The Physics of Everything

The Physics of Everything
Reported by
Don Monroe

Posted October 03, 2016

Presented By

The New York Academy of Sciences


During the second quarter of 2016, the Academy hosted a series of evening panel discussions boldly titled "The Physics of Everything." With grant support from The John Templeton Foundation, the six panels addressed how physics is addressing grand questions and the potential limitations on what we may be able to learn using physics to address those questions.

The first session posed the question "What Does the Future Hold for Physics: Is There a Limit to Human Knowledge?" Three physicists explored the frontiers of cosmology and quantum physics. There may always be limits to our understanding in areas such as string theory, dark matter, and multiverses. But the panelists agreed that it is best to proceed as if explanations based in physics can be found.

The second session asked "Where Do Physics and Philosophy Intersect?" The panelists argued that philosophy has much to offer physics, in contrast to the assertions of some prominent physicists. In such areas as the role of observations in quantum mechanics and the experimental validation of theory, the panelists illustrated the power of a critical, interdisciplinary approach to understanding not only the facts but the significance of physics ideas.

Physics has achieved its success in part by selecting problems that can be addressed by reducing them to simple parts following simple laws. Less amenable subjects were discussed in the third session, "Complexity: A Science of the Future?" Biology, ecology, and social phenomena such as cities are all not easily described with simple equations, but still show amazing regularities. The panelists, including two physicists, a philosopher, and a computer scientist, sketched their hopes for unifying principles that explain these regularities.

One complex phenomenon that has so far evaded physical analysis was the topic of the fourth session, "The Rise of Human Consciousness." Two of the panelists, a neuroscientist and a roboticist, did not even regard consciousness as a distinct phenomenon but rather as a byproduct of systems that model themselves. In contrast, a philosopher and a physicist felt that this behavioral description misses something essential about what consciousness "feels like." Both groups expressed hope for experimental tests of their views.

The fifth session addressed the question, "Are We Alone in the Universe?" Recent experimental observations of thousands of planets orbiting other stars bolster the expectation that ours is not the first civilization. The panelists explored how we might look for others and why we have not encountered any yet. One troubling possibility is that civilizations that achieve global scale inevitably encounter challenges such as climate change that they fail to overcome.

The sixth session was entitled "Did Einstein Kill Schrödinger's Cat? A Quantum State of Mind." Two physicists and a computer scientist discussed amazing developments at the intersection of quantum gravity (as manifested in black holes) and quantum information. Trying to reconcile these disparate observations has overturned the traditional view of black holes, with no clear resolution so far. At the same time, an abstract connection between gravitational models and quantum models in one fewer dimension is providing insight into both fields, including new methods for error correction in quantum computing as well as benchtop tests of black hole physics.

The wide ranging discussions of the six panels illustrate the power of physics to explain the universe as well as the vast frontiers of that universe that still remain to be explored.

Use the tabs above to find session summaries of and references for this series.

Grant Support from

  • John Templeton Foundation

The opinions expressed in this eBriefing are those of the panelists and authors and do not necessarily reflect the views of the John Templeton Foundation.

How to cite this eBriefing

The New York Academy of Sciences. The Physics of Everything. Academy eBriefings. 2016. Available at: www.nyas.org/PhysicsofEverything-eB

Panel 1: What Does the Future Hold for Physics — Is There a Limit to Human Knowledge?

Moderator: Jill North (Rutgers University)
  • 00:01
    1. Introduction
  • 05:58
    2. Recent examples of successes in physics; Limitations of knowledge
  • 34:49
    3. Criteria of knowledge
  • 44:38
    4. Going forward; Undecidable questions
  • 51:52
    5. The next big revelation
  • 55:47
    6. Audience Q and

Panel 2: Where do Physics and Philosophy Intersect?

Moderator: Kate Becker (Science Writer and Editor)
  • 00:01
    1. Introduction; The common origin of philosophy and physics
  • 08:51
    2. Regarding time
  • 18:00
    3. Testing the multiverse
  • 28:41
    4. What philosophy offers physics
  • 41:32
    5. The point of physics; The role of consciousness
  • 49:58
    6. The relationship between philosophers and physicists
  • 58:51
    7. Audience Q and

Panel 3: Complexity — A Science of the Future?

Moderator: George Musser (Journalist and Author)
  • 00:01
    1. Introduction; A rock as a complex system
  • 03:58
    2. The science of cities; Spatial complexity; Regarding time
  • 15:40
    3. Complicated and complex; Universality in physical laws
  • 24:21
    4. Downward causation; Regularity and simplicity
  • 35:44
    5. Scaling laws and alien life; The LUCA; Beyond scaling laws
  • 51:45
    6. The state of biological study
  • 58:35
    6. Audience Q and

Panel 4: The Rise of Human Consciousness

Moderator: George Musser (Journalist and Author)
  • 00:01
    1. Introduction; Opening remarks by panelists
  • 08:34
    2. Dualism and modern science; Conciousness as a state of matter
  • 15:00
    3. Building and understanding; The role of information
  • 25:06
    4. Cognition and feeling; The attention schema theory
  • 34:32
    5. Defining consciousness
  • 44:20
    6. Robotic body schema; Fear of A.I. and moral implications
  • 55:11
    7. Audience Q and

Panel 5: Are We Alone in the Universe?

Moderator: Ira Flatow (PRI's Science Friday®)
  • 00:01
    1. Introduction; Considering the Drake Equation
  • 16:07
    2. Considering planetary colonization and extraterrestrial intelligence
  • 29:08
    3. Taking SETI seriously; Contaminating other planets
  • 42:58
    4. Climate change and terraforming; Space law
  • 51:10
    5. A blank check; Audience Q and

Panel 6: Did Einstein Kill Schrödinger's Cat? A Quantum State of Mind

Moderator: George Musser (Journalist and Author)
  • 00:01
    1. Introduction; Quantum gravity 101
  • 14:10
    2. Quantum computation
  • 38:23
    3. Entanglement, scrambling, and the butterfly effect
  • 50:33
    4. Practical applications; An anecdote
  • 55:32
    5. Audience Q and



David Z. Albert, Quantum Mechanics and Experience (1994)

Scott Aronson, Quantum Computing since Democritus (2013)

Bernard Chazelle, The Discrepancy Method: Randomness and Complexity (2000)

Adam Frank, About Time: Cosmology and Culture at the Twilight of the Big Bang (2012)

Stephen M. Gardiner, A Perfect Moral Storm: The Ethical Tragedy of Climate Change (2011)

Marcelo Gleiser, A Tear at the Edge of Creation: A Radical New Vision for Life in an Imperfect Universe (2010)

Michael Graziano, Consciousness and the Social Brain (2013)

Jim Holt, Why Does the World Exist? An Existential Detective Story (2012)

Hod Lipson, Fabricated: The New World of 3D Printing (2013)

George Musser, Spooky Action at a Distance: The Phenomenon That Reimagines Space and Time—and What It Means for Black Holes, the Big Bang, and Theories of Everything (2016)

Louisa Preston, Goldilocks and the Water Bears: The Search for Life in the Universe (2016)

Michael Strevens, Bigger than Chaos: Understanding Complexity through Probability (2003)

Selected Journal Articles

Almheiri, A., Marolf, D., Polchinski, J. et al. 2013. Black holes: complementarity or firewalls? J High Energy Phys. 2013: 62.

Harlow, D. & Hayden, P. 2013. Quantum computation vs. firewalls. J High Energy Phys. 2013: 85.

Worth RJ, Sigurdsson S, and House CH 2013. Seeding Life on the Moons of the Outer Planets via Lithopanspermia. Astrobiology 13(12): 1155-1165.


Science Friday

13.7 Blog


Planet Hunters

Kickstarter for Tabby's star

Science Fiction

James S.A. Corey, Leviathan Wakes (2011) (The Expanse series)

Kim Stanley Robinson, The Mars Trilogy (2013)

Andy Weir, The Martian (2014)


Intra-galactic settlement simulations shown during Panel 5
(source: Adam Frank / Jonathan Carrol-Nellenback)

Three simulations of galactic settlement showing differing conditions for civilizations forming across an expanding galaxy. The different colors represent a civilization and its descendants colonizing star systems. In some of the simulations you can also see the "ships" moving from one system to the other:

Civilizations born and dying with each expanding across the galaxy in waves

One civilization (and its ships) expanding across the galaxy

Occasional exploding supernova wipe out regions of the galaxy, which then have to wait for the next civilization to evolve and start colonization


Lecture 1: What Does the Future Hold for Physics: Is There a Limit to Human Knowledge?

Vijay Balasubramanian, PhD

University of Pennsylvania

Vijay Balasubramanian is the Cathy and Marc Lasry Professor of Physics at the University of Pennsylvania. He pursues research in two different fields: string theory (including the physics of black holes and whether they destroy information) and theoretical neuroscience (including the computational principles underlying the architecture of the brain's neural circuits). He has also addressed problems in statistical inference and "Occam's Razor"—the trade-off between simple and accurate mathematical models. Born in Bombay and raised in India and Indonesia, Balasubramanian came to the United States for college. He earned degrees in physics and computer science at MIT, and received two patents in artificial intelligence. After completing a PhD in physics from Princeton, he became a Junior Fellow of the Harvard Society of Fellows. Vijay Balasubramanian spent 2012–2013 at the Ecole Normale Superieure in Paris and has been a visiting professor at the CUNY Graduate Center, Rockefeller University, and the Vrije Universiteit Brussel (Free University of Brussels) in Belgium.

Eva Silverstein, PhD

Stanford University

Eva Silverstein is a Professor of Physics at Stanford University. Silverstein's research includes predictive mechanisms for early-universe inflationary cosmology accounting for its sensitivity to quantum gravity, tested by current and near-term cosmic microwave background data. They have led to a more systematic understanding of the inflationary process and its range of observational signatures. She has also pursued the wider development of quantum field theory and string theory including its mechanisms for a cosmological constant, black hole horizon dynamics, and duality symmetries.

Neal Weiner, PhD

Center for Cosmology and Particle Physics, New York University

Neal Weiner received his undergraduate degree in Physics and Mathematics from Carleton College and a PhD in Physics from the University of California, Berkeley. After completing his postdoctoral training at the University of Washington, Dr. Weiner joined the faculty of the Department of Physics at NYU in 2004. He has broad interests in particle physics and cosmology.

Dr. Weiner's focus is generally on physics beyond the standard model. In this broad field, his work has included studies of dark matter, extra dimensions, supersymmetry, grand unification, flavor, neutrino mass, inflation and dark energy, as well as relationships between the different subjects. Recently, he has been actively engaged in the development of ideas related to "dark sectors" where dark matter has its own interactions beyond gravitational and the implications for the ongoing search for dark matter. Dr. Weiner is currently the Director of the Center for Cosmology and Particle Physics at NYU.

Jill North, PhD (moderator)

Professor of Philosophy, Rutgers University

Jill North is an Associate Professor in the Philosophy Department at Rutgers University–New Brunswick. Her research focuses on the philosophy and foundations of physics, especially the metaphysics of physics—what the world is ultimately like, according to our best physical theories. She is currently working on the questions of whether spacetime exists and whether it emerges from something else more fundamental (under a grant from the National Science Foundation), as well as whether different mathematical formulations of classical mechanics are genuinely theoretically equivalent. She earned her undergraduate degree in Physics and Philosophy at Yale in 1997 and her PhD in Philosophy at Rutgers in 2004. After finishing her PhD, she held a postdoctoral fellowship in the Philosophy Department at NYU, and then taught in the Philosophy Departments at Yale and Cornell before joining the faculty at Rutgers last fall.

Lecture 2: Where do Physics and Philosophy Intersect?

David Z. Albert, PhD

Director of the MA Program in The Philosophical Foundations of Physics at Columbia University

David Albert is the Fredrick E. Woodbridge Professor of Philosophy at Columbia University. He received his PhD in theoretical physics from the Rockefeller University in 1981, and has taught since then both in physics and philosophy departments in Tel Aviv University, the University of South Carolina, Harvard, Princeton, and Columbia. Most of his work has been focused on issues at the foundations of quantum mechanics, and on fundamental questions about the direction of time. He is the author of numerous scientific and philosophical articles and three books: Quantum Mechanics and Experience, Time and Chance, and After Physics (all of which are published by Harvard University Press).

Hans Halvorson, PhD

Professor of Philosophy, Princeton University

Hans Halvorson is professor of philosophy at Princeton University. He works at the intersection of mathematics, physics, and philosophy, with notable research achievements in quantum field theory, quantum information theory, category theory, and philosophy of science. Together with Rob Clifton and Jeffrey Bub, he demonstrated that quantum mechanics can be derived from information theory. He is a recipient of The Mellon New Directions Fellowship, and The Cushing Memorial Prize in the History and Philosophy of Physics.

Jim Holt

Writer and Essayist; author of Why Does the World Exist? An Existential Detective Story

Jim Holt is a longtime contributor to the New Yorker, where he has written on string theory, time, infinity, numbers, jokes, logic, and truth. He also writes regularly for the New York Times, the New York Review of Books, and the London Review of Books. His 2012 book Why Does the World Exist?: An Existential Detective Story was a National Book Critics Circle Award finalist for general nonfiction and was named one of the ten best books of the year by the New York Times Book Review. An international bestseller, Why Does the World Exist? has been published worldwide in 18 languages. His 2006 book Stop Me If You've Heard This: A History and Philosophy of Jokes was published in the U.S. and the United Kingdom, and in French, Italian, and German translation. Holt has taught mathematics at the University of Virginia, philosophy at Columbia University, and economics at the City University of New York.

Kate Becker, MS (moderator)

Science Writer and Editor

Kate Becker is a science writer and editor specializing in physics and astronomy. As the editor of The Nature of Reality, a blog about fundamental physics, she brought scientists, philosophers, and writers together to take on deep questions in physics. She spent eight years developing stories for the NOVA and NOVA scienceNOW science documentary series, and she has written The Visible Universe, an astronomy column for the Boulder Daily Camera, since 2007. She studied physics at Oberlin College and astronomy at Cornell University, and she's had the good fortune to observe with the Arecibo Observatory in Puerto Rico and the Very Large Array in New Mexico.

Lecture 3: Complexity: A Science of the Future?

Bernard Chazelle, PhD

Princeton University

Bernard Chazelle is Eugene Higgins Professor of Computer Science at Princeton University, where he has been on the faculty since 1986. His current research focuses on the "algorithmic nature" of living systems.

A professor at the Collège de France in Paris in recent years as well as a member of the Institute for Advanced Study in Princeton, he received his PhD in computer science from Yale University in 1980. The author of the book, The Discrepancy Method, he is a fellow of the American Academy of Arts and Sciences, the European Academy of Sciences, the Association for Computing Machinery, and the recipients of three Best-Paper awards from SIAM.

Marcelo Gleiser, PhD

Author of A Tear at the Edge of Creation; Professor of Physics and Astronomy, Dartmouth College

Marcelo Gleiser is a theoretical physicist at Dartmouth College specializing in particle cosmology—mixing the physics of the very smallest constituents of the universe with the physics of the universe as a whole. To make sense of the world and our place in the grand scheme of things, he studies the emergence of complex structures in nature, focusing on very fundamental questions related to what he calls the "three origins": cosmos, life, and mind.

Michael Strevens, PhD

Author of Bigger than Chaos; Professor of Philosophy at New York University

Michael Strevens was born and raised in New Zealand. He moved to the US in 1991 to undertake a PhD at Rutgers University; currently, he teaches philosophy of science at New York University. His academic work covers topics such as understanding, complexity, causation, and the social structure of science, as well as the philosophical applications of cognitive science.

Geoffrey West, PhD

Distinguished Professor and Past President, Santa Fe Institute

Geoffrey West is Distinguished Professor and former President of the Santa Fe Institute, and Associate Fellow of Oxford University's Martin School. His BA is from Cambridge and PhD from Stanford where he later joined the faculty. West is a theoretical physicist whose primary interests have been in fundamental questions ranging from elementary particles and their interactions to universal scaling laws in biology and developing a science of cities, companies and global sustainability. His work is motivated by the search for "simplicity underlying complexity." His research includes metabolism, growth, aging & death, sleep, cancer and ecosystems, the dynamics of cities and companies, rates of growth and innovation, and the accelerating pace of life. He has been featured widely across the media including The New York Times, The Economist, Financial Times, Wired, Scientific American, Nova, National Geographic and the BBC. His work was selected as a breakthrough idea by Harvard Business Review (2007) and for Time's 2006 list of "100 Most Influential People in the World."

George Musser, PhD (moderator)

Journalist and Author

George Musser is a contributing editor at Scientific American magazine, a Knight Science Journalism Fellow at MIT for 2014–2015, and the author of Spooky Action at a Distance (2015) and The Complete Idiot's Guide to String Theory (2008). Although he focuses on space science and fundamental physics, his writings range widely over the sciences. He has won numerous awards for his work, including the 2011 Science Writing Award from the American Institute of Physics and 2010 Jonathan Eberhart Planetary Sciences Journalism Award from the American Astronomical Society. As a Scientific American senior editor for 14 years, he was co-awarded the National Magazine Award in 2003 and 2011.

Lecture 4: The Rise of Human Consciousness

David Chalmers, PhD

Co-Director, Center for Mind, Brain and Consciousness, New York University

David Chalmers is University Professor of Philosophy and co-director of the Center for Mind, Brain, and Consciousness at New York University, and also holds a part-time position at the Australian National University. He is well-known for his work on consciousness, especially for his formulation of the "hard problem" of consciousness.

His 1996 book The Conscious Mind: In Search of a Fundamental Theory was successful with both popular and academic audiences. Chalmers co-founded the Association for the Scientific Study of Consciousness and has organized some of the most important conferences in the field. He also works on many other issues in philosophy and cognitive science, and has articles on the possibility of a "singularity" in artificial intelligence and on philosophical issues arising from the movie The Matrix.

Michael Graziano, PhD

Associate Professor, Department of Psychology, Princeton University

Michael Graziano is a professor of psychology and neuroscience at Princeton University. He is an author of novels, children's books, and books on the brain. He is known for his work on how the brain represents the space near the body and controls complex movement within that space. More recently he has worked on the brain basis of conscious experience, and his most recent book is Consciousness and the Social Brain.

Hod Lipson, PhD

Professor of Mechanical Engineering, Columbia University; author of Fabricated: The New World of 3D Printing

Max Tegmark, PhD

Professor, Department of Physics, Massachusetts Institute of Technology

Max Tegmark is a Professor of Physics at MIT, co-founder of the Future of Life Institute, and Scientific Director of the Foundational Questions Institute. His research has ranged from cosmology to the physics of cognitive systems, and is currently focused at the interface between physics, AI and neuroscience. He is the author of over 200 publications and the book Our Mathematical Universe: My Quest for the Ultimate Nature of Reality. His work with the Sloan Digital Sky Survey on galaxy clustering shared the first prize in Science magazine's "Breakthrough of the Year: 2003."

George Musser, PhD (moderator)

Journalist and Author

George Musser is a contributing editor at Scientific American magazine, a Knight Science Journalism Fellow at MIT for 2014–2015, and the author of Spooky Action at a Distance (2015) and The Complete Idiot's Guide to String Theory (2008). Although he focuses on space science and fundamental physics, his writings range widely over the sciences. He has won numerous awards for his work, including the 2011 Science Writing Award from the American Institute of Physics and 2010 Jonathan Eberhart Planetary Sciences Journalism Award from the American Astronomical Society. As a Scientific American senior editor for 14 years, he was co-awarded the National Magazine Award in 2003 and 2011.

Lecture 5: Are We Alone in the Universe?

Adam Frank, PhD

Professor of Physics and Astronomy, University of Rochester

Astrophysicist Adam Frank is an expert on astrophysical fluid dynamics, his computational research group at the University of Rochester develops advanced supercomputer tools for studying how stars form and how they die. A self-described "evangelist of science" he is also committed to showing others the beauty and power of science by exploring the proper context of science in culture. He is the co-founder of NPR's 13.7 Cosmos & Culture blog and is a regular commentator on All Things Considered. His work appears regularly in The New York Times and he is the author of three books. His new book, titled Our Fate in the Stars: What Science Tells Us About Life in the Cosmos and What It Means for the Human Future, is due out in 2017.

Louisa Preston, PhD

Astrobiologist and author, London

Dr. Louisa Preston is an astrobiologist and planetary geologist. She works in environments across the Earth, where life is able to survive our planet's most extreme conditions, using them as blueprints for possible extra-terrestrial life forms and habitats. Having worked on projects for NASA and the Canadian, European and UK Space Agencies, the only thing Louisa enjoys more than devising ways to find life on Mars is writing about it. She has published numerous articles and academic papers and her first book Goldilocks and the Water Bears is out through Bloomsbury in September. She is also an avid believer in the power of science communication, having regularly appeared on radio and television programmes, such as the BBC's The Sky at Night, and spoken about the search for life on Mars at the TED Conference in 2013, as a TED Fellow.

Stephen M. Gardiner, PhD

Professor of Philosophy, University of Washington

Stephen M. Gardiner is Professor of Philosophy and Ben Rabinowitz Endowed Professor of Human Dimensions of the Environment at the University of Washington, Seattle. His main areas of interest are ethical theory, political philosophy and environmental ethics. His research focuses on global environmental problems (especially climate change), future generations, and virtue ethics.

Steve is the author of A Perfect Moral Storm: the Ethical Tragedy of Climate Change (Oxford, 2011), the coordinating co-editor of Climate Ethics: Essential Readings (Oxford, 2010), and the editor of Virtue Ethics: Old and New (Cornell, 2005). His articles have appeared in journals such as Ethics, the Journal of Political Philosophy, Oxford Studies in Ancient Philosophy, and Philosophy and Public Affairs.

Steve has published on a diverse range of topics including intergenerational justice, the ethics of geoengineering, the precautionary principle, climate justice, Aristotle's account of the reciprocity of the virtues, Seneca's approach to virtuous moral rules, and Socrates' political philosophy. His most recent books are Debating Climate Ethics (Oxford, 2016), a "for and against" book on climate justice, with David Weisbach, and the Oxford Handbook on Environmental Ethics (Oxford, 2016), co-edited with Allen Thompson.

Jason Thomas Wright, PhD

Associate Professor of Astronomy and Astrophysics, Pennsylvania State University; Principal Investigator at NASA's Nexus for Exoplanet Systems Science (NExSS)

Jason Wright is an associate professor of astronomy & astrophysics at the Center for Exoplanets and Habitable Worlds at the Pennsylvania State University. He is an observational astronomer, studying stars and the planets that orbit them, with special emphasis on stars like the Sun and giant planets. More recently, he has also been involved in the search for extraterrestrial intelligence (SETI), with an emphasis on the observable consequences of highly advanced civilizations on their stellar and galactic environments. Professor Wright spent his early childhood living outside of Seattle, and teenage years near Boston. He earned his undergraduate degree at Boston University and his PhD at the University of California, Berkeley. He lives near State College, PA with his wife (who is also an astronomer at Penn State) and two young children.

Ira Flatow (moderator)

Host of PRI's Science Friday®

Lecture 6: Did Einstein Kill Schrödinger's Cat? A Quantum State of Mind

Scott Aaronson, PhD

Associate Professor of Electrical Engineering and Computer Science, Massachusetts Institute of Technology

Scott Aaronson is an Associate Professor of Electrical Engineering and Computer Science at MIT. Beginning this fall, he'll be David J. Bruton Centennial Professor of Computer Science at the University of Texas at Austin. He studied at Cornell and UC Berkeley, and did postdocs at the Institute for Advanced Study as well as the University of Waterloo. His research focuses on the capabilities and limits of quantum computers, and more generally on computational complexity and its relationship to physics. His first book, Quantum Computing Since Democritus, was published in 2013 by Cambridge University Press.

Aaronson has written about quantum computing for Scientific American and The New York Times, and writes a popular blog. He's received the National Science Foundation's Alan T. Waterman Award, the United States PECASE Award, and MIT's Junior Bose Award for Excellence in Teaching.

Daniel Harlow, PhD

Postdoctoral Fellow, Harvard University Center for the Fundamental Laws of Nature

Daniel Harlow is a theoretical physicist specializing in black holes, cosmology, and quantum gravity. He is one of the leaders of a new field that applies ideas from the theory of quantum computation to black hole physics, and is perhaps best known for his work for his work reformulating a key relationship of quantum gravity (the AdS/CFT correspondence) as a quantum error correcting code, demonstrating a novel link between quantum gravity and information theory.

Dr. Harlow has a PhD in physics from Stanford University, and prior to coming to Harvard was a postdoctoral fellow in the Princeton Center for Theoretical Science. Starting in 2017 he will be an assistant professor in the Center for Theoretical Physics at the Massachusetts Institute of Technology.

Brian Swingle, PhD

Postdoctoral Research Fellow, Stanford Institute for Theoretical Physics

Brian received his BS in Physics from Georgia Tech in 2005 and his PhD in Theoretical Physics from MIT in 2011. He was a Simons Fellow at Harvard from 2011 to 2014 and then joined the Stanford Institute for Theoretical Physics in the fall of 2014. In the summer of 2015 he became an It From Qubit Fellow as part of the new "It From Qubit" Simons Collaboration. His interests span a variety of topics at the intersection of many-body physics, quantum information, and quantum gravity. He introduced the use of tensor networks in quantum gravity and is one of the originators of the idea that quantum entanglement holds spacetime together.

George Musser, PhD (moderator)

Journalist and Author

George Musser is a contributing editor at Scientific American magazine, a Knight Science Journalism Fellow at MIT for 2014–2015, and the author of Spooky Action at a Distance (2015) and The Complete Idiot's Guide to String Theory (2008). Although he focuses on space science and fundamental physics, his writings range widely over the sciences. He has won numerous awards for his work, including the 2011 Science Writing Award from the American Institute of Physics and 2010 Jonathan Eberhart Planetary Sciences Journalism Award from the American Astronomical Society. As a Scientific American senior editor for 14 years, he was co-awarded the National Magazine Award in 2003 and 2011.

Don Monroe

Don Monroe is a science writer based in Boston, Massachusetts. After getting a PhD in physics from MIT, he spent more than fifteen years doing research in physics and electronics technology at Bell Labs. He writes on physics, technology, and biology.


Grant Support from

  • John Templeton Foundation

The opinions expressed in this eBriefing are those of the panelists and authors and do not necessarily reflect the views of the John Templeton Foundation.


Jill North, Moderator

Rutgers University

Vijay Balasubramanian

University of Pennsylvania

Eva Silverstein

Stanford University

Neal Weiner

New York University


Physics has successfully explained much of the unobservable universe, but some aspects of the predictions may be inherently untestable.

Although physicists seek organizing principles, these will never explain the specific details of the world we find ourselves in.

The human mind may not be capable of comprehending a complete theory.

Looking for answers outside of the scientific process could impede further scientific discovery.

Finite horizons

The first session of The Physics of Everything, held on April 5, 2016, was titled "What Does the Future Hold for Physics: Is There a Limit to Human Knowledge?" Three physicists discussed the boundaries of our current view of the most-basic features of the universe and whether there are clear horizons beyond which physics will never see.

Over the past century, physics has pushed our understanding to the distant edges of the universe and to its very first moments. "It's just amazing how much has been learned," said Eva Silverstein. She noted that the extraordinary uniformity of the cosmic microwave background radiation leftover from the early universe is now attributed to an early "inflationary" period driven by a new quantum variable. Unfortunately, she said, "that variable can't help but have quantum fluctuations. It's those fluctuations that we think seeded the structure that we see," such as galaxies, as well as the tiny deviations from uniformity of the microwaves. "But at the same time, those fluctuations mean that that all we see is one realization," essentially a cosmic roll of the dice that determined the specific structure we live in.

Using the cosmic background and other data, said Neil Weiner, "we can determine the constituents of the universe very precisely: 4% is atoms, about a quarter is dark matter. and the balance is dark energy." Dark energy's uniform effects leave few clues as to its nature, but dark matter is more promising because it reveals its clumpy distribution through its gravitation. Still, "how can you look for it if you don't know what it is?" Weiner wondered. "I don't know that we'll ever know."

The traditional view of black holes was that they left essentially no clues to what had fallen in. These days, however, said Vijay Balasubramanian, "the dominant view is that there is a mistake in the reasoning concluding that information is lost." However, this information is not in a form that would be useful to an observer, he said. "It's an in-principle question." [For further discussion on this topic, see Lecture 6.]

Human limitations

For some modern physics theories, such as string theory, it is hard to envision experimental tests, raising the question of whether experimental validation should be required. It is hard to answer this question in the abstract, Weiner said. He cited the recent evidence for the Higgs boson, which had been predicted a half century earlier. If the experiment had not been done, "would you say you know there's a Higgs boson?" he asked. "I'd probably say no."

A similar problem arises for the "multiverse": multiple non-communicating universes that are predicted by many inflation models. "An optimistic scenario would be that we test our theory of physics that we can observe locally," Silverstein said, which would then inspire confidence if "that theory has the further consequence of a multiverse. We're not there yet." [For a philosophical perspective, see Lecture 2.]

"We're not going to measure everything," Balasubramanian noted. "The scientific method is making some small number of measurements, making a relatively simple model that we can contain within our heads, and then betting that works over a much larger range." The question is "whether that kind of extrapolation has limits," he said.

"We get most excited when it's about principles," Silverstein said. Balasubramanian agreed, citing the Jorge Luis Borges story "Funes the Memorious," about a boy who knows every fact but cannot truly think. In saying "to know," he said, "we don't actually mean knowing all these things separately. We mean a sort of categorical knowledge of the principles," he said.

"This is discounting the wonderful things that you can see in phenomenology, which you would not have seen as a principle," Weiner objected. "There's the laws that maybe we hope that we can get to from a finite set of experiments," he said, "and then there are some questions about what is actually out there. There are different types of limits for these different types of questions."

Our brain architecture could also limit our comprehension. "A cat can't understand calculus," Balasubramanian said. "There's no a priori reason why the human brain is capable of producing the representations of knowledge necessary to understand the universe." The fate of information in black holes, said Silverstein, is an example "where we are currently limited by our lack of intelligence."

Known unknowables?

In developing theories, "the challenge is that you can make many more-complex theories that will fit the limited data," said Balasubramanian. "So we use Occam's razor [which favors the simplest workable theory] as a rule of thumb to guide us through this maze. I'm constantly surprised it works to the extent that it does."

This approach can fail, though cautioned Weiner. "If you stick to this idea of simplicity then sometimes you can miss" an accurate picture. As an example, he imagined that we lived in the dark matter sector and tried to come up with a model for ordinary matter. "You would never come up with the description of the universe that actually is. You would insist on something which is very, very simple," he said. "We're going to describe it as simply as we can, but that doesn't mean that's actually right."

In spite of these challenges, the three theorists saw no benefit to invoking a deity or something else beyond physics. "I don't know really what 'going outside physics' means," Silverstein said. Religion seems to fill a deep psychological need for some people, Balasubramanian said, but "I don't think it's useful, at least to me, as a discovery method in science."

"Actually it could be dangerous," Weiner said. "Even if it ends up being wrong that we can explain things by the scientific method or by physical principles, you should at least pursue it as if you can, because if you don't then you run the risk of missing out on really tremendous stuff."


Kate Becker, Moderator

Science Writer and Editor

David Z. Albert

Columbia University

Jim Holt


Hans Halvorson

Princeton University


The critical analysis provided by philosophy can help understand what physical theories mean.

A realistic description of the external world may still be possible.

Science should not limit itself to predicting experimental outcomes.

People from a variety of backgrounds working in different disciplines advance the philosophy of science.

A long relationship

The second session of The Physics of Everything, held on April 25, addressed the relationship between physics and philosophy. These subjects have a long and constructive history together, but some prominent physicists have expressed disdain for philosophy's usefulness. The panelists saw things differently.

"We have physicists, who profess to have no interest in philosophy, actually doing quite a lot of philosophy, often without knowing it," said author Jim Holt. He cited Steven Weinberg's dismissal of positivism [which demands that physical theories refer to observable things] as "dangerous," while Stephen Hawking "proudly calls himself a positivist." These two highly eminent physicists "seem to have a philosophical disagreement," Holt observed. "The question is, are they're doing it well or poorly?"

Recent contributions of philosophy to physics, said David Z. Albert, include the question of "why is it that measurements have determinant outcomes?" In that instance, "physicists were not at all surprising us with the range of questions that they could answer. They were surprising us in a different way: with the narrowness of the questions they were willing to consider." Philosophers showed that "there was a genuine scientific question," he said. "This I think was very productive."

Need for interpretation

"Physicists can be surprisingly good at answering difficult questions," said Hans Halvorson. "Philosophy's role is not to police physics but to make sense of it." As an example he mentioned the use of the word "particle." In the current theory, "there are quantum fields, but there are not really particles," he said. Philosophers "had the luxury to sit back and say, I wonder if there really are particles?"

Albert discussed the interpretation of time, noting that physics has long "spatialized" time, treating it as a "fourth component of the address of an event." By this gesture, he said, physics "cut itself off from the possibility of confronting what was really interesting about the temporal," so that any discussion of time "flowing" sounds like "meaningless gibberish." However, "our experience of the temporal coordinate is very, very different from our experience of the spatial coordinates," he said, and "physics needs to give an account of how the way that mass and energy is distributed gives rise to this."

Quantum mechanics, Albert said, has threatened the long-standing aspiration in the West "to achieve some kind of understanding of the world as a separate, real, free-standing object," with properties independent of whether anybody is looking. This vision came to be regarded "as naïve, as presumptuous, and at any rate as false." But recently, he said, philosophers and others have shown that this conclusion was "reached too fast, that the original aspiration of producing an understandable, realistic account of what was going on, rather than just a prescription for predicting the outcomes of experiments, is now widely considered much more alive and well than it had been through most of the previous century."

Because observations are central in quantum mechanics, "physicists are much more willing to talk about consciousness as an integral part of physical processes," said Holt. In contrast, "contemporary philosophers tend to be much more realistic, in the metaphysical sense."

Goals of physics

Philosophers are also well equipped to examine the nature of scientific deductions, the role of experiments, and the significance of models. In Niels Bohr's framing of quantum mechanics, for example, "science is no longer trying to describe an objective external world," Halvorson said, but simply to predict the outcome of experiments. "Philosophers were very instrumental in saying the task of science can't be put so simply."

A more recent challenge comes from the independent universes that naturally emerge in some cosmological theories. "The multiverse often gets accused a little too quickly of being something unverifiable," Albert said. "It may be in some principled way, not possible to, say, directly look at the other universes, but that doesn't mean that we won't come to have good empirical reasons of all sorts of other kinds to believe these theories." If other predictions are verified and increase our confidence in the multiverse framework, "it doesn't seem a departure from traditional scientific standards of confirmation."

Still, Holt noted that the predicted universes could have completely different laws of physics, different fundamental constants, and even different numbers of spatial dimensions. "Suddenly the ability of physics to predict things is greatly eroded, because things that looked like they're fundamental parts of our universe—the constants that define the standard model, for example—become contingent, like local weather." For some people, "that's a cheap way of explaining too much, and it's not in the best tradition of science."

An ongoing challenge for collaboration between physics and philosophy, Halvorson said, is that "people become tagged by their departments. These boundaries should be more fluid than they are." In spite of recent public criticisms, physicists' defensiveness about fundamental questions "is on the decline," Albert said. "One of the chief engines of intellectual progress is that people die. Young people find it a more natural extension of what they are doing."


George Musser, Moderator

Journalist and Author

Bernard Chazelle

Princeton University

Marcelo Gleiser

Dartmouth College

Michael Strevens

New York University

Geoffrey West

Santa Fe Institute


Many phenomena, including those of biology and social systems, are not amenable to the reductive analysis of traditional physics.

Many complex systems show mathematical regularities such as scaling laws.

Organization can occur for systems that are somewhat optimized, whether through explicit information exchange or through natural selection.

A general theory of complex adaptive systems may not meet the expectations drawn from physics.

Classifying complexity

The third session of The Physics of Everything, held on May 9, 2016, addressed the science of complexity as a way of understanding topics that have remained inaccessible to traditional physics. An immediate challenge is that complexity is partially in the eye of the beholder. Philosopher Michael Strevens gave the example of a rock. Although outwardly uninteresting, he said, at a microscopic level "the atoms are all moving around crazily."

Systems like the rock are still simple in some fundamental way, though, said physicist Geoffrey West. Planetary motions, for example, can be precisely described in a few equations. "To actually solve those equations in arbitrary situations can be incredibly complicated, even though you can write it all on one page," he said. "But you can't imagine doing anything like that for a city, or cancer, or the weather, or the financial markets." To do so misses "all the dynamics, all of the evolution, all of the emergent behavior, and all its adaptive qualities."

"In computer science we have a very specific definition of complexity," said Bernard Chazelle. That definition hinges on the properties of algorithms that recapitulate the behavior of a system, to "classify problems as to how difficult they are, how easy they are." He has been applying this notion to the quintessential adaptive complex systems of biology, whose examples dominated the discussion. "Biology is a historical science," he said. Because they incorporate history, "algorithms are simply different beasts" from equations, Chazelle said. "That requires a new language."

Emergent principles

The panelists repeatedly contrasted complex systems with expectations raised by physics, which makes extremely precise predictions for the few problems it can solve exactly. "Good physicists are able to strip the physical problem down to its bare elements, solve it, and still get something that makes sense for the whole complicated system," said physicist Marcelo Gleiser. But even slight complications require approximations. "As you start putting more components together, the way they interact generates different, novel behavior, which you cannot understand from the bottom up."

"In physics it's very important to be able to separate time scales or length scales," Chazelle said. "if you move to a different scale, you have a different language that allows you to completely ignore what happens below. In biology this rarely happens."

In spite of the difficulty of isolating scales, many biological and social systems show astonishing regularities, West said. For example, for a wide range of species, on average, the "metabolic rate scales with the mass of the organism as a power law of 3/4," he said. "That comes from analyzing the universal properties of networks that support life: the respiratory system, the circulatory system."

West has found that similar laws characterize cities, with their hierarchical networks for transporting people and goods. "It's much more, and to some extent less, than the sum of its parts, and it has all kinds of interesting emergent properties." The prevalence of these scaling laws, West said, supports the idea that these disparate systems are in some sense as efficient as possible. Even in biological evolution, which is not goal-directed, "the continuous feedback mechanisms that are implicit in natural selection leads to something being optimized."

Strevens put it more generally: "the adaptiveness of life is very strong evidence that regularity and simplicity and stability is more the rule than the exception in complex systems." The reason for the stability may even be comprehensible, he speculated. "For an explanation to have such a wide range of applicability in explaining stability, it must be, in some sense a kind of simple explanation."

"You don't need predictive power to have an explanation," Strevens countered. "We understand a lot of things in human history, have a real grasp of why they came out the way they did, but we would never have been able to predict them ahead of the fact."

A general theory?

"The bigger question is: is there a general theory of complexity, and in particular of adaptive systems?" West said. "I'm actually quite skeptical. Even though there have to be principles and laws, maybe to put it into the analog to equations is something that we will never do."

"There's only certain things one brain can comprehend," Chazelle said. Although physics has been very successful in addressing its chosen problems, "in biology, or other fields, it's not obvious to me that that has to be the case." Nonetheless, in view of the success of evolution, he said, "it's hard to believe that there are not some graspable principles that we have not yet gotten our hands on."

Something deserving the name of "law," West said, should make verifiable predictions. But "maybe that's not what biology is about," Gleiser cautioned, because "it depends on historical contingencies. It may be that you just can't do it."

"You don't need predictive power to have an explanation," Strevens countered. "We understand a lot of things in human history, have a real grasp of why they came out the way they did, but we would never have been able to predict them ahead of the fact." The description may even be comprehensible, he speculated. "For an explanation to have such a wide range of applicability in explaining stability, it must be, in some sense a kind of simple explanation."

Extending this framework to the social interactions that really matter to us, "love, and hate, and envy, and excitement, and awe," is a huge challenge, Wests admitted. "How can we ever imagine that we're going to have a scientific theory that explains all of that?" Nonetheless, he added, "it would be a mistake not to try."


George Musser


Journalist and Author

Max Tegmark

Massachusetts Institute of Technology

David Chalmers

New York University

Michael Graziano

Princeton University

Hod Lipson

Columbia University


Some researchers think that the subjective experience of feeling conscious demands a special explanation.

Other researchers regard consciousness as the self-image of a system that is modeling itself.

Experiments may be able to test some of these theoretical ideas.

Consciousness in artificial intelligence is not necessarily dangerous, but could eventually raise ethical questions.

What does it feel like to be conscious?

The fourth session of The Physics of Everything, held on May 23, 2016, addressed the nature of consciousness. Moderator George Musser began by challenging the four panelists to first define consciousness, and got some concise but widely different answers.

For two panelists, the crux of consciousness is what it feels like. "Consciousness is the subjective experience of the mind and of the world," said philosopher David Chalmers, and "subsumes all those phenomena that feel like something from the first person point of view." Unlike philosophers like Descartes, who regarded the mind and body as separate, he noted, "these days, the scientific and philosophical consensus is that there is no evidence for a nonphysical soul or ego." Nonetheless, "it is not the case that consciousness has been fully explained in terms of physics or in terms of the brain." Chalmers said. In particular, why the complex processing performed by our brains "should feel like something from the inside is still a pretty big mystery. There's nothing like a consensus theory," he said. "This is the explanatory gap that structures a lot of work in this field."

Cosmologist Max Tegmark viewed the issue similarly. "Consciousness is the way information feels when it's being processed in a certain very complex way. To me the great mystery is what precisely are these complex ways? What is it that's required?" Tegmark regards feeling as compelling evidence "I have this subjective experience. I know that even more directly than I know that I'm made of quarks. What principle it is that the motion of my quarks satisfy, for there to be that subjective experience?"

Chalmers cautioned that "our best theories of the mind go with our best, latest technologies, so at one point the model of the mind is a steam engine, or is a telephone exchange. These days we're in the information age, so it's very natural to try to understand consciousness in terms of information. But I think there's more to it than that," he said, noting that the informational structure of the world seems to correlate with states of the brain.

No magic in the machine

For the other two panelists, the subjective experience of consciousness, of a seemingly independent "I," is probably a byproduct of oversimplified self-models. "There is no metaphysical gap" that requires explanation, said neuroscientist Michael Graziano. "Our question is not how do you get the inner feeling but how do you get brains that insist they have an inner feeling?"

Source: illustratedprogress.com
Credit: Cornell University

Graziano described a patient of a therapist friend who "thought he had a squirrel in his head instead of a brain." No rational explanations could dissuade him, Graziano said, "because they didn't explain how the squirrel got there." Although most our self-models are not as bad as that, "the way the brain models that or simulates or describes that to itself is a little bit cartoonish, and we describe it as having an inner feeling," Graziano said. "The machine mistakenly thinks it has magic inside of it."

Mechanical engineer Hod Lipson has built robots with varying degrees of self-simulation, which he sees as a working definition of consciousness.

"The notion of feeling is exactly aligned with self-simulation," he said. "It doesn't prove that it's the same thing, but I don't think it's a contradiction."

Lipson emphasizes that self-modeling can vary widely in accuracy. "There's a whole continuum," he said, and even models that are wrong "allow these robots to make the right decision." There's no reason to regard human consciousness is the endpoint of this continuum, he stressed, and focusing on it could be a mistake. Modeling more primitive consciousness is "pretty simple," he said. "We're trying to bite off too much trying to explain human consciousness right away."

Experimental tests

Both Tegmark and Graziano think that their views about consciousness can be experimentally tested. Tegmark has been working with Giulio Tononi of the University of Wisconsin on "integrated information theory," which Tegmark describes as "the first time I actually saw someone write down an equation that information processing was supposed to satisfy to have this magic, subjective quality." With modern neuroscience, he said, "you can now compare what your brain is doing and what you're reporting that you are aware of to test the model's predictions."

These tools are still limited to subjects who can report their subjective experience, however. As Chalmers cautioned, "we don't have direct access to consciousness in any case except for ourselves."

Graziano described consciousness theoretically in terms of a simplified "attention schema," and said the absence of attention leads to specific types of errors in motion control. "This is the kind of test we're doing," he said. "Thus far the theory has failed to be falsified."

Conscious artificial intelligence?

Lipson said artificial-intelligence systems perform better when they include self-modeling—his definition of consciousness. "They are much more resilient. They can act. They can learn faster. They can recover from damage. They can anticipate things before they happen."

But consciousness is irrelevant to whether humans should worry about artificial intelligence, Tegmark said. "You don't really care if that heat-seeking missile coming after you is having an inner experience or not," he quipped. Still, "it matters from an ethical point of view: whether we should treat them also at some level as having ethical rights."

Graziano said consciousness could actually make such systems safer, because it could induce a degree of empathy. "A conscious computer is a computer that could potentially attribute consciousness to us, and therefore it is less likely to go around killing us all."


Ira Flatow, Moderator

Science Friday

Adam Frank

University of Rochester

Stephen M. Gardiner

University of Washington

Louisa Preston

University of London

Jason Thomas Wright

Pennsylvania State University


Observations have found huge numbers of planets orbiting nearby stars.

The chances seem high that technological civilizations have arisen many times in our galaxy.

Unless every advanced species destroys itself, there ought to be many around that we might contact.

Physically colonizing beyond our solar system will probably not happen for millennia, if ever.

Exoplanets are real, and abundant

The fifth session of The Physics of Everything, on June 13, 2016, explored the question "Are We Alone in the Universe?" The four panelists examined the prospects for communication with extraterrestrial life and whether the absence of contact, so far, bodes poorly for the survival of advanced civilizations.

To frame this issue, researchers use the "Drake equation," named for radio astronomer Dr. Frank Drake, which expresses the probability of contact as a product of many factors. Some factors quantify the planets where life could arise, while others describe the probability that it did so and that it evolved into intelligent, technological creatures who are willing and able to communicate with us.

Recent direct observations, for example from the Kepler space telescope, have clarified the first, physical factors by revealing exoplanets orbiting other stars. "Pretty much every star you see in the sky has at least one planet going around it," said astrophysicist Adam Frank. Moreover, many of these planets are likely to be in the "Goldilocks Zone" where temperatures are not too hot or cold. "The closest planet that's habitable is almost certainly within ten light years," said astrophysicist Jason Thomas Wright.

In view of this results, "it's time to stop snickering about aliens," Frank said. "We can really start asking questions about life (not necessarily intelligent life)." Researchers are beginning to measure chemical makeup of exoplanet atmospheres, and "within 30 years we may have data that tells that there are biospheres," he said.

Astrobiologists like Louisa Preston expect that extraterrestrial life will probably be carbon-based. But rather than a society-changing encounter with a super-advanced civilization, she said, "chances are the first life we find will be a lipid or a bit of protein on Mars. So all the scientists will be out at the bar celebrating," but the rest of the world may be unimpressed.

Intelligent life

What excites non-scientists is the possibility of extraterrestrial intelligence. One challenge is that we might not know it when we see it. "We haven't thought very hard about the ways that potentially very advanced civilizations might be detectible," Wright said. "We should be on the lookout for anomalies." He mentioned intriguing recent Kepler observations, flagged by citizen scientists, that suggest enormous structures passing in front of a star. He referred to one such star as "Tabby's star," after Tabetha Boyajian of Yale University, who showed that the anomalies were real. "We don't have any good ideas for what's going on," Wright said.

The panelists largely agreed that attributing this puzzle to an intelligent civilization demands extraordinary proof. "You've got to exhaust [other possibilities] until your fingers have gone bloody before you would go for the aliens," Frank said. Nonetheless, he thinks that intelligent civilizations are highly likely, based on the Drake equation and the new exoplanet data. Frank and collaborator Woody Sullivan recently calculated that if the odds of life forming on a random planet are "better than one in ten billion trillion," then "we're not the first time that an intelligent civilization has occurred."

But the most important factor in the Drake equation may be the average lifetime of advanced civilization. "If that number is 10,000 years," Frank said. "the galaxy is pretty sterile." To achieve long-term survival, civilizations have to find their way past challenges such as climate change, which he referred to as like our "final exam": "If we make it, we get to go on."

Philosopher Stephen Gardiner agreed that "there are certain kinds of tests that evolving species face," with climate change perhaps being only one. Such "genuinely global, strongly intergenerational" challenges require "a level of sophistication that's not only scientific but also institutional and ethical," and which our response to climate change has not so far exhibited. Gardiner also suggested that extraterrestrials could be waiting for us to show not only technological skills but moral advancement. "They might expect us to reach some ethical threshold before they would permit contact," because contact with us "might be like giving an Uzi to a two-year-old."

Still, even if only a small number of civilizations survive for hundreds of millions of years, they will dominate the average, Wright stressed. "It's really hard for me to imagine that it's inevitable that every species will destroy itself forever." Moreover, once a species has developed settlements on other planets or around other stars, he said, it becomes "pretty immortal."

Leaving Earth

Several panelists commended science-fiction writers for addressing issues such as colonization, but the challenges are daunting. "Interstellar travel is a long, long, long way off," Frank said. "The solar system is home for the next few thousand years."

Even our local neighborhood is extremely inhospitable, and "terraforming" Mars, for example, would likely take thousands of years, Preston noted. Ironically, "to make Mars earthlike we actually need to cause climate change on it," she said, although she also wondered if we have the right to do so. Wright also cautioned that we have yet to establish any settlements in Antarctica, "and it's much more hospitable than any place off the planet."


George Musser


Journalist and Author

Scott Aaronson

Massachusetts Institute of Technology

Daniel Harlow

Harvard University

Brian Swingle

Stanford University


Studying what happens to quantum information in black holes has led to insights spanning widely different fields of physics and computing.

The conclusion that space and time are destroyed at the surface of a black hole might not be justified, because of limits on computational complexity.

A surprising connection between quantum models and gravity models in a different dimension could lead to new error-correcting codes.

It may be possible to test black hole physics in tabletop experiments.

When fields collide

Compared to the other sessions of The Physics of Everything, the final session on June 29 followed a more formal presentation format reflecting its more technical content, but the participants strove to convey their messages to a broad audience.

Quantum mechanics and general relativity have never been harmonized, but this is rarely a problem because they usually pertain to the very small or the very large, respectively. For black holes and the big bang, however, these theories cannot simply agree to disagree. In the past few years, as physicists have explored the fate of quantum information in black holes, they have been forced to introduce profoundly revolutionary ideas.

"There are a lot of surprising connections in this area," said computer scientist Scott Aronson. He described one such connection, based on limits he had found on the complexity of calculations quantum computers could do. These limits, surprisingly, may have important implications for black holes, so "I had no choice but to become interested," he said. Aronson traced the ideas to Stephen Hawking's widely accepted proposal that black holes emit radiation that leads them to evaporate. "For a black hole the mass of our sun, you would merely have to wait 1067 years or so," he said. At that point, any information in the matter that formed the black hole would be not merely inaccessible, but destroyed. "This was a serious problem," Aronson said, and even Hawking eventually agreed that this information must have been carried away by quantum correlations, or entanglement, among the radiated photons.

"This led to a whole sequence of problems," he continued, culminating in 2012 in what is called "the firewall paradox." An influential paper [see Resources] argued that once most of a black hole had evaporated, the radiation it emits would be strongly entangled both with all the preceding radiation and with the quantum fields just inside the event horizon. This would violate a principle known as "monogamy of entanglement" that no entity can be entangled with two others.

"Everyone in the community seems to agree that this is not a problem, but they completely disagree about why it's not a problem," Aronson said. The "firewall" paper proposed that space and time cease to exist at the event horizon, which contradicts the longstanding belief that nothing strange happens to spacetime there.

An alternative proposal noted that revealing the entanglement would require an enormous computation. Aronson had previously shown that this type of calculation would be hard even with a quantum computer, so the black hole would have long since evaporated. "If you believe that there are reasonable limits of what a quantum computer can do, then creating this firewall is exponentially hard," he said, "and maybe we should worry about it less than you would have thought."

Soup-can physics

Daniel Harlow co-authored the paper that proposed this calculational escape route, but he spoke on this panel about a different, surprising connection involving quantum information. This work builds on the mapping, identified almost 20 years ago, between certain gravity-free quantum theories, known as conformal field theories (CFTs), and some gravitational models. The correspondence has only been established for universes that have a negative value for the cosmological constant (roughly equivalent to "dark energy"), unlike its apparently positive value in our universe. "This is a theory of somebody's quantum gravity, but it's not our quantum gravity," Harlow said. Nonetheless, theorists expect that many of the results for this "anti-de Sitter" space (AdS) will apply more generally.

Intriguingly, the two connected models differ by one spatial dimension. Harlow used the analogy of a can of soup. "The soup of quantum gravity is actually equal to a theory which just lives on the metal part of the can. But it's a theory without gravity." Harlow stressed that knowing what is happening deep in the "soup" requires an extensive description at the surface.

Surprisingly, the relationship between bulk and surface is similar to that exploited in error-correcting codes that could enable robust quantum computation. "The mathematics people developed to describe how to protect your quantum computation is the same mathematics as this AdS-CFT correspondence," Harlow said. "Maybe the reason a quantum computer will work is because it's simulating quantum gravity." "This is a connection between things that seem like they should have nothing to do with one another," Harlow observed. Moreover, it could produce codes "that have different properties than those coding people have already come up with. For some things they're more efficient."

Experimental tests

AdS/CFT allows researchers to learn about gravitational systems by studying the corresponding quantum systems. But Brian Swingle started out using the correspondence in the other direction. "I was on the boundary," he said, studying quantum electronic systems. In this case, looking deep inside the "soup" gives information about entanglement between "electrons that are farther apart, or larger groups of electrons."

"I was doing quantum gravity without even knowing it." Swingle said. "This was very exciting, because it opened up the possibility that we could actually do experiments to study some of these crazy things that Daniel and Scott have been talking about," Swingle said.

As one example, in modeling the random radiation emitted by black holes, theorists assume that new information is rapidly dispersed. "It's believed that black holes do this as fast as possible; that nothing in nature can scramble information more rapidly," Swingle said. To test this experimentally, researchers simply have to take the corresponding quantum system back in time, perturb it slightly, and then return it to the present to see how extensive the changes are. In this way, "we can understand something about how the black hole scrambles information," he said.

"That may sound kind of crazy," but by using controlled low-temperature systems, "we can actually do things like this." Quantum-optics researchers, for example, routinely apply pulses of light or microwaves to effectively reverse the dynamics of quantum degrees of freedom. "You can, in effect, rewind and fast forward the tape all you want," Swingle said. "The very exciting possibility is that we could actually learn about black holes in a tabletop experiment sometime soon."