The Swedish theoretical physicist Oskar Klein is known for two important ideas, both of which appeared in the same 1926 paper. One was the Klein-Gordon equation, the earliest wave equation consistent with the laws of special relativity, which describes particles with no intrinsic angular momentum (spin). However, all the elementary particles discovered over the next eight decades were found to have a non-zero spin, so this equation appeared to be a theoretical abstraction unconnected to nature. Yet it was studied as a “toy model” in diverse contexts, and invariably taught to students of relativistic quantum theory because of its mathematical simplicity. Then, in 2012, a spinless elementary particle — the Higgs boson — was discovered, and everything fell into place: The Klein-Gordon equation is the correct framework for this particle.This year, the centenary of Klein’s paper has seen one of the world’s leading exponents of string theory — Professor Ashoke Sen — receiving the prestigious Oskar Klein Medal. He is the first Indian to feature among a stellar list of awardees that includes C N Yang, Hans Bethe and Stephen Hawking. Sen, who turned 70 this year, has had an illustrious career spanning over four decades during which he has made several influential contributions to theoretical particle physics. He also happens to be a close friend, colleague and collaborator who I have known since the late 1970s when we were both PhD students at Stony Brook University.AdvertisementTo appreciate the significance of Sen’s research, it is useful to keep in mind the Klein-Gordon story. What started as an elegant construction consistent with the known symmetries of Physics found direct application to elementary particles almost a century later, but it did not languish in the interim — it was a rich source of theoretical possibilities, phenomena and effects, and was studied in thousands of papers. String theory is an inheritor of this tradition. The foundational idea was to formulate a relativistic quantum theory where the basic objects are extended strings rather than point particles. Somewhat surprisingly, this postulate led to a more tightly constrained formalism than standard quantum theory. String theory emerged as a consistent framework for a quantum theory of gravity coupled to interactions of electromagnetic and nuclear type, something that had resisted conventional approaches for decades.Also Read | In season of protests, on the costs of speaking out, consequences of silenceSince its discovery, string theory has attracted intense theoretical interest, even as direct experimental application has remained out of reach. It remains to be seen whether and when it will be experimentally verified, but like the Klein-Gordon equation, it has featured in thousands of theoretical papers due to the rich possibilities and phenomena that it incorporates.Early in his career, Sen asked himself the question: “Is there a consistency condition for relativistic strings propagating in a generic curved spacetime?” He came up with a simple but profound result — the condition is precisely the original Einstein equation, supplemented by calculable corrections. Many decades later, this discovery still looks truly remarkable.AdvertisementWhile he was already recognised internationally as an outstanding young theoretical physicist, it was in 1994 that Sen’s work on electric-magnetic duality propelled him to the next level of fame. I recall Ashoke coming up to me in the corridor of the Tata Institute, looking unusually excited, and saying: “I’ve found a self-dual harmonic two-form on monopole moduli space.” In that moment, I understood each of the words separately, but not their impact. This mathematical result, on which he had laboured for months, provided striking evidence for electric-magnetic duality — a symmetry under which the roles of electricity and magnetism are exchanged. Many physicists had speculated about it within familiar quantum theories, but it had proved difficult to find a concrete example. Sen found substantial evidence for a specific example, and it lay within string theory. At a conference that I attended in Paris in 1994, American physicist Edward Witten described Sen’s result as “the last straw that broke the camel’s back” — the result that finally convinced him about duality.The outcome of his discovery has been a unified picture of string theory as an interconnected web of possibilities for physical gravitating systems in space-time dimensions ranging from 2 to 11. While low-dimensional quantum systems are familiar, dimensions higher than our own have remained the stuff of science fiction. Interestingly, though, among the first to speculate on their existence was none other than Klein — and this is the second idea referred to at the beginning of this article. Kaluza-Klein theory, as it is known, is the beautiful observation that the equations of gravity in a hypothetical five-dimensional spacetime reduce in a suitable limit to the familiar equations of both gravity and electromagnetism in our world. Thus, two basic forces of nature can be explained in terms of a single one operating in a higher dimension. String theory realises this idea in a more precise form than its inventors could have envisioned, and Sen’s work paved the way to understand how apparently different string theories are linked by a complex web of dualities, suggesting that the formalism is not just useful but also unique.you may likeSen’s more than 300 published papers are marked by their clarity, simplicity, originality, and austere discipline. He typically starts with a small, deceptively simple premise and applies his penetrating intellect to convert it into a concrete result. Thanks to his work, today we understand more deeply the paradoxes regarding the entropy and information of black holes. He has repeatedly demonstrated that embedding outstanding physical problems into string theory gives us a better chance of understanding them, a testament to the range of possibilities inherent in this framework.The history of Physics is replete with theoretical ideas that were confirmed long after being proposed. Yet, string theory has faced some negativity among the broader community of Indian physicists, stemming perhaps from a lack of comprehension of how frontier physics works. Fortunately, Sen worked at the Tata Institute of Fundamental Research, the Harish-Chandra Research Institute and the International Centre for Theoretical Sciences, all autonomous institutes of the Department of Atomic Energy (DAE), in an atmosphere that was largely supportive of this sort of theoretical research. Indeed, the founder of the DAE, Homi J Bhabha, would likely have recognised it as a logical continuation of his own research. He — and the leaders who succeeded him — created an environment with the freedom to think and an enabling and constructive ecosystem. Ashoke Sen’s achievements are a powerful vindication of that vision.The writer, a theoretical physicist, has worked at the Tata Institute of Fundamental Research, Mumbai and IISER, Pune