About

Leonard Susskind is the Felix Bloch Professor of Theoretical Physics at Stanford University. His research interests include string theory, quantum field theory, quantum statistical mechanics, and quantum cosmology. He is widely regarded as one of the fathers of string theory, having independently introduced the idea that particles could be states of excitation of a relativistic string, along with Yoichiro Nambu and Holger Bech Nielsen. Susskind was the first to introduce the concept of the string theory landscape in 2003. He holds a Ph.D. from Cornell University in Physics (1965) and a B.S. from City College of New York in Physics (1962). He is a member of the National Academy of Sciences of the USA, the American Academy of Arts and Sciences, an associate member of the faculty of Canada's Perimeter Institute for Theoretical Physics, and a distinguished professor of the Korea Institute for Advanced Study.

Research topics

• Political Science
• Computer Science
• Mathematics
• Law
• Quantum mechanics
• Theoretical physics
• Physics

Selected publications

• More About the Spontaneous Breaking of Time Reversal in de Sitter Space

  ArXiv.org · 2026-01-04

  articleOpen access1st authorCorresponding

  It is widely thought that the quantum theory of de Sitter space requires the existence of a physical observer in the static patch. What exactly is meant by an observer is unclear; it could be anything from a few photons with energy just above the Gibbons-Hawking temperature to a gravitationally bound cluster of galaxies. In a recent note I explained that the need for observers can arise from the spontaneous breaking of time-reversal symmetry. This longer paper expands on the subject, filling in conceptual arguments that were implicit but not explicitly stated in the earlier paper.

  PublisherOA PDF

• Is Time Reversal in de Sitter Space a Spontaneously Broken Gauge Symmetry?

  arXiv (Cornell University) · 2026-03-12

  articleOpen access1st authorCorresponding

  I'll begin with some well-deserved acknowledgements: I am grateful to Daniel Harlow for discussions of time-reversal holonomies. I have also benefited from a long ongoing correspondence with Edward Witten, but frankly in both cases I can't tell whether they agree with me or not. I have often been accused of imprecision, especially toward the later parts of a paper, where I expect that my readers have ``caught on." That does eventually happen -- the readers catching on and I thank them -- but I'm now almost 86 and I can't wait. So I've tried to maintain a level of conceptual if not mathematical rigor throughout. Mathematical rigor(mortis) can sometimes be the enemy of conceptual clarity. I thank my friend Richard Feynman for reminding me of that lesson. Finally I thank the chatbot who gave me the definition of scaffold in section \ref{Scaff}. It was better than anything I was able to do. Symmetries of a Holographic theory; whether continuous or discrete, local or global, are gauge symmetries of the bulk. This includes discrete space-time symmetries such as C and P. But time-reversal is sufficiently different from other symmetries that we may question the standard wisdom and ask whether symmetries involving T should be gauged in the bulk. Harlow and Numasawa \cite{Harlow:2023hjb} say yes; time-reversal is a gauge symmetry. Witten \cite{Witten:2025ayw} says no: time reversal is different and does not manifest as a gauge symmetry of the bulk. My view is -- yes -- but with a twist: Time-reversal is indeed a gauge symmetry; but it is hidden by spontaneous symmetry breaking. In this paper I will review the case for spontaneous symmetry breaking of time-reversal and explain the ``smoking gun" -- a closed curve and a holonomy which flips forward-going clocks to backward going clocks, and vice versa.

  PublisherOA PDF

• More About the Spontaneous Breaking of Time Reversal in de Sitter Space

  arXiv (Cornell University) · 2026-01-04

  preprintOpen access1st authorCorresponding

  It is widely thought that the quantum theory of de Sitter space requires the existence of a physical observer in the static patch. What exactly is meant by an observer is unclear; it could be anything from a few photons with energy just above the Gibbons-Hawking temperature to a gravitationally bound cluster of galaxies. In a recent note I explained that the need for observers can arise from the spontaneous breaking of time-reversal symmetry. This longer paper expands on the subject, filling in conceptual arguments that were implicit but not explicitly stated in the earlier paper.

  PublisherDOI

• Is Time Reversal in de Sitter Space a Spontaneously Broken Gauge Symmetry?

  arXiv (Cornell University) · 2026-03-12

  preprintOpen access1st authorCorresponding

  I'll begin with some well-deserved acknowledgements: I am grateful to Daniel Harlow for discussions of time-reversal holonomies. I have also benefited from a long ongoing correspondence with Edward Witten, but frankly in both cases I can't tell whether they agree with me or not. I have often been accused of imprecision, especially toward the later parts of a paper, where I expect that my readers have ``caught on." That does eventually happen -- the readers catching on and I thank them -- but I'm now almost 86 and I can't wait. So I've tried to maintain a level of conceptual if not mathematical rigor throughout. Mathematical rigor(mortis) can sometimes be the enemy of conceptual clarity. I thank my friend Richard Feynman for reminding me of that lesson. Finally I thank the chatbot who gave me the definition of scaffold in section \ref{Scaff}. It was better than anything I was able to do. Symmetries of a Holographic theory; whether continuous or discrete, local or global, are gauge symmetries of the bulk. This includes discrete space-time symmetries such as C and P. But time-reversal is sufficiently different from other symmetries that we may question the standard wisdom and ask whether symmetries involving T should be gauged in the bulk. Harlow and Numasawa \cite{Harlow:2023hjb} say yes; time-reversal is a gauge symmetry. Witten \cite{Witten:2025ayw} says no: time reversal is different and does not manifest as a gauge symmetry of the bulk. My view is -- yes -- but with a twist: Time-reversal is indeed a gauge symmetry; but it is hidden by spontaneous symmetry breaking. In this paper I will review the case for spontaneous symmetry breaking of time-reversal and explain the ``smoking gun" -- a closed curve and a holonomy which flips forward-going clocks to backward going clocks, and vice versa.

  PublisherDOI

• Double-scaled SYK, QCD, and the flat space limit of de Sitter space

  Journal of High Energy Physics · 2025-10-16 · 4 citations

  articleOpen accessSenior author

  A bstract A surprising connection exists between double-scaled SYK at infinite temperature, and large N QCD. The large N expansions of the two theories have the same form; the ’t Hooft limit of QCD parallels the fixed p limit of SYK (for a theory with p -fermion interactions), and the limit of fixed gauge coupling g ym — the flat space limit in AdS/CFT — parallels the double-scaled limit of SYK. From the holographic perspective fixed g ym is the far more interesting limit of gauge theory, but very little is known about it. DSSYK allows us to explore it in a more tractable example. The connection is illustrated by perturbative and non-perturbative DSSYK calculations, and comparing the results with known properties of Yang Mills theory. The correspondence is largely independent of the conjectured duality between DSSYK and de Sitter space, but may have a good deal to tell us about it.

  PublisherOA PDFDOI

• DSSYK at infinite temperature: the flat-space limit and the ’t Hooft model

  Journal of High Energy Physics · 2025-11-18 · 2 citations

  articleOpen accessSenior author

  A bstract In the limit of infinite radius de Sitter space becomes locally flat and the static patch tends to Rindler space. A holographic description of the static patch must result in a holographic description of some flat space theory, expressed in Rindler coordinates. Given such a holographic theory how does one decode the hologram and determine the bulk flat space theory, its particle spectrum, forces, and bulk quantum fields? In this paper we will answer this question for a particular case: DSSYK at infinite temperature and show that the bulk theory is a strongly coupled version of the ’t Hooft model, i.e., (1+1)-dimensional QCD, with a single quark flavor. It may also be thought of as an open string theory with mesons lying on a single Regge trajectory.

  PublisherOA PDFDOI

• The Black Hole Information Paradox

  Springer Series in Astrophysics and Cosmology · 2025-01-01

  book1st authorCorresponding
  PublisherDOI

• Where is the Entropy in DSSYK-de Sitter? Correction to a wrong claim

  ArXiv.org · 2025-11-14

  preprintOpen access1st authorCorresponding

  A question arises in the holographic description of the static patch of de Sitter space: Where does the entropy reside? The answer of course is in the stretched horizon, but how far from the mathematical horizon is the stretched horizon? In recent papers and lectures I argued that the entropy in DSSYK/JT-de Sitter resides at a string distance from the horizon. That conclusion was based on misconception about the confinement-deconfinement transition in the 't Hooft model. When corrected the right answer is of order the Planck distance (which differs from the string distance by a factor of order $\sqrt{N}).$

  PublisherOA PDFDOI

• Why do we Need Observers? Spontaneous Breaking of Time-Reversal in de Sitter Space

  ArXiv.org · 2025-12-15

  preprintOpen access1st authorCorresponding

  In this paper I explain the relation between the need for observers in de Sitter space and the spontaneous breakdown of time-reversal symmetry.

  PublisherOA PDFDOI

• Double-Scaled SYK, QCD, and the Flat Space Limit of de Sitter Space

  ArXiv.org · 2025-01-16

  preprintOpen accessSenior author

  A surprising connection exists between double-scaled SYK at infinite temperature, and large N QCD. The large N expansions of the two theories have the same form; the 't Hooft limit of QCD parallels the fixed p limit of SYK (for a theory with p-fermion interactions), and the limit of fixed gauge coupling g -- the flat space limit in AdS/CFT -- parallels the double-scaled limit of SYK. From the holographic perspective fixed g is the far more interesting limit of gauge theory, but very little is known about it. DSSYK allows us to explore it in a more tractable example. The connection is illustrated by perturbative and non-perturbative DSSYK calculations, and comparing the results with known properties of Yang Mills theory. The correspondence is largely independent of the conjectured duality between DSSYK and de Sitter space, but may have a good deal to tell us about it.

  PublisherOA PDFDOI

Recent grants

• Research in Theory of Elementary Particles and Cosmology

  NSF · $4.2M · 2008–2013

• Research in Particle Theory, Cosmology, and Quantum Gravity

  NSF · $2.4M · 2017–2020

• Research in Particle Theory, Cosmology, and Quantum Gravity

  NSF · $2.5M · 2020–2023

• Research in Theory of Elementary Particles and Cosmology

  NSF · $3.8M · 2003–2008

• Research in Theory of Elementary Particles and Cosmology

  NSF · $1.8M · 2013–2017

Frequent coauthors

• Adam R. Brown

  39 shared

• John B. Kogut

  28 shared

• Lárus Thorlacius

  23 shared

• Henry W. Lin

  22 shared

• Sepehr Nezami

  Google (United States)

  17 shared

• Grant Salton

  17 shared

• Michael Walter

  17 shared

• Igor R. Klebanov

  Princeton University

  16 shared

Labs

• Leonard Susskind LabPI

Education

• B.S., Physics

  University of California, Berkeley

  1962

• Ph.D., Physics

  Stanford University

  1967