Leiden theoretical physicists have now, for the first time, used string theory to describe a physical phenomenon. Their discovery has been reported in Science Express, and is the first time that a calculation based on string theory has been published in Science,
'This is superb. I have never experienced such euphoria.' Jan Zaanen says enthusiastically. Together with Mihailo Cubrovic and Koenraad Schalm, he has successfully managed to shed light on a previously unexplained natural phenomenon 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, 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 adds that, '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 viable solution.
'There have always been a lot of expectations surrounding string theory,' Zaanen explains. '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'. String theory has been plagued for years because, in spite of its excellent mathematics, it was never able to make a concrete link with the physical reality - the world around us. In other words, no experiment could be devised in which to test string theory.
But now, Zaanen, together with his colleagues Cubrovic and Schalm, is applying string theory to a phenomenon called 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', where the electrons exhibit the same behavior at small quantum mechanical scale or at macroscopic human scale.
Zaanen and Schalm used the aspect of string theory known as AdS/CFT correspondence, simply put it is a bridge between string theory and quantum mechanics. This allows situations in a large relativistic world to be translated into a description at minuscule quantum physics level. This correspondence bridges the gap between these two different worlds. By applying the correspondence to the situation where a black hole vibrates when an electron falls into it, they arrived at the description of electrons that move in and out of a quantum-critical state.
'We hadn't expected it to work so well,' says a delighted Zaanen. 'The maths was a perfect fit; it was superb. When we saw the calculations, at first we could hardly believe it, but it was right.'
Although the mystery of high temperature super-conductivity is only partially resolved, the findings do show that major problems in physics may be addressed using string theory. Zaanen believes, 'AdS/CFT correspondence now explains things that colleagues who have been beavering away for ages were unable to resolve, in spite of their enormous efforts. There are a lot of things that can be done with it. We don't fully understand it yet, but I see it as a gateway to much more.'
Source: Leiden University