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Jul 15 2018 | Quantum Information

When Shannon formulated his groundbreaking theory of information in 1948, he did not know what to call its central quantity, a measure of uncertainty. It was von Neumann who recognized Shannon’s formula from statistical physics and suggested the name entropy. This was but the first in a series of remarkable connections between physics and information theory. Later, tantalizing hints from the study of quantum fields and gravity, such as the Bekenstein-Hawking formula for the entropy of a black hole, inspired Wheeler’s famous 1990 exhortation to derive “it from bit.” That three-syllable manifesto asserted that to properly unify the geometry of general relativity with the indeterminacy of quantum mechanics, it would be necessary to inject fundamentally new ideas from information theory. Wheeler’s vision was sound, but it came twenty-five years early. Only now is it coming to fruition, with the twist that classical bits have given way to the qubits of quantum information theory.

This talk will provide a tour of some of the recent developments at the intersection of quantum information and fundamental physics that are the source of this renewed excitement.

May 8 2018 | Cosmology

None of us were consulted when the universe was created. And yet it is tempting to ask not only how the universe evolves, but also why, and could it be different? Our universe weighs more than 1050 tons. Could it be created “on the cheap”? Would it require a comprehensive project plan, and if so, where was this plan written before the universe was born? Can we study the evolution of the universe by cosmological observations, and then “play the movie back” to the origin of time, or will something unavoidably prevent us from doing it? Why do we live in a 4-dimensional space-time? Why is the universe comprehensible? We will try to approach these and other similar questions in the context of the theory of the inflationary multiverse.

Reverse Engineering The Universe presentation

Many mathematical concepts trace their origins to everyday experience, from astronomy to mechanics. Remarkably, ideas from quantum theory turn out to carry tremendous mathematical power too, even though we have little intuition dealing with elementary particles. The bizarre quantum world not only represents a more fundamental description of nature, it also inspires a new realm of mathematics that might be called “quantum mathematics” that turns out to be a powerful tool to solve deep outstanding mathematical problems. Similarly, new mathematical ideas address some of the most fundamental questions in physics, such as the Big Bang, black holes, and the ultimate fate of space, time, and matter.

Robbert Dijkgraaf, Director of the Institute for Advanced Study and Leon Levy Professor since 2012, is a mathematical physicist who has made significant contributions to string theory and the advancement of science education. He is President of the InterAcademy Partnership, a past President of the Royal Netherlands Academy of Arts and Sciences, and a distinguished public policy adviser and advocate for science and the arts. For his contributions to science, he has received the Spinoza Prize, the highest scientifc award in the Netherlands, and has been named a Knight of the Order of the Netherlands Lion. He is a member of the American Academy of Arts and Sciences and the American Philosophical Society. He is also a trained artist, writer, and popular lecturer.

Quantum Mathematics and the Fate of Space, Time, and Matter presentation

Professor Eva Silverstein of the Stanford Institute for Theoretical Physics (SITP) discusses the physics of horizons, black holes, and string theory.

Black hole and cosmological horizons -- from which nothing can escape according to classical gravity -- play a crucial role in physics. They are central to our understanding of the origin of structure in the universe, but also lead to fascinating and persistent theoretical puzzles. They have become accessible observationally to a remarkable degree, albeit indirectly. These lectures will start by introducing horizons and how they arise in classical gravity (Einstein's general relativity). In the early universe, the uncertainty principle of quantum mechanics in the presence of a horizon introduced by accelerated expansion (inflation) leads to a beautifully simple, and empirically tested, theory of the origin of structure. Its effects reach us in tiny fluctuations in the background radiation we observe from the time when atoms first formed.

This theory, and the observations, are sensitive to very high energy physics, including effects expected from a quantum theory of gravity such as string theory. Modeling the early universe within that framework helps us better understand the inflationary process and its observational signatures. Analyzing the `big data' from the early universe -- which continues to pour in -- is a major effort. This provides concrete tests of theoretical models of degrees of freedom and interactions happening almost 14 billion years ago.

Our understanding breaks down if we push further back in time, or into black hole horizons. This challenges us to determine more precisely how and why our existing theories fail. I will explain these basic puzzles, and conclude with some of the latest results on this question in string theory, which exhibits interesting new effects near black hole horizons.

Lecture presentation

Professor Eva Silverstein of the Stanford Institute for Theoretical Physics (SITP) discusses the physics of horizons, black holes, and string theory.

Black hole and cosmological horizons -- from which nothing can escape according to classical gravity -- play a crucial role in physics. They are central to our understanding of the origin of structure in the universe, but also lead to fascinating and persistent theoretical puzzles. They have become accessible observationally to a remarkable degree, albeit indirectly. These lectures will start by introducing horizons and how they arise in classical gravity (Einstein's general relativity). In the early universe, the uncertainty principle of quantum mechanics in the presence of a horizon introduced by accelerated expansion (inflation) leads to a beautifully simple, and empirically tested, theory of the origin of structure. Its effects reach us in tiny fluctuations in the background radiation we observe from the time when atoms first formed. 

This theory, and the observations, are sensitive to very high energy physics, including effects expected from a quantum theory of gravity such as string theory. Modeling the early universe within that framework helps us better understand the inflationary process and its observational signatures. Analyzing the `big data' from the early universe -- which continues to pour in -- is a major effort.  This provides concrete tests of theoretical models of degrees of freedom and interactions happening almost 14 billion years ago.  

Our understanding breaks down if we push further back in time, or into black hole horizons. This challenges us to determine more precisely how and why our existing theories fail. I will explain these basic puzzles, and conclude with some of the latest results on this question in string theory, which exhibits interesting new effects near black hole horizons.

Lecture presentation

Mar 3 2017 | Cosmology

John Carlstrom gives the plenary lecture at the New Horizons in Inflationary Cosmology Templeton Conference organized by the Stanford Institute for Theoretical Physics.

Our understanding of the origin, evolution and make-up of the Universe has undergone dramatic and surprising advances over the last decades. Much of the progress has been driven by measurements of the fossil light from the big bang, called the cosmic microwave background radiation, which provides us with a glimpse of the Universe as it was 14 billion years ago. This talk will discuss what we know about the Big Bang and how we learned it. We will also talk about the new questions we are asking about the origin of the Universe and the experiments being pursued to answer them, peering back to the beginning of time.

Feb 6 2017 | Condensed Matter

The Majorana fermion is a hypothetical fermionic particle which is its own anti-particle.  Intense research efforts focus on its experimental observation as a fundamental particlein high energy physics and as a quasi-particle in condensed matter systems.  Professor Zhang discusses the theoretical prediction and the experimental discovery of the chiral Majorana fermion in a topological state of quantum matter.

For more details, please see the following announcement:

Discovery of The Chiral Majorana Fermion

Jan 30 2017 | Condensed Matter

For the past 60 years, progress in information technology has been governed by Moore's law, which states that the number of transistors on a semiconductor chip doubles every 18 months. However,  this remarkable trend is drawing to a close, mostly because the electrons that carry current in chips move like cars driving through a crowded marketplace, swerving around obstacles and dissipating too much of their energy as heat. The recent discovery of a new state of matter "the  topological insulator" may  lead to a new paradigm of information processing, in which electrons moving in opposing directions are separated into well-ordered lanes, like automobiles on a highway. This talk will explain the basic principles behind this amazing discovery.

Nov 14 2016 | Quantum Information

Professor Patrick Hayden of the Stanford Institute for Theoretical Physics (SITP) introduces the science of quantum information.

Over the past sixty years, computers have shrunk, networks have spread and flickering bits of information have ever more thoroughly infiltrated all aspects of our lives. The boundary between the virtual world of information and the physical world we ultimately inhabit has been slowly fading to the point that it is becoming hard to tell where one ends and the other begins. But deep down, we know there is a difference. Information is an invented abstraction: engineered, processed and repackaged but not the basic stuff of reality. Or perhaps not. The fundamental laws of physics, in the form of quantum mechanics, force physicists to wrestle with the very meaning of information. If Schrodinger’s cat can be both alive and dead, then the familiar “bit” isn’t up to the task of describing her state.

With gathering speed, scientists have been developing the science of truly quantum mechanical information. Not only is it strange, it has proven to be useful. Quantum computers could solve problems no digital computer will ever be able tackle. Quantum cryptosystems could only be cracked by violating the laws of physics. In these lectures, we’ll explore the nature of quantum information and how to use it. We’ll end by applying those pragmatic ideas to the nature of spacetime itself, finding that the boundary between the virtual and physical worlds is far fuzzier than we could have imagined.

Nov 7 2016 | Quantum Information

Professor Patrick Hayden of the Stanford Institute for Theoretical Physics (SITP) introduces the science of quantum information.

Over the past sixty years, computers have shrunk, networks have spread and flickering bits of information have ever more thoroughly infiltrated all aspects of our lives. The boundary between the virtual world of information and the physical world we ultimately inhabit has been slowly fading to the point that it is becoming hard to tell where one ends and the other begins. But deep down, we know there is a difference. Information is an invented abstraction: engineered, processed and repackaged but not the basic stuff of reality. Or perhaps not. The fundamental laws of physics, in the form of quantum mechanics, force physicists to wrestle with the very meaning of information. If Schrodinger’s cat can be both alive and dead, then the familiar “bit” isn’t up to the task of describing her state.

With gathering speed, scientists have been developing the science of truly quantum mechanical information. Not only is it strange, it has proven to be useful. Quantum computers could solve problems no digital computer will ever be able tackle. Quantum cryptosystems could only be cracked by violating the laws of physics. In these lectures, we’ll explore the nature of quantum information and how to use it. We’ll end by applying those pragmatic ideas to the nature of spacetime itself, finding that the boundary between the virtual and physical worlds is far fuzzier than we could have imagined.

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