Upamanyu MoitraU. Moitra Contact
Upamanyu Moitra

Theoretical physicist, University of Amsterdam

Upamanyu Moitra

Quantum gravity, string theory, black holes and beyond.

Lumos extrema, JHEP 2025

Welcome

I am Upamanyu Moitra, a theoretical physics 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. My interests in physics are wide-ranging – from formal theory to sophisticated experiments. My present research interests include string theory, quantum field theory, gravitation and cosmology. 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.

Latest paper

Edge Modes on Stringy Horizons

Physical Review Letters, 2026. With Atish Dabholkar and E. Harris.

See it in the list

Writing

The other side of the blackboard

Essays on cinema, literature, culture, history, society and science since 2008: Nolan reappraised, Hitchcock interrogated, Tagore in conversation with Einstein and the vanishing islands of the Sundarbans.

Read the essays

Interactive

Four things to play with

Entanglement in AdS₃, a near-extremal throat, the Duffing oscillator, and the causal structure of a black hole.

Open the demos

Profile

Per aspera ad astra

Postdoctoral researcher at the University of Amsterdam, after ICTP Trieste and a PhD at TIFR Mumbai.

Research

Rerum cognoscere causas

Ad fontes, back to the sources: each line of my work below comes with the question it set out to answer, what it established and when, and where it has led. 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?

Collaborative work

A charged or spinning black hole can be cooled all the way to zero temperature, a state called extremality. As it gets there, the region just outside its horizon stretches into a long throat with a simple geometry, \(\mathrm{AdS}_2\). In a series of collaborative papers, we showed that the black hole’s behaviour at low temperature, both its thermodynamics and its response to being disturbed, can be read off from that throat, where a two-dimensional theory of gravity, Jackiw–Teitelboim (JT) 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. The description we set out in August 2018 (the time-reparametrisation mode together with the phase modes of the gauge fields, with the higher partial waves kept in view) set the stage for all subsequent calculations in the quantum statistical mechanics of near-extremal black holes, from the question of a mass gap above extremality to the logarithmic corrections to the entropy. In May 2019 we derived the Schwarzian description that governs these quantum corrections for every black hole whose throat carries an \(\mathrm{SL}(2,\mathbb{R})\) 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 its starting point in these papers.

On the record

  1. 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; at somewhat higher energies, higher partial waves can be cumulatively significant. The results apply to any \(\mathrm{AdS}_2 \times S^d\) throat, in asymptotically AdS and flat spacetimes.

    arXiv:1808.08239 with Trivedi, Vishal; JHEP 07 (2019) 055

  2. We showed that JT gravity gives the correct low-temperature free energy 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, resolving a puzzle in the literature. We verified this explicitly for rotating black holes in four and five dimensions, near-extremal Kerr among them, and pointed to the Kerr black holes found in nature as a place where the description could prove useful.

    arXiv:1905.10378 with Sake, Trivedi, Vishal; JHEP 11 (2019) 047

  3. We analysed JT gravity coupled to many conformal fields in the semi-classical limit. Two choices of boundary conditions give interestingly different thermodynamics, and in both the second law holds for the generalised entropy, which counts the entanglement of the matter fields across the horizon.

    arXiv:1908.08523 with Sake, Trivedi, Vishal; JHEP 04 (2020) 199

  4. We showed that, contrary to earlier expectations, a fluid–gravity correspondence exists for near-extremal black branes even when the temperature is the smallest scale. The constitutive relations become non-local in time beyond first order, yet perturbation theory remains sensible.

    arXiv:2005.00016 with Sake, Trivedi; JHEP 02 (2021) 021

  5. We formulated the JT path integral directly in terms of the metric and the dilaton, in anti-de Sitter and de Sitter space, agreeing with the first-order formalism in the asymptotic limit. With bosonic matter, the double-trumpet path integral diverges as the wormhole neck shrinks, a Casimir effect; for fermions, suitable boundary conditions avoid it.

    arXiv:2101.00596 with Sake, Trivedi; JHEP 10 (2021) 204

  6. We showed that the de Sitter JT path integral gives the Hartle–Hawking wavefunction of universes tunnelling “from nothing” and the amplitudes for contracting universes to tunnel into expanding ones, with random matrix theory as a hologram. With matter and suitably twisted boundary conditions, the tunnelling avoids a big bang or big crunch singularity.

    arXiv:2202.03130 with Sake, Trivedi; JHEP 06 (2022) 138

Can entanglement across a horizon be made well defined and finite?

Collaborative work

In quantum field theory the entanglement entropy across a horizon is infinite, because modes of arbitrarily short wavelength straddle it; with gauge fields, even its definition is subtle. We have addressed both problems. In gauge theories, we showed that the classical part of the entropy drops out of physical quantities such as the relative entropy in the continuum limit. In string theory, the orbifold method, a stringy analogue of the replica trick, yields an entanglement entropy that is finite and calculable.

Why it matters

Entanglement entropy that is finite in both the ultraviolet and the infrared is what a consistent quantum theory of gravity should deliver, and its finiteness bears directly on the black hole information paradox. 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.

On the record

  1. In gauge theories the entanglement entropy splits into a classical and a quantum term. We argued that in the continuum limit the classical term is dominated by modes at the cut-off scale, and so drops out of the relative entropy and the mutual information for states of finite energy. We extended this to \(p\)-form theories and discussed electric–magnetic duality.

    arXiv:1811.06986 with Soni, Trivedi; JHEP 08 (2019) 059

  2. In ten-dimensional Type II string theory, we summed the tachyonic contributions to the orbifold partition function and continued them to the physical region \(0 < N \le 1\), obtaining a finite and calculable one-loop entanglement entropy.

    arXiv:2306.00990 with Dabholkar; Phys. Rev. D 109, L121901 (2024), Letter

  3. We showed that the entropy 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

  4. We computed the exact one-loop partition function of orbifolds of a black hole horizon in string theory, as a function of the black hole’s mass and spin.

    arXiv:2312.14253 with Dabholkar; JHEP 06 (2024) 053

  5. We constructed the orbifolds of the ten-dimensional heterotic strings, where modular invariance requires the gauge symmetry to be broken by a Wilson line, and classified every pattern of breaking. In all cases the answer continues to a finite one for \(0 < N \le 1\).

    arXiv:2407.17553 with Dabholkar; JHEP 12 (2024) 012

  6. We defined a quantum entanglement index: ultraviolet divergent in field theory, but finite in both the ultraviolet and the infrared in string theory. It diagnoses the entanglement structure of string theory without any analytic continuation in \(N\).

    arXiv:2507.15939 with Dabholkar, Harris; JHEP 02 (2026) 136

  7. We obtained the partition function of edge modes on a stringy horizon: modular invariant, ultraviolet finite and open to state counting.

    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?

Single-author and collaborative work

In holography, gravity in anti-de Sitter space is equivalent to a quantum field theory living on its boundary, so a question that is hard on one side can be easy on the other. The papers here use that dictionary in both directions: charged operators on the boundary register the contest between gravitational attraction and electric repulsion in the bulk; light circling a black hole sets how fast the field theory relaxes; boundary counter-terms decide what holography means at a finite cut-off; and boundary correlators measure how much a black hole’s horizon fluctuates.

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, by tying it to orbits in the bulk. The 2026 work on BTZ black holes gives what is, to our knowledge, the most rigorous evidence to date of gauge-invariant fluctuations far above the Planck scale within perturbative quantum gravity.

On the record

  1. 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 reproduces the thermodynamics of dyonic black holes locally on the cut-off surface.

    arXiv:2212.12545 with Jatkar; PTEP 2023, 083B03

  2. I studied near-circular orbits of charged particles around charged black holes in anti-de Sitter space of any dimension, where Newtonian attraction competes with Coulomb repulsion; in the dual field theory this is the light-cone bootstrap for charged operators. I related the result to the weak gravity conjecture for AdS black holes of all sizes.

    arXiv:2305.08907; Phys. Rev. D 109, L041903 (2024), Letter

  3. Turning to massless particles, 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 \(i\epsilon\) prescription for the JWKB approximation; it yields the eikonal quasi-normal modes, reveals a new relaxation timescale set by the chemical potential near extremality, and makes superradiant instabilities easy to study.

    arXiv:2408.08308; JHEP 03 (2025) 168

  4. With the same counter-term, we showed that the three-sphere partition function in \(\mathrm{AdS}_4\) is independent of the cut-off, even when the cut-off surface is deformed, and that the renormalised entanglement entropy can be too; it integrates out fewer degrees of freedom than other counter-terms.

    arXiv:2505.09675 with Jatkar; JHEP 02 (2026) 171

  5. 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?

Single-author work

General relativity comes with promises that are hard to prove: that the past determines the future even inside a black hole, that gravitational collapse unfolds in an orderly way, and that a black hole can hold only so much charge for its mass. String theory adds corrections that can move these boundaries. I have examined each of them in settings simple enough to solve exactly: the inner horizon of a two-dimensional black hole, self-similar collapse in two dimensions, and charged black holes in two-dimensional heterotic string theory, where T-duality pins the corrections down.

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 universal \(\mathrm{AdS}_2\) throat. 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.

On the record

  1. I showed that a massive scalar wave-packet near the Cauchy horizon of an \(\mathrm{AdS}_2\) black hole becomes infinitely differentiable for smooth initial data, whatever the parameters of the spacetime or the field: strong cosmic censorship fails at the classical level.

    arXiv:2011.03499; Phys. Rev. D 103, L081502 (2021), Letter

  2. I began the systematic study of continuously self-similar gravitational dynamics in two dimensions. Self-similarity turns out to fix how the dilaton couples to curvature, singling out models related to two-dimensional string theory and Liouville gravity; singularities are generic, with hints of a Choptuik-like scaling law.

    arXiv:2211.01394; JHEP 06 (2023) 194

  3. I wrote the effective theory of heterotic strings in two dimensions in a formalism inspired by double field theory, manifestly consistent with T-duality, showed that the classification of higher-derivative corrections extends to the heterotic case, and found novel features of the charged black hole.

    arXiv:2511.21687; JHEP 05 (2026) 165

  4. I found the duality-invariant higher-derivative corrections to the charged black hole of two-dimensional heterotic string theory, including the corrected charge-to-mass ratio of extremal black holes and their entropy from the attractor mechanism.

    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.

Talks

Have Chalk, Will Talk

Every talk, seminar, poster and meeting, with recordings where they exist. Each place carries its city and country, so either will find it.

Spoke or presentedAttended

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.”

Thesis examiner

“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 […]”

Thesis examiner

“The research is original, of high quality […] Upamanyu has written a good thesis.”

Thesis examiner

Read it chapter by chapter

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 AdS₂ geometry and the associated SL(2, 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 AdS₂ 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.

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 page 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 page 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.

Examples

More to come!

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

Beyond physics

Photography, film and radio.

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.

Research

Four threads, one horizon

Where gravity, quantum information and string theory meet. Each thread links to the papers that belong to it.

Papers

Publications

Newest first. Search by title, co-author, year or arXiv number; filter by theme; copy BibTeX from any row.

Demos

Things you can play with

Each solves the real equations in your browser. Small windows onto problems from the papers.

Entanglement from a geodesic

In AdS₃/CFT₂ the entanglement entropy of a boundary interval is a quarter of the length of the bulk geodesic hanging below it, in Planck units. Drag the interval and watch the geodesic and the entropy respond.

Entropy S
Complement S̄
Geodesic depth
Purity check

S(θ) is symmetric about θ = π because the global state is pure: the entropy of a region equals that of its complement.

Growing an AdS₂ throat

A charged black hole cooled towards extremality grows an ever longer neck. That neck is where two-dimensional Jackiw–Teitelboim gravity takes over.

Outer horizon r₊
Inner horizon r₋
Temperature T
Entropy S
Throat length
S − S₀ vs 4π²T

Units G = M = 1. Near extremality the entropy above its extremal value approaches 4π²T, the linear specific heat Jackiw–Teitelboim gravity predicts.

Where chaos starts

The subject of a master's project at Jadavpur: a spring that is not quite Hooke's, driven periodically. It goes from orderly to chaotic as you turn a single dial.

Drive periods
Poincaré points

The trace is the trajectory in the (x, ẋ) plane. Dots are the Poincaré section, one per drive period: a few dots means order, a cloud means chaos.

The shape of a black hole

The causal structure of an eternal black hole, drawn so that light always travels at 45°. Click anywhere to send out a pair of light rays.

Inside the horizon every ray ends on the singularity.

Code

Computation

Theoretical physics is not done with a pencil alone: solvers, symbolic algebra, and the numerics behind the figures.

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Read in your language

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