Ashoke Sen of Bengaluru Is the First Indian Physicist to Win the Oskar Klein Medal, Announced on 24 October 2026 for His Work on String Theory and Black Holes

Ashoke Sen, a string theorist at the International Centre for Theoretical Sciences (ICTS) in Bengaluru, has been awarded the 2026 Oskar Klein Medal. ICTS and the Press Trust of India describe him as the first Indian physicist to receive it. He gave the Oskar Klein Memorial Lecture, “Gravity from strings and strings from gravity,” at the AlbaNova University Centre in Stockholm on 22 October 2026, and ICTS announced the medal on Saturday 24 October.

The medal is awarded by Stockholm University together with the Nobel Committee for Physics of the Royal Swedish Academy of Sciences. ICTS cites three strands of Sen’s work: S-duality, the precise counting of black hole entropy, and string field theory.

What the Oskar Klein Medal is

The medal was established in 1988 and is named after the Swedish theoretical physicist Oskar Klein. According to the PTI report carried by ThePrint, the organising committee of the Oskar Klein Memorial Lectures invites a distinguished researcher each year to give a memorial lecture and to receive the medal.

The first three recipients were Chen-Ning Yang, Steven Weinberg and Hans Bethe, in that order. The PTI list gives Bethe the 1990 medal and adds Stephen Hawking (2003), Peter Higgs (2009) and Kip Thorne (2016).

ThePrint’s account of the Stockholm lecture says Sen described how string theory leads to a consistent quantum theory of gravity and took up open puzzles in the quantum theory of black holes. String theory, in the same report’s plain description, proposes that the basic constituents of matter are tiny one-dimensional strings and not point particles.

The 1994 paper behind S-duality

Physicists can calculate reliably when the forces in a theory are weak and struggle when they are strong. A duality is a mapping between two descriptions of the same physics. Bhavana, the Indian Physics Association’s magazine, explains S-duality as a symmetry that maps a strongly interacting description of a theory onto a weakly interacting one, so that a hard calculation becomes an easier one.

Proving S-duality outright would mean solving both sides exactly. Sen instead collected evidence. In a paper submitted to the arXiv preprint server on 7 February 1994, he argued that if S-duality holds in certain string and gauge theories, bound states of monopoles and dyons (particles carrying magnetic charge, or electric and magnetic charge together) must exist. He then built the mathematical object that such states require, a self-dual harmonic form, on the space of two-monopole configurations, which establishes the bound states in the two-monopole case.

The Royal Society’s profile credits him with predicting and confirming these dyon-monopole bound states, and says the result set off activity in both physics and mathematics. The 2012 Breakthrough Prize in Fundamental Physics, one of the first awarded, cited him for uncovering evidence of strong-weak duality in certain supersymmetric string theories and gauge theories, and for opening the path to the realisation that “all string theories are different limits of the same underlying theory.”

Counting what a black hole is made of

The Bekenstein-Hawking relation sets a black hole’s entropy at one quarter of the area of its horizon. String theory offers a way to check that figure by counting the microscopic states behind it.

Andrew Strominger and Cumrun Vafa, in a paper submitted on 9 January 1996, derived that relation for a class of five-dimensional extremal black holes in string theory by counting the degeneracy of certain bound states. Sen’s lecture notes, submitted on 9 August 2007, cover the entropy of extremal black holes through what he calls the entropy function formalism. They also present a precise microscopic count of a class of quarter-BPS dyons in N=4 supersymmetric string theories and compare it with the black hole entropy, expanded in inverse powers of the electric and magnetic charges. The Royal Society’s profile describes this strand as the application of string theory methods to the quantum properties of black holes.

In 2014 the International Centre for Theoretical Physics (ICTP) in Trieste gave its Dirac Medal to Sen, Strominger and Gabriele Veneziano for contributions to string theory. ICTP’s announcement called Sen’s research on black holes and on the symmetries of string theory highly influential.

Black holes are also studied from the outside, with instruments. tuput has written about the gravitational-wave detections of colliding black holes and about the first image of a black hole.

String field theory and the brane-antibrane paper

The third strand is string field theory. On 26 May 1998 Sen submitted a paper on a D-brane (an extended object in string theory) paired with an anti-D-brane. It shows that the pair carries a tachyonic mode, and that at the minimum of the tachyon’s potential the negative energy cancels the combined tension of the two branes, leaving zero energy density and restoring spacetime supersymmetry.

A review by Sen and Barton Zwiebach, submitted on 29 May 2024, states that consistent, gauge-invariant string field theories now exist describing all string theories. The authors list tachyon condensation and the resolution of ambiguities in the standard worldsheet calculation among its applications.

The career behind the medal

Sen was born in 1956, according to ThePrint. He studied at Presidency College in Kolkata and took his master’s degree at IIT Kanpur. He earned his PhD at Stony Brook University in the United States in 1982, worked at Fermilab and at the Stanford Linear Accelerator Center, and returned to India in 1988 to join the Tata Institute of Fundamental Research in Mumbai. He later moved to the Harish-Chandra Research Institute, then in Allahabad (now Prayagraj), which is the affiliation the Breakthrough Prize lists for 2012. Profiles date the move to 1995 and to 1997, so this article gives no year. Since then he has joined ICTS, where he holds the ICTS-Infosys Madhava Chair.

ICTS lists his earlier honours as the ICTP Prize (1989), the Shanti Swarup Bhatnagar Prize (1994), Fellowship of the Royal Society (1998), the Padma Shri (2001), the Infosys Prize in Mathematical Sciences (2009), the Breakthrough Prize (2012), the Padma Bhushan (2013) and the Dirac Medal (2014). It also announced on 24 April 2026 that he had been elected an International Honorary Member of the American Academy of Arts and Sciences.

Two earlier Indian physicists whose names are attached to results are covered in the Chandrasekhar limit and Satyendra Nath Bose and the boson.

ICTS director Rajesh Gopakumar said in the medal announcement that it had been wonderful having Sen at the institute and that he had set the tone for excellence there.