Describing the interactions on a more fundamental level the concepts of relativistic quantum field theories are employed. With the advent of quantum mechanics in the first decades of the 20th century it was realized that the electromagnetic field, including light, is quantized and can be seen as a stream of particles, the photons. This implies that the interaction between matter is mediated by the exchange of photons. In turn, matter particles are excitations of further quantum fields which pervade all of spacetime and which produce incessant vacuum fluctuations.

Light scatters light

An example of a pure quantum field theoretical phenomenon is the scattering of light by light, which is impossible in the classical theory of electromagnetism. Quantum fluctuations can turn photons fleetingly into electron-positron pairs which can be exchanged with those of other photons. Light-by-light scattering has only recently been observed directly (in ultrarelativistic heavy-ion collisions at CERN), but it also contributes indirectly to other quantum field theoretical phenomena, in particular to the anomalous magnetic moment of elementary particles. The latter can be measured to such high accuracy that it can be used to test the limits of the present Standard Model of particle physics, because even particles that are much too heavy to be produced in present-day particle colliders make their appearance in vacuum fluctuations.

Two Feynman-Diagrams

Light-by-light scattering (photons represented by wavy lines, charged particles by lines with arrows), and virtual light-by-light scattering in higher-order corrections to the anomalous magnetic moment of charged particles.


Recent research topics at our institute include hadronic light-by-light scattering where the photon couples to strongly interacting particles. The interactions of the latter are too complex to be captured by perturbation theory and Feynman diagrams, requiring new techniques such as gauge-gravity correspondence. The latter is also known as holography, because it is based on the description of a strongly interacting quantum field theory in flat spacetime as sort of a hologram in a higher-dimensional curved spacetime.

A theoretically clean and well studied example of gauge-gravity duality is the so-called AdS/CFT correspondence, where the higher dimensional spacetime is a maximally symmetric negatively curved (anti de Sitter) space and the quantum field theory in one dimension less is a conformal field theory (CFT). Conformal field theories also play important roles in string theory.



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