Ashoke Sen helped show that five string theories are one.
In October 2026 the theoretical physicist Ashoke Sen became the first Indian to receive the Oskar Klein Medal in Stockholm. It is the latest honour for four decades of work that changed how physicists see string theory, almost all of it done in India.
This is that work, cut from paper: a 1994 duality, branes that decay, black holes whose entropy can be counted, and the string field theory he is still writing at 70.
Four decades, eight lines of work.
Sen began in the physics of quarks, moved to string theory in the mid-1980s and has not left it. Each strip is one theme, spanning the years his papers on it appeared: S-duality in 1994, unstable branes from 1998, black-hole entropy across two decades, and string field theory since 2014.
What Sen worked on, and when
Spans are vizmaya's reading of his publication record, so their ends are soft. Notches mark key papers: 1994 dyons, 2002 rolling tachyon, 2005 entropy function, 2015 with Witten, 2021 D-instantons. Sources: Sen's research summary; arXiv.
A physics teacher's son from Kolkata, trained on quarks in the US.
Sen was born in Kolkata in July 1956, the elder son of Anil Kumar Sen, who taught physics at Scottish Church College. He took his BSc at Presidency College in 1975 and his MSc at IIT Kanpur in 1978.
He did his PhD at Stony Brook under George Sterman, finishing in 1982 with work on quantum chromodynamics, the theory of quarks and gluons. Postdocs followed at Fermilab from 1982 and at SLAC from 1985. There he turned to string theory: his 1985 paper on the heterotic string in an arbitrary background field has about 540 citations.
Some exam-prep sites name Barton Zwiebach as his PhD adviser. He was not: Sen names George Sterman, and Zwiebach was a later collaborator.
In 1994 he showed that a hard theory can be an easy one in disguise.
In some theories, particles interact so strongly that the usual approximations break down. S-duality says such a theory can be exactly equivalent to another one in which electric and magnetic charges swap roles and the interaction is weak, so a hard problem becomes an easy one.
Claus Montonen and David Olive had proposed the idea for field theory in the 1970s, and many physicists doubted it. In 1994 Sen gathered the evidence in string theory and predicted bound states of magnetic monopoles and dyons that the duality required. Then he showed that they exist.
The work helped set off what the field calls the second superstring revolution. That led to Edward Witten's 1995 proposal that the five string theories are limits of a single theory.
Sen has said these checks made the community take the duality more seriously. His 2012 Fundamental Physics Prize citation credits him with opening the path to the idea that all string theories are limits of one underlying theory.
One theory, two descriptions
Schematic. g is the coupling strength. Papers: hep-th/9402002, hep-th/9402032.
He counted a black hole's hidden states two ways, and the answers matched.
A black hole has entropy, a measure of how many internal states it could be in, and gravity fixes it from the area of the horizon. If string theory is right, counting the string states with the same mass and charges should give the same number.
In 1995 Sen showed that both answers grow with mass, charge and coupling in the same way, up to a factor he could not compute. Andrew Strominger and Cumrun Vafa soon found exact agreement in other systems, and Atish Dabholkar later fixed Sen's missing factor.
In 2005 his entropy function gave one way to compute the entropy of extremal black holes, even with stringy corrections. From 2011 he and his collaborators computed quantum, logarithmic corrections, which again agree with the microscopic count.
With Justin David and Dileep Jatkar he also showed that the exact counts of certain charged states are generated by a special mathematical function, confirming an earlier conjecture by Dijkgraaf, Verlinde and Verlinde.
Two ways to count, one answer
Schematic. Dark slips: Sen's papers. Papers: hep-th/9504147, hep-th/0506177, 1106.0080.
His most-cited paper, about a brane rolling away, has 1,622 citations.
D-branes are surfaces on which open strings end. Some are unstable: they carry a tachyon, a field perched at the top of a hill. From 1998 Sen conjectured that when the tachyon rolls down, the energy released exactly cancels the brane's tension and leaves empty space, and that kinks in the field are smaller branes. Physicists now call these Sen's conjectures.
In 2002, in “Rolling tachyon” and “Tachyon matter”, he followed the decay in real time. On Google Scholar they are his two most-cited papers, with 1,622 and 1,382 citations, ahead of the 1998 brane–antibrane paper with 920.
Citation counts differ by database and change daily. These are Google Scholar's merged counts as captured for this story. The 1994 S-duality papers were not in that snapshot, so they are missing here.
Sen's most-cited papers, Google Scholar citations
Saffron: papers on tachyons and unstable branes. Source: Google Scholar profile, counts as captured in October 2026.
A teacher in Kolkata and Prayagraj
Sen still talks to undergraduates, including at Scottish Church College in Kolkata, where his father taught. For 26 years he worked at the Harish-Chandra Research Institute in Prayagraj, alongside his wife, the condensed-matter theorist Sumathi Rao, whom he met at Stony Brook.
In his late fifties he went back to fix string theory's foundations.
String theory is usually calculated as a series of approximations, and for superstrings that series had a long-standing technical gap. In 2014 Sen closed it with a method called vertical integration, which he and Edward Witten set out systematically in 2015.
He then built a field theory covering every sector of superstring theory and used it to prove unitarity and crossing symmetry: in plain terms, that probabilities add up and the theory behaves as a sensible quantum theory should.
The same tools reach further. With Alok Laddha and Biswajit Sahoo he derived classical soft graviton theorems, which describe the faint gravitational waves from a collision. In 2021 he computed D-instanton effects whose size S-duality had predicted, and his result agreed. He is still publishing in 2026.
His 2024 review with Barton Zwiebach, his collaborator on tachyons a quarter-century earlier, surveys the whole of string field theory.
The last decade, paper by paper
- 2015Filling the gaps in superstring perturbation theory, with Witten.
1504.00609 - 2018Gravity waves from the soft theorem, in any dimension, with Laddha.
1801.07719 - 2019Logarithmic terms in the soft theorem in four dimensions, with Sahoo.
1808.03288 - 2021Normalising D-instanton amplitudes; the result agrees with S-duality.
2104.11109 - 2024String field theory: a review, with Zwiebach.
2405.19421 - 2026Still publishing, with students and collaborators at ICTS.
2610.03708
Years are journal or arXiv years. Source: arXiv; Sen's research summary.
He came home in 1988 and did nearly all of this in India.
In 1988 he joined the Tata Institute of Fundamental Research in Mumbai. From 1995 to 2021 he was at the Harish-Chandra Research Institute in Prayagraj, then called the Mehta Research Institute, and helped make it a centre for string theory. The S-duality, tachyon and black-hole papers were all written from these two institutes. Since November 2021 he has held the Infosys Madhava Chair at ICTS-TIFR in Bengaluru.
In 2012 he was the only scientist from a developing country among the nine winners of the first Fundamental Physics Prize. In 2014 he became the first scientist working in India to win the ICTP's Dirac Medal; Deepak Dhar became the second in 2026.
In 2012 Sen said he wanted to use the prize money to fund children's education. Later articles say he gave much of it away; we found no primary record of how much.
The recognition followed the work, from age 33 to 70.
He won the ICTP Prize in 1989, at 33, and the Shanti Swarup Bhatnagar Prize at 38. At 42 he was elected a Fellow of the Royal Society. India gave him the Padma Shri in 2001 and the Padma Bhushan in 2013.
In 2012 he won the first Fundamental Physics Prize, worth US$3 million. The Dirac Medal followed in 2014, shared with Andrew Strominger and Gabriele Veneziano. In October 2026, at 70, he gave the Oskar Klein Memorial Lecture in Stockholm, “Gravity from strings and strings from gravity”, and received its medal, the first Indian to do so, according to ICTS.
Ages are the age he reached in the award year. Sources disagree on when he became an INSA fellow (1995 or 1996). We could not verify a reported US National Academy membership, so it is left out. The 2024 Breakthrough Prize, sometimes credited to him, went to John Cardy and Alexander Zamolodchikov.
Sen's age at each major honour
Sources: Sen's CV (ICTS); ICTP; ICTS.
A career in six slips.
papers on his main Semantic Scholar profile, and counting.
Semantic Scholar, 1 Nov 2026citations to them; h-index 95.
Semantic Scholar, 1 Nov 2026citations for “Rolling tachyon” (2002), his most-cited paper.
Google Scholaryears at the Harish-Chandra Research Institute, 1995–2021.
Sen's HRI pagehis 2012 Fundamental Physics Prize: about 2.7 times a full Nobel that year.
IAS; NobelPrize.org; ChemistryViewshonorary doctorates, from Calcutta University to IIT Bombay.
Sen's CVCitation totals depend on the database: Scopus counts 22,754 citations and an h-index of 77. In 2018 a study of INSPIRE, the high-energy physics database, ranked him 27th of all its authors by co-author-adjusted citations. More science on vizmaya: how a pond alga gave us a light switch for the brain, the evidence that an asteroid killed the dinosaurs, and every vizmaya story.
“This is certainly the biggest award I have got.”