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October 15, 2015

MIT Physics Colloquium

The Upside Of Noise

Photon Torpedoes, QED Pinwheels, and the first 70 years of fluctuational electrodynamics

Noise is the sworn enemy of experimental scientists and a perennial headache for engineers, who spend much of their lives trying to mitigate its effects. It is thus somewhat surprising that there exists a branch of physics---and, increasingly, technology---in which noise plays the role of friend. This is fluctuational electrodynamics, the study of random microscopic fluctuations that mediate macroscopic transfers of energy and momentum. Although the first theoretical studies of fluctuation-induced interactions---Casimir forces and London dispersion forces---date back over 70 years, it has only been in the past 20 years that experimental breakthroughs and theoretical advances have combined to usher in the present golden age of fluctuation physics, a field now wide open for exploration and discovery.

In this talk, I will first review the theoretical underpinnings of fluctuational electrodynamics and briefly trace its history in the 20th century. I will then discuss the rapid progress of the past decade in predictive methods---both analytical and numerical---that have transformed the science of predicting Casimir forces on complex-shaped bodies from a forbidding intellectual challenge into an almost routine modeling procedure. This will bring us to the bleeding edge of the field today: the physics of non-equilibrium fluctuations, with applications including near-field radiative heat transfer and thermal self-propulsion of nanoscale bodies. I will discuss state-of-the-art computational approaches for tackling these formidable problems, then present new predictions for spontaneous self-propulsion and self-rotation of warm asymmetric bodies---photon torpedoes and QED pinwheels---in cold environments. The talk will also cover new theoretical tools, inspired by electrical engineering, for problems in quantum field theory, including entanglement entropy and constrained path integrals.

Content File
  • Talk Slides, 10/15/2015
  • PDF
  • Audio recording, 10/15/2015
  • MP3

    January 23, 2013

    Determinants that Count

    The Kastelyn method for counting dimer configurations and graph matchings and the Gessel-Viennot method for counting vertex-disjoint path systems

    How many ways can you cover a chessboard with dominoes? How many ways can 4 salesmen visit 17 cities without overlapping? How many ways can a grid of microscopic magnets align or misalign with each other? Astonishingly, all of these questions can be answered by writing down a simple matrix of integers and computing its determinant. This talk, prepared for the undergraduate math lecture series during MIT's Independent Activities Period, introduces you to these powerful methods of counting, explains what they have to do with the physics of ferromagnetics and diatomic molecules, and shows you some elegant tricks for computing certain large determinants analytically to yield compact closed-form expressions.
    Content PDF
  • Talk Slides, 1/23/2013
  • PDF

    December 9, 2010

    PhD Thesis Defense

    Fluctuating Surface Currents:

    A New Algorithm for Efficient Prediction of Casimir Interactions among Arbitrary Materials in Arbitrary Geometries

    They said the day would never come. (They were almost right.) But I finally defended my PhD thesis and lived to tell the tale.
    Content PDF
  • Thesis Defense Slides, 12/09/2010
  • PDF

    January 17, 2008

    On The Number Of Primes Below A Given Magnitude:

    The Riemann Zeta Function and How To Use It

    Here's an informal talk I put together for our student-led group talk series on various topics of interest in the ``higher mathematics.'' Riemann discovered in 1867 how to use complex analysis to solve an age-old problem in number theory, namely, how to write down an analytical formula for the number of primes below a given magnitude. The ideas involved in this development are just so totally amazingly cool, and the techniques one learns along the way of such immense practical utility, as entirely to overwhelm whatever humiliation we physicists and engineers might feel at putting time and effort into what would otherwise seem a purely abstract esoteric mathematical pursuit.
    Content PDF Audio Recording
  • A first definition of the Riemann Zeta function
  • What does Zeta(s) have to do with the primes?
  • A second definition of the Riemann Zeta function
  • The number of primes below a given magnitude

    February 9, 2006

    Quantum Chemistry, DFT, Quantum Transport, and Nanoscale Device Modeling For Nonspecialists

    This was a three-part series of talks in which I attempted to explain my research to the other members of my research group. The idea was to extract the central physical ideas of nonrelativistic quantum mechanics, to introduce them -- with some motivating background -- to an audience of extensive mathematical maturity but no knowledge of post-1867 physics, and to present the most up-to-date mathematical formulations of the problems as they are attacked by computational physicists today.

    My assessment of the success of this program is pessimistic. I don't think I did very well by the audience, succeeding in conveying neither a sense of how enticingly simple these problems really are to formulate, nor an appreciation of how profound would be the impact on society (health care! energy sources!) of any progress on efficient solvers. Still, for my own selfish purposes I found it a useful exercise to think through what are the key, irreducible, necessary and sufficient physical ideas of quantum mechanics without which nothing works and from which all else flows (I claim to have reduced it all to two principles?!), and maybe other physicists who need to give this kind of talk might get some ideas, perhaps on what not to do, from the slides.

    Part Content PDF Audio Recording
    • Invitation: Why Hydrogen Sticks Together
    • Quantum Mechanics: Classical Physics Plus Two New Ideas
    • Quantum Mechanics Of Multi-electron Systems
    • Hartree Method
    • Hartree-Fock, Configuration Interaction, Quantum Monte Carlo
    • Homogeneous Electron Gas
    • Density Functional Theory
    • Practical DFT Calculations
    • Invitation: Quantized Conductance in QPCs
    • Landauer Conductance Theory
    • Computing the Transmission Coefficient
    • Putting It All Together: Modern Device Modeling

    Let me also give shout-outs to some of the astounding freeware tools that enable this and all other work I ever do: , , , latex-beamer, and speex.

    December 4, 2005

    Poster: Simulating Quantum Transport In Carbon Nanotube FETs

    Here's the poster version of my memo on modeling carbon nanotube FETS,, compiled for an IFC/MARCO workshop in December '05.

    April 20, 2005

    Simulating Quantum Transport in CNTFETs

    Here are the slides from the first talk I gave on nanoscale device modeling to my new research group.

    Homer Reid's Research Talks Page, by Homer Reid
    Last Modified: 11/16/16