A contribution by Michael Leissner
(Field Theory Group, Institute of Physics, University of Oldenburg)

Symmetries in nature have always fascinated mankind. Initially admired primarily for their aesthetics, they were later used to simplify mathematical descriptions of nature. In the 19th century, however, there was a rethink in physics that elevated symmetries beyond the status of a purely mathematical tool. The requirement that laws of nature must be formulated in a highly symmetrical way is practically THE cornerstone of modern physics. Large areas of physics can be excellently described by assuming symmetrical laws of nature.

However, towards the middle of the 20th century, experimental findings from elementary particle physics suggested that nature apparently only approximately exhibited the symmetries that experts had previously been convinced of.

This year's Nobel Prize winner Yoichiro Nambu was instrumental in finding a way out of this dilemma. In 1960, he showed that field theories, such as those used to describe elementary particles, are also suitable for demonstrating "spontaneous symmetry breaking". In this concept, which has long been used in solid-state physics, scientists assume that the laws of nature themselves possess the required symmetries, but that nature chooses a state of lesser symmetry within this symmetrical set of rules.

An illustrative image is a roulette wheel whose high symmetry is "broken" when a ball thrown into it chooses one of the 37 subjects.

The fact that we exist and can think about the universe is based on this kind of symmetry breaking in nature: matter and antimatter are only almost perfect mirror images of each other.

The tiny deviation from perfect symmetry first observed in a particle accelerator in 1964 is most likely the reason why a surplus of matter over antimatter was created during the Big Bang. Equal amounts of matter and antimatter would have disintegrated into pure energy in a very short time, but the excess matter allowed our universe to come into being.

The two Nobel Prize winners Makoto Kobayashi and Toshihide Maskawa were responsible for harmonising the symmetry breaking responsible for this with the existing, experimentally well-established theories. At that time, in 1972, the existence of three types of quarks (components of elementary particles) was regarded as certain and the existence of a fourth as highly probable.

For their model, however, the two scientists needed two more types of quark, whose existence they therefore assumed. The fact that these quarks could actually be detected experimentally between 1974 and 1994 confirms the Kobayashi-Maskawa theory in an impressive way.