Physics Homework 131 Answers Of An Alien

Department Colloquia

Martin Weiner Lecture Series
Department of Physics Colloquium
4 pm, Abelson 131
Refreshments at 3:30 pm outside Abelson 131

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Fall 2017 Colloquia

Tuesday, September 5, 2017
Margaret Johnson, Johns Hopkins University
"Protein self-assembly in the cell: Spatial and temporal control through membrane localization"
Host: Michael Hagan
Abstract: Cell division, endocytosis, and viral budding would not function without the localization and assembly of protein complexes on membranes. What is poorly appreciated, however, is that by localizing to membranes, proteins search in a reduced space that effectively drives up concentration. We have derived an accurate and practical analytical theory to quantify the significance of this dimensionality reduction in regulating protein assembly on membranes. We define a simple metric, an effective equilibrium constant, that allows for quantitative comparison of protein-protein interactions with and without membrane present. We find that many of the protein-protein interactions between pairs of proteins involved in clathrin-mediated endocytosis in human and yeast cells can experience enormous increases in effective protein-protein affinity (10-1000 fold) due to membrane localization. By developing new methods for reaction-diffusion simulation of protein structures, we further characterize the non-equilibrium dynamics of these assembly processes, with reaction parameters defined from experiment. Both theory and simulations highlight the power of membrane localization in triggering robust protein-protein binding, suggesting that it can play an important role in controlling the timing of endocytic protein coat formation.

Tuesday, September 12, 2017
Michael Hagan, Brandeis University
Host: Physics Department
"How to control the size of a self-assembling, self-filling shell"
Abstract: The self-assembly of a protein shell around a cargo is a common mechanism of encapsulation in biology. For example, many viruses assemble an icosahedral protein shell (capsid) around the the viral nucleic acid. Some viruses then acquire an additional exterior coating by budding through a cell membrane. Similarly, bacterial microcompartments (BMCs) are large icosahedral protein shells that assemble around collections of enzymes to act as organelles inside of bacteria.

In this talk I will use coarse-grained computational models and simple scaling calculations to illuminate the factors that control such a self-assembly process. I will particularly focus on how material properties (such as nucleic acid electric charge, membrane bending modulus, or enzyme cohesive forces) control assembly pathways and the size of the assembled shell.

Tuesday, September 19, 2017
Daniel Whiteson, University of California, Irvine & Jorge Cham, PHD Comics
"We Have No Idea"
Host: Bjoern Penning
Abstract: Jorge and Daniel will talk about the big unsolved mysteries of the Universe, including dark matter, dark energy, and the behavior of cats. A fun presentation that combines science, humor, and live drawing, inspired by their new book.

Tuesday, September 26, 2017
Dapeng Bi, Northeastern University
"Mechanics of Epithelial Tissues: Structure, Rigidity and Fluidity"
Host: Bulbul Chakraborty
Abstract: Cells must move through tissues in many important biological processes, including embryonic development, cancer metastasis, and wound healing. Often these tissues are dense and a cell's motion is strongly constrained by its neighbors, leading to glassy dynamics. Although there is a density-driven glass transition in particle-based models for active matter, these cannot explain liquid-to-solid transitions in confluent tissues, where there are no gaps between cells and the packing fraction remains fixed and equal to unity. I will demonstrate the existence of a new type of rigidity transition that occurs in confluent tissue monolayers at constant density.  The onset of rigidity is governed by a model parameter that encodes single-cell properties such as cell-cell adhesion and cortical tension. I will also introduce a new model that simultaneously captures polarized cell motility and multicellular interactions in a confluent tissue and identify a glassy transition line that originates at the critical point of the rigidity transition. This work suggests an experimentally accessible structural order parameter that specifies the entire transition surface separating fluid tissues and solid tissues. 

Tuesday, October 3, 2017

No colloquium (Brandeis Thursday)

Tuesday, October 10, 2017
Tulika Bose, Boston University
"Unlocking the mysteries of the Universe with the CMS experiment at the Large Hadron Collider"
Host: Bjoern Penning
Abstract: The discovery of the Higgs Boson at the Large Hadron Collider (LHC) in 2012 was a ground-breaking event in particle physics history. The LHC has restarted recently at an unprecedented center of mass energy of about 13 TeV and the data collected by the CMS experiment is expected to help fully understand the nature of electroweak symmetry breaking and potentially discover new physics. In this talk, I will review recent results from the CMS experiment with special focus on searches for physics beyond the Standard Model.

Tuesday, October 17, 2017
Elizabeth Blanton, Boston University
"Extragalactic Jets as Probes of Clusters of Galaxies"
Host: John Wardle
Abstract: I will present multi-wavelength (X-ray, optical, infrared, and radio) observations of clusters of galaxies, including in-depth study of nearby objects and a survey of distant systems. Cooling of the hot, gaseous intracluster medium in cluster centers can feed the supermassive black holes in the cores of the dominant cluster galaxies leading to active galactic nucleus (AGN) outbursts. This AGN feedback can reheat the gas, stopping cooling and large amounts of star formation. Most relaxed, cool core clusters host powerful AGN in their central galaxies and these AGN can significantly affect the distribution of e.g., temperature and abundance on cluster scales. AGN heating can come in the form of shocks, buoyantly rising bubbles that have been inflated by radio jets and lobes, and sound wave propogation. Sloshing of the cluster gas, related to minor, off-center interactions with galaxy sub-clusters or groups also affects the distribution of temperature and abundance on large scales. This sloshing gas can interact with the AGN's radio-emitting jets and lobes causing them to bend. This bending is also found in AGN jets and lobes embedded in clusters undergoing major, head-on cluster, cluster mergers. Since this bending is a signature of interaction within clusters, bent, double-lobed AGN observed in the radio can be used as beacons for clusters of galaxies at high redshifts. I will describe our large sample of high-redshift, bent-double radio sources that were observed in the infrared with the Spitzer Space Telescope and in the optical with the Discovery Channel Telescope and that have yielded approximately 200 new, distant clusters of galaxies. The clusters in our survey ("COBRA," Clusters Occupied by Bent Radio AGN) will serve as important laboratories for studying galaxy evolution.

Tuesday, October 24, 2017
Stephon Alexander, Brown University
"The Jazz of Physics: The Link Between Music and The Structure of the Universe"
Host: Bjoern Penning
Abstract: In this talk Alexander revisits the interconnection between music and the evolution of astrophysics and the laws of motion. He explores new ways music, in particular jazz music, mirrors modern physics, such as quantum mechanics, general relativity, and the physics of the early universe. Finally, he discusses ways that innovations in physics have been and can be inspired from "improvisational logic" exemplified in Jazz performance and practice.

Tuesday, October 31, 2017
Huajian Gao, Brown University
"Mechanics of Cell-Nanomaterials Interaction"
Host: Michael Hagan
Abstract: Nanomaterials, including various types of nanoparticles, nanowires, nanofibers, nanotubes, and atomically thin plates and sheets have emerged as candidates as building blocks for the next generation electronics, microchips, composites, barrier coatings, biosensors, drug delivery, and energy harvesting and conversion systems. There is now an urgent societal need to understand the biological interactions and environmental impact of nanomaterials which are being produced and released into the environment by nearly a million tons per year. This talk aims to discuss mechanics as an enabling tool in this emerging field of study. The discussions will touch on some of the recent experimental, modelling and simulation studies on the mechanisms of cell uptake of low-dimensional nanomaterials and their effects on subcellular vesicles and damage.

Tuesday, November 7, 2017
Narayanan Menon, UMass Amherst
Host: Michael Hagan
"Flexibility and Form: Emergence of Shape in Thin Sheets"
Abstract: Thin sheets assume a rich diversity of shapes in the natural world, ranging from folds on the earth’s crust, to the wavy shapes of leaves and flowers, down to more microscopic biomembranes and synthetic thin films. We have used thin polymer films floating on the surface of a fluid as a venue in which to study the emergence of complex shapes via successive elastic instabilities. Understanding these patterns required new notions of ‘thinness’ or bendability of a sheet, which define regimes in which textbook theories of post-buckling fails. I will end by describing opportunities for wrapping and encapsulation in this new regime of highly-bendable materials.

Tuesday, November 14, 2017
Julie Williams-Byrd, NASA Langley Research Center
"Decision Analysis Methods Used to Make Appropriate Investments in Human Exploration Capabilities and Technologies"
Host: Anique Olivier-Mason
Abstract: NASA is transforming human spaceflight. The Agency is shifting from an exploration-based program with human activities in low Earth orbit (LEO) and targeted robotic missions in deep space to a more sustainable and integrated pioneering approach. Through pioneering, NASA seeks to address national goals to develop the capacity for people to work, learn, operate, live, and thrive safely beyond Earth for extended periods of time. However, pioneering space involves daunting technical challenges of transportation, maintaining health, and enabling crew productivity for long durations in remote, hostile, and alien environments. Prudent investments in capability and technology developments, based on mission need, are critical for enabling a campaign of human exploration missions. There are a wide variety of capabilities and technologies that could enable these missions, so it is a major challenge for NASA’s Human Exploration and Operations Mission Directorate (HEOMD) to make knowledgeable portfolio decisions. It is critical for this pioneering initiative that these investment decisions are informed with a prioritization process that is robust and defensible. It is NASA’s role to invest in targeted technologies and capabilities that would enable exploration missions even though specific requirements have not been identified. To inform these investments decisions, NASA’s HEOMD has supported a variety of analysis activities that prioritize capabilities and technologies. These activities are often based on input from subject matter experts within the NASA community who understand the technical challenges of enabling human exploration missions.

This paper will review a variety of processes and methods that NASA has used to prioritize and rank capabilities and technologies applicable to human space exploration. The paper will show the similarities in the various processes and showcase instances where customer specified priorities force modifications to the process. Specifically, this paper will describe the processes that the NASA Langley Research Center (LaRC) Technology Assessment and Integration Team (TAIT) has used for several years and how those processes have been customized to meet customer needs while staying robust and defensible.

Tuesday, November 21, 2017

No colloquium (Thanksgiving week)

Monday, November 27 to Wednesday, November 29, 2017
Eisenbud Lecture Series in Mathematics and Physics
James Sethna, Cornell University
Host: Bulbul Chakraborty

Monday, November 27, 2017 (Lecture I), 4:00pm, Gerstenzang 121
"Sloppy Models, Differential Geometry, and How Science Works"
James P. Sethna, Katherine Quinn, Archishman Raju, Mark Transtrum, Ben Machta, Ricky Chachra, Ryan Gutenkunst, Joshua J. Waterfall, Fergal P. Casey, Kevin S. Brown, Christopher R. Myers
Abstract: Models of systems biology, climate change, ecosystems, and macroeconomics have parameters that are hard or impossible to measure directly. If we fit these unknown parameters, fiddling with them until they agree with past experiments, how much can we trust their predictions? We have found that predictions can be made despite huge uncertainties in the parameters – many parameter combinations are mostly unimportant to the collective behavior. We will use ideas and methods from differential geometry to explain what sloppiness is and why it happens so often. We show that physics theories are also sloppy – that sloppiness may be the underlying reason why the world is comprehensible.

Tuesday, November 28, 2017 (Lecture II), 4:00pm, Abelson 131
"Crackling Noise"
James P. Sethna
Abstract: A piece of paper or candy wrapper crackles when it is crumpled. A magnet crackles when you change its magnetization slowly. The earth crackles as the continents slowly drift apart, forming earthquakes. Crackling noise happens when a material, when put under a slowly increasing strain, slips through a series of short, sharp events with an enormous range of sizes. There are many thousands of tiny earthquakes each year, but only a few huge ones. The sizes and shapes of earthquakes show regular patterns that they share with magnets, plastically deformed metals, granular materials, and other systems. This suggests that there must be a shared scientific explanation. We shall hear about crackling noise and that it is a symptom of a surprising truth: the system has emergent scale invariance – it behaves the same on small, medium, and large lengths. 

Wednesday, November 29, 2017 (Lecture III), 10:00am, Abelson 333
"Normal Form for Renormalization Groups: The Framework for the Logs"
James P. Sethna, Archishman Raju, Colin Clement, Lorien Hayden, Jaron Kent-Dobias, Danilo Liarte, and Zeb Rocklin
Abstract: Ken Wilson’s renormalization group solved for the behavior of phase transitions by mapping statistical mechanics into a differential equation in the space of all Hamiltonians, as we examine them on different length scales. This mapping from complex physical systems to simple differential equations has allowed us to explain scale invariance that emerges in everything from crackling noise to the onset of chaos. The results of the renormalization group are commonly advertised as the existence of power law singularities near critical points. This classic prediction is often violated, with logarithms and exponentials that pop up in the most interesting cases. Mathematicians have developed normal form theory to describe the likely behaviors of differential equations. We use normal form theory to systematically group these seeming violations into universality families. We recover and explain the existing literature, predict the nonlinear generalization for universal homogeneous functions, and show that the procedure leads to a better handling of the singularity even for the classic 4-d Ising model.

Tuesday, December 5, 2017
Roxanne Guenette, Harvard University
Host: Bjoern Penning
"Neutrinos: from zeros to heroes?"
Abstract: The Standard Model, that describes extremely well the particles and their interactions, predicts that neutrinos are massless and only interacts via weak interaction. These properties made neutrinos some of the least interesting particles of the model... until the discovery that they oscillate. This groundbreaking result implies that neutrinos are massive particles and opens the door to physics beyond the Standard Model- the holy grail of particle physicists. In addition, it seems that neutrinos could hold the key to many great mysteries of physics, such as the imbalance in the Universe between matter and anti-matter, and these are now within the reach of the next generation of neutrino experiments. After reviewing the intriguing properties of neutrinos, I will present the open questions in neutrino physics and describe how current and future neutrino experiments, focusing on Liquid Argon experiments, can bring new answers. 

Spring 2018 Colloquia

Tuesday, January 16, 2018

No colloquium

Tuesday, January 23, 2018
Jesse L. Silverberg, Harvard University
"Soft, Structured, Living Materials"
Host: Seth Fraden
Abstract: The central narrative of contemporary biology is that DNA encodes all relevant information for an organism's function and form. While this genotype-to-phenotype framing is appealing for its reductionish simplicity, it has a substantial problem. Between nanometer-scale DNA and organismal-scale phenotype sits a gap of 5 to 9 orders of magnitude in length. This gap covers everything from active protein diffusion, and macromolecular self-assembly, to biopolymer networks and pattern forming mechanical instabilities. In other words, the story of how organisms get their function and form starts with genes, but rapidly transitions to the language of soft matter physics as we examine larger and longer length scales.
In this talk, I'll address technological challenges and solutions for studying multiscale biophysics in an experimental setting. Along the way, I will discuss how cm-scale cartilage tissue achieves its remarkable mechanical properties through biopolymer self-organization, and how advances in big data and cloud computing can be leveraged for visualizing this nm-scale structure. I will continue to develop the theme of multiscale biophysics in the context of cell-cell fusion, a remarkably common yet mysterious processes in which individual cells fuse together to increase their size ~1,000-fold while decreasing metabolic costs by ~75%. I will also briefly touch on current work studying embryonic morphogenesis where gradients in cell growth lead to the geometric nonlinearities driving epidermal pattern formation. The physics of phase transitions, instabilities, and networks will become reoccurring themes that appear in surprising and unexpected ways as we work to close the genotype-to-phenotype gap.

Thursday, January 25, 2018
Qin Xu, ETH Zurich
"Surface tension and surface elasticity of soft solids"
Host: Seth Fraden
Abstract: Surface tension, also known as surface stress, is a fundamental physical property of any interface. However, surface tensions of solids are normally overlooked as they are too weak to significantly deform bulk solids. Measurements of solid surface tension in traditional engineering materials, such as metals and oxides, have proven to be very challenging. Consequently, our understanding of solid capillarity relies heavily on untested theories. Here, we take the advantage of high compliance and large deformability of a soft polymeric gels to directly measure solid surface tension under different deformation conditions. Under biaxial stretch, we find the surface tension depends on the strain via a surface modulus, which remarkably, is many times larger than the zero-strain surface tension. Under uniaxial stretch, solid surface tension becomes anisotropic and orientation dependent. In these experiments, we decompose the surface modulus into surface shear and surface bulk components. Further, we try to understand the origin of surface elasticity of soft gels by studying compression induced surface instability. These results suggest that solid surface tension, as a strain-dependent tensor, can play a dominant role in solid mechanics at much larger length scales than previously anticipated.

Tuesday, January 30, 2018
Daniel Harlow, Massachusetts Institute of Technology
Host: Matthew Headrick
"Black Holes, Holography, and Quantum Error Correction"
Abstract: Constructing a theory of quantum gravity is one of the grand challenges of theoretical physics.  In recent years considerable progress has been made on this problem in the special (and unfortunately unphysical) context of a negative cosmological constant, using the powerful tools of the anti de Sitter/ conformal field theory (AdS/CFT) correspondence.  In this talk I will explain how this correspondence can be naturally reformulated using the theory of quantum error correction, which was originally invented to protect quantum computers from decoherence.  I will also explain how this reformulation clarifies several old puzzles in quantum gravity.  Although the subject material may sound difficult, the talk should be accessible to beginning graduate students.

Tuesday, February 6, 2018

No colloquium

Thursday, February 8, 2018
Rodrigo E. Guerra, NYU
Host: Seth Fraden
"Entropic Unjamming of Emulsions"
Abstract: Like many yield-stress fluids, the mechanical properties of emulsions are controlled by two seemingly irreconcilable energy scales. In a compressed emulsion, droplets are pressed together by an external osmotic pressure and held in place by the balance of repulsive contact forces: forming solids with elastic moduli proportional to the ratio of interfacial tension to droplet size, σ/R ​​. In dilute emulsions, by contrast, droplets are free to slide past each other and diffuse: composing viscoelastic fluids with osmotic moduli proportional to 3 kB T/{ 4π R3}​​. Naively, the transition between a dense fluid and an amorphous solid should mark the balance between these energy scales; however, since σ/R is typically 106 to 1010 times larger than 3 kB T/4π R3 ​​, it is not clear how such an enormous gap is bridged. Ignoring thermal fluctuations entirely or treating them as a perturbation reduces this question to a geometric packing problem that can be studied in simulation. Here we show, however, that thermal fluctuations play a fundamental role in this transition, and that the balance of elasticity and thermal agitation defines a critical osmotic pressure, Π*​​, below which the solid is too weak to resist the thermal excitations of its constituent droplets. Measurements of the elastic moduli of barely-compressed emulsions confirm values of Π*​​ predicted by a simple balance of thermal fluctuations and local yielding, and show that the fragility and softness of emulsions amplify effects of thermal fluctuations: requiring values of Π* ∼ 105 ⋅ 3 kB T/4π R3 ​​ to contain them.

Tuesday, February 13, 2018
Scott Russell Waitukaitis, AMOLF
Host: Seth Fraden
"The elastic Leidenfrost effect:  coupling vapor release and elastic deformations to power sustained bouncing"
Abstract: The Leidenfrost effect occurs when an object near a hot surface vaporizes rapidly enough to lift itself up and hover. Although well-understood for liquids and stiff sublimable solids, little is known about the interaction of vaporizable soft solids with hot surfaces.  In this talk, I will introduce a new phenomenon that occurs with vaporizable soft solids: the elastic Leidenfrost effect. By dropping vaporizable hydrogel spheres onto hot surfaces I will show that, rather than hovering, they energetically bounce several times their diameter for minutes at a time. With high-speed video during a single impact, one sees high-frequency microscopic gap dynamics at the sphere-substrate interface.  Solving for the dynamics of a simplified system with numerical simulations, I will reveal how these otherwise-hidden agitations constitute work cycles that harvest mechanical energy from the vapor and sustain the bouncing.  Quite literally, the hydrogel sphere behaves like a soft engine, where nearly all of the components are embedded into a single object made from a single material.  These findings suggest a novel strategy for injecting mechanical energy into soft materials, with potential relevance in fields ranging from soft robotics to active matter. 

Thursday, February 15, 2018
Guillaume Duclos, Brandeis
Host: Seth Fraden
"Active nematic liquid crystals in biological materials: from multicellular tissues to active bio-polymers"
Abstract:  Active nematics describes a phase of matter where active particles that consume energy to produce mechanical work assemble at high density in a state with orientational order but no positional order. In this talk, I will show how the active nematic framework allows us to better understand aspects of the collective behaviors that emerge in biological materials.
I will focus on two model systems:

  1. A tissue-based liquid crystal composed of elongated cells
  2. A bio-polymer liquid crystal composed of elongated viruses, microtubules and molecular motors

First, I will describe the emergence of nematic order in multicellular tissues composed of elongated cells and show how the interplay between confinement, topological defects and activity controls the self-organization and the emergence of collective flows in 2D active nematics. In a second part, I will present our recent efforts to describe the emergence of flows in biomimetic 3D active gels and 3D active liquid crystals. Although most out-of-equilibrium collective phenomena in living cells and their potential engineering applications take place in complex 3D environments, majority of the experimental and theoretical work exploring self-organization of active biological materials has been restricted to 2D systems. Here, I will explore how active fluids composed of biological polymers and molecular motors behave and self-organize in 3D. I will first describe the generic bend instability that emerges in a flow-aligned 3D active gel and show how the interplay between activity, nematic elasticity and confinement controls the wavelength of this activity driven instability. I will then present current work on the emergence of flows and topological defect loops in 3D with a system composed of a passive colloidal liquid crystal doped with active microtubules.

Tuesday, February 20, 2018

No Colloquium (Midterm Recess)

Tuesday, February 27, 2018
Philip Nelson, University of Pennsylvania
Host: Jané Kondev
"The physics, biology, and technology of resonance energy transfer"
Abstract: Resonance energy transfer has become an indispensable experimental tool for single-molecule and single-cell biophysics, and a conceptual tool to understand bioluminescence and photosynthesis. Its physical underpinnings, however, are subtle: It involves a discrete jump of excitation from one molecule to another, and so we regard it as a strongly quantum-mechanical process. And yet its first-order kinetics differ from what many of us were taught about two-state quantum systems; quantum superpositions of the states do not seem to arise; and so on. The key step involves acknowledging quantum decoherence.
Ref: P C Nelson, Biophys J in press (2018).

Tuesday, March 6, 2018

No colloquium (APS)

Tuesday, March 13, 2018
Rachel Mandelbaum, Carnegie Mellon University
Host: Marcelle Soares-Santos
"Cosmology with the Hyper Suprime-Cam (HSC) survey"
Abstract: Hyper Suprime-Cam (HSC) is an imaging camera mounted at the Prime Focus of the Subaru 8.2-m telescope operated by the National Astronomical Observatory of Japan on the summit of Maunakea in Hawaii. A consortium of astronomers from Japan, Taiwan and Princeton University is carrying out a three-layer, 300-night, multiband survey from 2014-2019 with this instrument. In this talk, I will focus on the HSC survey Wide Layer, which will cover 1400 square degrees in five broad bands (grizy), to a 5 sigma point-source depth of r~26. We have covered 240 square degrees of the Wide Layer in all five bands, and the median seeing in the i band is 0.60 arcseconds. This powerful combination of depth and image quality makes the HSC survey unique compared to other ongoing imaging surveys. In this talk I will describe the HSC survey dataset and the completed and ongoing science analyses with the survey Wide layer, including galaxy studies, strong and weak gravitational lensing, but with an emphasis on weak lensing. I will demonstrate the level of systematics control, the potential for competitive cosmology constraints, some early results, and describe some lessons learned that will be of use for other ongoing and future lensing surveys.

Tuesday, March 20, 2017
David Pritchard, Massachusetts Institute of Technology
Host: Matthew Headrick
"How 10 Years of Education Research Revealed My 40 Years of Bad Assumptions"
Abstract: Once upon a time, I thought our final exam measured what students should learn. But further investigations of exactly what students learned and what they learned it from, how much they remembered as seniors, the role of homework copying, the limitations of partial credit grading, and the disparity between what physics teachers want to teach and what their students want to learn have been disquieting. I shall discuss how we can help students learn what they should learn, and describe a new classroom pedagogy that helps students to become more expert. Then I’ll describe how education research, development, and online learning might be combined to spread better learning universally.

Tuesday, March 27, 2018
Anushya Chandran, Boston University
Host: Matthew Headrick
"Schrodinger's clowder: entanglement in many-body physics"
Abstract: Nearly 80 years after Schrodinger introduced his famous cat, quantum entanglement has fuelled a conceptual revolution in many-body quantum physics. What Einstein, Podolsky and Rosen once found anathema, we now understand as a quantitative tool for classifying the allowed quantum phases of matter, analyzing the performance of quantum simulations and diagnosing thermalization. In this talk, I will re-introduce you to entanglement from this many-body point of view. We will see how the entanglement entropy organizes quantum phases of matter in the absence of local order parameters and how the entanglement spectrum probes the bulk-boundary correspondence in such quantum phases. Finally, we will turn to the dynamics of entanglement and how it has offered a phenomenological understanding of the newly discovered many-body localized phase.

Tuesday, April 3, 2018

No colloquium: Passover and spring recess

Tuesday April 10, 2018
Washington Taylor, Massachusetts Institute of Technology
Host: Matthew Headrick

Tuesday, April 17, 2018
Suri Vaikuntanathan, University of Chicago
Host: Aparna Baskaran

Tuesday, April 24, 2018
Stefan Söldner-Rembold, University of Manchester


“Ours is a world of nuclear giants and ethical infants. We know more about war that we know about peace, more about killing that we know about living.” –Omar N. Bradley

Nuclear physics is one of the most daunting, emotionally charged phrases in all of science. You can hardly say the words without the image of a mushroom cloud popping into most people’s heads, followed by the devastations of radiation sickness and lingering radioactivity.

Image credit: National Archives image (208-N-43888), Charles Levy, of the Nagasaki bomb.

But — as a physicist — that’s not what I think of at all.

Think down to all the basic constituents of matter, down beneath your cells, your organelles, the molecules that make them up all the way down to the individual atoms that make up the elements of everything on Earth.

Image credit: CC 3.0, via

At the heart of every atom is an atomic nucleus made up of some combination of protons and neutrons, which in turn are made up of even more fundamental particles known as quarks and gluons. When I think of nuclear physics, I think of tremendous numbers of these little guys — Avogadro’s Number‘s worth of them — and how they combine together on the smallest scales.

Image credit: Lawrence Berkeley National Laboratory.

At the most fundamental level (that we know of), the quarks and gluons bind together, with three quarks making up each and every nucleon, where a nucleon is a general term for either a proton or a neutron.

Neutrons and protons aren’t just made up of three quarks apiece, though, and you might have guessed that if you looked up the masses of the quarks and the masses of protons and neutrons.

Image credit: (L), Fermilab / D0 (R).

We typically say that a proton is made up of two up and one down quark, and that a neutron is made up of one up and two downs. But an up quark has a mass of about 4-5 MeV (in natural units) and a down has a mass of 7-8 MeV; based on that, you might think a proton has a mass of around 17 MeV and a neutron at around 19 MeV.

Those guesses are reasonable, and yet they’re only about 2% of the actual proton and neutron masses, which are about 938 MeV and 940 MeV, respectively. Where does the rest of that mass come from?

Image credit: Alex Dzierba, Curtis Meyer and Eric Swanson.

From binding energy. Those numbers I gave you for quark masses are for theoretically free quarks, which aren’t bound at all to anything else. But — at least in the Universe’s current state — free quarks can’t exist, thanks to the rules of Quantum Chromodynamics, the laws that govern the strong force! Quarks can only stably exist in bound states, and the only bound states of quarks that are stable for longer than a microsecond are protons and neutrons.

So most of the energy that we observe as “mass” in a proton or neutron doesn’t come from the masses of the three quarks that define the nucleon themselves, but rather from the field of the strong force that keeps them bound together. Another way of looking at it is to consider the gluons and the sea of virtual particles (quarks and antiquarks of all types) that make up each nucleon as what really makes the mass of a proton or neutron as heavy as it is.

Image credit: Matt Strassler of

Which is to say, to have something just as simply as one proton or one neutron, there’s a lot more involved than just three quarks. In fact those three quarks that you hear of as making up a proton or neutron are more specifically known as valence quarks, and all the other quarks-and-antiquarks inside are known as sea quarks, which carry some 98% of the masses of these particles.

But with the sole exception of a single proton (which serves as a common hydrogen nucleus), these nucleons don’t exist in isolation in nature.

Image credit: CERN / European Organization for Nuclear Research,

They exist in states where they’re bound to one another. That’s what makes atoms interesting: the fact that they have different numbers of protons (which makes for different elements) and different numbers of neutrons (which makes for different isotopes). And unsurprisingly, each unique combination of protons and neutrons has a unique binding energy, and only a very select few combinations are stable.

Image credit: I. Cullen at

The simplest combination — one proton and one neutron — is known as a deuteron, and is about 2.2 MeV lighter than a free proton and neutron alone. Start adding more, like two protons and two neutrons (to make Helium-4), and you’re suddenly 28 MeV lighter than those four free particles. The most stable of all the elements is Iron-56, with 26 protons and 30 neutrons, and which has a mass that’s a full 492 MeV lighter than 26 free protons and 30 free neutrons.

Heavier elements and isotopes may have a larger total binding energy, but no element has a higher binding-energy-per-nucleon than this isotope of iron.

Image credit: Pearson Prentice Hall, modified by University of Wisconsin Stevens Point.

Low-mass particles are easy to fuse into higher-mass ones; they emit a lot of energy when they do so. So long as you can achieve sufficient temperatures and densities, this is something that happens spontaneously, and is both the great hope of commercial nuclear fusion and also how all the elements in the Universe heavier than lithium were produced: in the nuclear fusion furnaces of stars!

Image credit: Catherine Michelle Deibel’s Ph.D. Thesis.

On the other hand, (mostly heavier) particles that have too little binding-energy-per-nucleon can spontaneously undergo one of three radioactive decays to reach a more stable state:

  1. Gamma decay, where a nucleus emits a gamma-ray (high-energy photon) to form a slightly lower-mass nucleus with the same number of protons and neutrons;
  2. Beta decay, where a nucleus emits an electron (and an antineutrino) to form a nucleus with a slightly lower mass, with one more proton and one fewer neutron than its parent nucleus;
  3. and Alpha decay, where a nucleus emits a Helium-4 nucleus (two protons and two neutrons, known as an alpha particle), and results in a nucleus with a lower mass, two fewer neutrons and two fewer protons than its parent.

These processes may be a one-off, as in the daughter nuclei they give rise to may be stable, or they may be part of a radioactive decay chain, which can require many steps until something stable is reached.

Image credit: Wikimedia Commons user Eugene Alvin Villar.

What’s particularly interesting is that:

  • Each combination of protons and neutrons has a specific binding energy and therefore a specific rest mass,
  • but there are only three types of radioactive decay, each which gives rise to a photon (massless), electron (of a small, given mass) or an alpha particle (of a larger, given mass), and therefore
  • each radioactive element produces radioactive decays with specific, characteristic energies (and timescales) associated with them!

Each type of particle — alpha, beta and gamma — takes different types of material to stop them, and to shield sensitive things (like you and me) from them.

Image credit:

Alpha particles are mostly harmless; they can be stopped by a sheet of paper, and even if they actually reach your body, are stopped by the outer one-or-two layers of skin cells on your epidermal layers.

Beta particles can do some damage; they can penetrate your skin and, in large doses, can give you radiation sickness and kill you.

But gamma particles are the most deadly: it takes a full foot (30 cm) of lead to effectively shield you from gamma radiation, and most cases of radiation sickness related to, say, the Hiroshima bomb came from gamma radiation.

But that’s not all cases of radiation sickness.

Images credit: Alistair Fuller/AP (L), PA (R).

Alexander Litvinenko was famously poisoned by being forced to ingest a radioactive isotope of Polonium (Po-210), which is an alpha-emitting radioactive particle. Although alpha decay is harmless if it takes place outside of your body, inside of you, all of that radiation is absorbed internally, and death is inevitable within a matter of days. (This is true for ingesting sufficient quantities of any radioactive material with a short half-life!)

But there are some specific characteristics that allow us to determine just what it was that poisoned him.

Image credit: Nuclear Physics Laboratory, University of Cyprus.

When an alpha particle is emitted from a nucleus, it will have a specific amount of kinetic energy that’s determined by the decay parent (Po-210 in this case) and the large, daughter nucleus (Pb-206, lead, which is stable). So when you want to determine whether there’s a particular radioactive substance, you look for alpha particles with a certain energy, and that’s a smoking gun for the radioactive parent.

There’s a question burning up around the world right now: is this how Yasser Arafat died?

Image credit: Arabs48, via

Although at this point I don’t think anyone can say for sure, this is the type of detective work that will uncover the answer: nuclear physics! It’s an incredibly powerful thing; used irresponsibly, it can kill hundreds of thousands in an instant or poison a targeted individual slowly and painfully, but used responsibly, it can be a practically infinite source of power for mankind.

It’s to be respected and valued, and only feared in the wrong hands. No matter what, it’s a great opportunity to understand our natural world a little better, and how matter on the smallest scales can impact the largest things we know!

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