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String theory has come under fire in recent years. Promises have been made that have not been lived up to. Leiden (The Netherlands) theoretical physicists have now for the first time used string theory to describe a physical phenomenon. Their discovery has been reported in Science Express.
'This is superb. I have never experienced such euphoria.' Jan Zaanen makes no attempt to hide his enthusiasm. Together with Mihailo Cubrovic and Koenraad Schalm, he has successfully managed to shed light on a previously unexplained natural phenomeon using the mathematics of string theory.
Electrons can form a special kind of state, a so-called quantum critical state, that plays a role in high-temperature super-conductivity. Super-conductivity at high temperatures has long been a 'hot issue' in physics. In super-conductivity, discovered by Heike Kamerlingh Onnes in Leiden, electrons can zoom through a material without meeting any resistance. In the first instance, this only seemed possible at very low temperatures close to absolute zero, but more and more examples are coming up where it also occurs at higher temperatures. So far, nobody has managed to explain high temperature super-conductivity. Zaanen: 'It has always been assumed that once you understand this quantum-critical state, you can also understand high temperature super-conductivity. But, although the experiments produced a lot of information, we hadn't the faintest idea of how to describe this phenomenon.' String theory now offers a solution.
This is the first time that a calculation based on string theory has been published in Science, even though the theory is widely known. 'There have always been a lot of expectations surrounding string theory,' Zaanen explains, having himself studied the theory to satisfy his own curiosity. 'String theory is often seen as a child of Einstein that aims to devise a revolutionary and comprehensive theory, a kind of 'theory of everything'. Ten years ago, researchers even said: 'Give us two weeks and we'll be able to tell you where the big bang came from. The problem of string theory was that, in spite of its excellent maths, it was never able to make a concrete link with the physical reality - the world around us.'
But now, Zaanen, together with his colleagues Cubrovic and Schalm, are trying to change this situation, by applying string theory to a phenomenon that physicists, including Zaanen, have for the past fifteen years been unable to explain: the quantum-critical state of electrons. This special state occurs in a material just before it becomes super-conductive at high temperature. Zaanen describes the quantum-critical state as a 'quantum soup', whereby the electrons form a collective independent of distances, where the electrons exhibit the same behaviour at small quantum mechanical scale or at macroscopic human scale.
whereby the electrons form a collective independent of distances, where the electrons exhibit the same behaviour at small quantum mechanical scale or at macroscopic human scale.