Martin A. Fisher School of Physics

January 31, 2023

Bulbul Chakraborty, Brandeis

Abstract: Diversity in the natural world emerges from the collective behavior of large numbers of interacting objects. Statistical physics provides the framework relating microscopic to macroscopic properties.  A fundamental assumption underlying this approach is that we have complete knowledge of the interactions between the microscopic entities.  But what if that, even though possible in principle, becomes impossible in practice ?  Can we still construct a framework for describing their collective behavior ?    Dense suspensions and granular materials are two often quoted examples where we face this challenge.  These are systems where because of the complicated surface properties of particles there is extreme sensitivity of the interactions  to particle positions.  In this talk, I will present a perspective based on notions of constraint satisfaction that provides a way forward.  I will focus on our recent work on the emergence of elasticity in the absence of any broken symmetry, and sketch out other problems that can be addressed using this perspective.

February 14, 2023

Katie Elliott, Brandeis

Abstract: My book manuscript, Chance Explanation, argues for a novel account of scientific explanation.  On my picture, the direct explanation of an event’s occurrence is its chance of occurring, while all other information about the present and past helps to explain an event’s occurrence only indirectly, by explaining the event’s chance of occurring.  Though some aspects of this novel view are counterintuitive, the first part of my talk shows that it can be motivated from within a popular broad conception of scientific explanation: Peter Railton’s mechanistic conception.

The second part of my talk shows how to extract an otherwise mysterious feature of chance’s predictive role from my account of chance’s explanatory role.  Chance is a kind of predictive bottleneck between the past and future; there is a great deal of evidence that one might acquire about whether an event will occur before it does, but an event’s chance of occurring is as good or better grounds for one’s expectations about whether the event will occur as is all of that other evidence.  I show that this unique aspect of chance’s predictive role follows from my account of chance’s explanatory role.

February 28, 2023

Elias Most, Princeton/Institute for Advanced Study

Announcing the dawn of a new era of multi-messenger astrophysics, the gravitational wave event GW170817 – involving the collision of two neutron stars – was detected in 2017. In addition to the gravitational wave signal, it was accompanied by electromagnetic counterparts providing new windows into the different physics probed by the system. Since then, several gravitational wave events involving neutron stars have been discovered, with many more expected over the next years.
To understand and interpret the physics of these events, it is necessary to model the intricate dynamics of such systems before, during and after the merger, including the amplification of strong magnetic fields and the formation of hot and dense nuclear matter. Due to its strong non-linear nature, a modeling of the post-merger phase will only be possible with cutting-edge numerical approaches that combine strong gravity, nuclear physics and plasma astrophysics.

In this talk, I will discuss recent advances in the multi-physics modeling of a neutron star coalescence. With the help of several examples, I will show how future gravitational wave detections of the post-merger phase might allow to systematically uncover the properties of hot dense matter.  Along those lines, I will also show how neutrino-driven viscosity can naturally arise during the collision.

Lastly, I will show how cutting-edge numerical simulations can help to uncover new mechanisms for the production fast radio precursors emitted shortly before the collision of two neutron stars.

 I will conclude by discussing how such a multi-physics approach will enable a next generation modeling of the engines of multi-messenger gravitational events.

Alison Sweeney, Yale  

Postponed untl Fall 2023

March 28, 2023

Eva Silverstein, Stanford

Click here for the Zoom link.

April 18, 2023

Hugo Ribeiro, U Mass Lowell

September 13, 2022

Presentions by Berko Prize Recipients:

Francesca Capocasa: Not blinded by the light: how ATLAS will keep seeing particles during the HL-LHC Run Kanaya Malakar: Origin of polarization correlation in tissues? Bibi Najma: Multiscale investigation of the emergent behaviors in biomimetic active matter

 

September 20, 2022

Joonas Nättilä, Columbia University, Flatiron CCA

Abstract: Neutron stars are one of the most extreme objects in the Universe: they are dense stars with a radius of ~12km and a mass of ~1.5 times the mass of the Sun. They rotate with periods from seconds to milliseconds and have exterior magnetic field strengths from 10^8 to 10^15 Gauss. Their rotating magnetospheres are dynamic and evolving; the magnetic topology undergoes explosive reconfigurations similar to that of our Sun. These magnetic eruptions can lead to intense turbulent flares that require coupling between the kinetic plasma physics of the gas in the magnetosphere and quantum electrodynamics of the energized particles and intense radiation field. In my talk, I will discuss the extreme plasma astrophysics of neutron stars. I will also discuss how our recent kinetic simulations of collisionless plasmas in strong magnetic fields start to shed light on many of the observed phenomena from various types of neutron stars like pulsars, magnetars, and neutron star binary mergers.

October 11, 2022

Jörn Dunkel, MIT

Abstract: Over the last two decades, major progress has been made in understanding the self-organization principles of active matter.  A wide variety of experimental model systems, from self-driven colloids to active elastic materials, has been established, and an extensive theoretical framework has been developed to explain many of the experimentally observed non-equilibrium pattern formation phenomena. Two key challenges for the coming years will be to translate this foundational knowledge into functional active materials and to identify sparse quantitative models that can inform and guide the design and production of such materials. Here, I will describe joint efforts with our experimental collaborators to realize self-growing bacterial materials, and to implement computational model inference schemes for active and living systems dynamics.

October 25, 2022

Rob Phillips, Cal Tech

Abstract: One of the most important scientific developments in modern history is the realization that the evolution of the Earth is deeply intertwined with the evolution of life. Perhaps the most famous example of this intimate relationship is the chemical transformation of Earths’ atmosphere following the emergence of photosynthesis resulting in the oxygenic atmosphere we depend upon today.   Over the past 10,000 years, humans have become a similarly influential force of nature, directly influencing the rise and fall of ecosystems, the temperature and volume of the oceans, the composition of terrestrial biomass, the planetary albedo and ice cover, and the chemistry of the atmosphere to name just a few of many such examples. The breadth of human impacts on the planet is so diverse that it penetrates nearly every scientific discipline.  In this talk, I will use the style of thinking attributed especially to EnricoFermi and known as Fermi problems to conduct a series of estimates to be compared to corresponding data on a myriad of human impacts ranging from the ratio of artificially fixed nitrogen to its natural counterparts to estimating the basis of the more than 80 billion chickens slaughtered each year to comparing the total human made mass to the entire planetary biomass.  The emphasis is on a broad and coherent picture of the factual backdrop on human impacts as revealed in the recently released anthroponumbers.org database based at the same time upon simple, but careful estimates of a series of dimensionless ratios that capture some of the key ways in which humans interact with the planet.

November 1, 2022

Joseph Cimpian, NYU

Abstract: Unconditionally, males—specifically, straight-cisgender males—are over-represented in the majors of physics, engineering, and computer science (PECS), while straight-cisgender females are over-represented in nursing. Of course, some portion of this over-representation is attributed to differences in factors like reported interest and career aspirations. But these gender and sexuality/gender-identity gaps are also susceptible to gender and sexuality stereotyping. Using longitudinal, nationally-representative data, we explore the extent to which individuals pursue stereotypical and counter-stereotypical majors, depending on their interests and demonstrated abilities, as well as by the socio-political environment they were raised in. For example, the male-female gap in PECS pursuit is both initially smaller and fully explained by student covariates among high-STEM achieving students; among low-STEM achieving students, males pursue PECS more often, and this cannot be explained by a wide range of factors. The sexuality/gender-identity gaps are, again, smaller and better explained among high-STEM achievers, but this extends through a wider range of majors; among average- and low-STEM achievers, major pursuit follows more stereotypical patterns. Additionally, LGBTQ students raised in more liberal environments more often pursue counter-stereotypical majors, reaching near parity with non-LGBTQ levels. Overall, this work suggests that student ability and the socio-political environment play unique roles in moderating the student factors predicting representation across college majors, suggesting new directions for research and interventions.

November 8, 2022

Ben Rogers, Brandeis

November 29, 2022

Marianna Linz, Harvard