Aspen Center for Physics

    2020 Heinz R. Pagels
    FREE Physics Talks

    Thursdays via Zoom

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    5:30 to 6:30 PM Public Talks

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  • June 18, 2020
    Particle Physics: What We Know We Don't Know
    Speaker: André de Gouvêa, Northwestern University
    It is the job of particle physicists to identify the fundamental building blocks of matter, learn their properties, and describe how they talk to one another. In spite of fantastic progress over the past several decades, there is still much we don’t understand. This evening, hosted online by the Aspen Center for Physics, Professor de Gouvêa will discuss some of the big questions - and there are only a few of them - that remain unanswered. He will concentrate on two puzzles: Why do neutrinos have nonzero masses? What is most of the universe made of?
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  • June 25, 2020
    Macrocosm in the Microcosm:  Analogies Between Materials and Particle Physics
    Speaker: Peter Armitage, Johns Hopkins University
    One of the continuing, but remarkable themes in physics is that concepts and mathematical ideas are repeated in different contexts across vastly different scales of length and time.   For instance, there are deep connections between the underlying equations that describe elementary particles and those that describe the physics of materials like superconductors and magnets.  Examples abound.  For instance, the Higgs mechanism that generates mass was first identified as the phenomenon that prevents magnetic fields from penetrating superconductors.  The effects of electric and magnetic fields on a newly discovered class of materials called topological insulators is described by equations that may describe the dark matter that permeates the universe.  We also find phenomena in materials that are "like" those of free space, but differ in essential ways.  Echoing the multiverse of the string theorists, every material presents its own set of physical laws that may not have an analogy in the world of our experience.   In this regard, insight into materials teaches us something deep about the space of possibilities of the kinds of physical laws that can exist.
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  • July 2, 2020
    The Black Hole Information Paradox: A Resolution on the Horizon?
    Speaker: Netta Engelhardt, MIT
    Can information escape from a black hole? General Relativity, which describes the behavior of black holes, and quantum mechanics, which describes the behavior of information, do not agree on the answer. This disagreement is the essence of the famous 45-year-old Black Hole Information Paradox. Understanding the resolution of this problem is a central pillar in the quest for quantum gravity, a theory that describes the universe at the smallest scales by unifying General Relativity and quantum mechanics. In the past year, there has been an unprecedented amount of progress towards a resolution. I will describe the origin of the paradox and the current status in light of the new developments.
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  • July 9, 2020
  • Darkly Changed Dark Matter
    Speaker: Lisa Randall, Harvard University
    Dark matter by its very nature is elusive. Despite the abundant evidence for its existence, its nature remains a mystery. The challenge to theorists is to imagine the many ways in which it can hide in order to anticipate ways in which it might be discovered. In this talk, I discuss dark matter and how we might hope to find it – even if its interactions with our matter are limited to gravitational. I'll even discuss some more speculative possibilities for how dark matter could have impacted evolution on Earth.
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  • July 16, 2020
    The Evolutionary "Design" of Protein Machines
    Speaker: Rama Ranganathan, University of Chicago
  • Proteins are the nanoscale machines in cells that carry out nearly all of the functions necessary for life. Examples include antibodies that can bind to specific targets with great selectivity, enzymes that catalyze complex chemical reactions, and signaling molecules that process and transmit information about the external world. We call them “machines” because just like high-performance machines in our ordinary experience, they do their job through cycles of motions that seem finely tuned for their biological functions. But unlike man-made machines, proteins are evolved materials, built through a process that we do not yet understand and with designs for which we do not yet have good models. In this talk, I will present our understanding of the evolutionary “design” of proteins and will discuss both fundamental and practical insights that have emerged. Ultimately, the goals are to explain exactly what kind of machines proteins are, how they work, and why they are built the way they are through the process of evolution.
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  • July 23, 2020
    Giant Black Holes Devouring Stars: Extreme Cosmic Events Illuminating Black Hole Spacetimes
    Speaker: Jane Dai, University of Hong Kong
  • In the center of every big galaxy, there is an enormous black hole with billions of stars orbiting around it. Astrophysicists want to study how these black holes grow together with galaxies through the cosmic history and how they affect the surrounding spacetime. However, this is a hard task, since these black holes are usually dark and faraway.

    Recently, a lot of progress has been made in studying how such massive black holes can tear apart stars wandering too close. Such phenomena, called tidal disruption events, can produce very bright flares which temporarily illuminate the black holes. In this talk, we will show how exactly massive black holes destroy and eat stars, why general relativity is important in the whole process, and how the properties of massive black holes can be probed through such events. We will also discuss how the frontier of this research topic has been pushed forward by using modern telescopes and large-scale computer simulations.
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  • July 30, 2020
    Time Reversal Symmetry and Unconventional Superconductors
    Speaker: Aharon Kapitulnik, Stanford University
    Let’s imagine we film a movie of two billiard balls colliding on a perfectly smooth frictionless table. Now run the movie forward or backwards – can you tell which one is which? Probably not. Let’s repeat the experiment with squishy balls, run the movie forward and backwards, can you tell the difference? of course you can, since the squishy balls move much slower after the collision (some of that kinetic energy got lost in the collision), and running the movie backwards will make no sense. We will therefore say that the first experiment with the billiard balls is symmetric under the reversal of time – or – time reversal invariant, while the second experiment is not. In fact, our everyday lives are much more like the second experiment. Time has an arrow, which we notice as a consequence of loss of disposable energy just like the squishy balls.

    However, the microscopic world is mostly time reversal invariant. For example, in classical physics, Newton’s law of motion: “mass x acceleration = Force” remains the same if time direction is reversed. This observation carries to most quantum mechanical systems, particularly solids (unless we subject the solid to effects that break this symmetry such as magnetic field).

    Superconductivity is an emerging phenomenon in a solid, where below some critical temperature the electrical resistance of the material vanishes, and magnetic fields are expelled from the interior of the material. Conventional superconductivity was explained by the celebrated BCS theory (due to Bardeen, Cooper, and Schrieffer). The main ingredient of this theory is the pairing of opposite spin electrons to create a condensate of so-called Cooper-pairs. Such a condensate continues to respect time reversal symmetry.

    Unconventional superconductors are materials which exhibit superconductivity that cannot be simply explained by BCS theory. This will typically point to some novel type of pairing, and may exhibit new emergent effects, such as violation of time reversal symmetry. This in turn has shown to be important for future applications, particularly in quantum information systems.

    In this talk I will review the concept of time reversal symmetry, its relation to solid state physics, particularly magnetism, and then the novel discovery of this effect in unconventional superconductors. In particular I will describe an extremely sensitive apparatus that can be used to detect the tiny effects of time reversal symmetry breaking and possible implications to future applications.
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  • August 6, 2020
    In Hot Water: Our Changing Polar Oceans
    Speaker: Andrew Thompson, Caltech
    During recent decades the signatures of a warming climate have been particularly visible in Earth's polar regions.  In addition to warming atmospheric and oceanic temperatures, changes in sea-ice extent and concentration as well as the thickness of floating Antarctic ice shelves have also been observed.  Many of these changes have been linked to intricate ocean circulation patterns at the high latitudes and the ability of these currents to deliver heat to various components of the cryosphere:  sea ice, ice shelves and glaciers. Deciphering the physical processes influencing ocean-ice interactions and accurately modeling them relies on the collection of persistent measurements that have been historically challenging in these environments.  This presentation will provide an overview of key research areas aimed at improving predictions of the future evolution of Earth's poles, with a particular focus on novel approaches for observing and modeling the ocean.
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  • August 13, 2020
    The Hubble Conundrum: A Potential Hint of New Physics in the Universe's Oldest Light
    Speaker: Colin Hill, Columbia University
    The talk will focus on the discrepancy in measurements of the universe's current expansion rate (the Hubble constant), and potential new-physics scenarios that could resolve this discrepancy. I will focus mostly on the early universe and the cosmic microwave background, including a discussion of forthcoming new results from the Atacama Cosmology Telescope, of which I am a collaboration member.
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  • August 20, 2020
    Quantum Mechanics for Quantum Materials: The Quantum Revolution in Solid State Physics and Materials Science
    Speaker: Andrew Millis, Columbia University
    Quantum mechanics is very strange: entanglement, superposition, Einstein’s "spooky action at a distance” are so far outside the realm of our classical intuition that they are hard to get our minds around. While many of the important behaviors of materials (semiconductors, superconductors, magnets, optical amplifiers, to name a few) are fundamentally quantum mechanical in origin, until recently our understanding has been firmly based classical concepts: putting quantum phenomena into a classical straitjacket. In this talk, I will show how this is changing: how the community is coming to grips with intrinsically quantum concepts and how this new understanding is transforming our ability to understand and calculate the properties of the world around us—both in general and in the area of technologically useful materials.
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"I found the general atmosphere [at the Aspen Center for Physics] very stimulating. All practical matters were taken care of in a pragmatic and effective way, all time was available for discussions and self-study. The beautiful surroundings did not distract, but stimulated creative thinking. It is too bad that life cannot always be so simple and pleasant."