# LOOPS 05 - Physicists in search of the Holy Grail of physics

What happens when space and time are no longer valid?

For the first time, the internationally most important LOOPS conference will take place in Germany: LOOPS 05 from 10 to 14 October 2005 at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Potsdam.

One of the greatest challenges in theoretical physics is to reconcile general relativity (ART) and quantum theory. Only when the seemingly insurmountable gap between them can be bridged will we understand what "holds the world together at its core". To establish a quantum theory of ART - quantum gravity for short - is therefore called the "Holy Grail" of modern physics by many physicists. If it succeeds in developing it, it will revolutionize our understanding of nature.

A promising approach is the so-called Loop Quantum Gravity (LQG). Its name derives from the fact that in this theory space is made up of the smallest quanta, namely tiny, interlinked and intersecting loops that can pass and arise over time. According to the LQG, for example, the space taken up by a DIN A4 sheet of paper is made up of about 1000000000000000000000000000000000000000000000000000000000000 of these loops (a one with 68 zeros!). The LQG can be seen as a consistent further development of the ART.

## Background

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

But the general theory of relativity does not take quantum effects into account. 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 centre of black holes. "One of the greatest challenges facing theoretical physics therefore continues to be bringing quantum theory and general relativity together," says Prof. Dr. Hermann Nicolai, Director at the Max Planck Institute for Gravitational Physics.

## Promising candidates for this are LQG and string theory

Compared 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 much closer to Einstein's theory. - is itself quantized and is thus 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. This principle, known as background dependence, 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 QLL, however, are convinced that only a background dependent theory can make the decisive breakthrough.

A constantly growing number of researchers worldwide are working on QLG and alternative background independent approaches to quantum gravity. There are currently about 300, and about 150 of them, including all the leading minds, are coming to LOOPS 05 in Potsdam.

The Max Planck Institute for Gravitational Physics (Albert Einstein Institute) in Golm/Potsdam and the Perimeter Institute in Waterloo/Canada are the institutes worldwide where research in both QLG and string theory is prominent. The exchange between both directions is cultivated and found its expression among other things in the conference "Strings meets Loops", which caused quite a stir in 2003.

## The search for a comprehensive theory

For the scientific layman, physics seems to deal with seemingly completely different topics. Mechanics, thermodynamics, optics, electricity, magnetism, atomic physics and nuclear physics are examples. But already in 1873 the British physicist Maxwell was able to describe electrical, magnetic and optical phenomena of physics in a uniform way.

In the 21st century, with Einstein's theory of general relativity and the Standard Model of elementary particle physics, we now have physical theories that correctly describe natural processes over a huge range of distances - from the diameter of a proton to the diameter of the visible universe. And this is done by means of mathematical formulas that fit on a piece of paper!

However, the physicists' search for a comprehensive theory encounters a hard mathematical limit when trying to reconcile gravity or general relativity and quantum theory. There are various approaches to overcoming them, the best known of which are string theory and loop quantum gravity/loop quantum gravity.

The physicists' search for this theory also touches on metaphysics. The fascination that this holy grail of physics exerts not only on researchers is unbroken. With the enormous ranges of magnitude that physics spans, it reaches the limits of what the human mind can even grasp.