retinal degenerative diseases lead to blindness due to loss of the "image capturing" photoreceptors, while neurons in the "image-processing" inner retinal layers are relatively well preserved. Information can be reintroduced into the visual system using electrical stimulation of the surviving inner retinal neurons. Some electronic retinal prosthetic systems have been already approved for clinical use, but they provide low resolution and involve very difficult implantation procedures.

We developed a photovoltaic subretinal prosthesis which converts light into pulsed electric current, stimulating the nearby inner retinal neurons. Visual information is projected onto the retina from video goggles using pulsed near-infrared (~880nm) light. This design avoids the use of bulky electronics and wiring, thereby greatly reducing the surgical complexity. Optical activation of the photovoltaic pixels allows scaling the implants to thousands of electrodes.
In preclinical studies, we found that prosthetic vision with subretinal implants preserves many features of natural vision, including flicker fusion at high frequencies (>20 Hz), adaptation to static images, center-surround organization and non-linear summation of subunits in receptive fields, providing high spatial resolution. Initial results of the clinical trial with our implants (PRIMA, Pixium Vision) having 100mm pixels, as well as preclinical measurements, confirm that spatial resolution of prosthetic vision can reach the sampling density limit.

For a broad acceptance of this technology by millions of patients who lost central vision due to age-related macular degeneration, visual acuity should exceed 20/100, which requires pixels smaller than 25mm. I will describe the fundamental limitations in electro-neural interfaces and 3-dimensional configurations which should enable such a high spatial resolution. Ease of implantation of these wireless modules, combined with high resolution opens the door to highly functional restoration of sight.

Aut. Qtr. Colloq. committee: R. Blandford (Chair), A. Kapitulnik, R. Laughlin, L. Senatore

Location: Hewlett Teaching Center, Rm. 200