Researcher, University of Amsterdam
Upamanyu Moitra
I draw immense satisfaction from finding things out. Blessed with natural stupidity, I chase the mechanism behind what I see (and what I don’t). I like to turn my thoughts into equations and equations into numbers.
Welcome
I am Upamanyu Moitra, a postdoc at the Institute of Physics, University of Amsterdam, Netherlands. I obtained my PhD from TIFR India and had my first postdoctoral stint at ICTP Trieste in Italy. I have been involved in activities promoting science awareness since my childhood.
My blog, Where the Mind is Without Fear . . ., includes reports on my outreach activities and my articles that have appeared in newspapers and magazines.
Writing
The other side of the blackboard
Essays on science, cinema, literature, culture, history, society and environment since 2008: Nolan reappraised, Hitchcock interrogated, Tagore in conversation with Einstein and the vanishing islands of the Sundarbans.
Read the essaysProfile
Origin story
Per aspera ad astra
Postdoctoral researcher at the University of Amsterdam, after ICTP Trieste and a PhD at TIFR Mumbai.
Stories
Stories with Code
If there’s a common thread connecting all my passions, it is storytelling. I love to hear, and tell, stories. Each paper, each equation, each line of code that I write should be in the service of a story. It has been a lament of mine that not many academic publications are written like stories. Indeed, not all stories these days are written like stories.
I sincerely believe that whatever I do, no matter how technical it is, can be communicated to anyone who has curiosity but no formal training. The point of this series is to explore alongside you, dear reader, tales that combine science, markets and the arts. These will be narrated, out of compulsion, from my own point of view, because that is (usually) the only one I have. To make this expedition accessible to a wider audience, I will tell each story with Python code that you can run yourself.
Please let me know if you want a specific story told here.
New stories by email
One short email when a new story is out, and nothing else. Unsubscribe any time.
Research
Getting to the bottom of things
Rerum cognoscere causas
Ad fontes, back to the sources: the main directions of my research below come with the questions, what they established and when, and the path they have led to. My work includes both collaborations, which I have very much enjoyed, and single-author papers. For citation records, use the Google Scholar and INSPIRE buttons at the top of the page.
What does a black hole do when it is (almost) as cold as it can be?
A charged or spinning black hole can be cooled all the way to zero temperature, a state called extremality. As the black hole gets there, the region outside the horizon expands to a long throat with a simple geometry, \(\mathrm{AdS}_2\). In a series of collaborative papers, we showed that the low-temperature features of the black holes, both the thermodynamics and the response to being disturbed, can be deduced from that throat, where the Jackiw–Teitelboim (JT) theory of gravity takes over.
Where it has led
This work showed, in full generality, that the low-temperature physics of near-extremal black holes is governed by the \(\mathrm{AdS}_2\) throat and the associated JT model. The description we wrote down in August 2018 (the time-reparametrisation mode together with the phase modes of the gauge fields) set the stage for the consideration of the quantum statistical mechanics of near-extremal black holes. In May 2019, we derived the Schwarzian description that governs these quantum corrections for every black hole carrying a near-horizon \(\mathrm{SL}(2,\mathbb{R})\) symmetry, rotating ones included: it underlies the quantum corrections to near-extremal thermodynamics and holds universally across all near-extremal black holes in generic supergravity theories. Anyone entering that literature will find a natural starting point in these papers.
Paper trail
-
We showed that the behaviour of extremal and near-extremal black holes at low energies and low temperatures is determined by the \(\mathrm{AdS}_2\) throat, going beyond the S-wave approximation and including charged matter. The leading behaviour comes from the time-reparametrisation mode and from phase modes of the gauge fields. This paper, applying to asymptotically AdS and flat spacetimes alike, established a near-\(\mathrm{AdS}_2\)–near-\(\mathrm{CFT}_1\) correspondence.
arXiv:1808.08239 with Trivedi, Vishal; JHEP 07 (2019) 055
-
This showed that JT gravity gives the correct low-temperature thermodynamics of every black hole whose throat has an \(\mathrm{AdS}_2\) factor and the associated \(\mathrm{SL}(2,\mathbb{R})\) symmetry, rotating black holes included, and explained why, therefore resolving a puzzle in the literature.
arXiv:1905.10378 with Sake, Trivedi, Vishal; JHEP 11 (2019) 047
-
We analysed JT gravity with matter fields in the semi-classical limit. This had interesting consequences for the information problem.
arXiv:1908.08523 with Sake, Trivedi, Vishal; JHEP 04 (2020) 199
-
We showed that, contrary to earlier expectations, a fluid–gravity correspondence exists for near-extremal black branes even when the temperature is the smallest physical scale.
arXiv:2005.00016 with Sake, Trivedi; JHEP 02 (2021) 021
-
We formulated the JT path integral directly in terms of the metric and the dilaton, in anti-de Sitter and de Sitter space. With certain matter fields, the path integral diverges.
arXiv:2101.00596 with Sake, Trivedi; JHEP 10 (2021) 204
-
We showed that the dS JT path integral gives the wave function of universes. With matter and suitable boundary conditions, one can avoid a big bang or crunch.
arXiv:2202.03130 with Sake, Trivedi; JHEP 06 (2022) 138
Can entanglement across a horizon be made well defined and finite?
In quantum field theory, the entanglement entropy across a horizon is infinite, because modes of arbitrarily short wavelength straddle it. Even the definition of entanglement is subtle in many cases, including string theory. I have addressed the problem both in field theory and string theory with my collaborators. The orbifold method in string theory, an analogue of the replica trick, yields an entanglement entropy that is finite and calculable.
Why it matters
A nice quantum theory of gravity should give an entanglement entropy that is finite in both the ultraviolet and the infrared. This finiteness bears directly on the black hole information problem. The orbifold method offers a way to define this quantity in string theory and to compute it order by order in perturbation theory. The latest step, on the edge modes of stringy horizons, appeared in Physical Review Letters in 2026 and was reported by Phys.org.
Paper trail
-
The entanglement entropy in gauge theories splits into classical and quantum parts. We argued that in the continuum limit the classical term is dominated by modes at the cut-off scale, and so drops out of certain UV-safe measures. We extended this to \(p\)-form theories and discussed electric–magnetic duality.
arXiv:1811.06986 with Soni, Trivedi; JHEP 08 (2019) 059
-
In ten-dimensional Type II string theory, we resummed the tachyonic contributions to the orbifold partition function and analytically continued them to the physically relevant region, eventually obtaining a finite and calculable one-loop entanglement entropy in perturbative string theory.
arXiv:2306.00990 with Dabholkar; Phys. Rev. D 109, L121901 (2024), Letter
-
We showed that the entropy, as defined above, stays finite in compactifications to lower dimensions, where it depends on the moduli, suggesting that finiteness is a generic feature of string theory.
arXiv:2310.13735 with Dabholkar; JHEP 06 (2024) 209
-
We calculated the exact one-loop partition function of orbifolds of a black hole horizon in string theory relevant for holography.
arXiv:2312.14253 with Dabholkar; JHEP 06 (2024) 053
-
We constructed the orbifolds of the ten-dimensional heterotic strings, with new complications arising because of gauge symmetry, and classified the patterns of symmetry breaking. The answer continues to a finite one in the physically interesting regime.
arXiv:2407.17553 with Dabholkar; JHEP 12 (2024) 012
-
We defined a quantum entanglement index which is ultraviolet divergent in field theory, but finite in string theory. It elucidates the entanglement structure of string theory without any analytic continuation in \(N\).
arXiv:2507.15939 with Dabholkar, Harris; JHEP 02 (2026) 136
-
We obtained the partition function of edge modes on a stringy horizon: modular invariant and ultraviolet-finite.
arXiv:2601.13131 with Dabholkar, Harris; Phys. Rev. Lett. 136, 221501 (2026)
What can the boundary of spacetime tell us about the gravity inside it?
Gravity in AdS space is equivalent to a quantum field theory living on its boundary in the framework of holography. Amazingly, an extremely hard question on one side can be fairly easy on the other. The papers here use that dictionary in both directions. Operators on the boundary encode the competition between gravitational and electric forces in the bulk. Light going around a black hole sets how fast things are dragged on the field theory side. Boundary counter-terms determine what holography means at a finite radial cut-off. Boundary correlators measure the extent to which the horizon of a black hole fluctuates and also suggest how to detect them.
What it opened
The 2023 Letter was the first to pose the light-cone bootstrap question for charged operators in holography, with a specific prediction in the appropriate regime and opened up the weak gravity conjecture for large charged AdS black holes, until then largely unexplored. The 2026 work on BTZ black holes gives what is, to our knowledge, the most rigorous evidence to date of gauge-invariant fluctuations parametrically larger than the Planck scale within perturbative quantum gravity.
Paper trail
-
We showed that the Dirac–Born–Infeld-like counter-term performs complete background subtraction for four-dimensional gravity with both AdS and flat asymptotics and correctly gives the thermodynamics of dyonic black holes locally on the cut-off surface.
arXiv:2212.12545 with Jatkar; PTEP 2023, 083B03
-
I studied near-circular orbits of charged particles around charged black holes in AdS space of any dimension, where Newtonian attraction competes with Coulomb repulsion. In the dual CFT, this is related to the light-cone bootstrap for charged operators. I connected the result to the weak gravity conjecture for AdS black holes of all sizes.
arXiv:2305.08907; Phys. Rev. D 109, L041903 (2024), Letter
-
I showed that the photon ring of a near-extremal charged AdS black hole can reach into the near-horizon region, in high enough dimension or for charged particles. I proposed a simple \(\mathrm{i}\epsilon\) prescription for the JWKB approximation. I discovered a new relaxation timescale set by the chemical potential near extremality.
arXiv:2408.08308; JHEP 03 (2025) 168
-
With the same counter-term as above, we showed that the three-sphere partition function in \(\mathrm{AdS}_4\) is topological and that the renormalised entanglement entropy can be too.
arXiv:2505.09675 with Jatkar; JHEP 02 (2026) 171
-
We defined a precise, gauge-invariant quantum width for the horizon of a BTZ black hole and related it to boundary correlators through holography. It is typically of order \((G_\mathrm{N} L_\mathrm{AdS}^3)^{1/4}\), parametrically larger than the Planck scale.
arXiv:2606.28160 with Freivogel; Preprint
What does gravity allow, and what does it forbid?
General relativity comes with natural assumptions that are notoriously difficult to prove. Some of these natural assumptions include the past uniquely determining the future even inside a black hole, and the phenomenon of gravitational collapse unfolding in an orderly way. I love to explore such questions by considering extreme scenarios in gravitational physics. I have undertaken explorations of such scenarios in settings simple enough to give interesting physics.
What it opened
The analysis of 2020 gave a single, parameter-independent reason for the violations of strong cosmic censorship that had been found for particular nearly extremal black holes. The study of self-similar collapse opened a two-dimensional laboratory for critical phenomena in gravity. The duality-invariant work shows how tightly T-duality constrains the stringy corrections to charged black holes, down to the charge-to-mass ratio of extremal ones, a quantity central to the weak gravity conjecture.
Paper trail
-
I showed that a massive scalar wave-packet near the Cauchy horizon of a black hole in JT gravity becomes infinitely differentiable for smooth initial data, whatever the parameters of the spacetime or the field: strong cosmic censorship fails at the classical level. I further extended this result in my thesis.
arXiv:2011.03499; Phys. Rev. D 103, L081502 (2021), Letter
-
I began the systematic study of continuously self-similar gravitational dynamics in two dimensions. I studied strong gravitational effects with high-precision code.
arXiv:2211.01394; JHEP 06 (2023) 194
-
I studied heterotic strings in two dimensions in a formalism inspired by double field theory. I extended the classification of higher-derivative corrections to the heterotic case and found novel features of the charged black hole.
arXiv:2511.21687; JHEP 05 (2026) 165
-
I found the duality-invariant higher-derivative corrections to the charged heterotic stringy black hole. I found surprising results for the corrected charge-to-mass ratio of extremal black holes and their entropy.
arXiv:2603.24658; Phys. Rev. D 114, L061902 (2026), Letter
The papers are grouped into four themes for brevity. The themes overlap, and several papers could sit just as well under another heading.
Thesis
Aspects of Quantum Gravity, Holography and Entanglement
My PhD thesis, Tata Institute of Fundamental Research, Mumbai (2021). Thesis advisor: Prof. S. P. Trivedi.
My doctoral thesis was highly praised by the examiners. The comments of the examiners include:
“This thesis contains very high quality work at the frontiers of quantum field theory, general relativity and gauge-gravity duality. The issues addressed are of fundamental interest and the results are solid. The papers included here have already made a significant impact in the field. It is particularly noteworthy that the candidate has published a single author paper in a top journal […] In my opinion, this thesis far exceeds the expectations from a Ph.D. thesis in this field.”
“Upamanyu Moitra’s thesis is a compendium of several interesting results. I believe it is worth three Ph.D. theses […] [T]his thesis contains three different important results […]”
“The research is original, of high quality […] Upamanyu has written a good thesis.”
Read it chapter by chapter
- AcknowledgementsPDF
- Table of contentsPDFWith the list of publications and of figures
- Chapter 1: IntroductionPDF
- Chapter 2: Rotating Black Holes and Jackiw–Teitelboim GravityPDF
- Chapter 3: Fluid Dynamics for Near-Extremal Black BranesPDF
- Chapter 4: The Strong Cosmic Censorship Conjecture in Two DimensionsPDF
- Chapter 5: Quantum Information-Theoretic Measures in Gauge Theories and DualityPDF
- Chapter 6: CodaPDF
- Appendix: Conventions and Useful FormulæPDF
- BibliographyPDF
Abstract
I explore topics in quantum gravity, holography and quantum entanglement in this thesis and obtain several novel results in these topics.
This thesis shows why Jackiw–Teitelboim (JT) gravity reproduces the correct low-temperature thermodynamic behaviour of nearly extremal black holes, resolving a puzzle in the literature. The arguments are shown true for black holes in general supergravity theories with a near-horizon \(\mathrm{AdS}_2\) geometry and the associated \(\mathrm{SL}(2, \mathbb{R})\) isometry, including rotating black holes. These arguments are explicitly verified for some general examples of rotating black holes in four and five dimensions. This work could have interesting phenomenological implications.
A careful analysis in this thesis shows that, contrary to previous expectations, one can formulate a fluid-gravity correspondence even in the regime where the temperature is the smallest scale. Surprisingly, the constitutive relations become non-local in time beyond first order. The analogues of the hydrodynamic modes are obtained from first order fluid dynamics. In spite of the non-locality, the validity of a sensible perturbation theory is demonstrated. A novel feature of the time evolution of near-extremal fluid dynamics is established. Some time-independent solutions in an extremal background are also studied in relation to the attractor mechanism – two classes of bulk solutions are found that are dual to forced fluids.
The question of the validity of the strong cosmic censorship conjecture is studied for \(\mathrm{AdS}_2\) black holes. For neutral probe scalar fields, the conjecture is shown to be severely violated; a similar violation is also demonstrated for charged scalar fields. This work explains why some near-extremal black holes violate the conjecture, as pointed out in some recent studies.
The continuum-limit behaviour of the classical term of entanglement entropy in gauge field theories is explored and it is shown that the classical contributions to certain quantum information-theoretic quantities vanish in the continuum limit, in a wide class of gauge theories. Furthermore, the general behaviour of the algebra of observables in gauge theories under electric-magnetic duality is examined. In some special cases, the entanglement spectra on two sides of the duality are argued to coincide. Two different definitions of centres are argued to give identical answers for some ultraviolet-safe quantities in the continuum limit via duality, in general.
Talks
Have Chalk, Will Talk
Every talk, seminar, poster and meeting, with publicly available recordings. Search by institution or place.
Spoke or presentedAttended
Teaching
Docendo discimus
I am a firm believer in the principle that the best way to learn is to teach. After all, what is learning if not teaching oneself?
I love to learn and to teach. Other students have complimented me on my teaching. This section includes a few examples of my efforts to teach other students (and myself) a thing or two about physics. Some of the examples might look very simple and some, not so much. I will maintain this list with the hope that its contents will help other students like me. Please let me know if you find it useful. You could also suggest some problems that could appear here.
Guidance and mentorship
I have experience in guiding and mentoring younger students on their projects. I have co-supervised the Diploma Thesis of an ICTP Postgraduate Diploma student.
Writing
Notes and essays
Essays on cinema, literature, culture, history, society and science, written since 2008 for newspapers, magazines and my blog.
Older posts are on my blog, Where the Mind is Without Fear . . .
Personal
The rest of me…
…would be impossible to distill. Anyway, here are a couple of things.
Photography and film
I take an active interest in photography and filmmaking. I have been associated with the making of several documentary films which won acclaim in various international film festivals, including the International Film Festival of India (IFFI) in Goa.
In 2019, a photograph of mine was exhibited at the Embassy of Japan in India.
Radio
I have participated in talk shows on All India Radio.
Languages
Bengali, English, Hindi, Sanskrit, Italiano, Nederlands.
Code
Computation
Theoretical physics is not done with a pencil alone: solvers, symbolic algebra, and the numerics behind the figures.