# Top German researchers honored with prestigious Xanthopoulos Award

Prof. Dr. Thomas Thiemann from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Potsdam-Golm and Dr. Martin Bojowald from the Institute for Gravitational Physics and Geometry at Pennsylvania State University in Philadelphia/USA have been awarded the internationally renowned “Basilis Xanthopoulos Prize 2007”. They were jointly honored for their groundbreaking and complementary contributions to the development of a background independent quantum gravity at the 18^{th} International Conference on General Relativity and Gravitation in Sydney, Australia.

## Thomas Thiemann

has developed significant and highly advanced work on the mathematical foundations and formulations in the field of loop quantum gravity. Thiemann discovered the “Thiemann Identities” named after him and the construction of coherent stages. Both approaches have the potential to significantly improve a consistent interpretation of Hamilton's principle and to combine loop quantum gravity with Einstein's equations.

Thomas Thiemann is a researcher at the Albert Einstein Institute in Potsdam-Golm in the department of Quantum Gravitation and Unifying Theories. He is one of the world's leading scientists in the field of Loop Quantum Gravity, a promising approach to unify General Relativity with Quantum Mechanics. Central to this approach is the concept of background independence (BU). BU is a fundamental property of Einstein's theory: Space-time is not a rigid structure on which matter propagates, but a dynamic geometry that curves in proportion to the existing matter. Matter and geometry interact. As a result, the space-time structure, which is smooth according to classical quantum theory, is now granular, quantized. The singularities of the classical kind are possibly avoided with far-reaching consequences for cosmology. Thiemann's results have given enormous impetus to the studies of background independent quantum gravity. He is therefore a worthy recipient of the prize.

## Martin Bojowald

has achieved completely new and profound progress in combining quantized gravity with classical equations and concrete physical phenomena. By showing how ideas of symmetry can be incorporated into the theory of loop quantum gravity, Bojowald opened a window to a new approach to quantum cosmology. His “effective equations”, which represent a semi-classical approach to loop quantum cosmology, have already led to astonishing results that contradict the existence of cosmic singularities and early inflation of the universe. Since 2006, Martin Bojowald, who was previously also at the Albert Einstein Institute in Potsdam, has been conducting research at the Institute for Gravitational Physics and Geometry at Penn State University in Philadelphia. Bojowald's work has set a great stone in motion in the new field of quantum cosmology. He too is a worthy recipient of the award.

## Der Basilis Xanthopoulos Preis

has been established as a memorial prize by the Foundation for Research and Technology - Greece (FORTH). Since it was first awarded in 1991, the prize has been awarded every three years during the General Relativity and Cosmology Conference. The prize winners are proposed and elected by a committee. They must be outstanding researchers in the field of gravitational physics and cosmology and must not be older than 40 years. The Xanthopoulos Prize is endowed with 10,000 US$.

Previous award winners include Eanna Flanagan, Gary Horowitz, Juan Maldecena, and Carlo Rovelli.

## Basilis Xanthopoulos

was a Greek scientist in the field of theoretical physics, known for his contributions to general relativity and his findings on colliding plane waves. Born in Drama, Xanthopoulos studied at the University of Thessaloniki, Greece and the University of Chicago, USA. There he also did research on colliding plane waves together with Nobel Prize winner Subrahmanyan Chandrasekhar, which have been named after them ever since. He returned to the University of Thessaloniki, later University of Crete in Iraklion, where he taught and researched as a professor until he lost his life in a tragic assassination attempt in 1990.

## Background

With his Special Theory of Relativity, Albert Einstein presented a theory of space and time in 1905 that did not fit with Newton's description of gravity. Only 10 years later he succeeded in including gravity by defining it as an effect of the curvature of space and time – the idea, valid until then, that space and time were flat, was finally discarded.

But general relativity does not consider quantum effects. These only become effective at unimaginably small distances, for example in strong gravitational fields such as those that existed shortly after the Big Bang, or at the center of black holes. Therefore, one of the greatest challenges in theoretical physics is still to bring quantum theory and general relativity together.

## Promising candidates for this are LQG and string theory.

In comparison to the better known string theory, LQG is much closer to Einstein's theories. For example, as described above, the geometry of space and time – i.e. lengths, areas, volumes, etc. – is itself quantized and is therefore subject to Heisenberg's uncertainty relation. In string theory, on the other hand, the geometry of space and time must be specified. This contradicts the basic principles of Einstein's equations, according to which geometry cannot be predetermined. Instead, it must be determined dynamically as a function of the existing matter. In short: Matter bends space; the curvature of space causes matter to accelerate its motion. This principle, known as background independence, is one of the essential foundations of LQG. In string theory, however, this principle is violated. Here, the principles that led to the unification of the three other forces of nature are more likely to be followed. The advocates of LQG, however, are convinced that only a theory that is independent of the background can achieve the decisive breakthrough.

A constantly growing number of researchers around the world are working on LQG. The founders of this theory are Abhay Ashtekar, Jurek Lewandowski, Carlo Rovelli, Lee Smolin and Thomas Thiemann.

The Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam-Golm is one of the institutes worldwide where both QLG and string theory are prominently researched. The exchange between both directions is cultivated and finds its expression in regular conferences.