String theory pioneer Ashoke Sen wins Oskar Klein Medal

The Oskar Klein Medal is an honour for significant contributions to theoretical physics. Stockholm University and the Nobel Committee for Physics of the Royal Swedish Academy of Sciences awarded it to Ashoke Sen in 2026. The medal also includes an accompanying memorial lecture.

Sen delivered that lecture in Stockholm on October 22. Its title was “Gravity from Strings and Strings from Gravity.” The award recognises work that has shaped attempts to understand quantum gravity, especially through string theory, duality, and black-hole physics.

Sen is the first Indian laureate to receive the medal. His research helped connect equations for moving strings with equations for surrounding physical fields. He also proposed the Sen conjectures and developed methods for studying black holes’ microscopic states and entropy. These contributions have advanced theoretical tools for understanding quantum gravity, although string theory still lacks experimentally confirmed predictions using today’s technology.

The award is especially notable for India because Sen is the first Indian laureate to receive the Oskar Klein Medal. He is part of a strong Indian community of string theorists working at institutions such as TIFR Mumbai, ICTS Bengaluru, HRI Prayagraj, and IISER Pune.

It matters for theoretical physics because Sen’s research addresses one of its deepest problems: reconciling quantum physics with Einstein’s theory of relativity. His work linked string equations with field equations, established S-duality, introduced the Sen conjectures, and clarified black-hole microstates and entropy.

The recognition also reflects the broad influence of his ideas. Edward Witten called one of Sen’s papers “short but brilliant” and said it convinced him that a duality had to be right. Sen’s work has inspired further progress in understanding subatomic particles and quantum gravity, even though string theory remains experimentally unconfirmed.

Sen became a leading figure by solving important conceptual problems in string theory. He explored how string theory might describe matter and fundamental forces within one framework, while also investigating its links with quantum gravity. His work reached beyond string theory into black-hole physics.

In the 1980s, Sen helped connect equations for hypothetical strings moving through space with equations describing surrounding physical fields. In the 1990s, he showed that two apparently different descriptions of physical forces could be equivalent, an idea called S-duality. In 1999, he proposed three related ideas known as the Sen conjectures.

Sen also developed mathematical methods for understanding black holes’ microscopic states. These methods helped account for their observable, macroscopic characteristics, including entropy. His contributions earned major honours, including the Breakthrough Prize, the Dirac Medal, and the Padma Bhushan, and influenced later work on particle interactions and quantum gravity.

In standard physics, four fundamental forces are recognised: gravity, electromagnetism, the strong force, and the weak force. String theory aims to describe matter and these fundamental forces within one framework, called quantum gravity. The article specifically highlights gravity as part of this unifying goal.

Gravity is the hardest force to reconcile with quantum physics because Einstein’s theory of relativity describes gravity and spacetime differently from quantum theory’s description of microscopic phenomena. String theory replaces point-like particles with hypothetical strings. The equations governing those strings can be related to equations describing physical fields.

This approach offers a possible bridge between the two frameworks. It has produced sophisticated mathematics and important insights, including dualities and black-hole methods. However, the article notes that string theory has not yet produced predictions confirmed by experiments using today’s technologies. Its unifying promise therefore remains a theoretical possibility rather than an experimentally established theory.

S-duality is the idea that two descriptions which look physically different can nevertheless be equivalent. In other words, they may use different mathematical variables or perspectives while describing the same physical forces and outcomes. This matters because one description can make a difficult problem easier to understand.

During the 1990s, Sen helped show that two apparently different ways of describing physical forces were really equivalent. The article identifies this idea as S-duality. His work helped physicists recognise deeper relationships within string theory rather than treating separate formulations as unrelated theories.

S-duality strengthened string theory’s role as a framework for exploring quantum gravity. Edward Witten said one of Sen’s papers convinced him that the duality had to be right. Such equivalences have also inspired work leading to further breakthroughs in understanding how subatomic particles interact, although string theory remains without experimentally confirmed predictions using current technology.

Black holes were known to possess entropy, but physicists lacked a convincing explanation for its origin within their physical theories. Entropy describes information about microscopic possibilities behind a system’s larger, observable behaviour. Understanding it is important because it links quantum descriptions to the macroscopic features of black holes.

Sen developed mathematical methods for studying a black hole’s microscopic states. These states represent the hidden configurations that can produce the same broad black-hole characteristics. His methods connected those microscopic possibilities with observable, macroscopic properties, including entropy.

This work gave physicists a more coherent way to investigate black-hole structure within string theory and quantum gravity. It did not make string theory experimentally confirmed, but it advanced the mathematical framework used to study quantum gravity. Alongside his work on dualities and conjectures, Sen’s black-hole research helped move the field closer to explaining how quantum information could underlie cosmic objects.

Quantum gravity is difficult because two successful frameworks currently remain separate. Quantum physics describes microscopic matter and interactions, while Einstein’s theory of relativity describes gravity. A complete theory would need to make these descriptions work together without losing their established insights.

String theory attempts this by treating fundamental objects as hypothetical strings rather than point-like particles. It seeks equations that describe matter and nature’s fundamental forces, including gravity, within one framework called quantum gravity. Sen helped connect equations for strings moving through space with equations describing the physical fields around them.

The approach has generated advanced mathematics, including dualities, conjectures, and methods for studying black-hole microstates. These results have considerably advanced theories of quantum gravity. Yet the article notes a major limitation: string theory has not made predictions confirmed by experiments using today’s technologies. Its connection between quantum physics and relativity therefore remains unfinished.