What were the first five string theories

String theory

Do we live in a universe made of particles or of strings? And how many dimensions does this universe actually have? String theory offers an alternative description of our world - how the strong force between the quarks can be described with its help is being investigated by the physicists, for example in the theory group at the DESY research center.

Quarks from strings?

The strong force - which is particularly important for understanding the proton collisions at the LHC - works as if the individual quarks were connected to one another by elastic threads. In fact, the theorists tried very early on to determine the properties of hadrons, i.e. particles made up of quarks and gluons, through the vibration behavior of one-dimensional threads or strings. strings) to explain. The first string theories emerged from such attempts around 1970; however, its application to the strong force was not very successful at first.

It took almost thirty years of intensive research before string theorists made a long-awaited breakthrough in describing hadron physics. According to string theory, the basic building blocks of nature are not in the form of point-like particles, but rather behave like one-dimensional strings. In order for the whole thing to be mathematically consistent, the universe of string theory must expand into ten spacetime dimensions. However, some of these dimensions can be "rolled up" in such a way that we do not perceive them directly as actually existing spatial directions. The possibility of building a universe from such elementary strings has attracted a lot of attention, mainly because string theory resolves long-known theoretical incompatibilities between quantum physics and Albert Einstein's general relativity theory of gravity.

A question of perspective

A question of projection

A world of strings would be indistinguishable from our four-dimensional universe - at least if you don't look too closely. In particular, strings can also be used to create objects that are similar to black holes. Because black holes are excellent for studying the quantum aspects of gravity, they were intensively studied in the 1990s. As part of these studies, the string theorists made a remarkable discovery: They found that quantum chromodynamics, i.e. the theory of quarks and gluons in three-dimensional space, by no means provides the only possible description of hadronic physics. In fact, the researchers discovered completely new models with which hadron physics can be described as a string theory in a five-dimensional space-time. (Five spatial directions of the nine-dimensional string universe must therefore be rolled up.) This may seem strange at first glance. Because it follows that we cannot fundamentally differentiate whether our real world is four- or five-dimensional - everything depends on whether we regard it as a world made of particles or strings.

As surprising as the existence of two completely different descriptions of the same reality may seem at first, the phenomenon is not entirely unusual. Something similar happens, for example, with a photograph, which can be stored either via the chemistry of a conventional film or as a sequence of bits - i.e. zeros and ones - in modern digital cameras. Although the underlying image is the same, how it looks in the camera could hardly be more different. Of course, the two representations can be converted into one another with the help of the appropriate technology.

String theory on the rise

Particles and strings

Each of the two well-known descriptions of hadron physics - quantum chromodynamics or some of its supersymmetric relatives as well as string theory - has its own advantages. Quantum chromodynamics offers sophisticated tools for the study of hadronic systems that are manageable as long as the distance between the quarks remains small. The intuitive image of the strong interaction as spring force suggests that the methods of string theory can be used to make predictions, especially in the area of ​​the otherwise difficult to access large inter-quark distances, again only with paper and pencil. The intermediate range of average quark distances is currently reserved primarily for the supercomputers of the lattice theorists. However, there are now some spectacular examples in which string theory calculations could be carried out for any distance between the quarks, without the support of high-performance computers.

Recent experiments on heavy ion accelerators indicate that string theory does indeed describe large distances between the quarks very efficiently. When two heavy ions collide, the numerous quarks and gluons in their nuclei form a drop of “quark-gluon soup”, which then evaporates in the form of a large number of hadrons. However, before it evaporates, the drop behaves almost like a liquid. Their viscosity, i.e. their toughness, can be measured experimentally. It is now clear that the quark intervals in such droplets are too great to be able to reliably calculate the viscosity with the usual approximation methods of quantum chromodynamics. On the other hand, the predictions of the string theory models agree relatively well with the experimental measured values.

At present it is still very difficult to calculate the properties of hadrons using string theory. While the computational techniques in quantum chromodynamics have been intensively developed over decades, the corresponding development of string theory still requires a lot of work. The DESY string theory group is dedicated to this exciting task in collaboration with numerous partners around the world.